Role of nitric oxide in the contraction of circular muscle in the rat portal vein

The role of nitric oxide in the electrical and mechanical activities of the rat portal vein was examined in circular muscle preparations with intact e ndothelium that were isolated from the longitudinal muscle layer. In contra st to the longitudinal muscle preparation, the circular muscle preparation did not show spontaneous phasic contraction. Inhibition of nitric oxide syn thesis by N-omega-nitro-L-arginine (L-NNA) induced a tonic contraction. The contraction was inhibited by L-arginine, sodium nitroprusside or nifedipin e. L-NNA did not induce contraction in endothelium-damaged preparations. Th e membrane potential of smooth muscle cells recorded in endothelium-intact preparations showed sporadic action potentials. L-NNA increased the frequen cy of action potentials without changing the resting membrane potential. Th e action potentials were inhibited by nifedipine. In the presence of L-NNA, sodium nitroprusside decreased the frequency of the action potentials with out changing the resting membrane potential. These results indicated that c ontraction of rat portal vein circular muscles is inhibited tonically by ni tric oxide, at least partly through inhibition of electrical activity.