Segmental heterogeneity of epithelial transport in rat large intestine

Abstract

Functionally isolated segments of rat colon and rectum were perfused in situ in a closed loop system. Rectum was defined as the lower 25--35% of the length of large intestine (cecum excluded). Perfusion conditions were optimized at 0.5 ml.min-1 and 3 cm H2O luminal pressure. Variation of perfusion rate between 0.2 and 2 ml.min-1 did not influence net volume transport (JNV). Luminal distension following elevation of hydrostatic pressure to 18 cm H2O reversibly increased Jnv. Under control conditions Jnv and Na+-transport rates (JnNa) of colon were 2--3 times higher than those of rectum. In colon transepithelial electrical potential difference (psims) was time independent --12 mV (lumen negative) whereas rectal psims increased with time from --6 mV, reaching a plateau of --67 mV within 6 h. Amiloride 10(-4) mol.l-1 had no effect on psims, Jnv, and JnNa in colon but did slightly depress K+-secretion in colon descendens. In contrast, psims in rectum was dose-dependently depressed, being reversed to +7 mV at 10(-4) mol.l-1. Jnv and JnNa were decreased by half. Acetazolamide in addition to amiloride lowered the positive post-amiloride rectal psims by half. Adrenalectomy had no effect on colonic psims, but abolished psims of the rectum. A single dose of 40 microgram.kg-1 b.w. aldosterone during the experiment restored the typical time course of rectal psims, but did not affect psims in colon. It is concluded that aldosterone induces an amiloride-sensitive Na+-pathway only in rectum, but not in colon, and that colon and rectum differ basically in their transport properties, quantitatively as well as qualitatively, as do the kidney distal convoluted tubule and the cortical collecting duct.

Ion transport in proximal colon of the rat. Sodium depletion stimulates neutral sodium chloride absorption.

“…Differences in ion transport processes in the rat colon have been reported (6,9,10,13,42), and significant differences in function appear to exist in various colonic segments of several other species including humans (1,2,7,8,43,44). Thus, there may be more differences than similarities in the function ofvarious segments ofthe large intestine.…”

“…Further, in vitro studies of ion transport have until recently been primarily performed in the distal portion of the colon (4,5). Recently, however, several reports have demonstrated that electrolyte transport is not identical in all portions of the mammalian large intestine and that the characteristics of both basal ion transport and the effect of aldosterone on sodium absorption may differ substantially in various colonic segments (5)(6)(7)(8)(9)(10).…”

“…Amiloride also diminished net chloride absorption by -4.0±1. 6 ,eq/ h a cm2. In contrast, 0.1 mM mucosal bumetanide had no effect on either net sodium or net chloride transport (Table IV).…”

Electrogenic sodium absorption in rabbit cecum in vitro.

“…Recent studies from our laboratory and others have established that there are important differences between the proximal and distal colon (1)(2)(3)(4). Additionally, there now appear to be differences within the rabbit proximal colon between the segment immediately distal to the cecum (the triple haustrated segment) and the region 10 cm beyond the cecum (the single haustrated segment) (4,5).…”

Segregation of Na/H exchanger-3 and Cl/HCO3exchanger SLC26A3 (DRA) in rodent cecum and colon

“…Similar results were obtained in a subset of experiments by using a miniature glass pH combination electrode with a 0.75-mm-diameter tip and internal reference (MI-415, Microelectrodes). In some experiments, the external reference electrode was anchored subcutaneously in the inguinal region with a suture, without correction for transmucosal potential differences (6 -12 mV) (30), to allow comparison to previous data obtained in the same manner (56). Electrodes were connected to a pH meter (Orion 720A) and calibrated against phosphate pH buffer standards (4.0 -10.0) or [Na ϩ ] standards (0 -100 mM in 20 mM K-acetate).…”