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 3Hinositol, and 3Hinositol phosphate formation was assessed after the addition of lithium chloride and norepinephrine. Norepinephrine increased the accumulation of 3Hinositol phosphate by approximately 5-fold, giving a maximal response at approximately 30 microM and a half-maximal response at approximately 1 microM. There was a significant increase in 3Hinositol phosphate formation in response to norepinephrine at 5 minutes, and the response was linear over 40 minutes. Norepinephrine-stimulated 3Hinositol phosphate formation was not blocked by propranolol (1 microM) or yohimbine (0.1 microM) but was completely antagonized by the alpha 1-selective antagonist prazosin (0.1 microM). Muscarinic cholinergic receptor activation by carbachol also stimulated 3Hinositol phosphate formation in rat ventricular myocytes. The maximal effect of carbachol (approximately 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 3Hinositol phosphate formation. Neither the calcium ionophore A23187 nor the calcium channel blockers verapamil and nifedipine had any effect on basal or hormone-stimulated 3Hinositol 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.