LV Myocyte Research Articles

Targeting liver angiotensinogen using a GalNAc-siRNA improves cardiac remodeling in spontaneously hypertensive rats

Abstract Background Dysregulation of the Renin-Angiotensin-Aldosterone System (RAAS) leads to hypertension and cardiac hypertrophy. Angiotensinogen (AGT) is the only precursor of all angiotensin peptide products. Liver-specific AGT silencing has been proven to be a promising treatment option in lowering blood pressure (BP) in patients, characterized by full RAAS inhibition and little RAAS escape phenomena. However, the long-term effects of AGT knockdown on cardiac remodeling is still unclear. Purpose To explore the long-term effects of a GalNAc-conjugated siRNA drug R0797070 on AGT inhibition, BP and cardiac functions in Spontaneously Hypertensive Rats (SHRs). Methods In vitro inhibitory activities of siRNAs were assessed in primary rat hepatocytes. The selected siRNA was conjugated with GalNAc based on RIBO-GALSTAR liver targeting system and named R0797070. In vivo effects of repeated doses of R0797070 were evaluated in SHR model. The animals were treated with R0797070 (3 mg/kg, every 8 weeks; subcutaneous injection) or PBS (5 ml/kg, every 8 weeks; subcutaneous injection) for 7 months with the first dose (day1) at the 3 months of age. Captopril (100 mg/kg per day; oral administration) was taken as a positive drug control and Wistar rats were used as normotensive control group. Blood pressure was monitored by tail-cuff method after an initial acclimatization training period. Serum AGT protein was measured by ELISA and liver mRNA expression was detected by qPCR. Echocardiography was used to determine the changes of cardiac morphology and functions，and wheat germ agglutinin (WGA) staining was used to measure myocyte cross-sectional size. Results R0797070 showed 90-99% inhibitory activities on hepatic AGT mRNA at different doses. In SHRs, a single dose of R0797070 at 3 mg/kg resulted in a maximum inhibition of 97% for serum AGT protein with 27 mmHg mean blood pressure (MBP) reduction on Day 15. Daily administration of Captopril could reduce MBP by 33 mmHg on average, while the duration of drug efficacy of R0797070 could reach 2 months with the average 25 mmHg MBP reduction. At the end of study, the echocardiographic results showed that R0797070 significantly reduced left ventricular hypertrophy in the 10-month-old SHRs. Consistently, R0797070 treatment reduced the heart weight by 10%，and LV myocyte size by 17%. Conclusion We showed the long-term effects of a GalNAc-siRNA R0797070 on silencing angiotensinogen expression, reducing blood pressure and left ventricular hypertrophy. Compared to daily oral administration of Captopril and its effect on blood pressure R0797070 demonstrated significant effect on cardiac hypertrophy.Figure 1Figure 2

Abstract 9578: Chronic Phosphodiesterase Type 5 Inhibition by Sildenafil Causes Regression of Heart Failure: Insights into Molecular and Cellular Mechanisms

Background: Sildenafil (SIL), a selective inhibitor of PDE5 has been shown to exert profound beneficial effects in heart failure (HF). However, whether and by what mechanisms SIL can stop the progression of HF, or maybe even reverse the clinical route remain unknown. We tested the hypothesis that chronic SIL would improve intrinsic myocyte function and [Ca 2+ ] i regulation and reverse HF-induced detrimental alterations on cardiac SERCA2a, β-adrenergic receptors (AR) and nitric oxide synthase (NOS) isoforms, thus playing a salutary role in HF. Methods: We evaluated LV and myocyte function and the protein levels of myocyte β 1 - and β 3 - AR, SR Ca 2+ -ATPase (SERCA2a), phospholamban (PLB), and 3 NOS in 3 groups of male rats (8/group) for a period of 3 months (M): 1) HF , 3 M after receiving isoproterenol (ISO) (170 mg/kg sq for 2 days); 2) HF/SIL , 2 M after ISO, then SIL (70 μg/kg/day sq via mini pump) was initiated and given for 1 M; and 3) Controls . Results: Versus C, ISO-treated rats progressed to severe HF at 3 M with significantly reduced LV contractility (E ES , 0.7 vs 1.1 mmHg/μl), and slowed LV relaxation. LV myocyte contraction (dL/dt max , 77 vs 136 μm/sec), relaxation (dR/dt max , 62 vs 104 μm/sec) and [Ca 2+ ] iT (0.15 vs 0.24) were significantly decreased accompanied by a diminished myocyte inotropic response to ISO (10 -8 M) superfusion. These abnormalities were associated with concomitant significant decreases in myocyte protein levels of β 1 -AR (0.23 vs 0.64), SERCA2a (0.46 vs 0.80), PLB Ser16 /PLB ratio, and eNOS (0.28 vs 0.46), but increased protein levels of β 3 -AR (0.23 vs 0.10) and iNOS (0.18 vs 0.08). Chronic SIL treatment prevented HF-caused reduced E ES (1.3 mmHg/μl) and EF (62%) and retained normal dL/dt max (131μm/s), dR/dt max (106 μm/s), and [Ca 2+ ] iT (0.24), and restored normal myocyte contractile response to ISO stimulation. With SIL, protein levels of myocyte β 1 - and β 3 -AR, SERCA2a were normalized to control values, but eNOS was significantly elevated (0.77). Conclusions: Chronic SIL prevents HF-caused downregulation of cardiac β 1 -AR and reverses contrast changes between iNOS and β 3 -AR with SERCA2a and eNOS expression, restores normal myocyte function, [Ca 2+ ] iT , and β-adrenergic reserve, and leads to regression of LV dysfunction in a rat model of progressive HF.

Abstract 9032: Restoration of Normal Cardiomyocyte Basal and β-Adrenergic Receptor (AR) Subtype Modulation in Diabetic Cardiomyopathy by Chronic Selective Beta3-AR Antagonist in a Mouse Model of Type 2 Diabetes

Background: We have shown that L-748,337 (L-7), a selective β 3 -adrenergic receptor (AR) antagonist reversed LV dysfunction and β-AR desensitization in diabetes (DM) caused diabetic cardiomyopathy (DCM). The mechanism is unclear. Recent evidence suggests that normal myocardial performance depends on the balance in myocyte β 3 -, β 1 -, and β 2 -AR. Pivotal restructuring of β-AR system resulting in the decline of β-AR reserve plays a crucial role in DCM. We assessed the hypothesis that chronic L-7 would prevent DCM-induced abnormalities of β-AR subtype-stimulated regulation on intrinsic LV myocyte function and [Ca 2+ ] i regulation, thus restoring cardiac function. Methods: Studies were conducted in 3 groups of female mice: Type 2 DM (T2) (n=9), induced with high-fat diet (HFD) intake for 14 weeks (W), but after fed HFD for 4 W receiving streptozotocin (STZ, 40 mg/kg/day, i.p. for 5 days); T2/L-7 (n=7), T2 mice at 10 W received L-7 (10 -7 M/kg/day, mini-pump) for 4 W; and Controls (C)(n=9). We compared LV myocyte contractile and [Ca 2+ ] iT responses to β-AR subtype stimulation by random exposure of myocytes to Isoproterenol (ISO,10 -8 M) or a selective β 1 -, β 2 -, or β 3 -agonist, Norepinephrine (NE, 10 -7 M), Zinterol (ZIN, 10 -5 M) and BRL-37,344 (BRL, 10 -8 M), respectively, during drug perfusion. Results: T2 and T2/L-7 mice had mean blood glucose levels of 388 mg/dl and 369 mg/dl, respectively. Versus C, only in T2, basal LV myocyte contractility (dL/dt max , 76 vs 133 μm/s), relaxation and [Ca 2+ ] iT (0.16 vs 0.21) were significantly decreased. ISO-stimulated dL/dt max (40% vs 58%) was attenuated accompanied by a diminished NE-mediated increase in dL/dt max (28% vs 44%), but enhanced BRL-induced decreases in dL/dt max (29% vs 12%).The response of dL/dt max (24% vs 15%) to ZIN was increased. Importantly, in T2/L-7 myocytes, the basal dL/dt max (128 μm/s) and [Ca 2+ ] iT (0.21) remained close to control values with preserved β-stimulated positive modulation on cell contraction. The increases in dL/dt max in response to ISO (57%), NE (40%) or ZIN (15%) were similar to that in control myocytes, respectively. Conclusions: Chronic β 3 -AR antagonist reverses β-AR signaling defects, resensitizing the β-AR subtype system modulation on LV myocytes function, thus playing a salutary role in DCM.

Abstract 9051: Gender Differences and Aging on Beta-Adrenergic Receptor Subtypes Expression and Function in Murine Left Ventricular Myocytes

Background: Sex-specific modulation of β-adrenergic receptor (AR) stimulation plays a pivotal role in cardiac function and in the manifestation of heart disease, including aging. However, the sex and age influences on 3 cardiac β-AR subtypes have not been systematically evaluated. Their effects on β 1 -AR have yielded mixed results. Virtually no previous studies have examined sex and age-related changes on the functional cardiac β 3 -AR. We hypothesized that sex differences exist in aging-induced changes in myocyte β 1 - and β 3 -AR. Aged males may exhibit a greater selective β 1 -AR downregulation with a more pronounced decline of β-AR reserve. Methods: LV myocytes were obtained from young (Y) (6 mo) and aged (A) (24 mo) (6/group) male (M) and female (F) wild-type mice. We simultaneously compared LV myocyte protein levels of β 1 -, β 2 -, and β 3 -AR, and the responses of cell contractile function (dL/dt max ) to β-AR subtype stimulation by random exposure of myocytes to isoproterenol (ISO) (10 -8 M) or a selective β 1 -, β 2 -, or β 3 -agonist, Norepinephrine (NE, 10 -7 M), Zinterol (ZIN,10 -5 M) and BRL-37,344 (BRL,10 -8 M), respectively. Results: In Y mice, no sex differences were found in myocyte protein levels of β 1 - (YM: 0.73 vs YF: 0.74), β 2 - (0.28 vs 0.29) and β 3 -AR (0.19 vs 0.17). Consistently, there were similarly significant increases in dL/dt max in responses to ISO (YM: 73%, 209.5 vs121.1; YF: 72%, 210.7 vs122.5 μm/s), NE (48% vs 45%), ZIN (18% vs 20%) and decrease after BRL (8.1% vs 7.9%). Compared with sex-matched Y, in both AM and AF, aging caused a ~58% significant increase in β 3 (AM: 0.30 vs AF: 0.27) and slightly reduced β 2 (11% vs 6%) (P=NS). Aging caused ~30% decline in basal cell contractility. No sex differences in dL/dt max of aging myocyte response to ZIN (increased AM: 17% vs AF: 20%), and BRL (decreased 19% vs 21%). However, in AM, β 1 decreased 33% (0.48) which was significantly greater than that in AF (26%, 0.55). Versus AF, in AM, ISO caused significantly less increase in dL/dt max (34% vs 48%) accompanied by attenuated NE-mediated increase in dL/dt max (17% vs 26%). Conclusions: Aging is associated with LV myocyte β 1 -AR downregulation and β 3 -AR upregulation. Sex affects age-related β 1 -AR downregulation, with males exhibiting a more profound decrease with increasing age.

Abstract 9026: Endogenous Ca2+/Calmodulin-Dependent Protein Kinase II Overactivation Exacerbates Cardiac Dysfunction and Beta-Adrenergic Desensitization in a Mouse Model With Type 2 Diabetes

Background: Emergent evidence indicates that CaMKII overactivity in diabetes (DM) plays a crucial role in the pathophysiology of diabetic cardiomyopathy (DCM) by targeting a diverse array of proteins involved in cardiac electrical, structural and functional remodeling. CaMKII has been proposed to be a therapeutic target for DCM. However, although chronic inhibition of CaMKII has been associated with cardioprotection in several cardiovascular diseases, the direct cardiac effects of CaMKII activation in type 2 (T2) DM are unclear. Moreover, whether and how DCM alters cardiac responses to CaMKII are undefined. We tested the hypothesis that in DCM, CaMKII is upregulated, which alters cardiac contractile behavior. Endogenous CaMKII activation may produce direct depressions in LV myocyte basal function and β-adrenergic reserve. Acute inactivation of CaMKII with KN93 would have a beneficial impact in DCM. Methods: LV myocyte CaMKIIδ expression and activity and myocyte functional responses were compared in 2 groups (n=9/group) of wild-type female mice: Vehicle Control (C) , animals fed chow for 14 weeks (W); T2DM, i nduced with a high-fat diet (HFD) intake for 14 W, but after fed HFD for 4 W receiving streptozotocin (STZ, 40 mg/kg/day, i.p. for 5 days). In T2DM, we further assessed myocyte contractile and [Ca 2+ ] i transient ([Ca 2+ ] iT ) responses with and without acute treatment of myocytes with KN93 (10 -6 M, 30 min). Results: Versus C, in T2DM myocytes, the CaMKIIδ protein levels (28%, 0.51 vs 0.40) and CaMKIIδ phosphorylation (79%, 0.52 vs 0.29) were significantly increased. These changes were followed by significantly reduced cell contraction (dL/dt max , 76.0 vs 137.3 μm/s), relaxation (dR/dt max , 61.7 vs 116.0 μm/s) and [Ca 2+ ] iT (0.16 vs 0.21). ISO-stimulated increases dL/dt max (39% vs 58%), dR/dt max (35% vs 54%) and [Ca 2+ ] iT (20% vs 30%) were also significantly reduced. Moreover, in T2DM myocytes, superfusion with KN93 significantly improved myocyte basal contraction (104.2 μm/s), relaxation (79.3 μm/s) and [Ca 2+ ] iT (0.21). Conclusions: In T2 diabetes-induced DCM, upregulation of CaMKIIδ has adverse functional effects. Acute endogenous CaMKII activation exacerbates LV myocyte dysfunction. The inhibition of CaMKII has a significant beneficial impact.

Abstract 9022: Chronic Beta3-Adrenergic Receptor Antagonist Therapy Rescues Diabetic Cardiomyopathy Through Ca 2+ /Calmodulin-Dependent Protein Kinase II Inhibition in a Type II Diabetes Murine Model

Background: Recent evidence showed that CaMKII inhibition with KN93 protects against diabetic cardiomyopathy (DCM). However, the off-target effects have limited its use. There are no CaMKII inhibitors available clinically. It may be best to use some complementary approaches for CaMKII inhibition. Previously we showed that diabetes produced upregulation of cardiac β 3 -AR-mediated inhibitory pathway promoting DCM progression. There is a significant crosstalk between β-AR signaling and CaMKII activation presenting CaMKII as a possible downstream mediator of detrimental β 3 -AR signaling in DCM. It is possible that the benefit of β 3 -AR antagonists (β 3 ANT) might involve modulation of CaMKII. We tested the hypothesis that chronic L-748-337, a selective β 3 ANT may inhibit CaMKII activity and limit DCM. Methods: LV myocyte CaMKIIδ expression and activity and myocyte function were determined in 3 groups (7/group) wild-type female mice over 14 weeks (W): Type 2 DM ( T2 ), 14 W fed high-fat diet (HFD), but after HFD for 4 W receiving streptozotocin (STZ, 40 mg/kg/day, i.p. 5 days); T2β 3 ANT , T2 mice at 10 W received L-748,337 (10 -7 M/kg/day, mini-pump) for 4 W; and Vehicle controls (C). Results: Versus C, T2 diabetes was induced in mice received HFD and low dose STZ with significantly elevated blood glucose. But only T2 myocytes had significantly increased protein levels of CaMKIIδ (T2: 29%, 0.53 vs C: 0.41) and CaMKIIδ phosphorylation (at Thr287) (PCaMKIIδ) (79%, 0.52 vs 0.29) followed by significantly reduced cell contraction (dL/dt max , 75 vs 134 μm/s), relaxation (dR/dt max , 60 vs 114 μ m/s) and [Ca 2+ ] iT (0.16 vs 0.21). The isoproterenol (ISO, 10 -8 M)-stimulated increases in dL/dt max (38% vs 58%), dR/dt max and [Ca 2+ ] iT (19% vs 30%) were also significantly reduced. By contrary, versus C, T2β 3 ANT myocytes had similar levels of CaMKIIδ (0.37) and PCaMKIIδ (0.31) with preserved cell contractility (131 μm/s), relaxation (109 μm/s) and [Ca 2+ ] iT (0.21). ISO caused increases in dL/dt max (57%), dR/dt max (52%) and [Ca 2+ ] iT (30%) were also closed to control values. Conclusions: Chronic β 3 ANT prevents Type II diabetes-caused upregulation of cardiac CaMKIIδ, thereby leading to the preservation of normal myocyte function, [Ca 2+ ] iT , and β-adrenergic reserve in a DCM murine model.

Abstract 9024: Molecular Mechanism of Chronic Beta3-Adrenergic Receptor Antagonist Caused Regression of Diabetic Cardiomyopathy in a Murine Model With Type 2 Diabetes

Background: Diabetic cardiomyopathy (DCM) is a serious complication of diabetes (DM). As yet, no effective therapeutic strategies exist. Recently, we found that DM-induced LV dysfunction was prevented by chronic treatment with a β 3 -adrenergic receptor (AR) antagonist (β 3 ANT). However, the molecular mechanism is unclear. We hypothesized that reversal of DM-induced detrimental alterations on the expressions of cardiac SR Ca 2+ -ATPase (SERCA2a), β-ARs, nitric oxide synthase (NOS) by β 3 ANT may play a key role for its salutary role in DCM. Methods: The protein levels of myocyte β 1 - and β 3 - AR, SERCA2a, NOS and myocyte functional responses were determined in 3 groups (7/group) wild-type female mice over 14 weeks (W): Type 2 DM ( T2 ), 14 W fed a high-fat diet (HFD), but after HFD for 4 W receiving streptozotocin (STZ, 40 mg/kg/day, i.p. 5 days); T2β 3 ANT , T2 mice at 10 W received L-748,337, a selective β 3 ANT (10 -7 M/kg/day, mini-pump) for 4 W; and Vehicle controls (C). Results: Versus C, T2DM was induced in mice received HFD and low dose STZ with significantly elevated blood glucose (~340 to 380 mg/dl). Only T2 mice had significantly decreased protein levels of LV myocyte β 1 -AR (T2: 0.35 vs C: 0.49) and SERCA2a (0.19 vs 0.29), but increased β 3 -AR (0.25 vs 0.14) and iNOS (0.25 vs 0.15) without significant changes in eNOS (0.14 vs 0.13) and nNOS. These changes were followed by significantly reduced basal cell contraction (dL/dt max , 75.4 vs 133.7 μm/s), relaxation (dR/dt max , 59.6 vs 113.8 μ m/s) and [Ca 2+ ] iT (0.16 vs 0.21). The isoproterenol (ISO, 10 -8 M)-stimulated increases in dL/dt max (38% vs 58%), dR/dt max (34% vs 55%) and [Ca 2+ ] iT (19% vs 30%) were also significantly reduced. By contrary, versus C, T2β 3 ANT myocytes had similar protein levels of β 1 -AR (0.52), SERCA2a (0.75) and iNOS (0.36) with preserved normal basal cell contractility (127.8 μm/s), relaxation (109.4 μm/s) and [Ca 2+ ] iT (0.21). ISO caused increases in dL/dt max (57%), dL/dt max (52%), and [Ca 2+ ] iT (30%) were also closed to control values. Conclusions: Chronic β 3 ANT prevents DCM-caused downregulations of cardiac β 1 -ARs and SERCA2a, and normalized upregulations of iNOS and β 3 -AR, thus leading to the preservation of normal myocyte contractile function, [Ca 2+ ] iT , and β-adrenergic reserve in a mouse model of T2DM.

Abstract 12731: Acute Calcium/Calmodulin-Dependent Protein Kinase II Inhibition Augments Cardiomyocyte Contractility and Improves β-Adrenergic Regulation in Experimental Hypertension of Transgenic Rats Expressing Human Angiotensinogen Gene

Background: Previously we showed that rats expressing the human angiotensinogen (AGT) gene (hAogen) exhibit sustained hypertension and cardiac dysfunction, associated with increased cardiac angiotensin II (Ang II). Recent evidence indicates that Ang II increases CaMKII activation which plays a crucial role in ANG II-related pathological processes associated with hypertension, hypertrophy and cardiac dysfunction. Although CaMKII inhibitors are being developed as safe and effective therapeutic agents, their mechanism of action remains to be fully understood. We tested the hypothesis that in hAogen rats, CaMKII is upregulated. Increased CaMKII activation may produce direct depression in LV myocyte basal function and β-adrenergic reserve. Acute inactivation of CaMKII with KN93 would have beneficial actions in hAogen. Methods: We compared LV myocyte CaMKIIδ expression and activity and myocyte functional performance in 10 Sprague Dawley (SD) control and 10 hAogen male rats. In hAogen, we further assessed myocyte contractile and [Ca 2+ ] i transient ([Ca 2+ ] iT ) responses to β-AR stimulation by isoproterenol (ISO, 10 -8 M) with and without pretreatment of myocytes with KN93 (10 -6 M, 30 min). Results: Versus SD, in hAogen myocytes, the CaMKIIδ protein levels (19%, 0.50 vs 0.42) and CaMKIIδ phosphorylation (at Thr 287 ) (35%, 0.27 vs 0.20) were significantly increased. These changes were followed by significantly reduced cell contraction (dL/dtmax, 104.9 vs 138.4 μm/s), relaxation (dR/dtmax, 93.0 vs 104.2 μm/s) and [Ca 2+ ] iT (0.15 vs 0.20). ISO-stimulated increases in dL/dtmax (43% vs 62%), dR/dtmax (32% vs 49%) and [Ca 2+ ] iT (19% vs 29%) were also significantly reduced. Moreover, in hAogen myocytes, pretreatment with KN93 markedly improved myocyte basal contraction (134.4 μm/s), relaxation (107.4 μm/s) and [Ca 2+ ] iT (0.20). ISO-induced increases in dL/dtmax (60%), dR/dtmax (50%), and [Ca 2+ ] iT (31%) were also significantly augmented. Conclusions: In rats harboring the human AGT gene, upregulation of CaMKIIδ has adverse functional effects. Acute endogenous CaMKII activation exacerbates LV myocyte dysfunction and β-AR desensitization. Inhibition of CaMKII has a significant beneficial impact in this established humanized model of hypertension.

Abstract 12677: Impaired Cardiomyocyte Contractility to Angiotensin-(1-12) in a Humanized Angiotensinogen Model of Hypertension

Background: Angiotensin-(1-12) [Ang-(1-12)] is a chymase-dependent source for Angiotensin II (Ang II) inotropic activity that may be impaired in a model of sustained hypertension with high cardiac Ang II content due to insertion of the human angiotensinogen (AGT) gene in the rat genome. Accordingly, we evaluated the effects of Ang-(1-12) and Ang II on myocyte contractility and [Ca 2+ ] i regulation in 9 adult male transgenic rats expressing the human AGT gene [TGR(hAGT)L1623)] and 9 SD controls. Methods: We compared LV myocyte contraction, relaxation and [Ca 2+ ] i transient ([Ca 2+ ] iT ) responses to Ang-(1-12) (4x10 -6 M) and Ang II (10 -6 M) in freshly isolated LV myocytes. Results: Myocyte contraction (dL/dt max , 109.6 vs 127.9 μm/s), relaxation (dR/dt max , 95.3 vs 107.5 μm/s) and [Ca 2+ ] iT (0.15 vs 0.24) were depressed in TGR(hAGT)L1623 rats compared to SD controls. Moreover, cell contractile and [Ca 2+ ] iT responses following exposure to Ang-(1-12) or Ang II were markedly blunted. In SD myocytes, versus baseline, Ang II or Ang-(1-12) superfusion produced significant increases in dL/dt max [Ang II: 44% vs Ang-(1-12): 34%], dR/dt max (33% vs 26%) and [Ca 2+ ] iT (31% vs 25%). Importantly, the magnitude of the responses to the two agents in TGR(hAGT)L1623 myocytes was significantly reduced. Versus the changes in SD myocytes, Ang-(1-12) caused significantly less increases in dL/dt max (22%), dR/dt max (16%) and [Ca 2+ ] iT (15%) in TGR(hAGT)L1623 myocytes . Ang II also caused similar significantly attenuated increases in dL/dt max (27%), dR/dt max (25%) and [Ca 2+ ] iT (23%). The Ang-(1-12)-induced inotropic effects were completely prevented in the presence of the inhibitory cAMP analog, Rp-cAMPS (10 –4 M, 2 hours) in both SD and TGR(hAGT)L1623 myocytes, but were further augmented only intransgenic rats after incubation of myocytes with the G i inhibitor, pertussis toxin (PTX, 2 μg/ml, 36°C, 5 hours). Conclusions: Ang-(1-12) stimulates LV myocyte contractile function and [Ca 2+ ] iT in both SD and TGR(hAGT)L1623 rats. Furthermore, we now show that the blunted inotropic responses to Ang-(1-12) and Ang II in rats expressing the human AGT gene is mediated through a cAMP-dependent mechanism that is coupled to both stimulatory G and inhibitory PTX-sensitive G proteins.

Abstract 13100: Rescue Myocardial Aging Caused a Progressive Decline in Cardiac Function and β-Adrenergic Reserve With a Selective Beta3-Adrenergic Receptor Antagonist, but Not Beta1-Adrenergic Receptor Blocker in a Murine Model

Background: Cardiac aging (CA) leads to the progressive decline in cardiac function- increasing risk for heart failure (HF) and cardiac mortality. Both down-regulation of β 1 -adrenergic receptors (AR) and up-regulation of a newly-described inhibitory pathway mediated by β 3 -AR may play a crucial role in the development of CA. β-AR blockers are a mainstay in the therapy of several cardiovascular diseases, including HF. However the therapeutic effects of different β blockers in CA are unknown. We tested the hypothesis that up-regulation of cardiac β 3 -AR with enhanced negative inotropic actions, rather than the desensitization of β 1 -AR, is the critical determinant of the dysfunctional β-AR regulation that occurs in CA. Thus, β 3 -AR antagonists (β 3 ANT), but not β 1 Blockade, may reverse the progression of age-associated deficits in LV myocyte functional performance. Methods: LV myocyte basal and isoproterenol (ISO)-stimulated functional and [Ca 2+ ] i transient ([Ca 2+ ] iT ) responses were compared in 4 groups (7/group) of wild-type (WT) mice: 1) YWT , young (Y, 6 mo); 2) Aβ 3 ANT , aged (A, 26 mo) treated with β 3 ANT (L-748,337, 10 -7 M/kg/day, mini-pump) for 12 weeks (W);3) Aβ 1 Blockade , metoprolol XL (1.5 mg/kg/day, po), a selective β 1 blocker was given for 12 W and 4) AWT , aged sham control. Results: Compared with YWT, AWT myocytes had about 30% decreases in basal cell contraction (dL/dt max , 85.9 vs 122.3 μm/s) and relaxation (dR/dt max , 69.4 vs 99.2 μm/s) with reduced the peak [Ca 2+ ] iT (0.17 vs 0.21). ISO (10 -8 M)-stimulated increases in dL/dt max (33% vs 69%), dR/dt max (31% vs 62%) and [Ca 2+ ] iT (15% vs 34%) also significantly reduced. In Aβ 1 Blockade group, metoprolol therapy failed to improve basal dL/dt max (86.3 μm/s), dR/dt max (68.9 μm/s) and [Ca 2+ ] iT (0.18). ISO-stimulated increases in dL/dt max (34%), dR/dt max (35%) and [Ca 2+ ] iT (17%) remained attenuated. By contrary, Aβ 3 ANT myocytes showed normal basal cell contraction (131.0 μm/s), relaxation (101.9 μm/s) and [Ca 2+ ] iT (0.21) with preserved ISO-stimulated increases in dL/dt max (68%), dR/dt max (59%) and [Ca 2+ ] iT (32%). Conclusions: Chronic β 3 -AR antagonist, but not β 1 -AR blocker leads to the preservation of myocyte function, [Ca 2+ ] iT , and β-adrenergic responsiveness in a murine model of cardiac aging.

Abstract 12808: Mechanism of Chronic Ranolazine Caused Regression of Cardiac Dysfunction in a Murine of Diabetic Cardiomyopathy: Beneficial Effects on Cardiomyocyte Contractile Function, [Ca 2+ ] i Regulation and Beta-Adrenergic Modulation

Background: Diabetic cardiomyopathy (DCM) increases the risk of heart failure. As yet, no effective therapeutic strategies exist. Recent evidence indicates that intracellular Na + concentration ([Na + ] i ) is augmented in the myocytes from diabetic hearts, where it causes oxidative stress, augments the sarcoplasmic reticulum Ca 2+ leak and contributes to electrical, structural and functional remodeling. Ranolazine (RAN), inhibiting persistent or late inward Na + current has been proposed to be a therapeutic choice for DCM. However, the role and mechanism of chronic RAN in DCM are unclear. We assessed the hypothesis that RAN improves myocyte function, [Ca 2+ ] i regulation, and β-adrenergic receptor (AR) signaling effectiveness, thus limiting DCM. Methods: We compared LV myocyte function, [Ca 2+ ] i transient ([Ca 2+ ] iT ) and responses to the stimulation of β-AR in 3 groups wild-type (WT) female mice over 10 weeks (W):1) DM (n=8), 10 W after receiving streptozotocin (STZ, 200 mg/kg, ip); 2) DM/RAN (n=6), 6 W after STZ, RAN (10 -5 M/kg/day, mini-pump) was initiated and was given for 4 W; and 3) Sham controls (C) (n=8). Results: Versus control, STZ-treated WT mice had DM with significantly elevated blood glucose levels (410 vs 128mg/dl) followed by LV myocyte dysfunction with decreases in myocyte contractility (dL/dt max ) (75.0 vs 140.1 μm/s), relengthening (dR/dt max ) (62.5 vs 116.6 μm/s) and [Ca 2+ ] iT (0.15 vs 0.22). In DM myocytes, the ability of β-AR agonist, isoproterenol (ISO, 10 -8 M) to increase cell contractility was blunted. Versus control, in DM myocytes, ISO-induced increases in dL/dt max (31% vs 60%), dR/dt max (23% vs 50%) and [Ca 2+ ] iT (15% vs 30%) were significantly reduced. By contrary, versus DM alone, DM/RAN myocytes showed normal basal cell contraction (137.8 μm/s), relaxation (117.2 μm/s) and [Ca 2+ ] iT (0.22) with preserved ISO-stimulated positive inotropic effect. Compared control, in DM/RAN, ISO caused similar increases in dL/dt max (62% vs 60%), dR/dt max (52% vs 50%) and [Ca 2+ ] iT (32% vs 30%). Conclusion: Chronic ranolazine leads to the preservation of myocyte function, [Ca 2+ ] iT and β-AR responsiveness in DCM. Thus, antagonizing myocyte [Na + ] i dysregulation might provide a new therapeutic strategy for DM-related decline in myocardial function.

Abstract 13451: Upregulation of CaMKII and Contrast Changes of Cardiac β 1 - and β 3 -Adrenergic Signaling Pathways in a Humanized Angiotensinogen Model of Hypertension

Background: The transgenic rats expressing the human AGT gene [TGR (hAGT) L1623] (hAogen) is an established humanized model of hypertension. Recently we further found that this model is associated with declines in cardiac contractile function and β-adrenergic receptor (AR) reserve. However, the molecular basis is unclear. It has been suggested that pivotal restructuring of β-AR system is a critical determinant of the dysfunctional β-AR regulation in several cardiovascular diseases, including hypertension. But the alterations of subtypes of β-ARs have not been systemically evaluated in this model. Furthermore, there is significant crosstalk between β-AR signaling and CaMKII activation presenting CaMKII as a possible downstream mediator of detrimental β-AR signaling in the abnormal contractile phenotype of cardiac hypertrophy. However, the changes in cardiac CaMKII in this model are unknown. We tested the hypothesis that in hAogen, CaMKII and β 3 -AR are upregulated in the myocardium, but β 1 -AR is down-regulated, which are important causes of cardiac dysfunction and β-AR desensitization. Methods: We compared LV myocyte CaMKIIδ expression and activity and β 1 - and β 3 -AR expression and determined myocyte functional performance and [Ca 2+ ] i transient ([Ca 2+ ] iT) response to β -AR stimulation in myocytes obtained from 6 Sprague Dawley (SD) control and 6 haogen male adult rats. Results: Compared with SD, in hAogen LV myocytes, β 3 -AR protein level significantly increased (26%, 0.29 vs 0.23), but β 1 -AR protein level (26%, 0.4 vs 0.54) significantly decreased. The CaMKIIδ protein levels (16%, 0.50 vs 0.43) and CaMKIIδ phosphorylation (at Thr287) (35%, 0.27 vs 0.20) were significantly increased. These changes were followed by significantly reduced cell contraction (dL/dt max , 102.1vs 138.8μm/s), relaxation (dR/dt max , 94.0 vs 104.7 μm/s) and [Ca 2+ ] iT (0.15 vs 0.21). ISO-stimulated increases in dL/dt max (42% vs 60%), dR/dt max (36% vs 51%) and [Ca 2+ ] iT (20% vs 31%) were also significantly reduced. Conclusions: The upregulation of LV myocyte CaMKIIδ and contrast changes in β 3 -AR and β 1 -AR may be the important cause of the abnormal contractile phenotype and β-AR desensitization in this humanized angiotensinogen model of hypertension of TGR (hAGT) L1623.

Abstract 13045: Chronic Beta3-Adrenergic Receptor Antagonist Therapy Rescues Diabetic Cardiomyopathy in a Mouse Model With Type 2 Diabetes: Insights into Cellular Mechanism

Background: Diabetic cardiomyopathy (DCM) leads to progressive decline in cardiac function, increasing the risk for heart failure. There are no known effective prevention approaches or therapeutic strategies. Recent evidence in diabetes mellitus (DM) human and animal hearts suggests that the up-regulation of β 3 -adrenergic receptor (AR)-mediated inhibitory pathway may be responsible for the progression of DCM. However, its precise role is still being debated. We hypothesize that β 3 -AR antagonists (β 3 -ANT) may rescue the detrimental effects of β 3 -AR activation, improve cardiomyocyte function, and preserve normal β-AR regulation, leading to the regression of DCM. Methods: We compared LV myocyte function, [Ca 2+ ] i transient ([Ca 2+ ] iT ) at baseline and responses to β-AR simulation with isoproterenol (ISO,10 -8 M) in 3 groups wild-type female mice over 14 weeks (W):1) Type 2 DM ( T2 ) (n=9), 14 W fed high-fat diet (HFD), but after HFD for 4 W receiving streptozotocin (STZ, 40 mg/kg/day, i.p. 5 days); 2) T2/β 3 -ANT (n=7), T2 mice at 10 W received L-748,337, a selective β 3 -ANT (10 -7 M/kg/day, mini-pump) for 4 W; and 3) Vehicle controls (C) (n=9). Results: Versus C, T2DM was induced in mice received HFD and low dose STZ with significantly elevated blood glucose levels (T2: 388, T2/β 3 -ANT: 369 vs C: 128 mg/dl). In T2, LV myocyte basal function and [Ca 2+ ] iT regulation were impaired measured as significantly decreased myocyte contractility (dL/dt max ) (76.8 vs 135.2 μm/s), relengthening (dR/dt max ) (62.1 vs 113.8 μ m/s) and [Ca 2+ ] iT (0.16 vs 0.21). Furthermore, versus C, in T2 myocytes, ISO-induced increases in dL/dt max (T2: 40% vs C: 58%), dR/dt max (35% vs 54%) and [Ca 2+ ] iT (19% vs 30%) were significantly reduced. By contrary, versus T2, T2/β 3 -ANT myocytes showed normal basal cell contraction (127.8 μm/s), relaxation (109.4 μm/s) and [Ca 2+ ] iT (0.21) with preserved ISO-stimulated positive inotropic effect. Versus C, in T2/β 3 -ANT, ISO caused similar increases in dL/dt max (57%), dR/dt max (52%) and [Ca 2+ ] iT (30%). Conclusion: Chronic β 3 -ANT leads to the preservation of LV myocyte function, [Ca 2+ ] iT , and β-AR responsiveness in a mouse model of type 2 diabetes. Thus, antagonizing β 3 -AR might provide a new therapeutic strategy for DM-related decline in myocardial function.

Abstract 12717: Enhance Negative Modulation of Beta 3 -Adrenergic Stimulation on Cardiomyocyte Functional Performance, [Ca 2+ ] i Regulation and β-Adrenergic Reserve in Rats Expressing Human Angiotensinogen Gene

Background: Transgenic rats expressing the human angiotensinogen gene [TGR (hAogen) 1623] (hAogen) exhibit sustained hypertension and systolic dysfunction associated with altered cardiac renin-angiotensin system and β- adrenergic receptor (AR) signal transduction systems. We have shown previously, in contrast to β 1 - and β 2 -ARs, β 3 -ARs are linked to G i proteins, and stimulation of β 3 -ARs inhibits cardiac contraction and relaxation. Hypertension is associated with increased cardiac expression of G i proteins. This may alter β 3 -AR stimulation and play an important role in the cardiac functional impairment of this humanized model of hypertension. However, the alteration and functional effect of β 3 -AR activation in this model are unknown. We tested the hypothesis that high cardiac tissue Ang II content in hAogen may cause an up-regulation of cardiac β 3 -AR, which exacerbates myocyte dysfunction and impairs calcium regulation, thereby directly contributing to the cardiac dysfunction. Methods: We compared LV myocyte β 3 -AR expression and myocyte functional and [Ca 2+ ] i transient ([Ca 2+ ] iT ) responses to β- and β 3 -AR stimulation in myocytes obtained from 8 Sprague Dawley (SD) control and 8 hAogen male adult rats. Results: Versus SD, in hAGT rats, basal myocyte contraction (dL/dt max , 107.1 vs 138.8 μm/s), relaxation (dR/dt max , 94.0 vs 103.9 μm/s) and [Ca 2+ ] iT (0.15 vs 0.21) were depressed. A non-selective β-AR agonist, isoproterenol (ISO) (10 -8 M) produced significantly smaller increases in dL/dt max , (42% vs. 61%), dR / dtmax (35% vs. 51%), and [Ca 2+ ] iT (20% vs. 30%). Moreover, versus SD, in hAogen rats, LV myocyte β 3 -AR protein level increased by 32% (0.29 vs 0.22) with resulted significantly altered myocyte functional response to β 3 -AR stimulation. Compared with the changes in SD myocytes, in hAogen myocytes, BRL (10 -8 M) produced a significantly greater decreases in dL/dt max , (27% vs 12%), dR/dt max (28% vs 11%), and [Ca 2+ ] iT (17% vs 10%). Conclusions: TGR (hAogen) 1623, this humanized model of hypertension is associated with an up-regulation of LV myocyte β 3 -AR, which enhances β 3 -AR-caused inhibition of myocyte contractile, relaxation and [Ca 2+ ] iT and exacerbates β-AR desensitization, thereby directly contributing to the cardiac dysfunction.

Abstract 13035: Beta3-Adrenergic-Receptor-Deficient Mice Protected From Diabetic Cardiomyopathy in Type 2 Diabetic Murine Model: Beneficial Effects on Cardiomyocyte Contractile Function, [Ca 2+ ] i Regulation and Beta-Adrenergic Modulation

Background: Diabetic cardiomyopathy (DCM) is the main cause of increased mortality in Diabetes mellitus (DM). There are no effective therapeutic strategies. Recently, we found that DM is associated with the upregulation of β 3 adrenergic receptor (AR) mediated cardiac inhibitory pathway. This suggests cardiac β 3 -AR activation may contribute to DCM progression and be a therapeutic target. We hypothesize that upregulation of β 3 -AR is maladaptive, and DM-caused progressive decline in cardiac function and β-adrenergic reserve will be prevented in β 3 -AR knockout (β 3 KO) mice. Methods: Studies were conducted in female mice of 2 Vehicle controls groups (n=8/group) of wild-type (CWT) and Cβ 3 KO, and 2 Type 2 DM (T2) of T2WT (n=8) and T2β 3 KO (n=6).T2 was induced by fed high-fat diet (HFD) for 14 weeks (W), but after HFD for 4 W receiving streptozotocin (STZ, 40 mg/kg/day, i.p. for 5 days). We compared LV myocyte contractile and [Ca 2+ ] iT responses to β-AR subtype stimulation by random exposure of myocytes to the superfusion of isoproterenol (ISO,10 -8 M) or a selective β 1 -, or β 3 -agonist, Norepinephrine (NE, 10 -7 M), and BRL-37,344 (BRL, 10 -8 M), respectively. Results: Mice received HFD plus STZ developed T2DM with elevated mean blood glucose from 128 mg/dl of control to 388 mg/dl and 382 mg/dl in T2WT and T2β 3 KO, respectively. However only T2WT mice developed DCM followed by significant decreases in myocyte contraction (dL/dt max , T2WT: 74.8 vs CWT:140.1 μm/s), relaxation (dR/dt max , 58.0 vs 117.9 μm/s) and [Ca 2+ ] iT (0.16 vs 0.22). ISO-stimulated increases in dL/dt max (37% vs 58%), dR/dt max (30% vs 53%), and [Ca 2+ ] iT (19% vs 30%) were attenuated accompanied by a diminished NE-caused increase in dL/dt max (28% vs 41%), but enhanced BRL-induced decrease in dL/dt max (29% vs 15%). By contrary, T2β 3 KO showed normal basal dL/dt max (137.6 μm/s), dR/dt max (111.4 μm/s) and [Ca 2+ ] iT (0.22). Importantly, T2β 3 KO myocytes showed preserved ISO-stimulated increases in dL/dt max (60%), dR/dt max (51%) and [Ca 2+ ] iT (32%) and restored normal dL/dt max responses to NE (41%). Conclusions: β 3 KO prevents T2DM-caused contrast changes in β 1 - and β 3 -AR-stimulated cardiac inotropic actions and leading to the preservation of normal myocyte function, [Ca 2+ ] iT , and β-AR responsiveness in DCM.

Abstract 13385: Protective Effect of Beta3-Adrenergic Receptor Deficiency on Diabetic Cardiomyopathy: Reversal of Type 2 Diabetes-Induced Contrast Changes of Cardiac SERCA 2a and Inducible Nitric Oxide Synthase

Background: We have shown that diabetes (DM)-induced cardiac dysfunction and β-adrenergic desensitization were prevented in β 3 -adrenergic receptor (AR) knockout (β 3 KO) mice. However, the molecular mechanism is unclear. We hypothesize that reversal of type 2 (T2) DM-induced alterations of cardiac SR Ca 2+ -ATPase (SERCA2a) and inducible nitric oxide synthase (iNOS) by β 3 KO may play a key role for the protective effect. Methods: LV myocyte protein levels of β 1 - and β 3 -AR, SERCA2a and iNOS, myocyte contractile and [Ca 2+ ] i transient ([Ca 2+ ] iT ) responses to isoproterenol (ISO, 10 -8 M) were compared in 4 groups (n=6) female mice: 2 Vehicle controls wild-type (CWT) and Cβ 3 KO; and 2 T2DM of T2WT and T2β 3 KO.T2 was induced by fed high-fat diet (HFD) for 14 weeks (W), but after HFD for 4 W receiving streptozotocin (STZ, 40 mg/kg/day, i.p. for 5 days). Results: Mice-treated with HFD plus STZ had high blood glucose (~350 to 380 mg/dl). Versus CWT, T2WT myocytes had significantly decreased protein levels of SERCA2a (0.18 vs 0.29), and β 1 -AR (0.35 vs 0.49), but increased iNOS (0.25 vs 0.15) and β 3 -AR (0.25 vs 0.14) with reduced cell contraction (dL/dt max ) (72.7 vs 140.9 μm/s), relaxation (dR/dt max ) (50.9 vs 117.3 μm/s) and [Ca 2+ ] iT (0.16 vs 0.22). Moreover, in T2WT myocytes, ISO caused significantly less increases in dL/dt max (37% vs 59%), dR/dt max (30% vs 53%) and [Ca 2+ ] iT (19% vs 30%). Versus CWT, T2β 3 KO did not alter basal myocyte contraction and relaxation and response to ISO stimulation, but had significantly increased protein levels of SERCA2a (0.38 vs 0.29). T2β 3 KO prevented DM-caused increase in iNOS (T2β 3 KO: 0.16 vs CWT: 0.15) and decrease in β 1 -AR (0.46 vs 0.49). Versus T2β 3 KO, Cβ 3 KO mice had similar alterations. Importantly, in contrast to T2WT, in T2β 3 KO myocytes, the increased SERCA2a and reduced iNOS correlated with normal basal cell contraction and relaxation with preserved ISO-stimulated inotropic response. ISO caused similar increases in dL/dt max (60% vs 59%), dR/dt max (51% vs 53%) and [Ca 2+ ] iT (32% vs 30%). Conclusions: β 3 KO prevents T2DM-caused downregulation of cardiac β 1 -ARs and reverses increased iNOS and decreased SERCA2a, leading to the preservation of myocyte function, [Ca 2+ ] iT , and β-adrenergic reserve. Thus, β 3 -AR may provide a new target for DCM.

Hypercaloric diet models do not develop heart failure, but the excess sucrose promotes contractility dysfunction.

Several diseases are associated with excess of adipose tissue, and obesity is considered an independent risk factor for the development of cardiac remodeling and heart failure. Dietary aspects have been studied to elucidate the mechanisms involved in these processes. Thus, the purpose was the development and characterization of an obesity experimental model from hypercaloric diets, which resulted in cardiac remodeling and predisposition to heart failure. Thirty- day-old male Wistar rats (n = 52) were randomized into four groups: control (C), high sucrose (HS), high-fat (HF) and high-fat and sucrose (HFHS) for 20 weeks. General characteristics, comorbidities, weights of the heart, left (LV) and right ventricles, atrium, and relationships with the tibia length were evaluated. The LV myocyte cross sectional area and fraction of interstitial collagen were assayed. Cardiac function was determined by hemodynamic analysis and the contractility by cardiomyocyte contractile function. Heart failure was analyzed by pulmonary congestion, right ventricular hypertrophy, and hemodynamic parameters. HF and HFHS models led to obesity by increase in adiposity index (C = 8.3 ± 0.2% vs. HF = 10.9 ± 0.5%, HFHS = 10.2 ± 0.3%). There was no change in the morphological parameters and heart failure signals. HF and HFHS caused a reduction in times to 50% relaxation without cardiomyocyte contractile damage. The HS model presented cardiomyocyte contractile dysfunction visualized by lower shortening (C: 8.34 ± 0.32% vs. HS: 6.91 ± 0.28), as well as the Ca2+ transient amplitude was also increased when compared to HFHS. In conclusion, the experimental diets based on high amounts of sugar, lard or a combination of both did not promote cardiac remodeling with predisposition to heart failure under conditions of obesity or excess sucrose. Nevertheless, excess sucrose causes cardiomyocyte contractility dysfunction associated with alterations in the myocyte sensitivity to intracellular Ca2+.

Overexpression of collagen type III in injured myocardium prevents cardiac systolic dysfunction by changing the balance of collagen distribution

ObjectiveLeft ventricular (LV) remodeling alters the contractile and relaxation properties and induces myocardial stiffness. As LV remodeling progresses, the amount of collagen type III (Col3) is gradually decreased, being replaced by collagen type I (Col1). We evaluated whether Col3 overexpression improved cardiac function and remodeling in a rat with ischemic cardiomyopathy (ICM). We also investigated the functional motif and mechanism of thrombin-cleaved N-terminal osteopontin (N-OPN) on cardiac remodeling. MethodsThe rats with ICM were divided into 3 groups: ligation only (Control) group and groups transplanted with nontransfected fibroblast sheets (normal Fb group) or with Col3-secretory fibroblast sheets (Col3 Fb group). A gelatin hydrogel containing the N-terminal fragment (N-OPN), N-OPN lacking the SVVYGLR sequence (⊿SV), the Arg-Gly-Asp (RGD) sequence (⊿RGD), RGD and SVVYGLR sequences (⊿RGD-SV), SVVYGLR alone (SV), or a random SV peptide was implanted into an ICM model rat. ResultsThe Col3 Fb group exhibited significantly attenuated LV systolic dysfunction. LV dilatation, myocyte hypertrophy, and LV fibrosis at the infarcted area were also attenuated by Col3 Fb implantation. Furthermore, N-OPN, ⊿RGD, and SV peptide suppressed the depression of cardiac function, LV dilatation, and myocyte hypertrophy, and also induced increased Col3 expression and reduction in the ratio of Col1 to Col3 in the infarcted and border areas. ConclusionsOverexpression of Col3 improved cardiac function by changing the balance of collagen distribution in LV remodeling. The SVVYGLR motif of the thrombin-cleaved N-OPN and SV peptide attenuated cardiac dysfunction by increasing Col3 and changing the pattern of collagen balance in the impaired area.

Abstract 12206: Altered Cardiomyocyte Inotropic Response to Chymase/Ang-(1-12) Pathway Activation in Heart Failure

Background: Angiotensin-(1-12) [Ang-(1-12)], a new member of the renin-angiotensin system (RAS) has been identified as a chymase-dependent source for Angiotensin II (Ang II) inotropic activity. Recent observations suggest that activation of this non-canonical chymase/Ang-(1-12) pathway may modulate cardiac function bypassing the inhibitory effects of RAS blockade with ACEI and ARBs in heart failure (HF). However, the direct cardiac effects of Ang-(1-12) and Ang II are unclear. Moreover, whether and how HF alters Ang-(1-12) and Ang II cardiac responses are undefined. We assessed the hypothesis that HF is associated with a reduced action of Ang-(1-12) on myocyte contractility and [Ca 2+ ] i regulation. Methods: We compared LV myocyte contractile and calcium transient ([Ca 2+ ] iT ) responses to angiotensin peptides in 12 SD rats with isoproterenol induced HF (2 months after isoproterenol 170 mg/kg sq for 2 days) and 16 age-matched controls. Results: In normal myocytes, versus baseline, Ang II (10 -6 M) superfusion significantly increased myocyte contraction (dL/dt max ) (40%, 184.6 vs 132.1 μm/s), relaxation (dR/dt max ) (34%, 141.3 vs 107.0 μm/s) and [Ca 2+ ] iT (29%, 0.31 vs 0.24). Superfusion of Ang-(1-12) (4x10 -6 M) caused similar increases in dL/dt max (34%) dR/dt max (25%) and [Ca 2+ ] iT (25%). Compared with the changes in normal myocytes, in HF myocytes, Ang II and Ang-(1-12) caused similar but significantly attenuated positive inotropic actions with about 42% to 50% less increases in dL/dt max , dR/dt max and [Ca 2+ ] iT . The Ang-(1-12)-mediated effects were abolished by prior exposure of myocytes to chymostatin in both normal and HF. The Ang II-induced inotropic effects were completely prevented in the presence of an inhibitory cAMP analog, Rp-cAMPS (10 –4 M 2 h) in both normal and HF myocytes, but were further augmented only in HF after the incubation of myocytes with the G i inhibitor, pertussis toxin (PTX, 2 μg/ml, 36°C, 5h). Conclusions: Ang-(1-12) stimulates LV myocyte contractile function and [Ca 2+ ] iT in both normal and HF rats through a chymase mediated action. Altered inotropic responses to Ang-(1-12) and Ang II in HF myocytes is mediated through a cAMP-dependent mechanism that is coupled to both stimulatory G and inhibitory PTX-sensitive G proteins.

Abstract 455: FGF23-induced Cardiac Hypertrophy is Reversible

Left ventricular hypertrophy (LVH) is a common feature of cardiovascular disease in chronic kidney disease (CKD) and affects up to 90% of patients by the time they reach dialysis. Serum levels of fibroblast growth factor (FGF) 23 continuously rise as patients progress to renal failure. We have previously shown that FGF23 can activate FGF receptor (FGFR) 4 and the PLCgamma/calcineurin/NFAT signaling cascade in cardiac myocytes and induce hypertrophy in vitro and in vivo. Administration of an isoform-specific FGFR4 blocking antibody in the 5/6 nephrectomy rat model of CKD immediately after surgery protects rats from developing LVH, and delivery of a pan-FGFR blocker in CKD rats two weeks after surgery reverses LVH. To further study the reversibility of cardiac FGF23 effects, we elevated serum FGF23 levels in mice by administration of a high phosphate (2%) diet for three months. Animals developed LVH, as evident by significantly increased LV wall thickness and myocyte cross sectional area. When mice were switch from high phosphate to normal chow, the LVH phenotype resolved and cardiac parameters were comparable to those of mice that constantly received a normal diet. Furthermore, isolated cardiac myocytes recovered within 24 hours from FGF23-induced hypertrophy upon removal of FGF23. Finally, the FGFR4 blocking antibody was capable of reversing FGF23-induced hypertrophy in vitro. Our data indicate that FGF23-induced LVH is reversible and treatable. Interfering with FGF23/FGFR4 signaling in the heart might provide a novel therapeutic strategy to tackle cardiac injury in CKD. We propose that progression and reversibility of cardiac injury might depend on the duration of cardiac FGF23/FGFR4 activation.