Effect of increased cAMP content on extracellular distribution of calcium and contractility in rabbit heart

Norepinephrine, dexamethasone, and papaverine hydrochloride caused a significant increase in myocardial cAMP level and an alteration of intracellular distribution of Ca, such as the reduction of Ca content on mitochondrial and microsomal fractions, with or without positive inotropic effects, in the rabbit. Positive inotropic effects evoked by norepinephrine were significantly increased under the elevated level of cAMP induced by dexamethasone and papaverine. On the other hand, verapamil showed no effects on cAMP concentration and intracellular Ca distribution of the heart. These results indicate that the accumulation of myocardial cAMP may result in the alteration of intracellular distribution of Ca, and it is suggested that there is an increase in the available Ca for contractile and metabolic mechanisms of the heart.