Frequency-dependent Acceleration Of Relaxation Research Articles

Reduced O-GlcNAcylation diminishes cardiomyocyte Ca2+ dependent facilitation and frequency dependent acceleration of relaxation

Ca2+ dependent facilitation (CDF) and frequency dependent acceleration of relaxation (FDAR) are regulatory mechanisms that potentiate cardiomyocyte Ca2+ channel function and increase the rate of Ca2+ sequestration following a Ca2+-release event, respectively, when depolarization frequency increases. CDF and FDAR likely evolved to maintain EC coupling at increased heart rates. Ca2+/calmodulin-dependent kinase II (CaMKII) was shown to be indispensable to both; however, the mechanisms remain to be completely elucidated. CaMKII activity can be modulated by post-translational modifications but if and how these modifications impact CDF and FDAR is unknown. Intracellular O-linked glycosylation (O-GlcNAcylation) is a post-translational modification that acts as a signaling molecule and metabolic sensor. In hyperglycemic conditions, CaMKII was shown to be O-GlcNAcylated resulting in pathologic activity. Here we sought to investigate whether O-GlcNAcylation impacts CDF and FDAR through modulation of CaMKII activity in a pseudo-physiologic setting. Using voltage-clamp and Ca2+ photometry we show that cardiomyocyte CDF and FDAR are significantly diminished in conditions of reduced O-GlcNAcylation. Immunoblot showed that CaMKIIδ and calmodulin expression are increased but the autophosphorylation of CaMKIIδ and the muscle cell-specific CaMKIIβ isoform are reduced by 75% or more when O-GlcNAcylation is inhibited. We also show that the enzyme responsible for O-GlcNAcylation (OGT) can likely be localized in the dyad space and/or at the cardiac sarcoplasmic reticulum and is precipitated by calmodulin in a Ca2+ dependent manner. These findings will have important implications for our understanding of how CaMKII and OGT interact to impact cardiomyocyte EC coupling in normal physiologic settings as well as in disease states where CaMKII and OGT may be aberrantly regulated.

Prolonged Ca2+ release refractoriness and T-tubule disruption as determinants of increased propensity to cardiac alternans in the hypertensive heart disease.

Cardiac alternans is a dynamical phenomenon linked to the genesis of severe arrhythmias and sudden cardiac death. It has been proposed that alternans is caused by alterations in Ca2+ handling by the sarcoplasmic reticulum (SR), in both the SR Ca2+ uptake and release processes. The hypertrophic myocardium is particularly prone to alternans, but the precise mechanisms underlying its increased vulnerability are not known. Mechanical alternans (intact hearts) and Ca2+ alternans (cardiac myocytes) were studied in spontaneously hypertensive rats (SHR) during the first year of age after the onset of hypertension and compared with age-matched normotensive rats. Subcellular Ca2+ alternans, T-tubule organization, SR Ca2+ uptake, and Ca2+ release refractoriness were measured. The increased susceptibility of SHR to high-frequency-induced mechanical and Ca2+ alternans appeared when the hypertrophy developed, associated with an adverse remodeling of the T-tubule network (6 mo). At the subcellular level, Ca2+ discordant alternans was also observed. From 6 mo of age, SHR myocytes showed a prolongation of Ca2+ release refractoriness without alterations in the capacity of SR Ca2+ removal, measured by the frequency-dependent acceleration of relaxation. Sensitizing SR Ca2+ release channels (RyR2) by a low dose of caffeine or by an increase in extracellular Ca2+ concentration, shortened refractoriness of SR Ca2+ release, and reduced alternans in SHR hearts. The tuning of SR Ca2+ release refractoriness is a crucial target to prevent cardiac alternans in a hypertrophic myocardium with an adverse T-tubule remodeling.

Effect of Cooling on Force-Frequency Relationship, Rest Potentiation, and Frequency-Dependent Acceleration of Relaxation in the Guinea Pig Myocardium

Effect of Cooling on Force-Frequency Relationship, Rest Potentiation, and Frequency-Dependent Acceleration of Relaxation in the Guinea Pig Myocardium

Stretching single titin molecules from failing human hearts reveals titin's role in blunting cardiac kinetic reserve.

Heart failure (HF) patients commonly experience symptoms primarily during elevated heart rates, as a result of physical activities or stress. A main determinant of diastolic passive tension, the elastic sarcomeric protein titin, has been shown to be associated with HF, with unresolved involvement regarding its role at different heart rates. To determine whether titin is playing a role in the heart rate (frequency-) dependent acceleration of relaxation (FDAR). W, we studied the FDAR responses in live human left ventricular cardiomyocytes and the corresponding titin-based passive tension (TPT) from failing and non-failing human hearts. Using atomic force, we developed a novel single-molecule force spectroscopy approach to detect TPT based on the frequency-modulated cardiac cycle. Mean TPT reduced upon an increased heart rate in non-failing human hearts, while this reduction was significantly blunted in failing human hearts. These mechanical changes in the titin distal Ig domain significantly correlated with the frequency-dependent relaxation kinetics of human cardiomyocytes obtained from the corresponding hearts. Furthermore, the data suggested that the higher the TPT, the faster the cardiomyocytes relaxed, but the lower the potential of myocytes to speed up relaxation at a higher heart rate. Such poorer FDAR response was also associated with a lesser reduction or a bigger increase in TPT upon elevated heart rate. Our study established a novel approach in detecting dynamic heart rate relevant tension changes physiologically on native titin domains. Using this approach, the data suggested that the regulation of kinetic reserve in cardiac relaxation and its pathological changes were associated with the intensity and dynamic changes of passive tension by titin.

SERCA is critical to control the Bowditch effect in the heart

The Bowditch effect or staircase phenomenon is the increment or reduction of contractile force when heart rate increases, defined as either a positive or negative staircase. The healthy and failing human heart both show positive or negative staircase, respectively, but the causes of these distinct cardiac responses are unclear. Different experimental approaches indicate that while the level of Ca2+ in the sarcoplasmic reticulum is critical, the molecular mechanisms are unclear. Here, we demonstrate that Drosophila melanogaster shows a negative staircase which is associated to a slight but significant frequency-dependent acceleration of relaxation (FDAR) at the highest stimulation frequencies tested. We further showed that the type of staircase is oppositely modified by two distinct SERCA mutations. The dominant conditional mutation SERCAA617T induced positive staircase and arrhythmia, while SERCAE442K accentuated the negative staircase of wild type. At the stimulation frequencies tested, no significant FDAR could be appreciated in mutant flies. The present results provide evidence that two individual mutations directly modify the type of staircase occurring within the heart and suggest an important role of SERCA in regulating the Bowditch effect.

Stretching Single Titin Molecules from Failing Human Hearts at Cardiac Cycle Reveals Titin's Role in Cardiac Kinetic Reserve

Diastolic dysfunction occurs not only in diastolic heart failure, but also ubiquitously in systolic heart failure. A main determinant of diastolic passive tension, the elastic sarcomeric protein titin, has been shown to be associated with heart failure, with unresolved mechanisms. Heart failure patients commonly experience symptoms only during physical activities or stress. To determine whether titin is playing a role in the mechanisms of heart failure in terms of its known impaired property of frequency‐dependent acceleration of relaxation (FDAR), we studied the FDAR responses in live left ventricular cardiomyocytes and single‐molecule force spectroscopy (SMFS) of titin elastic domains (PEVK‐distal Ig, distal Ig) from failing (n=11) and non‐failing (n=7) human hearts. To access SMFS on native titin, we developed a novel approach using atomic force microscopy to detect titin passive tension (TPT) based on frequency‐modulated cardiac cycle. Mean TPT was shown to be reduced upon an increased heart rate, which was significantly blunted in failing human hearts. The results from titin distal Ig domain were significantly correlated with the frequency‐dependent relaxation kinetics of human cardiomyocytes from the corresponding hearts, which was shown to be impaired in failing hearts. Correlation studies suggested that the higher the TPT, the faster the cardiomyocytes relaxed, but lower the potential of myocytes to speed up relaxation at a higher heart rate; such poorer FDAR response was also associated with a less reduction or a bigger increase in TPT upon a raised heart rate. Our study established a novel approach in detecting tension physiologically on native titin domains. The data from human hearts suggested that the regulation of kinetic reserve in cardiac relaxation and its pathological changes are associated with the intensity and dynamic changes of passive tension on titin.Support or Funding InformationThis project is supported by RO1HL113084 to Janssen PML, and T32NS779844 to Chen MPThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Abstract 71: Ca 2+ Reuptake is Reduced in the Absence of Insulin Receptors in Mouse Cardiomyocytes

Left ventricle diastolic dysfunction (LVDD), characterized by slow or incomplete relaxation of the ventricle during diastole, is an important contributor to heart failure development in diabetic patients. LVDD can be found in pre-diabetic patients as well as in almost half of asymptomatic patients with well-controlled diabetes. Reduced Ca 2+ reuptake into the sarcoplasmic reticulum (SR) by the SR Ca 2+ ATPase (SERCA) is an important contributor to the slow relaxation observed in diastolic dysfunction. Despite this there are still conflicting results with respect to SERCA expression and function in animal models of diabetes, due perhaps to differences in the duration and severity of diabetes and the presence of systemic complications that can per se alter SERCA expression and/or function. Here, we tested the hypothesis that the impaired relaxation observed in diabetic mice is a consequence of the direct effect of insulin on SERCA. We measured Ca 2+ transient relaxation in isolated cardiomyocytes from 8 weeks cardiomyocyte-selective insulin receptor knockout (CIRKO) mice (Cre-IR lox / lox ), , which lack systemic complications, and compared them with their WT littermates (IR lox / lox ) . Cardiomyocytes loaded with the Ca 2+ dye Fluo-4 were field stimulated at 2 and 3.3 Hz, at 37°C. The intracellular Ca 2+ was monitored using a custom-made epifluorescence system. SERCA and Na + /Ca 2+ exchanger (NCX) expression level was reported as the fold change relative to WT. We found that Ca 2+ transient relaxation was prolonged at both frequencies in CIRKO compared with WT, while retaining a normal frequency-dependent acceleration of relaxation. The exponential time constants were 104±4 (WT) vs. 190±16 (CK) ms at 3.3 Hz, n= 20/group, p<0.001 and 111±6 (WT) vs. 226±19 (CK) ms at 2 Hz, n= 20/group, p<0.001). We also found a reduction in SERCA expression (mRNA: 0.65±0.03, n=10, p<0.001; protein:0.62±0.11, n=4, p<0.05) with no difference in NCX expression (mRNA: 0.92±0.06 n=10/group, p=0.5; protein 0.96±0.05, n=6/group, p=0.2). In conclusion, our findings suggest that the slower Ca 2+ removal observed in the systemic models of diabetes is a direct effect of insulin on SERCA, which could contribute to the LVDD observed in diabetic patients.

Abstract 185: Chromogranin A is a Potent Prognostic Biomarker in Acute Heart Failure, and its Fragment Catestatin Regulates Cardiomyocyte Calcium Homeostasis by Camkiiδ Inhibition

Background: Circulating chromogranin A (CgA) levels have been found associated with clinical outcomes in cardiovascular disease, but whether the CgA fragment catestatin (CST) may directly modulate cardiomyocyte Ca 2+ handling is not known. Methods: The prognostic utility of circulating CgA levels were compared between patients with acute HF (n=143) and chronic acute obstructive pulmonary disease (COPD, n=84). Functional effects of CST were assessed in isolated cardiomyocyte and explanted hearts. Results: CgA levels were associated with mortality in acute HF patients, but not in acute COPD. (Figure; patients stratified according to CgA quartiles on hospital admission). Admission CgA levels were also associated with mortality after adjusting for other risk factors in multivariate analysis. We found CST to interact with Ca 2+ /calmodulin (CaM)-dependent protein kinase II δ (CaMKIIδ) and to inhibit CaMKIIδ activity. CST also reduced CaMKIIδ-dependent phosphorylation of the ryanodine receptor 2 and phospholamban. In line with CaMKIIδ inhibition, CST reduced Ca 2+ spark and wave frequency, attenuated Ca 2+ sparkdimensions, increased sarcoplasmic reticulum Ca 2+ content, andaugmented magnitude and kinetics of cardiomyocyte Ca 2+ transients and contractions. Frequency dependent acceleration of relaxation was most pronounced in the Control group, an indication of CaMKIIδ activation, and this was attenuated by CST. Conclusions: CgA regulates cardiomyocyte Ca 2+ handling via CaMKIIδ inhibition, which makes CgA an interesting CV biomarker and potential compensatory mechanism in situations of enhanced CaMKIIδ activity.

Physiologic force-frequency response in engineered heart muscle by electromechanical stimulation

A hallmark of mature mammalian ventricular myocardium is a positive force-frequency relationship (FFR). Despite evidence of organotypic structural and molecular maturation, a positive FFR has not been observed in mammalian tissue engineered heart muscle. We hypothesized that concurrent mechanical and electrical stimulation at frequencies matching physiological heart rate will result in functional maturation. We investigated the role of biomimetic mechanical and electrical stimulation in functional maturation in engineered heart muscle (EHM). Following tissue consolidation, EHM were subjected to electrical field stimulation at 0, 2, 4, or 6 Hz for 5 days, while strained on flexible poles to facilitate auxotonic contractions. EHM stimulated at 2 and 4 Hz displayed a similarly enhanced inotropic reserve, but a clearly diverging FFR. The positive FFR in 4 Hz stimulated EHM was associated with reduced calcium sensitivity, frequency-dependent acceleration of relaxation, and enhanced post-rest potentiation. This was paralleled on the cellular level with improved calcium storage and release capacity of the sarcoplasmic reticulum and enhanced T-tubulation. We conclude that electro-mechanical stimulation at a physiological frequency supports functional maturation in mammalian EHM. The observed positive FFR in EHM has important implications for the applicability of EHM in cardiovascular research.

While systolic cardiomyocyte function is preserved, diastolic myocyte function and recovery from acidosis are impaired in CaMKIIδ-KO mice

ObjectiveCaMKII contributes to impaired contractility in heart failure by inducing SR Ca2+-leak. CaMKII-inhibition in the heart was suggested to be a novel therapeutic principle. Different CaMKII isoforms exist. Specifically targeting CaMKIIδ, the dominant isoform in the heart, could be of therapeutic potential without impairing other CaMKII isoforms. RationaleWe investigated whether cardiomyocyte function is affected by isoform-specific knockout (KO) of CaMKIIδ under basal conditions and upon stress, i.e. upon ß-adrenergic stimulation and during acidosis. ResultsSystolic cardiac function was largely preserved in the KO in vivo (echocardiography) corresponding to unchanged Ca2+-transient amplitudes and isolated myocyte contractility in vitro. CaMKII activity was dramatically reduced while phosphatase-1 inhibitor-1 was significantly increased. Surprisingly, while diastolic Ca2+-elimination was slower in KO most likely due to decreased phospholamban Thr-17 phosphorylation, frequency-dependent acceleration of relaxation was still present. Despite decreased SR Ca2+-reuptake at lower frequencies, SR Ca2+-content was not diminished, which might be due to reduced diastolic SR Ca2+-loss in the KO as a consequence of lower RyR Ser-2815 phosphorylation. Challenging KO myocytes with isoproterenol showed intact inotropic and lusitropic responses. During acidosis, SR Ca2+-reuptake and SR Ca2+-loading were significantly impaired in KO, resulting in an inability to maintain systolic Ca2+-transients during acidosis and impaired recovery. ConclusionsInhibition of CaMKIIδ appears to be safe under basal physiologic conditions. Specific conditions exist (e.g. during acidosis) under which CaMKII-inhibition might not be helpful or even detrimental. These conditions will have to be more clearly defined before CaMKII inhibition is used therapeutically.

CaMKII effects on inotropic but not lusitropic force frequency responses require phospholamban

Increasing heart rate enhances cardiac contractility (force frequency relationship, FFR) and accelerates cardiac relaxation (frequency-dependent acceleration of relaxation, FDAR). The positive FFR together with FDAR promotes rapid filling and ejection of blood from the left ventricle (LV) at higher heart rates. Recent studies indicate that the multifunctional Ca2+/calmodulin-dependent protein kinase II (CaMKII) is involved in regulating FFR and FDAR. We used isolated perfused mouse hearts to study the mechanisms of FFR and FDAR in different genetic models, including transgenic myocardial CaMKII inhibition (AC3-I) and phospholmban knockout (PLN−/−). When the rate was increased from 360beats/min to 630beats/min in wild type mouse hearts, the LV developed pressure (LVDP) and the maximum rate of increase in pressure (dP/dt max) increased by 37.6±4.7% and 77.0±8.1%, respectively. However, hearts from AC3-I littermates showed no increase of LVDP and a relatively modest (20.4±3.9%) increase in dP/dt max. PLN−/− hearts had a negative FFR, and myocardial AC3-I expression did not change the FFR in PLN−/− mice. PLN−/− mouse hearts did not exhibit FDAR, while PLN−/− mice with myocardial AC3-I expression showed further frequency dependent reductions in cardiac relaxation, suggesting that CaMKII targets in addition to PLN were critical to myocardial relaxation. We incubated a constitutively active form of CaMKII with chemically-skinned myocardium and found that several myofilament proteins were phosphorylated by CaMKII. However, CaMKII did not affect myofilament calcium sensitivity. Our study shows that CaMKII plays an important role in modulating FFR and FDAR in murine hearts and suggest that PLN is a critical target for CaMKII effects on FFR, while CaMKII effects on FDAR partially require PLN-alternative targets.

Synergistic Effects between Phosphorylation of Phospholamban and Troponin I Promote Relaxation at Higher Heart Rate

We hypothesized that the extent of frequency-dependent acceleration of relaxation (FDAR) would be less than that of isoproterenol-(ISO-)dependent acceleration of relaxation (IDAR) at the same increment of heart rates, and ISO may improve FDAR. Cardiac function and phosphorylation of PLB and cTnI were compared in pacing, ISO treatment, and combined pacing and ISO treatment in isolated working heart. The increase in cardiac output and the degree of relaxation was less in pacing than in ISO treatment at the same increment of heart rates. The increasing stimulation frequency induced more significant relaxant effect in ISO perfusion than that in physiological salt perfusion. The pacing only phosphorylated PLB at Thr17, but ISO induced phosphorylation of cTnI and PLB at Ser16 and Thr17. Those results suggest that the synergistic effects of PLB and cTnI induce higher degree of relaxation which makes a sufficient diastolic filling of the ventricle at higher heart rate.

The cellular force‐frequency response in ventricular myocytes from the varanid lizard, Varanus exanthematicus

To investigate the cellular mechanisms underlying the negative force‐frequency relationship in the ventricle of the varanid lizard, we measured sarcomere and cell shortening, intracellular Ca2+ ([Ca2+]i), action potentials (APs) and K+ currents in isolated ventricular myocytes. Experiments were conducted between 0.2–1.0 Hz, which spans the physiological range of heart rates at 20–22°C for this species. As frequency increased, diastolic length, percent change in sarcomere length and relaxation time decreased significantly. Shortening velocity was unchanged. The rate of rise of [Ca2+]i increased, [Ca2+]i transient amplitude decreased, and diastolic [Ca2+]i increased. The first order time constant of the [Ca2+]i transient decay decreased at higher frequencies, indicating frequency dependent acceleration of relaxation, then leveled out and did not change above 0.5 Hz. The AP rate of rise was unaffected, but the AP duration (APD) decreased with increasing frequency. Peak depolarization was only significantly decreased at 1.0 Hz. The decrease in APD was not due to changes in IKr or Ito. Our results suggest that a negative FFR in varanid lizard ventricle is caused by decreased amplitude of the [Ca2+]i transient with an increase in diastolic [Ca2+]i, leading to incomplete relaxation between beats at high frequencies. This coincides with shortened action potential duration (APD) at higher frequencies.

The cellular force-frequency response in ventricular myocytes from the varanid lizard, Varanus exanthematicus

To investigate the cellular mechanisms underlying the negative force-frequency relationship (FFR) in the ventricle of the varanid lizard, Varanus exanthematicus, we measured sarcomere and cell shortening, intracellular Ca(2+) ([Ca(2+)](i)), action potentials (APs), and K(+) currents in isolated ventricular myocytes. Experiments were conducted between 0.2 and 1.0 Hz, which spans the physiological range of in vivo heart rates at 20-22 degrees C for this species. As stimulation frequency increased, diastolic length, percent change in sarcomere length, and relaxation time all decreased significantly. Shortening velocity was unaffected. These changes corresponded to a faster rate of rise of [Ca(2+)](i), a decrease in [Ca(2+)](i) transient amplitude, and a seven-fold increase in diastolic [Ca(2+)](i). The time constant for the decay of the Ca(2+) transient (tau) decreased at higher frequencies, indicating a frequency-dependent acceleration of relaxation (FDAR) but then reached a plateau at moderate frequencies and did not change above 0.5 Hz. The rate of rise of the AP was unaffected, but the AP duration (APD) decreased with increasing frequency. Peak depolarization tended to decrease, but it was only significant at 1.0 Hz. The decrease in APD was not due to frequency-dependent changes in the delayed inward rectifier (I(Kr)) or the transient outward (I(to)) current, as neither appeared to be present in varanid ventricular myocytes. Our results suggest that a negative FFR relationship in varanid lizard ventricle is caused by decreased amplitude of the Ca(2+) transient coupled with an increase in diastolic Ca(2+), which leads to incomplete relaxation between beats at high frequencies. This coincides with shortened APD at higher frequencies.

Blunted frequency-dependent upregulation of cardiac output is related to impaired relaxation in diastolic heart failure.

AimsWe tested the hypothesis that, in heart failure with normal ejection fraction (HFNEF), diastolic dysfunction is accentuated at increasing heart rates, and this contributes to impaired frequency-dependent augmentation of cardiac output.Methods and resultsIn 17 patients with HFNEF (median age 69 years, 13 female) and seven age-matched control patients, systolic and diastolic function was analysed by pressure–volume loops at baseline heart rate and during atrial pacing to 100 and 120 min−1. At baseline, relaxation was prolonged and end-diastolic left ventricular stiffness was higher in HFNEF, whereas all parameters of systolic function were not different from control patients. This resulted in smaller end-diastolic volumes, higher end-diastolic pressure, and a lower stroke volume and cardiac index in HFNEF vs. control patients. During pacing, frequency-dependent upregulation of contractility indices (+dP/dtmax and Ees) occurred similarly in HFNEF and control patients, but frequency-dependent acceleration of relaxation (dP/dtmin) was blunted in HFNEF. In HFNEF, end-diastolic volume and stroke volume decreased with higher heart rates while both remained unchanged in control patients.ConclusionIn HFNEF, frequency-dependent upregulation of cardiac output is blunted. This results from progressive volume unloading of the left ventricle due to limited relaxation reserve in combination with increased LV passive stiffness, despite preserved force–frequency relation.

Cardiac force-frequency relationship and frequency-dependent acceleration of relaxation are impaired in LPS-treated rats

IntroductionFrequency-dependent acceleration of relaxation (FDAR) ensures appropriate ventricular filling at high heart rates and results from accelerated sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) activity independent of calcium removal from the cell. Because lipopolysaccharide (LPS) challenge may induce aberrations in calcium trafficking and protein phosphorylation, we tested whether LPS would abolish FDAR in rats.MethodsFollowing LPS injection, changes in force-frequency relationship and FDAR were studied in cardiomyocytes, isolated hearts and in vivo by echocardiography. Calcium uptake and phosphatase activities were studied in sarcoplasmic reticulum (SR) vesicle preparations. Western blots of phospholamban and calcium/calmodulin-dependent protein kinase II, and serine/threonine phosphatase activity were studied in heart preparations.ResultsIn cardiomyocytes and isolated heart preparations, reductions in time constant of relaxation (τ) and time to 50% relaxation at increasing rate of pacing were blunted in LPS-treated rats compared with controls. Early diastolic velocity of the mitral annulus (Ea), a relaxation parameter which correlates in vivo with τ, was reduced in LPS rats compared with control rats. LPS impaired SR calcium uptake, reduced phospholamban phosphorylation and increased serine/threonine protein phosphatase activity. In vivo inhibition of phosphatase activity partially restored FDAR, reduced phosphatase activity and prevented phospholamban dephosphorylation in LPS rat hearts.ConclusionsLPS impaired phospholamban phosphorylation, cardiac force-frequency relationship and FDAR. Disruption of frequency-dependent acceleration of LV relaxation, which normally participates in optimal heart cavity filling, may be detrimental in sepsis, which is typically associated with elevated heart rates and preload dependency.

The cardiac force-frequency relationship and frequency-dependent acceleration of relaxation are impaired in lipopolysaccharide-treated rats: is the phospholamban-SERCA axis a therapeutic target?

Sepsis-induced myocardial dysfunction has traditionally been thought of as principally affecting systolic heart function. One of the primary reasons for this concept is that systolic dysfunction is relatively easy to conceptualize, visualize, and measure. With the advent of preload-independent measurements for diastolic function, both measurement and conceptual difficulties are being resolved, and a new realm of evidence is beginning to emerge regarding the aberrations that are found during cardiac relaxation in sepsis. A recent article in Critical Care brings this issue into sharper focus.

A Simulation Study on the Activation of Cardiac CaMKII δ-Isoform and Its Regulation by Phosphatases

Although the highly conserved Ca 2+/calmodulin-dependent protein kinase II (CaMKII) is known to play an essential role in cardiac myocytes, its involvement in the frequency-dependent acceleration of relaxation is still controversial. To investigate the functional significance of CaMKII autophosphorylation and its regulation by protein phosphatases (PPs) in heart, we developed a new mathematical model for the CaMKII δ isoform. Due to better availability of experimental data, the model was first adjusted to the kinetics of the neuronal CaMKII α isoform and then converted to a CaMKII δ model by fitting to kinetic data of the δ isoform. Both models satisfactorily reproduced experimental data of the CaMKII-calmodulin interaction, the autophosphorylation rate, and the frequency dependence of activation. The level of autophosphorylated CaMKII cumulatively increased upon starting the Ca 2+ stimulation at 3 Hz in the δ model. Variations in PP concentration remarkably affected the frequency-dependent activation of CaMKII δ, suggesting that cellular PP activity plays a key role in adjusting CaMKII activation in heart. The inhibitory effect of PP was stronger for CaMKII α compared to CaMKII δ. Simulation results revealed a potential involvement of CaMKII δ autophosphorylation in the frequency-dependent acceleration of relaxation at physiological heart rates and PP concentrations.

Reduced contractile response to α 1-adrenergic stimulation in atria from mice with chronic cardiac calmodulin kinase II inhibition

The sustained positive inotropic effect of α-adrenoceptor agonists in the heart is associated with a small increase in intracellular Ca 2+ transients together with a larger sensitization of myofilaments to Ca 2+. The multifunctional Ca 2+ and calmodulin-dependent protein kinase II (CaMKII) could contribute to this effect, either by affecting the Ca 2+ release (ryanodine receptor) or by an uptake mechanism (via phospholamban [PLB] and SR Ca 2+ ATPase). Here we examined the role of CaMKII in the positive inotropic effect of the α-adrenoceptor agonist phenylephrine in left atria isolated from a genetic mouse model of cardiac CaMKII inhibition (AC3-I). Compared to atria from wild-type (WT) or AC3-C (scrambled peptide), AC3-I atria showed the following abnormalities. PLB phosphorylation at Thr17, a known CaMKII target, was significantly lower (∼ 20%). Post-rest (30 s, 1 Hz, 37 °C) potentiation of force was absent (AC3-C, 190% of pre-rest amplitude). Basal force was ∼ 20% lower at 1.8 mM Ca 2+, but normal at high Ca 2+ concentration (> 4.5 mM). The maximal positive inotropic effect of phenylephrine, which was more pronounced at low frequencies in WT and AC3-C atria, lost its frequency dependence (1 Hz to 8 Hz). Thus, the effect of phenylephrine was reduced by ∼ 50% at 1 Hz, but was normal at 8 Hz. All three groups showed a negative force–frequency relation, and did not differ in the frequency-dependent acceleration of relaxation. Our data indicate a role of CaMKII in post-rest potentiation and the positive inotropic effect of α-adrenergic stimulation at low frequencies.

Temporal dissociation of frequency-dependent acceleration of relaxation and protein phosphorylation by CaMKII

Frequency-dependent acceleration of relaxation (FDAR) is an important intrinsic mechanism that allows for diastolic filling of the ventricle at higher heart rates, yet its molecular mechanism is still not understood. Previous studies showed that FDAR is dependent on functional sarcoplasmic reticulum (SR) and can be abolished by phosphatase or by Ca/CaM kinase (CaMKII) inhibition. Additionally, CaMKII activity/autophosphorylation has been shown to be frequency-dependent. Thus, we tested the hypothesis that CaMKII phosphorylation of SR Ca 2+-handling proteins (Phospholamban (PLB), Ca 2+ release channel (RyR)) mediates FDAR. Here we show that FDAR occurs abruptly in fluo-4 loaded isolated rat ventricular myocytes when frequency is raised from 0.1 to 2 Hz. The effect is essentially complete within four beats (2 s) with the τ of [Ca 2+] i decline decreasing by 42 ± 3%. While there is a detectable increase in PLB Thr-17 and RyR Ser-2814 phosphorylation, the increase is quantitatively small (PLB < 5%, RyR ∼ 8%) and the time-course is clearly delayed with regard to FDAR. The low substrate phosphorylation indicates that pacing of myocytes only mildly activates CaMKII and consistent with this CaMKIIδ autophosphorylation did not increase with pacing alone. However, in the presence of phosphatase 1 inhibition pacing triggered a net-increase in autophosphorylated CaMKII and also greatly enhanced PLB and RyR phosphorylation. We conclude that FDAR does not rely on phosphorylation of PLB or RyR. Even though CaMKII does become activated when myocytes are paced, phosphatases immediately antagonize CaMKII action, limit substrate phosphorylation and also prevent sustained CaMKII autophosphorylation (thereby suppressing global CaMKII effects).