A report on the positive inotropic action of amrinone.

The inotropic action of amrinone was evaluated on isolated guinea-pig atria and on specimens of human atrial myocardium. The drug elicited a positive inotropic response in both the tissues. On spontaneously beating guinea-pig atria the influence on contractility was higher than on the rate. The magnitude of the response was influenced by temperature. Propranolol or phentolamine did not affect the positive inotropic action of amrinone. The efficacy of amrinone was higher than ouabain, in normal as well as in low extracellular calcium. Amrinone showed a positive inotropic action also at high extracellular potassium. The positive staircase was not reversed by amrinone, which had a higher positive inotropic effect at higher frequency of driving; the drug did not significantly delay the decline of contractile force in zero calcium. It is possible that the inotropic effect of amrinone in mainly due to an effect on influx of calcium into the cells.