A MEASURABLE REDUCTION OF SR CA CONTENT FOLLOWS SPONTANEOUS CA RELEASE IN RAT VENTRICULAR MYOCYTES

The Ca content of the sarcoplasmic reticulum (s.r.) was measured in vo ltage-clamped rat ventricular myocytes from the integral of the Na-Ca exchange current evoked by applying caffeine to release the s.r. Ca co ntent. Following spontaneous release of Ca from the s.r., the s.r. Ca content was decreased. The magnitude of this decrease was equal to tha t of the amount of calcium directly measured to have been pumped out o f the cell during the spontaneous release. Following a spontaneous rel ease, the s.r. Ca content recovered linearly. These results are shown to be consistent with the hypothesis that the frequency of spontaneous release is determined by the time taken for the cell and s.r. to reac cumulate the Ca2+ ions pumped out of the cell during spontaneous relea se.