Modelling of colonic Cl- and K+ transport under resting and secreting conditions.

Transport of electrolytes, particularly of Cl- and K+, by the rat proximal colon was studied in vivo under conditions of high and low K+ diet and in the presence of transport inhibitors and secretagogues. The electrical parameters, PD, Isc, Rm, were monitored by direct clamping and via cable analysis. The present work reinforces own earlier work and that of others that - under control conditions - absorption of Na+ and Cl- is mainly electrically neutral. Low K+ diet reduced absorption of Na+ and Cl-, as well as secretion of K+, and increased HCO3- absorption. High K+ predominantly increased K+ secretion and reduced HCO3- absorption, these diet-induced changes being electrically neutral. Secretagogues such as PGE1 and theophylline reduced net absorption of Na+ and Cl-, increased the Isc and lowered the Rm. These effects could be reversed by the antidiarrheal drugs, loperamide and somatostatin, except the increase of K+ secretion, and reversed HCO3- transport absorption to secretion. A model consisting of absorptive columnar cells and secreting crypt cells is presented which encompasses the known and supposed aspects of colonic ion transport.