ALPHA(1)-ADRENOCEPTOR-MEDIATED INHIBITION OF CELLULAR CAMP ACCUMULATION IN NEONATAL RAT VENTRICULAR MYOCYTES

We studied adrenergic regulation of cellular cAMP in neonatal rat ventricular myocytes. Since cAMP content depends on synthesis, breakdown and egress, the contribution of each of these mechanisms was assessed. In the presence of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, cAMP accumulation stimulated by the beta-adrenoceptor agonist (-)-isoprenaline was diminished when the mixed alpha+beta adrenoceptor agonist (-)-noradrenaline was coincubated with (-)-isoprenaline. Moreover, adenylyl cyclase activation stimulated by (-)-isoprenaline was decreased by (-)-noradrenaline and by the selective alpha1-adrenoceptor agonists (-)-phenylephrine and methoxamine, suggesting that alpha-adrenoceptor agonism regulates cAMP metabolism through its effect on the synthetic pathway. Evidence for alpha1-adrenoceptor mediation of this response was enhancement of (-)-noradrenaline-induced cAMP generation by the selective alpha1-adrenoceptor antagonist terazosin (10 nmol/1). The selective alpha2-adrenoceptor antagonist yohimbine (10 nmol/1) had no effect. The alpha1-adrenoceptor mediated depression of (-)-isoprenaline-stimulated cAMP generation and adenylyl cyclase activation was prevented by terazosin and in separate experiments markedly enhanced by pertussis toxin pretreatment, suggesting involvement of a guanine-nucleotide regulatory protein in this process. Occupation of the alpha1-adrenoceptor by (-)-noradrenaline did not accelerate the rate of cAMP breakdown in the absence of phosphodiesterase inhibition. Furthermore, there was no enhancement of total phosphodiesterase activity by (-0-noradrenaline in the presence of (-)-propranolol. By contrast, pertussis toxin pretreatment augmented phosphodiesterase activity. Neither pertussis toxin nor (-)-noradrenaline increased cAMP egress. We conclude that in rat neonatal cardiac myocytes agonist occupation of the alpha1-adrenoceptor inhibits beta-adrenoceptor stimulated cAMP accumulation most likely by coupling to a guanine nucleotide inhibitory protein.