Antisecretory Effect of Loperamide in Colon Epithelial Cells by Inhibition of Basolateral K⁺ Conductance

Abstract

Background: The mechanism of the antisecretory effect of loperamide was investigated in cultured highly differentiated colon epithelial cells (HT-29/B6). Methods: Chloride secretion was stimulated via cAMP by forskolin (FSK, 10^-5 M), via Ca²⁺ by the muscarinic agonist carbachol (CCh, 10^-4 M), and 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 monolayers mounted in Ussing-type chambers. Results: Loperamide (5·10^-5 M) inhibited I_SC stimulated by FSK, CCh and PMA. The antisecretory action of loperamide was unaffected by preincubation with naloxone (10^-5 M). Furthermore, loperamide strongly inhibited basolateral ⁸⁶Rb efflux. Like loperamide, the calmodulin antagonist trifluoperazine (10^-4 M) inhibited I_SC induced by FSK, CCh or PMA. The Ca²⁺ channel blocker verapamil (5·10^-5 M), on the other hand, inhibited only PMA-stimulated I_SC, but had no effect on FSK or CCh-induced I_SC. Conclusions: Loperamide exerts a direct antisecretory action on chloride secretion of colon epithelial cells independently of the respective stimulatory signal transduction pathway. This antisecretory effect is not mediated by opiate receptors and reflects inhibition of basolateral K⁺ conductance.

• Calcium Calmodulin Cyclic Amp Chloride Secretion Diarrhoea Ht-29 Potassium Conductance Potassium Channel Protein Kinase C