Search

Defects in excitation-contraction coupling have been reported in failing hearts, but little is known about the relationship between these defects and the development of heart failure (HF). We compared the early changes in intracellular [Ca.sup.2+] cycling to those that underlie overt pump dysfunction and arrhythmogenesis found later in HF. Laser-scanning confocal microscopy was used to measure [Ca.sup.2+] transients in myocytes of intact hearts in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs) at different ages. Early compensatory mechanisms include a positive inotropic effect in SHRs at 7.5-9 mo compared with 6 mo. [Ca.sup.2+] transient duration increased at 9 mo in SHRs, indicating changes in [Ca.sup.2+] reuptake during decompensation. Cell-to-cell variability in [Ca.sup.2+] transient duration increased at 7.5 mo, decreased at 9 mo, and increased again at 22 mo (overt HF), indicating extensive intercellular variability in [Ca.sup.2+] transient kinetics during disease progression. Vulnerability to intercellular concordant [Ca.sup.2+] alternans increased at 9-22 mo in SHRs and was mirrored by a slowing in [Ca.sup.2+] transient restitution, suggesting that repolarization alternans and the resulting repolarization gradients might promote reentrant arrhythmias early in disease development. Intercellular discordant and subcellular [Ca.sup.2+] alternans increased as early as 7.5 mo in SHRs and may also promote arrhythmias during the compensated phase. The incidence of spontaneous and triggered [Ca.sup.2+] waves was increased in SHRs at all ages, suggesting a higher likelihood of triggered arrhythmias in SHRs compared with WKY rats well before HF develops. Thus serious and progressive defects in [Ca.sup.2+] cycling develop in SHRs long before symptoms of HF occur. Defective [Ca.sup.2+] cycling develops early and affects a small number of myocytes, and this number grows with age and causes the transition from asymptomatic to overt HF. These defects may also underlie the progressive susceptibility to [Ca.sup.2+] alternans and [Ca.sup.2+] wave activity, thus increasing the propensity for arrhythmogenesis in HF. calcium alternans; calcium transients; excitation-contraction coupling; heart failure; spontaneously hypertensive rats doi: 10.1152/ajpheart.00623.2010.

Cardiac cellular [Ca.sup.2+] transient (CAT) alternans and electrocardiographic T-wave alternans (TWA) often develop in myocardial ischemia, but the mechanisms for this relationship have not been elucidated. Acidosis is a major component of ischemia, but there is no direct evidence linking acidosis-induced cellular CaT alternans to ischemia-induced CaT alternans and TWA in whole heart. We used laser-scanning confocal microscopy to measure intracellular [Ca.sup.2+] ([Ca.sup.2+.sub.i]) cycling in individual myocytes of fluo-4 AM-loaded rat hearts and simultaneously recorded pseudo-ECGs to investigate changes in CaTs and late-phase repolarization, respectively, during baseline and rapid pacing under control and either globally acidic or globally ischemic conditions. Acidosis (hypercapnia; pH 6.6) increased diastolic [Ca.sup.2+.sub.i] levels, prolonged CaT duration, and shifted to slower heart rates both the development of pacing-induced acidosis-induced CaT alternans (both concordant and discordant) and of repolarization alternans (RPA, a measure of TWA in rat ECGs). The magnitudes of these shifts were equivalent for both CaT alternans and RPA, suggesting a close association between them. Nearly identical results were found in low-flow global ischemia. Additionally, ischemic preconditioning reduced the increased propensity for CaT altemans and RPA development and was mimicked by preconditioning by acidosis alone. Our results demonstrate that global acidosis or ischemia modifies [Ca.sup.2+.sub.i] cycling in myocytes such that the diastolic [Ca.sup.2+.sub.i] rises and the cellular CaT duration is prolonged, causing spatially concordant as well as spatially discordant cellular CaT alternans to develop at slower heart rates than in controls. Since RPA also developed at slower heart rates, our results suggest that acidosis is a major contributor to CaT alternans, which underlies the proarrhythmic state induced by myocardial ischemia and therefore may play a role in its modulation and prevention. arrhythmia; calcium cycling

Males and females show distinct differences in action potential waveform, ion channel expression patterns, and ECG characteristics. However, it is not known how sex-based differences in [Ca.sup.2+] cycling might contribute to these differences in electrophysiological activity. The goal of this study was to investigate the differences in cellular [Ca.sup.2+] transients in males and females and to examine how these might contribute to electrophysiological function. [Ca.sup.2+] transients were measured in individual myocytes within microscopic regions of the fluo-4 AM-loaded left ventricular epicardium of intact rat heart of both sexes (3 to 5 mo old). Pacing protocols were used to measure transient characteristics at a basic cycle length of 500 ms and during 10-s trains of rapid pacing delivered to the left ventricular apex. [Ca.sup.2+] transients were smaller in magnitude and longer in duration in females than in males. More importantly, the variability in [Ca.sup.2+] transient characteristics between myocytes in a microscopic recording site (heterogeneity index) was greater for females than males for characteristics related to transient duration. The rate sensitivity of [Ca.sup.2+] alternans development in individual myocytes was greater in females than in males, but there was also a greater heterogeneity in cellular responses to the rate dependence of alternans development in females. The longer [Ca.sup.2+] transients in females were also associated with slower restitution, which was likely to be responsible for the development of [Ca.sup.2+] and repolarization alternans at slower heart rates. These results demonstrate that there are distinct differences in cellular [Ca.sup.2+] cycling in male and female rat hearts. Not only is there slower reuptake of [Ca.sup.2+] in female rats, but there is greater local variability in [Ca.sup.2+] cycling at the microscopic level. These sex-based differences in [Ca.sup.2+] cycling could contribute to differences in ECG morphology and in arrhythmia sensitivity in males and females. calcium transients; intercellular heterogeneity; confocal microscopy; T-wave alternans; calcium alternans; restitution

Although the negative inotropic effects of both acute and chronic ethanol (EtOH) exposure are well known, little is known concerning the acute-to-chronic transition of such effects. In this study, our objective was to address this question by detailing the effects that acute EtOH exposure induces on cellular excitation-contraction (EC) coupling and, subsequently, comparing whether and how such changes translate to the early chronic EtOH condition in a rat model known to develop alcohol-induced cardiomyopathy. Acute EtOH exposure, as formerly reported, indeed induced dose-dependent negative inotropic changes in cellular EC coupling, manifest as reductions in cell shortening, [Ca.sup.2+] transient amplitude, [Ca.sup.2+] decay rate, and sarcoplasmic reticulum [Ca.sup.2+] content of isolated rat ventricular cardiac myocytes. Supplementary to this, we found [Ca.sup.2+] spark character not to be significantly affected by acute EtOH exposure. In contrast, the results obtained from cardiac myocytes isolated from rats fed a diet containing ~9% (vol/vol) EtOH for 1 mo revealed changes in these parameters reflecting positive inotropy, whereas at 3 mo, these parameters again reflected negative inotropy similar but not identical to that induced by acute EtOH exposure. No significant changes were evident at either 1- or 3-mo chronic EtOH administration in echocardiographic parameters known to be perturbed in alcoholic cardiomyopathy (ACM), thus indicating that we were examining an asymptomatic stage in chronic EtOH administration consistent with an acute-to-chronic transition phase. Continued study of such transition-phase events should provide important insight into which molecular-cellular components of EC coupling play pivotal roles in EtOH-induced disease processes, such as ACM. rat cardiac myocytes; calcium transients; calcium sparks; confocal calcium imaging doi:10.1152/ajpheart.00214.2006

Activation of cardiac ryanodine receptors by the calcium channel agonist FPL-64176. Am JPhysiol Heart Circ Physiol 283: H331-H338, 2002; 10.1152/ajpheart. 00788.2001.--We investigated the possibility that the [Ca.sup.2+] channel agonist FPL-64176 (FPL) might also activate the cardiac sarcoplasmic reticulum (SR) [Ca.sup.2+] release channel ryanodine receptor (RyR). The effects of FPL were tested on single channel activity of purified and crude vesicular RyR (RyR2) isolated from human and dog hearts using the planar lipid bilayer technique. FPL (100-200 [micro]M) increased single channel open probability ([P.sub.o]) when added to the cytoplasmic side of the channel ([P.sub.o] = 0.070 [+ or -] 0.021 in control RyR2; 0.378 [+ or -] 0.086 in 150 [micro]M FPL, n = 9, P < 0.01) by prolonging open times and decreasing closed times without changing current magnitude. FPL had no effect on [P.sub.o] when added to the trans (luminal) side of the bilayer (P.sub.o] = 0.079 [+ or - ] 0.036 in control and 0.103 [+ or -] 0.066 in FPL, n = 4, no significant difference). The bell-shaped [[Ca.sup.2+]] dependence of [[sup.3]H]ryanodine binding and of Po was altered by FPL, suggesting that the mechanism by which FPL increases channel activity is by an increase in [Ca.sup.2+]-induced activation at low [Ca.sup.2+] (without a change in threshold) and suppression of [Ca.sup.2+]-induced inactivation at high [[Ca.sup.2+]]. However, the fact that inactivation was restored at elevated [[Ca.sup.2+]] suggests a competitive interaction between [Ca.sup.2+] and FPL on inactivation. FPL had no effect on RyR skeletal channels (RyR1), where [P.sub.o] was 0.039 [+ or -] 0.005 in control versus 0.030 [+ or -]0.006 in 150 [micro]M FPL (no significant difference). These results suggest that, in addition to its ability to activate the L-type [Ca.sup.2+] channels, FPL activates cardiac RyR2 primarily by reducing the [Ca.sup.2+] sensitivity of inactivation. sarcoplasmic reticulum; heart; calcium release channel; human; dog

This study investigated the effects of cardiac glycosides on single-channel activity of the cardiac sarcoplasmic reticulum (SR) [Ca.sup.2+] release channels or ryanodine receptor (RyR2) channels and how this action might contribute to their inotropic and/or toxic actions. Heavy SR vesicles isolated from canine left ventricle were fused with artificial planar lipid bilayers to measure single RyR2 channel activity. Digoxin and actodigin increased single-channel activity at low concentrations normally associated with therapeutic plasma levels, yielding a 50% of maximal effect of ~0.2 nM for each agent. Channel activation by glycosides did not require MgATP and occurred only when digoxin was applied to the cytoplasmic side of the channel. Similar results were obtained in human RyR2 channels; however, neither the crude skeletal nor the purified cardiac channel was activated by glycosides. Channel activation was dependent on [[Ca.sup.2+]] on the luminal side of the bilayer with maximal stimulation occurring between 0.3 and 10 mM. Rat RyR2 channels were activated by digoxin only at 1 [micro]M, consistent with the lower sensitivity to glycosides in rat heart. These results suggest a model in which RyR2 channel activation by digoxin occurs only when luminal [[Ca.sup.2+]] was increased above 300 [micro]M (in the physiological range). Consequently, increasing SR load (by [Na.sup.+] pump inhibition) serves to amplify SR release by promoting direct RyR2 channel activation via a luminal [Ca.sup.2+]-sensitive mechanism. This high-affinity effect of glycosides could contribute to increased SR [Ca.sup.2+] release and might play a role in the inotropic and/or toxic actions of glycosides in vivo. digoxin; actodigin; digitalis; human heart