K+ transport by rat colon: adaptation to a low potassium diet

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Abstract
Recent studies with the isolated perfused rat kidney have demonstrated the existence of an intrinsic renal adaptation to conserve K+ in response to ingestion of a low K+ diet for 3 days. To determine whether the colon alters the K+ transport properties in a similar fashion, we measured transmural 86Rb fluxes across sheets of distal colonic epithelium under short-circuit conditions. Preliminary studies using a double-isotope technique demonstrated that 86Rb and 42K fluxes were similar; therefore 86Rb flux was considered equivalent to K+ flux. The distal half of the colon from each rat was divided into two segments, referred to as early and late distal colon. Experiments were carried out using rats fed a K+-free, control (0.15 mmol/g), and high K+ (1.13 mmol/g) powdered diet of otherwise identical electrolyte content. Net K+ secretion (Jnet) by the early distal colon was reduced from 0.45 in the controls to -0.02 .mu.eq .cntdot. cm-2 .cntdot. h-1 by a low K+ diet as a result of a decrease in serosal-to-mucosal flux (Jsm), with no change in mucosal-to-serosal flux (Jms). Conductance (GT) and short-circuit current (Isc) were unchanged. Jnet by the late distal colon averaged 0.17 in the controls and 0.01 .mu.eq .cntdot. cm-2 .cntdot. h-1 with a low K+ diet, but this difference was not significant statistically. In comparison with the controls, a high K+ diet had no effect on Jnet by the early distal colon (0.48 .mu.eq .cntdot. cm-2 .cntdot. h-1) but increased Jnet by the late distal colon substantially (0.77 .mu.eq .cntdot. cm-2 .cntdot. h-1), due entirely to an increase in Jsm. Thus a low K+ diet modifies the K+ transport properties of the rat colon, which is analogous to its adaptive effects on K+ handling by the kidney. Furthermore, different portions of the colon serve as the major sites for altered K+ transport in response to a low and high K+ intake.

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