¹²⁵Iodide as a Surrogate for ³⁶Chloride in Tracing Transepithelial Intestinal Chloride Transport

Abstract

Chloride (Cl−) transport by the large intestine is critically important, driving epithelial fluid secretion and absorption. Dysregulation of Cl− transport by the intestine results in diarrhea‐associated pathologies. Measurement of epithelial Cl− transport has long been achieved using the radioisotope 36Cl−. However, the price of 36Cl has sky‐rocketed, now costing over $100 per μCi compared to just $1.50 per μCi 15 years ago, making it prohibitively expensive and a barrier to future research. 125Iodide (125I−) has been used as an alternative to 36Cl− in some transport assays, including Cl− efflux and influx by cultured cells, but has never before been validated in parallel with 36Cl as an alternative for tracing transepithelial Cl− transport. The goal of this study was to examine whether 125I− can serve as a reliable surrogate for measuring Cl− transport by the intestinal epithelium. Under symmetrical short‐circuited conditions in the Ussing chamber, we simultaneously measured 36Cl− and 125I− fluxes across the cecum and distal colon from mice lacking the main Cl−/HCO3− exchanger, DRA (Slc26a3), relative to wild‐type controls. In addition, we used a cocktail of forskolin and 3‐isobutyl‐1‐methylxanthine (IBMX) to stimulate cAMP production and net Cl− secretion by these tissues. We found the unidirectional fluxes of 125I− correlated well with 36Cl− in all circumstances, even in the absence of DRA, with a significant R2 value of 0.86 across all fluxes, or higher (up to 0.93) when data was separated by genotype and intestinal segment. Unexpectedly, cAMP induced net Cl− secretion predominantly through a reduction in the absorptive, mucosal‐to‐serosal, Cl− flux rather than stimulating the secretory serosal‐to‐mucosal Cl− flux, which is noteworthy considering antidiarrheal drugs often target secretory processes. Also, rates of Cl− and I− absorption in DRA‐knockout mice were decreased by 79% and 50%, respectively, in the ceca, and 65% and 34% in the distal colon, suggesting this Cl− transporter may have a previously unrecognized role in I− uptake. In summary, we demonstrated 125I− is an adequate, cost‐effective alternative for tracing unidirectional Clfluxes across the intestinal epithelium. For example, the amount of 36Cl− used in this study cost $536 compared to a mere $4 worth of 125I−, thus making future research into epithelial Cltransport accessible. In addition, this study also provides insight into mechanisms of net Clsecretion and suggests that DRA may have a role in intestinal I− absorption.Support or Funding InformationThis study was supported by grants DK088892 and DK108755 to M. Hatch from the National Institutes of Health.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.