Voltage-independent changes in L-type Ca²⁺ current uncoupled from SR Ca²⁺ release in cardiac myocytes

To determine the effect of voltage-independent alterations of L-type Ca²⁺ current (I(Ca)) on the sarcoplasmic reticular (SR) Ca²⁺ release in cardiac myocytes, we measured I(Ca) and cytosolic Ca²⁺ transients (Ca(i)/²⁺; intracellular Ca²⁺ concentration) in voltage-clamped rat ventricular myocytes during 1) an abrupt increase of extracellular [Ca²⁺] (Ca(o)/²⁺) or 2) application of 1 ♂m FPL-64176, a Ca²⁺ channel agonist, to selectively alter I(Ca) in the absence of changes in SR Ca²⁺ loading. On the first depolarization in higher Ca(o)/²⁺, peak I(Ca) was increased by 46 ± 6% (P < 0.001), but the increases in the maximal rate of rise of Ca(i)/²⁺ (dCa(i)/²⁺/dt(max), where t is time; an index of SR Ca²⁺ release flux) and the Ca(i)/²⁺ transient amplitude were not significant. Rapid exposure to FPL-64176 greatly slowed inactivation of I(Ca), increasing its time integral by 117 ± 8% (P < 0.001) without significantly increasing peak I(Ca), Dca(i)/²⁺/dt(max), or amplitude of the corresponding Ca(i)/²⁺ transient. Prolongation of exposure to higher Ca(o)/²⁺ or FPL-64176 did not further increase peak I(Ca) but greatly increased Dca(i)/²⁺/dt(max), Ca(i)/²⁺ transient amplitude, and the gain of Ca²⁺ release (Dca(i)/²⁺/dt(max)/I(Ca), evidently due to augmentation of the SR Ca²⁺ loading. Also, the time to peak Dca(i)/²⁺/dt(max) was significantly increased in the continuous presence of higher Ca(o)/²⁺ (by 37 ± 5%, P < 0.001) or FPL-64176 (by 63 ± 5%, P < 0.002). Our experiments provide the first evidence of a marked disparity between an increased peak I(Ca) and the corresponding SR Ca²⁺ release. We attribute this to saturation of the SR Ca²⁺ release flux as predicted by local control theory. Prolongation of the SR Ca²⁺ release flux, caused by combined actions of a larger I(Ca) and maximally augmented SR Ca²⁺ loading, might reflect additional Ca²⁺ release from corbular SR.