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+ c urrent (I-Ca)on the sarcoplasmic reticular (SR) Ca2+ release in cardiac myo cytes, we measured I-Ca and cytosolic Ca2+ transients (Ca-i(2+); intracellu lar Ca2+ concentration) in voltage-clamped rat ventricular myocytes during 1) an abrupt increase of extracellular [Ca2+] (Ca-o(2+)) or 2) application of 1 mu M FPL-64176, a Ca2+ channel agonist, to selectively alter I-Ca in t he absence of changes in SR Ca2+ loading. On the first depolarization in hi gher Ca-o(2+), peak I-Ca was increased by 46 +/- 6% (P < 0.001), but the in creases 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 amp litude were not significant. Rapid exposure to FPL-64176 greatly slowed ina ctivation of I-Ca, increasing its time integral by 117 +/- 8% (P < 0.001) w ithout significantly increasing peak I-Ca, dCac(i)(2+)/ dt(max), or amplitu de of the corresponding Ca-i(2+) transient. Prolongation of exposure to hig her Ca-o(2+) or FPL-64176 did not further increase peak I-Ca but greatly in creased dCa(i)(2+)/ dt(max),Cac(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+)/dtmax was significa ntly 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 an d 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.