OUABAIN POTENTIATION OF Ca RELEASE FROM FRAGMENTED CARDIAC SARCOPLASMIC RETICULUM

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The present study was performed to determine the effect of ouabain on Ca release from fragmented sarcoplasmic reticulum (FSR) isolated from cardiac muscles by changing the ionic environment and by caffeine. FSR was isolated from cat, guinea pig and frog cardiac muscles as microsomal fractions obtained biochemically, namely, by means of ultra-centrifugation at 8000 × g-35000 × g. After sufficient Ca²⁺-uptake by FSR in a potassium methan-sulfonate (KMS) medium, one ml of the suspension was passed through a Millipore filter. By this treatment, almost all FSR adhered to the filter. Three ml of potassium chloride washing medium was added to alter the ionic environment. One min later the washing medium was passed through the filter. A KMS washing medium containing 30 mM caffeine was used to induce the Ca release by caffeine. In the case of ouabain, the medium containing ouabain (2×10^-6 M) was used. The time of ouabain treatment was 1 min. The grade of Ca release from FSR was almost the same among cat, guinea pig and frog cardiac FSR. Ouabain markedly potentiated Ca release from FSR by altering the ionic environment. These results suggest that the positive inotropic action of ouabain is dependent upon the potentiation of Ca release in SR vesicles of cardiac muscle.