RICE MAY INHIBIT INTESTINAL SECRETION BY A DIRECT EFFECT ON ACTIVATED CYSTIC FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR (CFTR) CHLORIDE CHANNELS

Abstract

81 The benefits of rice in treating acute diarrhea led us to studies in which an extract of boiled rice blocked the secretory response to adenosine 3'5 -- cyclic monophosphate (cAMP) in crypt epithelial cells (Lancet 346:90,1995). The experiments to be reported significantly extend these preliminary results using the same low molecular weight, lipophilic, non-peptide rice factor (RF) and a range of cellular and molecular techniques to characterize the specific anti-secretory impact of RF on the gut. We found: 1) RF (10 μg/ml) blocked adenosine 3'5 -- cyclic monophosphate (cAMP) activated Cl channels, but it failed to inhibit Cl channels activated by hypotonic dilution in fresh guinea pig jejunal crypt cell suspensions. 2) Added to the apical surface of T84 cell monolayers, RF (38 μg/ml) reduced cAMP-simulated Cl current by 24 ± 5% (p < 0.02). 3) In Baby Hamster Kidney (BHK) cells stably transfected with plasmid directing the high-level expression of wild type CFTR, RF (114μg/ml) inhibited forskolin-stimulated whole-cell CFTR chloride current from 2.83 to 0.61 nA measured at Vm = + 60mV. 4) In patches excised from cells expressing CFTR protein, cytoplasmic exposure to protein kinase A (60 nM) and MgATP (1 mM) caused generation of macroscopic chloride current, and addition of RF (60 μg/ml) caused marked 60 ± 1% inhibition of current (19.4± 2.7 to 7.6 ± 0.8 pA) within seconds. This inhibition was partially reversed by washout and restimulation with PKA/MgATP. 5) Finally, in a membrane patch containing a small number of CFTR channels, addition of 60μg/ml RF caused a rapid 78% reduction in single-channel open probability. We conclude that the anti-secretory action of this rice extract results from its direct action on the intestinal epithelial Cl channel, CFTR; it not only blocks activation of these channels, it can reverse existing CFTR activation. In addition to its theoretical capacity to enhance nutrient-linked absorption, ingested cooked rice may be capable of inhibiting those secretory states in the intestine such as cholera where diarrhea is associated with activation of CFTR chloride channels.