A Molecular Mechanism, of Myocardial alpha-Adrenoceptor-mediated Signaltransductio

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The signal transduction process subsequent to activation of myocardial alpha_1-adrenoceptors has been further complicated by the discovery of subtypes, alpha_<1A>, alpha_<1B> and alpha_<1C> whereas the functional role of alpha _<1C> in cardiac function is unknown at the moment. The present study was carried out to elucidate intracellular process mediated by cardiac alpha_1-adrenoceptors. Results obtained can be summarized as follows : (1)In the rabbit ventricle ca. 60% of alpha_1-adrenoceptors belong to alpha_<1B> subtype, which is coupled to the positive inotropic effect (PIE) and PI hydrolysis and is effectively antagonized by chlorethylclonidine (CEC). CEC at 0.1-10 uM inhibited the PIE of phenylephrine mediated by alpha_1-adrenoceptors in a concentration-dependent manner. (2)In the rabbit ventricle ca. 40% of alpha_1-receptors belong to alpha_<1A> subtype. There appear to be further two subclasses in this subtype : (1)WB 4101-sensitive one, coupled to PI hydrolysis and PIE ; (2) (+)-niguldipine-sensitive one, coupled to PIE without acceleration of PI hydrolysis. Denopamine and HV723 (in low concentration) inhibit selectively the (+)-niguldipine-sensitive subtype. (3)Activation of endothelin and angiotensin receptor in rabbit heart accelerates PI hydrolysis as alpha_1 stimulation does. In the present study it was found that the response to activation of these receptors showed a remarkable similarity to each other in following respects : (1) the PIE was associated with prolongation of isometric twitch contractions ; (2) the concentration-and time-dependent change in PIE and PI hydrolysis in response to receptor activation showed an excellent correlation ; (3) phorbol 12,13-dibutyrate that activates protein kinase C consistently inhibited the PIE and PI hydrolyis elicited through these receptors. These findings strongly indicate that PI hydrolysis may mediated the PIE induced by activation of