Cytosolic free calcium ion concentration ([Ca2+]i) and whole-cell L-type Ca2+ channel currents were measured during excitation-contraction (E-C) coupling in single voltage-clamped rat cardiac ventricular cells. The measurements were used to compute the total cellular efflux of calcium ions through sarcoplasmic reticulum (SR) Ca2+ release channels (FSR,rel) and the influx of Ca2+ via L-type Ca2+ channels (FICa). 2. FSR,rel was elicited by depolarizing voltage-clamp pulses 200 ms in duration to membrane potentials from -30 to +80 mV. Over this range, peak FSR,rel had a bell-shaped dependence on clamp pulse potential. In all cells, the 'gain' of the system, measured as the ratio, FSR,rel(max)/FICa(max), declined from about 16, at 0 mV, to much lower values as clamp pulse voltage was made progressively more positive. We named this phenomenon of change in gain as a function of membrane potential, 'variable gain'. At clamp pulse potentials in the range -30 to 0 mV, the gain differed from cell to cell, being constant at about 16 in some cells, but decreasing from high values (approximately 65) at -20 mV in others. 3. At clamp pulse potentials at which Ca2+ influx (FICa) was maintained, FSR,rel also had a small maintained component. When macroscopic Ca2+ influx was brief (1-2 ms, during 'tails' of FICa), FSR,rel rose rapidly to a peak after repolarization and then declined with a half-time of about 9 ms (typically). 4. The rising phase of [Ca2+]i transients could be interrupted by stopping Ca2+ influx rapidly (by voltage clamp). We therefore termed this phenomenon 'interrupted SR Ca2+ release'.(ABSTRACT TRUNCATED AT 250 WORDS)
