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The role of extracellular calcium in the positive inotropic effect of X537-A in dog's cardiac muscle was investigated. Ionophore (20 mug/ml) in the presence of external calcium increased the contractility by 67 +/- 12 percent within 2 min. In the absence of external calcium ionophore was ineffective in producing a positive inotropic effect in the muscle. Lanthanum, an inhibitor of calcium influx, in the concentrations of 2, 4, 6, and 8 mM produced a concentration dependent decrease in the contractility. Ionophore-induced increase in the contractility was inversely related to the concentration of lanthanum used. It is suggested that the release of calcium from intracellular stores under the effect of ionophore is not enough to maintain contractility. Also positive inotropic effect of ionophore appears to be mediated through the transportation of calcium across the sarcolemma. Such ionophore mediated transportation of calcium appears to be dependent upon surface binding of calcium.

EXTRACELLULAR CALCIUM AND POSITIVE INOTROPY OF IONOPHORE (X537-A) IN CARDIAC MUSCLE