Effect of loperamide on mucosal guanylyl cyclase activity in rat jejunum following Escherichia coli heat-stable toxin-induced fluid accumulation.

Loperamide has antidiarrhoeal activities against secretagogues with different mechanisms of action. Besides its opioid-like effect on intestinal motility and secretion it might exhibit additional antisecretory properties which may not be completely elucidated yet. Direct effects of loperamide on mucosal guanylyl cyclase have never been observed. We therefore investigated the effect of loperamide on intestinal fluid transport altered by heat-stable Escherichia coli enterotoxin which acts by stimulating mucosal guanylyl cyclase. Net fluid movement was determined during a 1 hr incubation period in ligated jejunal loops of anaesthetised female Wistar rats. Transport rates of net fluid movement were calculated from the loop contents measured gravimetrically at the beginning and the end of the experiments. Addition of heat-stable Escherichia coli enterotoxin to the luminal solution resulted in a net secretion of water which was significantly reversed into net absorption by loperamide. The specific activity of the particulate guanylyl cyclase was determined in mucosal scrapings of the jejunum without and with the addition of heat-stable Escherichia coli enterotoxin and/or loperamide. Additions of loperamide of up to 10 micromol/l did not change guanylyl cyclase activity. We conclude that the effect of loperamide counteracting heat-stable Escherichia coli enterotoxin induced changes of intestinal fluid transport does not involve a direct effect on guanylyl cyclase.