ALPHA-1-ADRENOCEPTOR STIMULATION INHIBITS THE ISOPROTERENOL-INDUCED EFFECTS ON MYOCARDIAL-CONTRACTILITY AND PROTEIN-PHOSPHORYLATION

In the present study the influence of alpha(1)-adrenoceptor stimulatio n on the beta-adrenoceptor agonist-induced increases in contractile pa rameters and protein phosphorylation was determined in isolated perfus ed hearts and isolated cardiac myocytes, respectively. Methoxamine inh ibited the isoproterenol-induced increases in left ventricular pressur e and heart rate dose dependently up to 90% and 75%, respectively; the EC(50) of this antiadrenergic effect was 4.4 mu M. The alpha(1)-adren oceptor antagonist, prazosin (1 mu M), greatly diminished methoxamine' s inhibitory action, confirming the alpha(1)-adrenoceptor-mediated mec hanism. The inotropic effect of glucagon was inhibited by methoxamine in a similar manner. Radioligand binding assays with [H-3]dihydroalpre nolol demonstrated that the antiadrenergic action of methoxamine is no t due to an unspecific beta-adrenoceptor blocking property. In an addi tional experimental series the effects of methoxamine and isoprotereno l on the protein phosphorylation pattern of isolated cardiac myocytes were investigated. Isoproterenol increased the phosphorylation state o f five proteins (6-kDa, phospholamban; 15-kDa; 28-kDa, troponin I; 97- kDa; 140-kDa) while in the experiments with methoxamine the 15-kDa pro tein was the only phosphorylated substrate. In the presence of methoxa mine the isoproterenol-induced phosphorylation of phospholamban, tropo nin I and the 97-kDa and 140-kDa protein was markedly inhibited while the phosphorylation state of the 15-kDa protein remained unaltered. Th e present study clearly demonstrated that alpha(1)-adrenoceptor stimul ation potently inhibits the beta-adrenoceptor-mediated changes in cont ractile force and phosphorylation of key regulatory proteins, most lik ely through modulation of cAMP metabolism