Effects of L-arginine on spontaneous contraction of the rat portal vein

The effects Of L-arginine on spontaneous contraction of endothelium-denuded longitudinal preparations of the rat portal vein were studied. L-Arginine increased the frequency of spontaneous contraction concentration-dependently between 10 muM and 1 mM. Changes in contraction amplitude and duration were not remarkable. D-Arginine had a negligible effect on spontaneous contraction. N-omega-nitro-L-arginine (1 mM) did not affect spontaneous contraction or the response to L-arginine. Addition of N-G-monomethyl-L-arginine (1 mM), L-lysine (1 mM) or N-ethymaleimide (0.1 mM) increased the frequency of spontaneous contractions and inhibited the. effect Of L-arginine. Glibenclamide (10 muM) did not affect spontaneous contraction or the response to L-arginine. Spontaneous increase in concentration of intracellular Ca2+, estimated as the ratio of Fura-PE3 fluorescence occurred synchronously with spontaneous contraction. Spontaneous increase in concentration of intracellular Ca2+ occurred more frequently in the presence Of L-arginine (I MM). L-Arginine (1 mM) also increased the number of action potential bursts/min in the longitudinal smooth muscle layer. L-Arginine (I mM) also depolarized cell membranes. This study indicates that L-arginine increases the frequency of spontaneous contraction of longitudinal muscle in the rat portal vein by membrane depolarization through mechanisms that do not involve, nitric oxide or the inhibition of ATP-sensitive K+ channels.