Effects of calcium on ouabain-induced inotropism in rat and guinea pig hearts.

: In experiments on isolated rat and guinea pig hearts, the influence of different calcium concentrations (0.45-3.6 mM) on ouabain action was studied. A different time course of the inotropic action of ouabain in rat heart (t1/2 about 15 sec) and guinea pig cardiac muscle (t1/2 about 4 min) was found. Furthermore, compared to guinea pig hearts, a considerably enhanced "sensitivity" of the rat myocardium to ouabain at increased calcium concentrations could be established. These results are explained on the basis of a different mode of calcium in modulating the contractility within these two species.