CELLULAR SITE OF ACTIVE K ABSORPTION IN THE GUINEA-PIG DISTAL COLONICEPITHELIUM

The mammalian distal colon, which is composed of different cell types, actively transports Na, K and Cl in absorptive and K and Cl in secret ory directions. To further characterize the K absorption process and t o identify the cells involved in K absorption, unidirectional Rb fluxe s and luminal Rb uptake into different epithelial cell types were dete rmined in isolated guinea-pig distal colon. Net Rb absorption (1.5-2.5 mu molh(-1).cm(-2)) was not influenced by inhibition of Na transport with amiloride or by incubating both sides of the epithelium with Na-f ree solutions, but was almost completely abolished by luminal ouabain, ethoxzolamide or by incubating both sides of the epithelium with Cl-f ree solutions. Luminal Rb uptake, blockable by luminal ouabain, prefer entially occurred in columnar surface and neck cells, to a lesser exte nt in surface goblet cells and to an insignificant degree in lower cry pt cells. Employing a luminal Rb-Ringer (5.4 mM Rb) the Rb concentrati on increased within 10 min in columnar surface and neck, surface goble t and lower crypt cells to 70, 32 and about 10 mmol.kg(-1) wet weight, respectively. The presence of 5.4 mM K in the luminal incubation solu tion reduced Rb uptake almost completely indicating a much higher acce ptance of the luminal H-K-ATPase for K than for Rb. The increase in Na and decrease in K concentrations in surface and neck cells induced by luminal ouabain might indicate inhibition of the basolateral Na-K-ATP ase or drastic enhancement of cellular Na uptake by the Na-H exchanger . Bilateral Na-free incubation did not alter Rb uptake, but bilateral Cl-free incubation drastically reduced it. Inhibition of net Rb absorp tion by ethoxzolamide and inhibition of both Rb absorption and Rb upta ke by bilateral Cl-free incubation support the notion that cellular CO 2 hydration is a necessary prerequisite for K absorption and that HCO, leaves the cell via a Cl-HCO3, exchanger. Since ouabain-inhibitable t ransepithelial Rb flux and luminal Rb uptake rate by surface and neck cells were about the same, Rb(K) absorption seems to be accomplished m ainly by columnar surface cells.