EFFECTS OF SARCOPLASMIC-RETICULUM CA2+ LOAD ON THE GAIN FUNCTION OF CA2+ RELEASE BY CA2+ CURRENT IN CARDIAC-CELLS

We studied the effects of variable sarcoplasmic reticulum (SR) Ca2+ loading on changes in the gain index of Ca2+ release from the SR, measured as the ratio of the amount of Ca2+ released to the magnitude of the Ca2+ current (I-Ca) integrated for the initial 20 ms of the depolarization, in whole cell voltage-clamped rat ventricular myocytes dialyzed with the Ca2+ indicator indo 1 salt at 23 degrees C. Changes in I-Ca were measured directly, and changes in the SR Ca2+ release were indexed by changes in the amplitudes and rates of rise of cytosolic Ca2+ (Ca-i(2+)) transients. The SR Ca2+ load was graded by the duration of conditioning voltage-clamp steps and verified by caffeine-dependent Ca-i(2+) transients. A train of abbreviated (from 100 to 20 ms) voltage-clamp depolarizations, which triggers SR Ca2+ release but fails to replenish the SR with Ca2+, diminished the SR Ca2+ load by 56 +/- 5%, did not alter peak I-Ca but reduced the amplitudes of the I-Ca-dependent Ca-i(2+) transients by 52 +/- 3%, and decreased the gain index by 60 i 3% (SE; n = 5 or 6). Changes in the amplitudes of Ca-i(2+) transients elicited by I-Ca and changes in the gain index were Linearly correlated (r(2) = 0.83 and 0.79, respectively; P < 0.001 for each) with changes in amplitudes of Ca-i(2+) transients elicited by caffeine pulses applied in lieu of the respective voltage-clamp pulses. The linear dependence of gain on SR Ca2+ loading provides further evidence against the existence of strong positive feedback of released Ca2+ on the Ca2+ release triggering mechanism and has theoretical implications for force-interval and staircase phenomena in the heart.