Background: Abnormalities in intracellular calcium (Ca) cycling during Ca overload can cause triggered activity because spontaneous calcium release (SCR) activates sufficient Ca-sensitive inward currents to induce delayed afterdepolarizations (DADs). However, little is known about the mechanisms relating SCR and triggered activity on the tissue scale. Methods and Results: Laser scanning confocal microscopy was used to measure the spatiotemporal properties of SCR within large myocyte populations in intact rat heart. Computer simulations were used to predict how these properties of SCR determine DAD magnitude. We measured the average and standard deviation of the latency distribution of SCR within a large population of myocytes in intact tissue. We found that as external [Ca] is increased, and with faster pacing rates, the average and SD of the latency distribution decreases substantially. This result demonstrates that the timing of SCR occurs with less variability as the sarcoplasmic reticulum (SR) Ca load is increased, causing more sites to release Ca within each cell. We then applied a mathematical model of subcellular Ca cycling to show that a decrease in SCR variability leads to a higher DAD amplitude and is dictated by the rate of SR Ca refilling following an action potential. Conclusions: Our results demonstrate that the variability of the timing of SCR in a population of cells in tissue decreases with SR load and is dictated by the time course of the SR Ca content.

Abstract-Optical mapping studies have suggested that intracellular Ca 2ϩ and T-wave alternans are linked through underlying alternations in Ca 2ϩ cycling-inducing oscillations in action potential duration through Ca 2ϩ -sensitive conductances. However, these studies cannot measure single-cell behavior; therefore, the Ca 2ϩ cycling heterogeneities within microscopic ventricular regions are unknown. The goal of this study was to measure cellular activity in intact myocardium during rapid pacing and arrhythmias. We used single-photon laser-scanning confocal microscopy to measure Ca 2ϩ signaling in individual myocytes of intact rat myocardium during rapid pacing and during pacing-induced ventricular arrhythmias. At low rates, all myocytes demonstrate Ca 2ϩ alternans that is synchronized but whose magnitude varies depending on recovery kinetics of Ca 2ϩ cycling for each individual myocyte. As rate increases, some cells reverse alternans phase, giving a dyssynchronous activation pattern, even in adjoining myocytes. Increased pacing rate also induces subcellular alternans where Ca 2ϩ alternates out of phase with different regions within the same cell. These forms of heterogeneous Ca 2ϩ signaling also occurred during pacing-induced ventricular tachycardia. Our results demonstrate highly nonuniform Ca 2ϩ signaling among and within individual myocytes in intact heart during rapid pacing and arrhythmias. Thus, certain pathophysiological conditions that alter Ca 2ϩ cycling kinetics, such as heart failure, might promote ventricular arrhythmias by exaggerating these cellular heterogeneities in Ca 2ϩ signaling. (Circ Res. 2006;99:e65-e73.) Key Words: calcium transients Ⅲ calcium alternans Ⅲ subcellular alternans Ⅲ arrhythmias O ne of the most important clues to the mechanisms responsible for repolarization alternans was derived from the fact that action potential duration (APD) alternans occurs at the cellular level in intact heart. 1-3 It is now widely accepted that T-wave alternans (TWA) on the surface ECG reflects tissue repolarization alternans at the level of the whole heart. In contrast to a purely electrophysiological explanation involving ion channel kinetics, 4,5 evidence suggests that APD and T-wave alternans are in fact associated with changes in intracellular Ca 2ϩ dynamics. 2,[5][6][7] The link between alternations in intracellular Ca 2ϩ dynamics and TWA has recently been summarized 2 as possibly arising from underlying alternans in Ca 2ϩ cycling. Intracellular Ca 2ϩ release enters into an alternating pattern based on the balance between the dynamics of Ca 2ϩ release, reuptake, and recovery rates that induce oscillations in APD as a result of Ca 2ϩ -sensitive conductances. Theoretically, a large contraction occurs as the result of a large release of Ca 2ϩ from stores in the sarcoplasmic reticulum (SR), which would in turn cause a large inward Na/Ca exchange current (I NCX ) and a long APD. Because the large SR Ca 2ϩ release would have the effect of temporary depletion of SR Ca 2ϩ content, the next beat would activ...

Although the development of abnormal myocardial mechanics represents a key step during the transition from hypertension to overt heart failure (HF), the underlying ultrastructural and cellular basis of abnormal myocardial mechanics remains unclear. We therefore investigated how changes in transverse (T)-tubule organization and the resulting altered intracellular Ca(2+) cycling in large cell populations underlie the development of abnormal myocardial mechanics in a model of chronic hypertension. Hearts from spontaneously hypertensive rats (SHRs; n = 72) were studied at different ages and stages of hypertensive heart disease and early HF and were compared with age-matched control (Wistar-Kyoto) rats (n = 34). Echocardiography, including tissue Doppler and speckle-tracking analysis, was performed just before euthanization, after which T-tubule organization and Ca(2+) transients were studied using confocal microscopy. In SHRs, abnormalities in myocardial mechanics occurred early in response to hypertension, before the development of overt systolic dysfunction and HF. Reduced longitudinal, circumferential, and radial strain as well as reduced tissue Doppler early diastolic tissue velocities occurred in concert with T-tubule disorganization and impaired Ca(2+) cycling, all of which preceded the development of cardiac fibrosis. The time to peak of intracellular Ca(2+) transients was slowed due to T-tubule disruption, providing a link between declining cell ultrastructure and abnormal myocardial mechanics. In conclusion, subclinical abnormalities in myocardial mechanics occur early in response to hypertension and coincide with the development of T-tubule disorganization and impaired intracellular Ca(2+) cycling. These changes occur before the development of significant cardiac fibrosis and precede the development of overt cardiac dysfunction and HF.

The mechanisms by which digitalis causes its therapeutic and toxic actions have been studied for nearly a half century, revealing a great deal about cardiac cell regulation of intracellular ions via the Na-K-ATPase (NKA) and how it is altered by cardiac glycosides. However, recent observations suggest that digitalis may have additional effects on cardiac cell function in both the short and long term that include intracellular effects, interactions with specific NKA isoforms in different cellular locations, effects on intracellular (including nuclear) signaling, and long-term regulation of intracellular ionic balances through circulating ouabain-like compounds. The purpose of this review is to examine the current status of a number of the newest and most interesting developments in the study of digitalis with a particular focus on cardiac function, although we will also discuss some of the new advances in other relevant cardiovascular effects. This new information has important implications for both our understanding of ionic regulation in normal and diseased hearts as well as for potential avenues for the development of future therapeutic interventions for the treatment of heart failure.

Although fast sodium current (INa) plays a major role in the generation and conduction of the cardiac impulse, the electrophysiological characteristics of INa in isolated human ventricular myocytes have not yet been fully described. We characterized the human ventricular INa of enzymatically isolated myocytes using whole cell voltage-clamp techniques. Sixty myocytes were isolated from ventricular specimens obtained from 22 patients undergoing open-heart surgery. A low temperature (17 degrees C) and Na+ concentration in the external solution (5 or 10 mM) allowed good voltage control and facilitated the measurement of INa. Cs+ was substituted for K+ in both internal and external solutions to block K+ currents, and F- was added to the internal solution to block Ca2+ current. INa was activated at a voltage threshold of approximately -70 mV, and maximal inward current was obtained at approximately -30 mV (holding potential = -140 mV). The voltage dependence of steady-state INa availability (h infinity) was sigmoidal with half inactivation occurring at -97.3 +/- 1.1 mV and a slope factor of 5.77 +/- 0.10 mV (n = 60). We did not detect any significant differences in these parameters in cells from patients with a variety of disease states, with or without congestive heart failure. The overlap in voltage dependence of h infinity and Na+ conductance suggested the presence of a Na+ "window" current. An inactivation time course was voltage dependent and was fitted best by the sum of two exponentials. The rate of recovery from inactivation also was voltage dependent and fitted by the sum of two exponentials.(ABSTRACT TRUNCATED AT 250 WORDS)