EFFECTS OF REST INTERVAL ON THE RELEASE OF CALCIUM FROM THE SARCOPLASMIC-RETICULUM IN ISOLATED GUINEA-PIG VENTRICULAR MYOCYTES

.Guinea pig cardiac myocytes were loaded with the fluorescent dye indo 1, and cell contraction was measured by a video edge-detection system. Ca2+ was released from the sarcoplasmic reticulum (SR) by rapidly cooling the myocytes or by rapid application of 10 mmol/L caffeine. Estimates of the amount of Ca2+ released from the SR after different rest intervals (ie, under different loading conditions) were obtained by measuring the current evoked by rapid application of 10 mmol/L caffeine, which we call Na+/Ca2+ exchange current. This current is completely inhibited by removal of extracellular Na+ and Ca2+ or by application of 5 mmol/L Ni2+. SR Ca2+ release after rest intervals of 5 to 120 seconds (assuming cell volume to be 30x10(-12) L) was estimated to be 57.8+/-5.7 to 25.7+/-4.5 mu mol/L accessible cell volume, respectively, equivalent to 23 to 10 mu mol/kg wet wt, respectively. There was an exponential decline in Ca2+ released from the SR after rest intervals of 2 to 120 seconds (rate constant, 0.029 s(-1); t(1/2), 24 seconds); thereafter, there remained a portion (56%) of Ca2+ releasable to caffeine application. We found a similar exponential decay (rate constant, 0.020 s(-1) t(1/2), 35 seconds) of the size of rapid cooling contractures with Increasing rest intervals. The time to peak of the Na+/Ca2+ exchange current in the presence of caffeine slowed at long rest intervals, ie, at smaller SR loads. A decrease in SR load of 50% increased the time to peak of the exchange current by 213+/-37% (n=6). The rate of generation of a rapid cooling contracture was faster after short rest intervals and was associated with a faster rise in the corresponding increase in indo 1 fluorescence. A decrease in SR load of 50% decreased the rate of cell shortening to 34+/-5% and decreased the rate of change in fluorescence to 54+/-2% (n=5).