Abstract
Investigations on the action of three glucocorticoids (cortisone, hydrocortisone and prednisolone) in isolated frog and rabbit hearts indicate that the glucocorticoids exert a biphasic action. The biphasic action was observed in the form of stimulation at low concentrations (10-8 to 10-5 g/ml) and inhibitory action at higher concentrations (10-5 to 10-3 g/ml) with considerable overlap in the stimulating and inhibiting concentration ranges. Biphasic action was also observed at the same acting concentration in some hearts. Moderately inhibitory doses frequently produced stimulation during recovery. The action of corticoids was more marked on isotropic response than on chronotropic. Stimulation of inotropic response caused up to 22% increase in ventricular contraction, while inhibition of inotropic response was 100% (cardiac asystole) at high concentrations. Electrocardiographic record showed conduction block and myocardial depression at inhibitory concentrations. The rabbit hearts were more sensitive to glucocorticoids than were the frog hearts. On the basis of cardiac effect, the three corticoids were in the ascending order of potency: hydrocortisone, cortisone and prednisolone.
