Although the kidney plays the major role in the regulation of systemic K+ homeostasis, the colon also participates substantively in K+ balance. The colon is capable of both K+ absorption and secretion, the magnitude of which can be modulated in response to dietary K+ intake. The H(+)-K(+)-adenosinetriphosphatase (H(+)-K(+)-ATPase) has been proposed as a possible mediator of K+ absorption in distal colon, but inhibitor profiles obtained in recent studies suggest that two, and perhaps more, distinct H(+)-K(+)-ATPase activities may be present in mammalian distal colon. We have developed highly specific probes for the catalytic alpha-subunits of colonic and gastric H(+)-K(+)-ATPase, alpha 1-Na(+)-K(+)-ATPase, and beta-actin, which were used in Northern analysis of total RNA from whole distal colon and stomach obtained from one of three experimental groups of rats: 1) controls, 2) chronic dietary K+ depletion, and 3) chronic metabolic acidosis. The probe for the colonic but not the gastric H(+)-K(+)-ATPase alpha-isoform hybridized to distal colon total RNA in all groups. A significant increase in colonic H(+)-K(+)-ATPase mRNA abundance was observed in response to chronic dietary K+ depletion but not to chronic metabolic acidosis. The alpha 1-isoform of Na(+)-K(+)-ATPase, which is also expressed in distal colon, did not respond consistently to either chronic dietary K+ depletion or chronic metabolic acidosis. The gastric probe did not hybridize to total RNA from distal colon but, as expected, hybridized to total stomach RNA. However, the abundance of gastric H(+)-K(+)-ATPase or Na(+)-K(+)-ATPase in stomach was not altered consistently by either chronic dietary K+ depletion or metabolic acidosis. Under the conditions of this study, it appears that the mRNA encoding the colonic alpha-isoform is upregulated by chronic dietary K+ restriction, a condition shown previously to increase K+ absorption in the distal colon.