Amount and turnover of calcium entering the cells of ventricular myocardium of guinea pig heart in a single excitation.

: Isolated perfused guinea pig heart ventricles were rendered quiescent by means of atrioventricular block and elevation of perfusate potassium concentration to 7.4 mM. Muscles were exposed to 45Ca2+ during 3 min of rest followed by a single or multiple excitations. Tissue analysis after a half minute of nonradioactive perfusion showed that the 45Ca content had increased after a single excitation by 0.183 mmol above control unstimulated muscles. The difference in calcium uptake between stimulated and unstimulated muscle was not decreased by exposure to 2 mM lanthanum during washout of 45Ca. No difference in uptake of radioactivity between the stimulated and control muscles was found if 63Ni2+ in a concentration 0.001 mM was used instead of 45Ca2+. The 45Ca uptake was strongly inhibited by 2 mM Ni2+ applied during exposure to this isotope. When the experimental muscles were subjected to rhythmic postrest stimulation, most of the 45Ca was taken up during the initial three beats, mostly during the first. A single excitation applied during washout of radioactivity resulted in rapid decrease in 45Ca content in the experimental muscles. It is concluded that about 0.18 mmol of Ca diffuses into myocardial cells at a single postrest excitation. Excitation-dependent extrusion of Ca2+ is postuated.