Both serotonin and a nitric-oxide donor cause chloride secretion in rat colonocytes by stimulating cGMP

Background. Previous studies have demonstrated that an antagonist of nitric oxide synthase inhibits neurally mediated chloride secretion in response to serotonin (5-HT). The purpose of this study was to demonstrate that chloride secretion in rat colonocytes that were caused by stimulation of neural. 5-HT receptors is mediated by way of a nitrergic pathway that involves the activation of guanylate cyclase. Methods. The nitric oxide (NO) donor, diethylenetriamine/NO (DNO), was added to an enriched suspension of rat colonocytes that were preloaded with Cl-36(-). In Parallel experiments, DNO (1 mu mol/L) was added to cells that were Preheated with the specific inhibitor of soluble guanylate cyclase, NS2028 (2 mu mol/L). In additional studies, the neural 5-HT3 receptor agonist, 2-methyl-5-HT (10 mu mol/L), was added to the serosal surface of muscle-stripped sheets of rat colonic mucosa that were mounted in Ussing chambers under voltage clamp conditions, both in the absence and presence of NS2028 (20 mum). Results. DNO induced 18.0% +/-8.0%greater Cl-36(-) efflux than controls (P<.05; n = 14 animals). This efflux was abolished by previous treatment with NS2028. In the chamber experiments, 2-methyl-5-HT induced electrogenic chloride secretion that was significantly inhibited by previous treatment with NS2028 (2.20.5 muA/cm(2) vs 13.1 +/-2.1 muA/cm(2); P<.001; n = 9 animals). Conclusions. The predominant secretomotor neurotransmitter that mediates the chloride secretory effects of 5-HT in vitro is nitric oxide. Both the secretory effect initiated at the 5-HT3 receptor on enteric neurons and at the NO- receptor on the rat colonocytes are mediated through the activation of intracellular guanylate cyclase and the production of cyclic guanosine monophosphate.