The mechanism of increasing Ca2+ responsiveness by alpha1-adrenoceptor stimulation in rat ventricular myocytes

We investigated the mechanism of alpha(1)-adrenoceptor stimulation on the myofibrillar Ca(2+) responsiveness at steady-state in intact rat ventricular myocytes. We produced tetanus, and an instantaneous plot of [Ca(2+)](i) vs. cell length (Ca-L trajectory) was constructed to estimate the Ca(2+) responsiveness. An alpha(1)-agonist, phenylephrine, dose-dependently shifted the Ca-L trajectory to the left, corresponding to sensitization of the myofilaments. An alpha(1)-antagonist, prazosin, and inhibition of the Na/H exchange by ethylisopropylamiloride (EIPA) completely reversed the phenylephrine-induced shift. Phenylephrine increased pH(i) (DeltapH(i) = +0.1), which was reversed by prazosin and EIPA. Chelerythrine, an inhibitor of protein kinase C (PKC), completely blocked the effects of phenylephrine on Ca(2+) responsiveness and pH(i). When pH(i) was increased (DeltapH(i) = +0.1) without phenylephrine by changing pH(o), the Ca-L trajectory was shifted to the same extent as that observed with phenylephrine. We conclude that alpha(1)-adrenoceptor stimulation activates Na/H exchange through a PKC-mediated pathway and that an increase in pH(i) is mainly responsible for the increase in Ca(2+) responsiveness.