α1-adrenoceptors stimulate a Gαs protein and reduce the transient outward K+ current via a cAMP/PKA-mediated pathway in the rat heart

alpha(1)-Adrenoceptor stimulation prolongs the duration of the cardiac action potentials and leads to positive inotropic effects by inhibiting the transient outward K+ current (I-to). In the present study, we have examined the role of several protein kinases and the G protein involved in Ito inhibition in response to alpha(1)-adrenoceptor stimulation in isolated adult rat ventricular myocytes. Our findings exclude the classic alpha(1)-adrenergic pathway: activation of the G protein G(alphaq), phospholipase C ( PLC), and protein kinase C (PKC), because neither PLC, nor PKC, nor G(alphaq) blockade prevents the alpha(1)-induced I-to reduction. To the contrary, the alpha(1)-adrenoceptor does not inhibit Ito in the presence of protein kinase A (PKA), adenylyl cyclase, or G(alphas) inhibitors. In addition, PKA and adenylyl cyclase activation inhibit Ito to the same extent as phenylephrine. Finally, we have shown a functional coupling between the alpha(1)-adrenoceptor and G(alphas) in a physiological system. Moreover, this coupling seems to be compartmentalized, because the alpha(1)-adrenoceptor increases cAMP levels only in intact cells, but not in isolated membranes, and the effect on Ito disappears when the cytoskeleton is disrupted. We conclude that alpha(1)-adrenoceptor stimulation reduces the amplitude of the Ito by activating a G(alphas) protein and the cAMP/PKA signaling cascade, which in turn leads to Ito channel phosphorylation.