Effects of Rest Interval on the Release of Calcium From the Sarcoplasmic Reticulum in Isolated Guinea Pig Ventricular Myocytes

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Abstract
Guinea pig cardiac myocytes were loaded with the fluorescent dye indo 1, and cell contraction was measured by a video edge-detection system. Ca 2+ 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 Ca 2+ 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 + /Ca 2+ exchange current. This current is completely inhibited by removal of extracellular Na + and Ca 2+ or by application of 5 mmol/L Ni 2+ . SR Ca 2+ release after rest intervals of 5 to 120 seconds (assuming cell volume to be 30×10 −12 L) was estimated to be 57.8±5.7 to 25.7±4.5 μmol/L accessible cell volume, respectively, equivalent to 23 to 10 μmol/kg wet wt, respectively. There was an exponential decline in Ca 2+ 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 Ca 2+ 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 + /Ca 2+ 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).