Pharmacological evidence for catecholamine-independent contractile effects of X-537A on the isolated working rat heart preparation.

The ionophore X-537A increased heart rate and contractility of the isolated, working rat heart preparation. The increased heart rate appeared to be caused solely by release of catecholamines as the response was completely eliminated by reserpine pretreatment or addition of propranolol to the perfusate. The inotropic response, however, had an apparent catecholamine-independent component as neither propranolol, nor propranol in combination with phentolamine, completely eliminated the inotropic response to X-537A. On the other hand, reserpine pretreatment did abolish the inotropic effect of the ionophore but this action appeared to be a nonspecific one as the responses to norepinephrine and to CaCl2 were substantially diminished.