PROCESSES THAT REMOVE CALCIUM FROM THE CYTOPLASM DURING EXCITATION-CONTRACTION COUPLING IN INTACT RAT-HEART CELLS

1. The processes that remove Ca2+ rapidly from the cytoplasm were studied in isolated rat ventricular myocytes subjected to whole-cell voltage clamp and internal perfusion with the Ca2+ indicator, indo-1. Na+-Ca2+ exchange was eliminated in most experiments by removing Na+ both internally and externally. 2. When the Ca2+-pumping ATPase of the sarcoplasmic reticulum (SRJ was inhibited with cyclopiazonic acid and ryanodine interfered with the release of Ca2+ from the SR, [Ca2+](i) transients rose slowly and declined extremely slowly. We concluded that transport of Ca2+ by mitochondria and the surface membrane Ca2+-pumping ATPase would be negligible over the time course of a single [Ca2+](i) transient.ially identical. The values of K-M, V-max and F-SR,F-leak that were obtained from the decline of[Ca2+](i) after voltage-clamp pulses to +10 mV were, respectively 0.28 +/- 0.04 mu M, 0.21 +/- 0.04 mM s(-1) and 0.018 +/- 0.0004 mM s(-1).