Early development of intracellular calcium cycling defects in intact hearts of spontaneously hypertensive rats

Abstract

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 Ca2+ cycling to those that underlie overt pump dysfunction and arrhythmogenesis found later in HF. Laser-scanning confocal microscopy was used to measure Ca2+ 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. Ca2+ transient duration increased at 9 mo in SHRs, indicating changes in Ca2+ reuptake during decompensation. Cell-to-cell variability in Ca2+ transient duration increased at 7.5 mo, decreased at 9 mo, and increased again at 22 mo (overt HF), indicating extensive intercellular variability in Ca2+ transient kinetics during disease progression. Vulnerability to intercellular concordant Ca2+ alternans increased at 9–22 mo in SHRs and was mirrored by a slowing in Ca2+ transient restitution, suggesting that repolarization alternans and the resulting repolarization gradients might promote reentrant arrhythmias early in disease development. Intercellular discordant and subcellular Ca2+ 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 Ca2+ 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 Ca2+ cycling develop in SHRs long before symptoms of HF occur. Defective Ca2+ 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 Ca2+ alternans and Ca2+ wave activity, thus increasing the propensity for arrhythmogenesis in HF.