On the subcellular localisation of calcium fraction correlating with contractile force of guinea-pig ventricular myocardium.

Cellular content of exchangeable Ca in the isolated, perfused ventricles of guinea-pig hearts was investigated by means of isotope 45Ca under conditions of complete equilibration of the muscle with radioactive perfusing solution. Content of 45Ca in the ventricles stimulated at the rate of 60 min-1 was 1.30 +/- 0.15 mmol/kg w.w. Six min of rest resulted in the drop of this content to 0.37 +/- 0.05 mmol/kg w.w. despite continued perfusion with 45Ca containing solution. The difference of 0.93 mmol/kg w.w. is equivalent to the rate-dependent fraction Ca2(6,7) and is labelled accordingly. Fraction Ca2 was hardly affected by caffeine (12.5mM). Protonofore CCCP in concentration 2 microM, carbonyl atractiloside in concentration of 20 microM and anoxia decreased 45Ca content in the stimulated ventricles to the resting levels. Omission of bicarbonate ions in the perfusing solution with constant pH = 7.35 resulted in the drop of fraction Ca2 to 0.55 mmol/kg w.w. The increase in the phosphate concentration from 0.42 mM to 3 mM resulted in increase of fraction Ca2 to 1.39 mmol/kg w.w. We propose that the likely site of subcellular localisation of fraction Ca2 are mitochondria.