DIRECT EVIDENCE FOR NITRIC-OXIDE STIMULATION OF ELECTROLYTE SECRETIONIN THE RAT COLON

Nitric Oxide (NO) is synthesized in the intestinal tract and may serve as a physiological regulator of intestinal ion transport and/or a pat hophysiologic mediator of secretory diarrhea associated with inflammat ory mucosal diseases. Indirect approaches, employing inhibitors of nit ric oxide synthase or compounds capable of donating NO in solution, ha ve been used to demonstrate the effects on gastrointestinal muscle and the mucosa. To determine directly whether nitric oxide itself is capa ble of stimulating electrolyte secretion we mounted muscle-stripped ra t distal colon in Ussing chambers and monitored short-circuit current (Isc), as an indicator of effects on mucosal ion transport. Comparison s were made to sodium nitroprusside (SNP). NO and SNP stimulated conce ntration-dependent (0.1 muM to 100 muM) increases in Isc, with NO bein g more potent than SNP. The EC50 for NO was approximately 8 muM compar ed to a value <20 muM for SNP. The response to NO was immediate. In co ntrast, SNP required a mean lag-time of 41 +/- 4 seconds, and a signif icantly longer time was required for SNP to reach its maximum effect. The response to both of these agonists was blocked by bumetanide, indi cating that they were stimulating a chloride ion secretory response. T he cyclooxygenase inhibitor piroxicam, the neurotoxin tetrodotoxin and the inhibitor of guanylate cyclase, methylene blue, all inhibited the response to both agonists. These studies demonstrate that NO itself c an stimulate chloride secretion by the rat colonic mucosa through a pr ostaglandin-dependent, and partially neural mechanism that may involve guanylate cyclase.