Physiological and pathological regulation of intestinal absorption by nitric oxide with special regard to the action of antidiarrheal drugs

As outlined in the summary of our project we were investigated the pro-absorptive role of nitric oxide in intestinal absorption. The effects of pharmacological modulation of the nitric oxide (NO) pathway on intestinal fluid transport were studied in a model of ligated jejunal loops of anaesthetized rats in vivo. Infusion of 5-hydroxytrypamine (5-HT) induced net fluid secretion. Intravenous infusion of the NO synthase inhibitor L-NAME reversed net fluid absorption in controls to net secretion and significantly enhanced 5-HT-induced fluid secretion. 5-HT-induced secretion was inhibited by intravenous infusion of L-arginine or SIN-1. Intraluminal instillation of cholera toxin induced net secretion, which was significantly enhanced by L-NAME and reduced by L-arginine. Another series of experiments was performed using a model of luminally perfused jejunal loops with cholera toxin. L-Arginine and sodium nitroprusside significantly enhanced net fluid absorption compared to controls and abolished the secretory effect of cholera toxin. Luminal perfusion with oral rehydration solution enhanced net absorption of fluid in controls and reversed cholera toxin-induced secretion to absorption. Infusion of L-arginine significantly enhanced further the antisecreotry effect of oral rehydration solution. These results give further support to the existence of an intestinal NO-mediated proabsorptive tone, which also downregulates fluid secretion elicited by different enterotoxins or mediators of secretion. With this mechanism we discovered a very important proabsorptive principle beside the sympathetic nervous regulation, which is also modulating the proabsorptive tone. These findings may lead to therapeutic consequences in the future. In a second part we investigated the role of the cyclooxygenases COX-1 and COX-2 in intestinal secretion mainly influenced by cholera toxin in our model. To elucidate the role of the different isoforms COX-1 and COX-2 in cholera toxin induced fluid secretion, the effects of COX-2 inhibitors and COX-1 inhibitors were studied. COX-1 and COX-2 mRNA expression were determined by RT-PCR and COX-2 protein by Western blot analysis in mucosal scrapings. The COX-2 inhibitor but not the COX-1 inhibitor inhibited CT-induced fluid secretion and PGE2 release. RT-PCR showed expression of COX-1 and COX-2 mRNA in controls and in CT treated rats. A band of COX-2 protein was also observed in controls. This means that COX-2 surprisingly is constitutively present in jejunal mucosa and that the hypothesis, COX-2 being only expressed after induction is wrong. CT enhanced COX-2 levels, and it can be assumed, that post-transscriptional modulation is responsible for CT-induced increase in COX-2 protein. Further studies are planned to elucidate the role of COX-2 in transmission of secretory responses in the gut. At present, another interesting candidate, TNF alpha and its role in mediation of secretion is investigated in our lab.