Characterization of the inhibitory effect of boiled rice on intestinal chloride secretion in guinea pig crypt cells

Background & Aims: When rice is incorporated into oral rehydration therapy for patients with secretory diarrhea, clinical outcomes improve. We have sh own that a factor purified from boiled rice (RF) blocks the secretory respo nse of intestinal crypt cells to adenosine 3',5'-cyclic monophosphate (cAMP ). Now we report that the cystic fibrosis transmembrane conductance regulat or (CFTR) chloride channel is the cellular target for this rice inhibitor. Methods: We used RF, the same previously described extract prepared from bo iled rice, to assess chloride channel activation in vitro, measuring (1) ce ll volume regulation of guinea pig intestinal crypt epithelial cell suspens ions using standard Coulter counter technology, (2) transepithelial chlorid e current in monolayers of T-84 cells mounted in Ussing chambers, and (3) w hole-cell and single-channel currents using the patch-clamp technique in ce lls transfected to express CFTR. Results: RF inhibited activation by cAMP o f CFTR chloride channels in all experimental preparations; RF did not block volume-stimulated Cl- secretion, suggesting that its effect might be speci fic for CFTR chloride channels. RF inhibited transepithelial cAMP-stimulate d Cl- current in T84 cells and inhibited forskolin (i.e., cAMP)-induced cur rent in cells transfected with CFTR. Excised patch and single-channel patch -clamp recordings supported the view that the response was a direct effect on CFTR rather than on cAMP signal transduction. Conclusions: RF exerts a s pecific inhibitory effect on CFTR chloride channels, blocking activation fr om the luminal surface of the cell and reversing established activation. Ma ny major diarrheal states are based on cAMP-induced CFTR activation, leadin g to excessive gut secretion; our findings could have clinical relevance.