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

Summary Ca²⁺ shifts in the isolated, perfused ventricular muscle of rabbit hearts were investigated with the aid of⁴⁵Ca under the conditions of complete equilibration of preparations with⁴⁵Ca containing solution. The cellular⁴⁵Ca content was calculated by subtraction of⁴⁵Ca²⁺ dissolved in the free water of extracellular space from the total tissue⁴⁵Ca²⁺ content. The cellular content of⁴⁵Ca 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 with⁴⁵Ca 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 of⁴⁵Ca and CF returned to the pre-rest values when stimulation was resumed.The difference between the⁴⁵Ca 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 mitochondrial 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.This study was supported by the grant CPBR, 11.6