Intra-sarcoplasmic reticulum free [Ca2+] and buffering in arrhythmogenic failing rabbit heart

Smaller Ca2+ transients and systolic dysfunction in heart failure (HF) can be largely explained by reduced total sarcoplasmic reticulum (SR) Ca2+ content ([Ca](SRT)). However, it is unknown whether low [Ca](SRT) is manifest as reduced: (1) intra-SR free [Ca2+] ([Ca2+](SR)), (2) intra-SR Ca2+ buffering, or (3) SR volume (as percentage of cell volume). Here we assess these possibilities in a well-characterized rabbit model of nonischemic HF. In HF versus control myocytes, diastolic [Ca2+](SR) is similar at 0.1-Hz stimulation, but the increase in both [Ca2+](SR) and [Ca](SRT) as frequency increases to 1 Hz is blunted in HF. Direct measurement of intra-SR Ca2+ buffering (by simultaneous [Ca2+](SR) and [Ca](SRT) measurement) showed no change in HF. Diastolic [Ca](SRT) changes paralleled [Ca2+](SR), suggesting that SR volume is not appreciably altered in HF. Thus, reduced [Ca](SRT) in HF is associated with comparably reduced [Ca2+](SR). Fractional [Ca2+](SR) depletion increased progressively with stimulation frequency in control but was blunted in HF (consistent with the blunted force-frequency relationship in HF). By studying a range of [Ca2+](SR), analysis showed that for a given [Ca](SR), fractional SR Ca2+ release was actually higher in HF. For both control and HF myocytes, SR Ca2+ release terminated when [Ca2+](SR) dropped to 0.3 to 0.5 mmol/L during systole, consistent with a role for declining [Ca2+](SR) in the dynamic shutoff of SR Ca2+ release. We conclude that low total SR Ca2+ content in HF, and reduced SR Ca2+ release, is attributable to reduced [Ca2+](SR), not to alterations in SR volume or Ca2+ buffering capacity.