MULTIPLE MECHANISMS IN THE SMOOTH-MUSCLE RELAXANT ACTION OF CALCITONIN-GENE-RELATED PEPTIDE (CGRP) IN THE GUINEA-PIG URETER

We have investigated the ability of human alpha CGRP (CGRP) to inhibit the electrically-evoked myogenic contractions of the guinea-pig urete r, in comparison with the K channel opener, cromakalim, and the adenyl ate cyclase activator, forskolin. CGRP (0.1 nM - 0.1 mu M) produced a concentration-dependent inhibition of the evoked contractions; its act ion was prevented by the CGRP receptor antagonist, CGRP(8-37) (1 mu M) , while it was unaffected by the nitric oxide (NO) synthase inhibitor, L-nitroarginine (30 mu M). The effect of CGRP was antagonized in a no ncompetitive manner (depression of E(max), no change in EC(50)) by gli benclamide (1-10 mu M), a blocker of ATP-sensitive potassium chan nels (K-ATP). A substantial fraction of the inhibitory effect of CGRP was glibenclamide-resistant, however. Glibenclamide also blocked the inhib itory action of cromakalim (0.1 - 10 mu M) without affecting the inhib ition produced by forskolin (0.1 - 30 mu M). When tested in a low-K me dium (extracellular K reduced from 5.9 to 1.2 mM), the inhibitory effe cts of CGRP, cromakalim and forskolin were enhanced. The inhibitory ef fect of forskolin was partly antagonized by glibenclamide when tested in a low-K medium. CGRP (0.1 mu M), cromakalim (3 mu M) and forskolin (10 mu M) inhibited the contractile response to KCl (80 mM), which is characterized by a distinct phasic and tonic component: cromakalim sel ectively inhibited the phasic response to KCl with CGRP and forskolin inhibited both components. The inhibitory effect of CGRP on the phasic contraction to KCl was partly glibenclamide (1 mu M) sensitive, while that on the tonic contraction was glibenclamide-resistant. The inhibi tory action of forskolin on both components of the response to KCl was unchanged by glibenclamide. The inhibitory effect of cromakalim on th e phasic response was prevented by glibenclamide. In sucrose gap elect rophysiological experiments, CGRP (0.3 mu M) produced hyperpolarizatio n of the membrane, increased the delay between stimulus application an d onset of the action potential, shortened the action potential durati on and eventually suppressed both action potential and phasic contract ions in 4 out 8 cases tested. The membrane hyperpolarization and the a bility of CGRP to suppress the evoked action potential were completely prevented by glibenclamide (1 mu M), but an inhibitory effect on the action potential duration and contractility persisted in the presence of glibenclamide. We conclude that activation of glibenclamide-sensiti ve K channels mediates part of the smooth muscle relaxant action of CG RP, and accounts for the ability of CGRP to suppress phasic contractio ns evoked through electromechanical coupling. However, a glibenclamide -resistant component also exists in the smooth muscle relaxant activit y of CGRP. The possible involvement of cAMP accumulation as an interme diate link between CGRP receptor occupation and activationof K channel s by CGRP remains speculative.