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 stud ied in isolated rat ventricular myocytes subjected to whole-cell volta ge clamp and internal perfusion with the Ca2+ indicator, indo-1. Na+-C a2+ exchange was eliminated in most experiments by removing Na+ both i nternally and externally. 2. When the Ca2+-pumping ATPase of the sarco plasmic reticulum (SRJ was inhibited with cyclopiazonic acid and ryano dine interfered with the release of Ca2+ from the SR, [Ca2+](i) transi ents rose slowly and declined extremely slowly. We concluded that tran sport of Ca2+ by mitochondria and the surface membrane Ca2+-pumping AT Pase would be negligible over the time course of a single [Ca2+](i) tr ansient.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).