ALPHA-1-ADRENERGIC AND MUSCARINIC CHOLINERGIC STIMULATION OF PHOSPHOINOSITIDE HYDROLYSIS IN ADULT-RAT CARDIOMYOCYTES

Cardiac .alpha.-adrenergic receptors mediate cellular responses to norepinephrine through an undefined series of molecular events. We examined the possibility that phosphoinositide hydrolysis was stimulated through .alpha.-adrenergic receptors in cardiomyocytes purified from adult rat ventricle. Phosphoinositide stores were labeled with [3H]inositol, and [3H]inositol phosphate formation was assessed after the addition of lithium chloride and norepinephrine. Norepinephrine increased the accumulation of [3H]inositol phosphate by approximately 5-fold, giving a maximal response at .apprx. 30 .mu.M and a half-maximal response at .apprx. 1 .mu.M. There was a significant increase in [3H]inositol phosphate formation in response to norepinephrine at 5 minutes, and the response was linear over 40 minutes. Norepinephrine-stimulated [3H]inositol phosphate formation was not blocked by propranolol (1 .mu.M) or yohimbine (0.1 .mu.M) but was completely antagonized by the .alpha.1-selective antagonist prazosin (0.1 .mu.M). Muscarinic cholinergic receptor activation by carbachol also stimulated) [3H]inositol phosphate formation in rat ventricular myocytes. The maximal effect of carbachol (.apprx. 2-fold) was always less than that of norepinephrine. The combined effects of norepinephrine and carbachol were additive, suggesting that the two hormones do not share a common rate-limiting step. Removal of extracellular calcium and addition of ethylene glycol bis(.beta.-amino ether)-N,N''-tetraacetic acid, attenuated, but did not abolish, norepinephrine- or carbachol-stimulated [3H]inositol phosphate formation. Neither the calcium ionophore A23187 nor the calcium channel blockers verapamil and nifedipine had any effect on basal or hormone-stimulated [3H]inositol phosphate formation. We suggest that some of the physiological and metabolic effects of adrenergic and cholinergic stimulation on the rat myocardium are secondary to receptor-mediated hydrolysis of phosphoinositides.