Role of PKC in the effects of alpha1-adrenergic stimulation on Ca2+ transients, contraction and Ca2+ current in guinea-pig ventricular myocytes.

The effects of alpha1-adrenoceptor stimulation on intracellular Ca2+ transients, contractility and L-type Ca2+ current (ICa,L) were studied in single cells isolated from ventricles of guinea-pig hearts. The aim of our study was to elucidate the mechanisms of the positive inotropic effect of alpha1-adrenergic stimulation by focussing on the role of protein kinase C (PKC). Phenylephrine, an alpha1-adrenergic agonist, at concentrations of 50-100 microM elicited a biphasic inotropic response: a transient negative inotropic response (22.9+/-6.0% of control) followed by a sustained positive inotropic response (61.0+/-8.4%, mean+/-SE, n=12). The Ca2+ transient decreased by 10.2+/-3.9% during the negative inotropic phase, while it increased by 67.7+/-10% (n=12) during the positive inotropic phase. These effects were inhibited by prazosin (1 microM), a alpha1-adrenergic antagonist. Phenylephrine increased the ICa,L by 60.8+/-21% (n=5) during the positive inotropic phase. To determine whether activation of PKC is responsible for the increases in Ca2+ transients, contractile amplitude and ICa,L during alpha1-adrenoceptor stimulation, we tested the effects of 4beta-phorbol 12-myristate 13-acetate (PMA), a PKC activator, and of bisindolylmaleimide I (GF109203X) and staurosporine, both of which are PKC inhibitors. PMA mimicked phenylephrine's effects on Ca2+ transients, contractile amplitude and ICa,L. PMA (100 nM) increased the Ca2+ transient, contractile amplitude and ICa,L by 131+/-17%, 137+/-25% (n=8), and 81.1+/-26% (n=5), respectively. Prior exposure to GF109203X (1 microM) or staurosporine (10 nM) prevented the phenylephrine-induced increases in Ca2+ transients, contractile amplitude and ICa,L. Our study suggests that during alpha1-adrenoceptor stimulation increase in ICa,L via PKC causes an increase in Ca2+ transients and thereby in the contractile force of the ventricular myocytes.