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Endocrinology Research Group, Clinical Research Centre, Harrow, Middlesex.

1. Potassium secretion by the distal colon was examined in relation to the secretion of chloride and absorption of sodium and to the epithelial turnover of 86Rb+ (as an analogue of potassium) in vivo in rats fed a standard, a potassium-rich or a sodium-depleted diet. 2. An acute intravenous potassium load stimulated potassium secretion two- to threefold. In rats fed the standard diet, sodium secretion was also increased but no significant change in the lumen-to-plasma sodium flux was detected. The potassium and sodium secretions were accompanied by increased chloride secretion which occurred even when the intravenous load contained no chloride. In rats fed the potassium-rich diet, there was a small increase in sodium absorption and a less marked increased of chloride secretion. In the sodium-depleted rats, however, about 70-80% of the increased potassium secretion was balanced by increased sodium absorption. 3. The epithelial turnover rate of 86Rb+ in the rats fed the potassium-rich (t1/2 12-14 min) and the sodium-depleted (t1/2 11-13 min) diets was faster than that of those of the standard diet (t1/2 22-27 min). 4. The epithelial potassium content was 304 +/- 9 nmol (mg dry weight)-1 and was not significantly altered despite considerable variations in potassium secretion rate induced by dietary changes and acute potassium loading. 5. It is concluded that epithelial potassium turnover rate is increased during chronic states of increased potassium secretion and that the rise in potassium secretion is consistent with increased activity of the basolateral Na(+)-K+ pump. Whether the increased potassium secretion is associated with chloride secretion or with increased sodium absorption appears to depend on the absence or presence of the amiloride-sensitive sodium pathway in the apical membrane.