Involvement of chloride channels in the receptormediated activation of longitudinal colonic muscle

In gastrointestinal smooth muscle intracellular Cl- is maintained at levels higher than its electrochemical equilibrium. Therefore, Cl- afflux through receptor-mediated opening of Cl- channels should result in membrane depola rization and may be sufficient to activate voltage-sensitive calcium channe ls (VSCCs). To determine the contribution of Cl- channels to receptor-media ted contraction of the longitudinal muscle layer of the rabbit distal colon , we studied the mechanical response of muscle strips to substance P, carba chol and potassium depolarization following the depletion of Cl-i(-), and i n the presence of the Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)-b enzoate (NPPB). A 60-min incubation of tissues in a HEPES-buffered solution in which NaCl had been replaced by Na isethionate (or Na gluconate) in equ imolar amounts resulted in disappearance of phasic contractions, and in a p artially reversible reduction of the tonic response to substance P and carb achol, but not to KCl depolarization. When the agonist was applied to tissu es in control solution, or to Cl--depleted tissues in a solution in which N a+ was acutely replaced in equimolar amounts by N-methyl-o-glucosamine, the mechanical response to substance P and carbachol was almost abolished. Acu te Na+ replacement alone without prior Cl- depletion did not abolish phasic contractions, but reduced the tonic response to substance P and carbachol. Similar to the effect of Cl- depletion, incubation of tissues in NPPB (6.6 x 10(-5)M) reduced the tonic response to substance P and carbachol, and ab olished phasic contractions. These findings are consistent with a contribut ion of a Cl- channel to the receptor-mediated activation of colonic smooth muscle. In addition, the data suggest that transient CI-channel mediated de polarizations may play a role in. the generation of phasic contractions.