Rest- and stimulation-dependent changes in exchangeable calcium content in rabbit ventricular myocardium.

Ca2+ shifts in the isolated, perfused ventricular muscle of rabbit hearts were investigated with the aid of 45Ca under the conditions of complete equilibration of preparations with 45Ca containing solution. The "cellular" 45Ca content was calculated by subtraction of 45Ca2+ dissolved in the free water of extracellular space from the total tissue 45Ca2+ content. The "cellular" content of 45Ca in stimulated (60 per min) preparation was 0.887 +/- 0.067 mmol/kg wet weight (w.w.). Six minutes of rest resulted in the drop of this content to 0.503 +/- 0.054 mmol/kg w.w. despite continued perfusion with 45Ca containing solution. Contractile force (CF) decreased at that time to 23% of control. The first post-rest contraction (RSC) resulted in a gain of 0.073 mmol45Ca/kg w.w. Both the content of 45Ca and CF returned to the pre-rest values when stimulation was resumed. The difference between the 45Ca content of post-rest stimulated and rested preparations (0.384 mmol/kg w.w.) is equivalent to Ca2 fraction (Ca2), previously described in guinea pig ventricular myocardium (17, 12). However, the volume of rabbit Ca2 is only about 42% of that in guinea pig. Caffeine in concentration 12.5 mM, which did not displace Ca2 from guinea pig ventricular muscle, decreased Ca2 in the rabbit ventricle by 44%. CCCP, a protonofore destroying the mitochondrial protone gradient essential for Ca2+ uptake and maintainability, displaced Ca2 completely from rabbit ventricles. These results, although far from conclusive, do suggest that both the mitochondria and sarcoplasmic reticulum might be the site of the rate-dependent Ca2 fraction. The physiological meaning of differences in Ca2 content between rabbit, guinea pig, and rat ventricular myocardium is discussed.