Left Atrial Muscle Research Articles

Abstract 1235: Molecular Basis for Sympathetic Remodeling in the Left Atrium in the Ventricular Tachypacing Canine Model of Congestive Heart Failure

Background: The sympathetic nervous system (SNS) is thought to play an important role in promoting atrial fibrillation (AF) initiation and maintenance, especially in the setting of congestive heart failure (CHF). However, it is unclear as to which components of the SNS are altered in atria isolated from failing hearts. These studies are essential, as they will delineate potential molecular targets that may be probed for therapeutic intervention in order to reduce AF pathogenesis. Methods : CHF was induced by ventricular tachypacing (240 beats/minute) for 4 weeks in hound dogs. A total of 4 CHF and 5 control dogs were used. Immunoblot analysis was performed on left atrial appendage (LAA) homogenates to examine the protein expression of Gsα, Gi2α, and the catalytic subunit of protein kinase A (PKA). Catalytic activities of PKA and protein phosphatase type 2a (PP2a) were determined using non-radioactive enzyme activity assay systems. Cyclic AMP (cAMP) accumulation was assessed by an enzyme linked immunoabsorbant assay and phosphorylation of cardiac troponin I (cTnI) was determined using a phosphospecific antibody against cTnI. Results: Gsαconcentrations were increased approximately 4-fold in failing left atrial preparations relative to control. In contrast, expression of Gi2αwas modestly increased ~1-fold in failing LAA relative to non-failing atrial muscles. PKA activity was significantly increased in failing LAA preparations compared to control (124.7 vs. 42.1 pmole phosphate/minute, P <0.05); while, PP2a activity was substantially decreased in failing LAA (4.60 vs. 7.06 pmole phosphate/μg protein/minute, P <0.05). cAMP concentrations were increased and expression of the catalytic subunit of PKA was upregulated (~1.5-fold increase) in failing LAA. Lastly, phosphorylation of cardiac troponin I (cTnI) was enhanced in failing LAA preparations. Conclusions: Our data are the first to characterize sympathetic remodeling in left atrial muscles isolated from an experimental model of CHF. Additionally, our results support the hypothesis that augmented sympathetic signaling and activation may contribute to the AF substrate in a canine model of CHF characterized by increased AF susceptibility.

Effect of calpain 1 on structural remodeling and contractile dysfunction in atrial fibrillation: experiment with dogs

To test the causal relationship between calpain activation and atrial structural changes during atrial fibrillation (AF). The tip of a spiral mono-polar pacing lead was fixed to the right atrial appendages of 15 dogs randomly divided into 3 equal groups: calpain inhibitor group, undergoing continuous pacing with the impulse of 600 beats/min for 3 weeks and intravenous injection of N-acetyl-Leu-Leu-Met (ALLM), a calpain inhibitor for 3 weeks; control group, undergoing continuous pacing and intravenous injection of dimethyl sulfoxide (DMSO; and sham operation group, given DMSO injection without pacing. Ultrasonography was used to observe the changes of the structures of left atrium and left atrial appendage and the heart function as well. Specimens of atrial muscles were obtained. Calpain 1 activity was detected by Suc-Leu-Leu-Val-Tyr-7-amino-4-methyl-coumarin method. HE staining was conducted to observe the myolysis. Western blotting was used to detect the protein expression of troponin I (TnI) and myosin. The left atrial ejection fraction (LAEF) of the ALLM group was (41 +/- 6)%, significantly higher than that of the control group [(34 +/- 9)%, P < 0.05]. The left atrial appendage ejection fraction (LAAEF) of the ALLM group was (41 +/- 6)%, significantly higher than that of the control group [(35 +/- 6)%, P < 0.05]. Myolysis was extensive in the control group [(71.5 +/- 10.2)%], relatively rare in the ALLM group [(12.3 +/- 16.5)%], and was not seen in the sham operation group, with significantly differences among the 3 groups (all P < 0.01). The calpain 1 activity was positively correlated with the degree of myolysis (r(s) = 0.90 961, P < 0.01). The TnI level of the control group was (43 +/- 12)% that of the sham operation group (P = 0.001), the TnI level of the ALLM group was (51 +/- 11)% that of the sham operation group (P = 0.002) and was significant higher than that of the control group (P = 0.01). The level of myosin of the control group was (51 +/- 11)% that of the sham operation group (P = 0.002), and that of the ALLM group was (149 +/- 33)% that of the control group (P = 0.005). Activation of and upregulation of expression of calpain participate in the structural remodeling of left atrial cardiac muscle and contractile dysfunction. Calpain inhibitor suppresses the increased calpain activity and reverses the structural remodeling of sustained atrial fibrillation. Calpain inhibition may therefore provide a possibility for therapeutic intervention in AF.

Coronary Sinus Activation Pattern in Patients with Atrioventricular Nodal Reentrant Tachycardia

Patterns of left atrial (far‐field signals) or coronary sinus (CS) muscle (near‐field) have been defined in CS recordings. The purpose of this study was to define the activation patterns from the coronary sinus in patients with anterior and posterior type of atrioventricular nodal reentrant tachycardia (AVNRT) circuits.Methods and Results: This retrospective study involved a total of 149 patients with 155 episodes of AVNRT which were divided into 3 patterns. In the anterior pattern (123 tachycardias), the atrial deflection from the His bundle electrogram preceded that from the proximal CS electrogram. In the posterior pattern (23 tachycardias), the proximal CS electrogram (CSp) was recorded earlier than the His bundle atrial electrogram. In the left atrial pattern (9 tachycardias), activation of distal CS sites preceded both proximal CS and atrial activation from the His bundle electrogram. A decapolar catheter with a 5‐mm interelectrode distance was used for CS recording. The CS electrograms were analyzed to determine the total signal duration as well as the duration of the initial component. An initial slow wave was defined as a duration exceeding 10 ms. 1) The duration of the initial component in patients with the anterior pattern was longer than in those with the posterior pattern in CSp (7.3 ± 3.1 ms vs. 4.5 ± 2.0 ms), CS7‐8 (7.4 ± 2.9 ms vs. 3.8 ± 1.5 ms), CS5‐6 (7.3 ms ± 3.3 ms vs. 4.4 ms ± 2.5 ms) and CS3‐4 (6.7 ms ± 2.4 ms vs. 4.5 ms ± 2.0 ms) (p &lt; 0.01). 2) Similarly the total electrogram duration in the CS was longer in patients with an anterior compared to a posterior pattern in CSp (38.3 ms ± 10.1 ms vs. 26.8 ± 6.1 ms), CS7‐8 (31.8 ms ±6.6 ms vs. 27.2 ms ± 6.0 ms), CS5‐6 (31.3 ms ± 6.8 ms vs. 26.5 ms ± 4.9 ms), and CS3‐4 (30.0 ms ± 6.3 ms vs. 25.0 ms ± 5.1 ms) (p &lt; 0.01). 3) The percentage of tachycardias showing an initial slow wave followed by rapid activation was higher for anterior pattern patients compared with posterior pattern patients in CSp (62% vs. 13%), CS7‐8 (79% vs. 4%), CS5‐6 (72% vs. 4%), CS3‐4 (54% vs. 9%) and distal CS (47% vs. 0%) (p &lt; 0.01).Conclusions: The pattern of an initial slow wave followed by a rapid wave in the CS was characteristic of an anterior AVNRT circuit and is explained by the initial involvement of far field left atrial components. In contrast, the predominant early rapid waves in the posterior AVNRT circuit are compatible with early CS activation from the right atrium.

Coronary Sinus Activation Pattern in Patients with Atrioventricular Nodal Reentrant Tachycardia

Patterns of left atrial (far-field signals) or coronary sinus (CS) muscle (near-field) have been defined in CS recordings. The purpose of this study was to define the activation patterns from the coronary sinus in patients with anterior and posterior type of atrioventricular nodal reentrant tachycardia (AVNRT) circuits. Methods and Results: This retrospective study involved a total of 149 patients with 155 episodes of AVNRT which were divided into 3 patterns. In the anterior pattern (123 tachycardias), the atrial deflection from the His bundle electrogram preceded that from the proximal CS electrogram. In the posterior pattern (23 tachycardias), the proximal CS electrogram (CSp) was recorded earlier than the His bundle atrial electrogram. In the left atrial pattern (9 tachycardias), activation of distal CS sites preceded both proximal CS and atrial activation from the His bundle electrogram. A decapolar catheter with a 5-mm interelectrode distance was used for CS recording. The CS electrograms were analyzed to determine the total signal duration as well as the duration of the initial component. An initial slow wave was defined as a duration exceeding 10 ms. 1) The duration of the initial component in patients with the anterior pattern was longer than in those with the posterior pattern in CSp (7.3 ± 3.1 ms vs. 4.5 ± 2.0 ms), CS7-8 (7.4 ± 2.9 ms vs. 3.8 ± 1.5 ms), CS5-6 (7.3 ms ± 3.3 ms vs. 4.4 ms ± 2.5 ms) and CS3-4 (6.7 ms ± 2.4 ms vs. 4.5 ms ± 2.0 ms) (p < 0.01). 2) Similarly the total electrogram duration in the CS was longer in patients with an anterior compared to a posterior pattern in CSp (38.3 ms ± 10.1 ms vs. 26.8 ± 6.1 ms), CS7-8 (31.8 ms ±6.6 ms vs. 27.2 ms ± 6.0 ms), CS5-6 (31.3 ms ± 6.8 ms vs. 26.5 ms ± 4.9 ms), and CS3-4 (30.0 ms ± 6.3 ms vs. 25.0 ms ± 5.1 ms) (p < 0.01). 3) The percentage of tachycardias showing an initial slow wave followed by rapid activation was higher for anterior pattern patients compared with posterior pattern patients in CSp (62% vs. 13%), CS7-8 (79% vs. 4%), CS5-6 (72% vs. 4%), CS3-4 (54% vs. 9%) and distal CS (47% vs. 0%) (p < 0.01). Conclusions: The pattern of an initial slow wave followed by a rapid wave in the CS was characteristic of an anterior AVNRT circuit and is explained by the initial involvement of far field left atrial components. In contrast, the predominant early rapid waves in the posterior AVNRT circuit are compatible with early CS activation from the right atrium.

Cardiac pheochromocytoma: A new case reported

1. Cina CS, Althani H, Pasenau J, Abouzahr L. Kommerell’s diverticulum and right-sided aortic arch: a cohort study and review of the literature. J Vasc Surg. 2004;39:131-9. 2. Kapustin AJ, Litt HI. Diagnostic imaging for aortic dissection. Semin Thorac Cardiovasc Surg. 2005;17:214-23. 3. Flachskampf FA. Assessment of aortic dissection and hematoma. Semin Cardiothorac Vasc Anesth. 2006;10:83-8. 4. Eltzschig HK, Rosenberger P, Lekowski RW Jr, Scott JD, Locke A, Shekar PS, et al. Role of transesophageal echocardiography in patients with suspected aortic dissection. J Am Soc Echocardiogr. 2005;18: 1221. 5. Ko SF, Ng SH, Fu M, Lo PH, Cheng YF, Lee TY. Dissection of retroesophageal aortic diverticulum and descending aorta in a patient with right aortic arch: magnetic resonance demonstration. Cardiovasc Intervent Radiol. 1996;19:438-41.

Phosphorylation of Titin Modulates Passive Stiffness of Cardiac Muscle in a Titin Isoform-dependent Manner

We investigated the effect of protein kinase A (PKA) on passive force in skinned cardiac tissues that express different isoforms of titin, i.e., stiff (N2B) and more compliant (N2BA) titins, at different levels. We used rat ventricular (RV), bovine left ventricular (BLV), and bovine left atrial (BLA) muscles (passive force: RV > BLV > BLA, with the ratio of N2B to N2BA titin, ∼90:10, ∼40:60, and ∼10:90%, respectively) and found that N2B and N2BA isoforms can both be phosphorylated by PKA. Under the relaxed condition, sarcomere length was increased and then held constant for 30 min and the peak passive force, stress-relaxation, and steady-state passive force were determined. Following PKA treatment, passive force was significantly decreased in all muscle types with the effect greatest in RV, lowest in BLA, and intermediate in BLV. Fitting the stress-relaxation data to the sum of three exponential decay functions revealed that PKA blunts the magnitude of stress-relaxation and accelerates its time constants. To investigate whether or not PKA-induced decreases in passive force result from possible alteration of titin–thin filament interaction (e.g., via troponin I phosphorylation), we conducted the same experiments using RV preparations that had been treated with gelsolin to extract thin filaments. PKA decreased passive force in gelsolin-treated RV preparations with a magnitude similar to that observed in control preparations. PKA was also found to decrease restoring force in skinned ventricular myocytes of the rat that had been shortened to below the slack length. Finally, we investigated the effect of the β-adrenergic receptor agonist isoprenaline on diastolic force in intact rat ventricular trabeculae. We found that isoprenaline phosphorylated titin and that it reduced diastolic force to a degree similar to that found in skinned RV preparations. Taken together, these results suggest that during β-adrenergic stimulation, PKA increases ventricular compliance in a titin isoform-dependent manner.

Effects of sodium selenate treatment on altered responses of left and right atria from streptozotocin-induced diabetic rats.

Experiments were designed to determine whether experimental diabetes alters inotropic and chronotropic effects of adrenergic, adenosinergic, and cholinergic agonists and whether the observed changes are prevented by sodium selenate therapy. Thirty-two rats were divided into four groups of eight subjects each. Diabetes induced by streptozotocin caused significant decreases in isoproterenol-invoked contraction of the left atria with preservation of the right atrial chronotropic responses. The diminished response of the left atrial muscle to isoproterenol did not respond to treatment with sodium selenate. The left atria adenosine-induced direct- and indirect inotropic responses were diminished in the diabetic rats. After treatment with sodium selenate the direct response was completely normalized, but the indirect response was only partially corrected. Adenosine-induced negative chronotropic effects are accompanied by changed responses in diabetic right atria that are corrected after treatment. The carbachol-induced inotropic and chronotropic responses were not altered in tests of the acetylcholine system. We conclude that in diabetic rats, sodium selenate treatment reverses the deficits of adenosine-induced negative inotropic responses of left and right atria, but not those of isoproterenol-induced positive inotropic responses.

Age-dependent biochemical and contractile properties in atrium of transgenic mice overexpressing junctin.

Junctin is a transmembrane protein of the cardiac junctional sarcoplasmic reticulum (SR) that binds to the ryanodine receptor, calsequestrin, and triadin 1. This quaternary protein complex is thought to facilitate SR Ca2+ release. To improve our understanding of the contribution of junctin to the regulation of SR function, we examined the age-dependent effects of junctin overexpression in the atrium of 3-, 6-, and 18-wk-old transgenic mice. The ratio of atrial weight and body weight was unchanged between junctin-overexpressing (JCN) and wild-type (WT) mice at all ages investigated (n=6-8). The protein expression of triadin 1 was decreased starting in 3-wk-old JCN atria (by 69%), whereas the expression of the ryanodine receptor was diminished in 6- (by 48%) and 18-wk-old (by 57%) JCN atria compared with age-matched WT atria. Force of contraction was decreased by 35% in 18-wk-old JCN compared with age-matched WT left atrial muscle strips, which was accompanied by a prolonged time of relaxation (48.1 +/- 0.9 vs. 44.2 +/- 0.8 ms, respectively, n=6-8, P <0.05). The spontaneous beating rate of isolated right atria was higher in 18-wk-old JCN mice compared with age-matched WT mice (389 +/- 10 vs. 357 +/- 6 beats/min, respectively, n=6-8, P <0.05). Heart rate was lower by 9% in telemetric ECG recordings in 18-wk-old JCN mice during stress tests. Three-week-old JCN atria exhibited a higher potentiation of force of contraction at rest pauses of 30 s (by 13%) and of 300 s (by 35%), suggesting increased SR Ca2+ content. This was consistent with the higher force of contraction in 3-wk-old JCN atria (by 29%) compared with age-matched WT atria (by 10%) under the administration of caffeine. We conclude that in 3-wk-old atria, junctin overexpression was associated with a reduced expression of triadin 1 resulting in a higher SR Ca2+ load without changes in contractility or heart rate. In 6-wk-old JCN atria, the compensatory downregulation of the ryanodine receptor may offset the effects of junctin overexpression. Finally, the progressive decrease in ryanodine receptor density may contribute to the decreased atrial contractility and lower heart rate during stress in 18-wk-old JCN mice.

Age-dependent effects induced by ouabain in rat heart muscles and cellular Ca2+ concentration.

Responsiveness to ouabain of the inotropic and chronotropic effects in rat atrial muscles during development (3-18 wks old) was examined. In spontaneously beating rat right atrial muscles, ouabain (3-30 microM) caused a potent positive inotropic effect in a concentration-dependent manner, but failed to have a chronotropic effect; at 30 microM, 78.6 +/- 3.4% (n = 14, p<0.01) in the contractile force and -1.1 +/- 2.3% (n = 14, p>0.05) in the sinus rate in 10-wk-old rats. The myocardium during development increased the responsiveness to ouabain (10 microM) by 27.6 +/- 2.1% (n = 14, p<0.01), 58.7 +/- 3.3% (n = 14, p<0.001), and 47.2 +/- 2.3% (n = 14, p<0.001) in 3-, 10-, and 18- wk-old rats, respectively. However, the response on the sinus rate was not modified in all of the developing stages. Higher frequencies of stimulation caused the more potent inotropic effect in left atrial muscles. In the experiments using a Ca2+-sensitive fluorescent dye (Fura-2), ouabain (10 and 30 microM) increased the cellular Ca2+ concentrations by 3.0 +/- 2.1% (n = 6, p>0.05) and 12.7 +/- 1.5% (n = 6, p<0.05) in 3-wk-old rats and by 13.0 +/- 2.7% (n = 6, p<0.05) and 42.9 +/- 3.1% (n = 6, p<0.01) in 18-wk-old rats, respectively. These results suggest that the ouabain-evoked response is enhanced during development (but tends to decrease from the maximum after maturing), presumably resulting from developmental degrees of cellular mechanisms such as Na+/K+ pump activity and Na+/Ca2+ exchange and is reflected by changes in the cellular Ca2+ concentration.

Vegetable oils used as vitamin E vehicle affect the electrical activity of rat heart.

The aim of this study is to define the possible effects of vegetable oils used as vitamin E vehicle on the electrical activity of the rat heart. To test the possible effects of vitamin E vehicles we studied the effect of i.p. injected corn oil, hazelnut oil or peanut oil on the action potential parameters recorded in both papillary and left atrial muscle strips. Four experimental groups were used. The control group was injected (i.p.) with distilled water, while the three remaining groups received injections of corn oil, hazelnut oil, or peanut oil for five weeks (in a dose of 0.4 ml/kg/day – minimum amount of oil in which vitamin E could be dissolved). We used borosilicated (15-20 M) capillary electrodes and intracellular action potentials (AP) were recorded in isolated papillary and left atrium muscle strips. While administration of three different types of vegetable oil had no significant effect on AP parameters of papillary muscle, they significantly prolonged the repolarization phase of AP in atrial strips. These results show that vegetable oils used as vitamin E vehicles may alter the electrical activity of the heart in a tissue-dependent manner. The present data indicate that the possible effect of vegetable oil vehicles should be kept in mind while evaluating the possible effects of in vivo vitamin E administration.

In-vivo electrophysiologic assessment of left atrial muscle sleeves in pulmonary veins with magnetic electroanatomic mapping in patients with paroxysmal atrial fibrillation Manisha S. Ashar, David J. Callans, Henry H. Hsia, Andrea M. Russo, Erica S. Zado, Francis E. Marchlinski. University of Pennsylvania Health Systems, Philadelphia, PA

In-vivo electrophysiologic assessment of left atrial muscle sleeves in pulmonary veins with magnetic electroanatomic mapping in patients with paroxysmal atrial fibrillation Manisha S. Ashar, David J. Callans, Henry H. Hsia, Andrea M. Russo, Erica S. Zado, Francis E. Marchlinski. University of Pennsylvania Health Systems, Philadelphia, PA

Doxorubicin affects the cardiac muscarinic system in the rat.

During the study on the mechanism of doxorubicin-induced cardiotoxicity, we observed that a long incubation (4 hr) with doxorubicin reduced the maximal negative inotropic effects of a muscarinic receptor agonist, carbachol. The mechanism responsible for this doxorubicin-induced reduction of the efficacy of carbachol was examined in isolated guinea pig hearts. In isolated left atrial muscle preparations, 1 hr incubation with 100 microM doxorubicin caused a parallel right-ward shift of the concentration-response curves for carbachol, but a longer (4 hr) incubation with this agent (30, 100 or 200 microM), caused a significant reduction of the magnitude of the negative inotropic effect of carbachol in addition to the concentration-dependent parallel right-ward shift. The 4-hr incubation with these concentrations of doxorubicin also reduced the maximal negative inotropic effect of an adenosine A1 receptor agonist, R-phenylisopropyl adenosine (R-PIA), without affecting the potency of this agonist. Doxorubicin (1 to 100 microM) reduced [3H]quinuclidinyl benzilate (QNB) binding in a concentration dependent manner, but failed to alter [3HIR-PIA binding. The decrease in the magnitude of the maximal negative inotropic effect by doxorubicin was caused by changes in the muscarinic system at steps common to the transduction of muscarinic and adenosine A1 receptor mechanisms.

Changes in Titin and Collagen Underlie Diastolic Stiffness Diversity of Cardiac Muscle

Small (N2B) and large (N2BA) cardiac titin isoforms are differentially expressed in a species-specific and heart location-specific manner. To understand how differential expression of titin isoforms may influence passive stiffness of cardiac muscle we investigated the mechanical properties of mouse left ventricular (MLV) wall muscle (expressing predominantly the small titin isoform), bovine left atrial (BLA) wall muscle (predominantly the large isoform), and bovine left ventricular (BLV) wall muscle (expressing small and large isoforms at similar levels). Results indicate that the overall passive muscle stiffness of the muscle types varies nearly ten-fold, with stiffness increasing in the following order: BLA, BLV and MLV. To investigate the basis of the variation in the overall muscle stiffness, the contributions of titin and collagen to muscle stiffness were determined. Results showed that increased muscle stiffness results from increases in both titin- and collagen-based passive stiffness, indicating that titin and collagen change in a co-ordinated fashion. The expression level of the small titin isoform correlates with titin's contribution to overall muscle stiffness, suggesting that differential expression of titin isoforms is an effective means to modulate the filling behavior of the heart.

Anti-muscarinic actions of mitoxantrone in isolated heart muscles of guinea pigs

A hypotheses that mitoxantrone is a competitive antagonist at muscarinic cholinergic receptors was examined in guinea-pig hearts. In isolated left atrial muscle preparations, electrically paced at 2 Hz, the muscarinic agonist, carbachol, caused a concentration-dependent decrease in developed tension. Mitoxantrone caused a parallel right-ward shift of the concentration–response curve for carbachol. Schild plots for the effect of mitoxantrone on the carbachol concentration–response relationship were linear with a slope of 0.88 which was not significantly different from the unity. The right-ward shift of the carbachol concentration–response relationship by mitoxantrone significantly reversed after an additional incubation with a mitoxantrone-free solution, although the reversal was incomplete after a 2-h incubation in the mitoxantrone-free solution. Mitoxantrone caused a concentration-dependent displacement of specific [ 3H]quinuclidinyl benzilate binding to membrane preparations obtained from ventricular muscles of guinea-pig hearts. These results indicate that mitoxantrone acts as a competitive antagonist for the muscarinic receptors.

Doxorubicin-induced late cardiotoxicity: delayed impairment of Ca2+-handling mechanisms in the sarcoplasmic reticulum in the rat

Doxorubicin treatment causes delayed development of cardiotoxicity. Whether the doxorubicin-induced impairment of cardiac functions reverses or progresses with time after the cessation of the treatment was examined. The rats were injected with doxorubicin (2.5 mg/kg, i.v., once a week for 3 weeks) and sacrificed at 1 (1W), 13 (13W), or 18 (18W) weeks after the final doxorubicin administration. The time to peak of twitch contraction observed at 2-Hz stimulation was not altered in left atrial or ventricular muscle preparations isolated from 1W rats, but it was prolonged in those from 13W and 18W rats. The reduction of the magnitude of postrest contraction and the alteration of force-frequency relationships in left atrial muscle preparations in 1W rats were not significant, but were intensified in the 13W and 18W groups. Alterations in the postrest contraction and the force-frequency relationships in ventricular muscle preparations isolated from doxorubicin-treated rat hearts were weaker, but the pattern of alteration was similar to that observed in left atrial muscle preparations. Caffeine-induced contraction observed in skinned fibers that were isolated from the 1W rats was not altered, but it was reduced in the 18W rats. The Ca2+ sensitivity of contractile proteins was not altered in doxorubicin-treated rat hearts in any of the groups. The K(d) values estimated from a [3H]ryanodine binding study were not altered, but the B(max) values were significantly lower in the 13W and 18W groups than those observed in control rats. These results suggest that the dysfunction of the sarcoplasmic reticulum progresses after the completion of doxorubicin treatment and contributes to the doxorubicin-induced late cardiotoxicity.

Doxorubicin-induced late cardiotoxicity: delayed impairment of Ca&lt;sup&gt;2+&lt;/sup&gt;-handling mechanisms in the sarcoplasmic reticulum in the rat

Doxorubicin treatment causes delayed development of cardiotoxicity. Whether the doxorubicin-induced impairment of cardiac functions reverses or progresses with time after the cessation of the treatment was examined. The rats were injected with doxorubicin (2.5 mg/kg, i.v., once a week for 3 weeks) and sacrificed at 1 (1W), 13 (13W), or 18 (18W) weeks after the final doxorubicin administration. The time to peak of twitch contraction observed at 2-Hz stimulation was not altered in left atrial or ventricular muscle preparations isolated from 1W rats, but it was prolonged in those from 13W and 18W rats. The reduction of the magnitude of postrest contraction and the alteration of force-frequency relationships in left atrial muscle preparations in 1W rats were not significant, but were intensified in the 13W and 18W groups. Alterations in the postrest contraction and the force-frequency relationships in ventricular muscle preparations isolated from doxorubicin-treated rat hearts were weaker, but the pattern of alteration was similar to that observed in left atrial muscle preparations. Caffeine-induced contraction observed in skinned fibers that were isolated from the 1W rats was not altered, but it was reduced in the 18W rats. The Ca2+ sensitivity of contractile proteins was not altered in doxorubicin-treated rat hearts in any of the groups. The K(d) values estimated from a [3H]ryanodine binding study were not altered, but the B(max) values were significantly lower in the 13W and 18W groups than those observed in control rats. These results suggest that the dysfunction of the sarcoplasmic reticulum progresses after the completion of doxorubicin treatment and contributes to the doxorubicin-induced late cardiotoxicity.

Adriamycin causes dual inotropic effects through complex modulation of myocardial Ca2+ handling.

Effects of adriamycin (ADR) on the twitch contraction of isolated guinea pig cardiac muscles were examined to elucidate its actions on intracellular Ca2+ mobilization. In right ventricular papillary muscles, ADR (100-300 micromol/L) caused positive inotropy when the muscles were constantly stimulated at low frequencies (0.1-0.5 Hz), whereas it caused negative inotropy when the muscles were stimulated at higher frequencies (2.0-3.0 Hz). Action potential duration was prolonged significantly by ADR, especially at the lower frequencies. The potentiation of twitch contraction of the first beat (B 1) following a short rest period (2-10 s) in ventricular muscles was inhibited by ADR. In untreated papillary muscles, B1 contraction showed time-dependent decay in response to a prolongation of the preceding rest period up to 120 s. ADR (300 micromol/L) caused ryanodin-like acceleration for the early B1 decay with rest period less than 20 s, but a substantial deceleration for the later B1 decay (> or =30 s). In left atrial muscles stimulated constantly, ADR had significant negative inotropy throughout the entire range of stimulation frequencies tested (0.1-4.0 Hz). The post-rest potentiation of B 1 contraction of atrial muscle in the presence of nifedipine was also inhibited by ADR. These findings suggest that ADR has dual inotropic effects through a complex modulation of myocardial Ca2+ handling, which may involve (1) an increase of Ca2+ influx through a prolongation of action potential duration, (2) ryanodine-like inhibition of Ca2+ release from the sarcoplasmic reticulum, and (3) inhibition of sarcolemmal Ca2+ extrusion probably through the Na+/Ca2+ exchange.

Left atrial systolic function assessed by left atrial ejection force in patients with sick sinus syndrome and paroxysmal atrial fibrillation.

We evaluated left atrial systolic function using left atrial ejection force (LAEF) in 19 patients with sick sinus syndrome (SSS) and in 20 with paroxysmal atrial fibrillation (Paf) whose ages ranged from 48 to 80 years. We also evaluated 35 normal individuals for comparison. The LAEF was calculated as 1/3 x mitral valve area x (peak velocity of A wave)2 using two-dimensional and pulsed-Doppler echocardiography according to Newton's law of motion and hydrodynamics. In normal individuals, LAEF positively correlated with age (r = 0.82, p < 0.01). Normal LAEF was calculated as 0.098 x age - 0.74 (kdynes) from the regression line. Because of this correlation, we used age-corrected LAEF (%LAEF) that was calculated as (measured LAEF / normal LAEF) x 100. The results showed that this value was 53+/-26% in patients with SSS and 54+/-26% in patients with Paf. Both were significantly lower than normal individuals (p < 0.001). Among SSS subtypes, %LAEF was lower in types II and III than in type I (51+/-14%, 37+/-19%, and 81+/-35%, respectively). In conclusion, left atrial systolic function is depressed in patients with Paf and SSS, in particular, types II and III. These results suggest that the pathological abnormalities extend not only to the sinus node but also to the left atrial muscle in patients with SSS and Paf.

Propofol-induced alterations in myocardial beta-adrenoceptor binding and responsiveness.

Propofol (iv) depresses cardiovascular function in both humans and animals. However, the mechanism underlying this action has not been well described. The present study was designed to test the hypothesis that this effect of propofol results in part from an antagonism of adrenergic control of the heart. Experiments examined effects of propofol on: 1) [3H]CGP12177 (a beta-adrenoceptor antagonist) binding in rat myocardial membranes; and 2) the inotropic and chronotropic actions of isoproterenol in rat left atrial muscle and right atria, respectively. Propofol (25-200 microM) increased the apparent dissociation constant for [3H]CGP12177 without affecting binding site density. Similarly, 200 microM propofol increased the 50% effective concentration values for the dose-dependent positive chronotropic and inotropic actions of isoproterenol in right and left atria, and depressed the maximum increase in spontaneous rate elicited by this beta-adrenoceptor agonist. Other experiments demonstrated that propofol does not alter muscarinic receptor binding as monitored using [3H]quinuclidi-nylbenzilate. In conclusion, these results indicate that propofol can decrease cardiac beta-adrenoceptor responsiveness; however, the concentrations of propofol required suggest that this action contributes to the cardiovascular depression produced by this anesthetic only during large-dose bolus injection. Experiments in membranes and cardiac preparations isolated from rat heart demonstrate that relatively high concentrations of propofol (25-200 microM) are required to antagonize beta-adrenoceptor binding and tissue responsiveness.

Effects of gender on the sensitivity of rat cardiac muscle to extracellular Ca 2+

Experiments were designed to determine if the inotropic response to increasing buffer calcium concentration differs in male and female cardiac muscle. Left atrial and papillary muscles were isolated from hearts of 3–4-month old male and female rats, bathed in Krebs–Henseleit solution (30°C), and stimulated at 1.5 Hz. Isometric developed tension was monitored continuously as extracellular Ca 2+ was increased in a cumulative fashion. When compared to male atrial muscle, female atrial preparations were more sensitive to the resulting positive inotropic action; EC 50 values were 2.89±0.22 and 1.86±0.21 mM in male and female atria, respectively. Two-way analysis of variance (ANOVA) also indicated that there was a significant gender-associated difference in the Ca 2+ dose–response curves in atrial muscle. In contrast, papillary muscle did not show a significant gender-related difference in EC 50 values (0.88±0.07 and 0.74±0.06 mM in males and females); however, the Ca 2+ dose–response curves obtained from male and female preparations were found to be significantly different when compared by ANOVA.