125 Iodide as a surrogate tracer for epithelial chloride transport by the mouse large intestine in vitro

What is the central question of this study? The tracer Cl , currently used to measure transepithelial Cl fluxes, has become prohibitively expensive, threatening its future use. Iodide, previously validated alongside Cl as a tracer of Cl efflux by cells, has not been tested as a surrogate for Cl across epithelia. What is the main finding and its importance? We demonstrate that I can serve as an inexpensive replacement for measuring Cl transport across mouse large intestine, tracking Cl transport in response to cAMP stimulation (inducing Cl secretion) in the presence and absence of the main gastrointestinal Cl -HCO exchanger, DRA. Chloride transport is important for driving fluid secretion and absorption by the large intestine, with dysregulation resulting in diarrhoea-associated pathologies. The radioisotope Cl has long been used as a tracer to measure epithelial Cl transport but is prohibitively expensive. Iodide has been used as an alternative to Cl in some transport assays but has never been validated as an alternative for tracing bidirectional transepithelial Cl fluxes. The goal of this study was to validate I as an alternative to Cl for measurement of Cl transport by the intestine. Simultaneous fluxes of Cl and I were measured across the mouse caecum and distal colon. Net Cl secretion was induced by the stimulation of cAMP with a cocktail of forskolin (FSK) and 3-isobutyl-1-methylxanthine (IBMX). Unidirectional fluxes of I correlated well with Cl fluxes after cAMP-induced net Cl secretion, occurring predominantly through a reduction in the absorptive mucosal-to-serosal Cl flux rather than by stimulation of the secretory serosal-to-mucosal Cl flux. Correlations between I fluxes and Cl fluxes were maintained in epithelia from mice lacking DRA (Slc26a3), the main Cl -HCO exchanger responsible for Cl absorption by the large intestine. Lower rates of Cl and I absorption in the DRA knockout intestine suggest that DRA might have a previously unrecognized role in iodide uptake. This study validates that I traces transepithelial Cl fluxes across the mouse large intestine, provides insights into the mechanism of net Cl secretion and suggests that DRA might be involved in intestinal iodide absorption.