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⁺ leading 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°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⁺ lead 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 falls to replenish the SR with Ca²⁺, diminished the SR Ca²⁺ lead 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 ± 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 (r² = 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.