Greater ATP dependence than sodium dependence of radiocalcium efflux in bullfrog ventricle

45Ca efflux was studied in intact bullfrog ventricles following a 2-h period of loading with radiocalcium-containing Ringer solution. The cannulated ventricle was placed in a closed air-filled container to which were applied rhythmic, electronically timed, positive- and negative-pressure pulsations, which induced ventricular volume excursions. The mechanical arrangement and timing circuitry made it possible for each period to be as short in duration as 15 s. By use of this technique, penetration of the extracellular space by [14C]inulin was found to be complete within 30 s, and recovery of the inulin proceeded with a time constant of 17-24 s, indicating a completeness of recovery of 98% within 90 s. Washout of added 45Ca was quantitatively quite close to that of inulin, and in addition the estimated rate of sequestration of the isotope was slow enough to introduce only a small error into the experimental results. 45Ca efflux was only slightly (15%) sensitive to replacement of extracellular sodium but was profoundly sensitive to the inhibitors of ATP synthesis, cyanide and 2-deoxy-glucose.