Antisecretory effect of loperamide in colon epithelial cells by inhibitionof basolateral K+ conductance

Background: The mechanism of the antisecrctory affect of loperamide was inv estigated in cultured highly differentiated colon epithelial cells (HT-29/B 6). Methods: Chloride secretion was stimulated via cAMP by forskolin (FSK, 10(-5) M), via Ca2+ by the muscarinic agonist carbachol (CCh, 10(-4) M), an d via protein kinase C by the phorbol ester PMA (5.10(-9) M). Stimulated Cl - secretion was quantified as short circuit current (I-SC) of HT-29/B6 mono layers mounted in Ussing-type chambers, Results: Loperamide (5.10(-5) M) in hibited Ise stimulated by FSK, CCh and PMA. The antisecretory action of lop eramide was unaffected by preincubation with naloxone (10(-5) M). Furthermo re, loperamide strongly inhibited basolateral Rb-86 efflux. Like loperamide . the calmodulin antagonist trifluoperazine (10(-4) M) inhibited I-SC induc ed by FSK, CCh or PMA. The Ca2+ channel blocker verapamil (5.10(-5) M), on the other hand, inhibited only PMA-stimulated Ise but had no effect on FSK or CCh-induced ISC Conclusions: Loperamide exerts a direct antisecretory ac tion on chloride secretion of colon epithelial cells independently of the r espective stimulatory signal transduction pathway. This antisecretory effec t is not mediated by opiate receptors and reflects inhibition of basolatera l K+ conductance.