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

Abstract

The model of sodium and chloride transport proposed for the colon is based on studies performed in the distal segment and tacitly assumes that ion transport is similar throughout the colon. In rat distal colon, neutral sodium-chloride absorption accounts for the major fraction of overall sodium absorption and aldosterone stimulates electrogenic, amiloride-sensitive sodium absorption. Since we have demonstrated qualitative differences in potassium transport in proximal and distal segments of rat colon, unidirectional 22Na and 'Cl fluxes were performed under shortcircuit conditions across isolated proximal colon of control and sodium-depleted rats with secondary hyperaldosteronism. In the control group, net sodium absorption (JN:) (7.4±0.5 Ateq/h * cm2) was greater than I,, (1.4±0.1 lueq/h cm2), and JPt was 0 in Ringer solution. Residual flux (J) was -5.2±0.5 teq/h cm2 consistent with hydrogen ion secretion suggesting that neutral sodium absorption may represent sodium-hydrogen exchange. 1 mM mucosal amiloride, which inhibits sodium-hydrogen exchange in other epithelia, produced comparable decreases in J" and JR (4.1±0.6 and 3.2±0.6 ;&eq/h * cm2, respectively) without a parallel fall in I,<. Sodium depletion stimulated JN, JA, and I., by 7.0±1.4, 6.3±1.9, and 0.8±0.2 ;eq/h cm2, respectively, and 1 mM amiloride markedly inhibited JN' and J by 6.0±1.1 and 4.0±1.6 ,ueq/h * cm2, respectively, with only a minimal reduction in I4.. Conclusions: the predominant neutral sodium-absorptive mechanism in proximal colon is sodium-hydrogen exchange. Sodium depletion stimulates electroneutral chloride-dependent sodium absorption (most likely as a result of increasing sodium-hydrogen and chloride-bicarbonate exchanges), not electrogenic chloride-independent sodium transport. The model of ion transport in the proximal colon is distinct from that of the distal colon.

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

“…In the rodent, as in the human, the proximal segment of the colon appears to have the greatest capacity for Na ϩ and water absorption per unit area, and much of this is probably associated with SCFA Ϫ absorption (75). Our finding that the proximal colon expresses abundant NHE3 without DRA is consistent with functional evidence that, in the absence of SCFA Ϫ , this segment absorbs Na ϩ rapidly via Na/H exchange without transporting significant quantities of Cl Ϫ or HCO 3 Ϫ (29,46,57). Another unique feature of the proximal colon is an acidic mucosal surface (rat: pH 6.2; mouse: pH 6.6) (46).…”

“…mouse; rat; intestine; electroneutral NaCl absorption TRANSPORT ACROSS THE COLONIC epithelium allows for efficient salvage of water and solutes [Na ϩ , Cl Ϫ , short-chain fatty acid (SCFA)], maintenance of the resident microbial ecosystem, and optimal fecal consistency. The absorptive movements of Na ϩ and Cl Ϫ in the mammalian colon are, to a large degree, mutually interdependent, electrically neutral, and associated with secretory movements of H ϩ and HCO 3 Ϫ (28,29,35,57,71). It is generally accepted that electroneutral NaCl absorption by the intestine largely reflects the tandem operation of apical Na/H exchange and Cl/HCO 3 exchange processes, and studies of rodent colon have established that these processes primarily involve the Na/H exchanger NHE3 (SLC9A3) and the Cl/ HCO 3 exchanger SLC26A3 (alias downregulated in adenoma, or DRA) (5,7,57,71,86).…”

“…Although it is well established that the rates and characteristics of Na ϩ and Cl Ϫ transport differ markedly between the proximal and distal segments of the colon (29,71), the spatial profile of electroneutral NaCl absorption along the cecocolonic axis remains poorly defined. To better understand the anatomical and functional compartmentation of the colon, and to reassess the applicability of the NHE3/DRA coupling concept in colonic electrolyte absorption, we determined the expression profiles of apical NHE3 and DRA proteins in the cecum and colon of rats and mice.…”

News from the End of the Gut-How the Highly Segmental Pattern of Colonic HCO3- Transport Relates to Absorptive Function and Mucosal Integrity

“…Na ϩ absorption occurs in an electroneutral manner 107) via NHE3, 38) leading to a relatively low mucosal surface pH.…”

Function of K+ Channels in the Intestinal Epithelium

“…Depending on the needs of metabolism and hormone concentrations, K + can be net-secreted via channels or reabsorbed by a luminal H+/K+-ATPase (Fig. 5) [10,34,89,120]. A detailed list of hormones and mediators regulating colonic transport is given in [133].…”