Contribution of both Ca2+ entry and Ca2+ sensitization to the alpha1-adrenergic vasoconstriction of rat penile small arteries

Abstract

Sympathetic adrenergic nerves maintain the flaccid state of the penis through the tonic release of norepinephrine that contracts trabecular and arterial smooth muscle. Simultaneous measurements of intracellular Ca(2+) concentration ([Ca(2+)](i)) and tension and experiments with alpha-toxin-permeabilized arteries were performed in branches of the rat dorsal penile artery to investigate the intracellular Ca(2+) signaling pathways underlying alpha(1)-adrenergic vasoconstriction. Phenylephrine increased both [Ca(2+)](i) and tension, these increases being abolished by extracellular Ca(2+) removal and reduced by about 50% by the L-type Ca(2+) channel blocker nifedipine (0.3 microM). Non-L-type Ca(2+) entry through store-operated channels was studied by inhibiting the sarcoplasmic reticulum Ca(2+)-ATPase with cyclopiazonic acid (CPA). CPA (30 microM) induced variable phasic contractions that were abolished by extracellular Ca(2+) removal and by the store-operated channels antagonist 2-aminoethoxydiphenyl borate (2-APB, 50 microM) and largely inhibited by nifedipine (0.3 microM). CPA induced a sustained increase in [Ca(2+)](i) that was reduced in a Ca(2+)-free medium. Under conditions of L-type channels blockade, Ca(2+) readmission after store depletion with CPA evoked a sustained and marked elevation in [Ca(2+)](i) not coupled to contraction. 2-APB (50 microM) inhibited the rise in [Ca(2+)](i) evoked by CPA and the nifedipine-insensitive increases in both [Ca(2+)](i) and contraction elicited by phenylephrine. In alpha-toxin-permeabilized penile arteries, activation of G proteins with guanosine 5'-O-(3-thiotriphosphate) and of the alpha(1)-adrenoceptor with phenylephrine both enhanced the myofilament sensitivity to Ca(2+). This Ca(2+) sensitization was reduced by selective inhibitors of PKC, tyrosine kinase (TK), and Rho kinase (RhoK) by 43%, 67%, and 82%, respectively. As a whole, the present data suggest the alpha(1)-adrenergic vasoconstriction in penile small arteries involves Ca(2+) entry through both L-type and 2-APB-sensitive receptor-operated channels, as well as Ca(2+) sensitization mechanisms mediated by PKC, TK, and RhoK. A capacitative Ca(2+) entry coupled to noncontractile functions of the smooth muscle cell is also demonstrated.