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

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 its 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 (J(net)) by the early distal colon was reduced from 0.45 in the controls to -0.02 μeq · cm^-2 · h^-1 by a low K⁺ diet as a result of a decrease in serosal-to-mucosal flux (J(sm)), with no change in mucosal-to-serosal flux (J(ms)). Conductance (G(T)) and short-circuit current (I(sc)) were unchanged. J(net) by the late distal colon averaged 0.17 in the controls and 0.01 μeq · cm^-2 · 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 J(net) by the early distal colon (0.48 μeq · cm^-2 · h^-1) but increased J(net) by the late distal colon substantially (0.77 μeq · cm^-2 · h^-1), due entirely to an increase in J(sm). 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.