Cellular site of active K absorption in the guinea-pig distal colonic epithelium

Abstract

The mammalian distal colon, which is composed of different cell types, actively transports Na, K and Cl in absorptive and K and Cl in secretory directions. To further characterize the K absorption process and to identify the cells involved in K absorption, unidirectional Rb fluxes and luminal Rb uptake into different epithelial cell types were determined in isolated guinea-pig distal colon. Net Rb absorption (1.5-2.5 micromol.h-1.cm-2) was not influenced by inhibition of Na transport with amiloride or by incubating both sides of the epithelium with Na-free solutions, but was almost completely abolished by luminal ouabain, ethoxzolamide or by incubating both sides of the epithelium with Cl-free solutions. Luminal Rb uptake, blockable by luminal ouabain, preferentially occurred in columnar surface and neck cells, to a lesser extent in surface goblet cells and to an insignificant degree in lower crypt cells. Employing a luminal Rb-Ringer (5.4 mM Rb) the Rb concentration increased within 10 min in columnar surface and neck, surface goblet 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 solution reduced Rb uptake almost completely indicating a much higher acceptance 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-ATPase 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 absorption by ethoxzolamide and inhibition of both Rb absorption and Rb uptake by bilateral Cl-free incubation support the notion that cellular CO2 hydration is a necessary prerequisite for K absorption and that HCO3 leaves the cell via a Cl-HCO3 exchanger. Since ouabain-inhibitable transepithelial Rb flux and luminal Rb uptake rate by surface and neck cells were about the same, Rb(K) absorption seems to be accomplished mainly by columnar surface cells.

Changes in element composition of A6 cells following hypotonic stress

“…(basolateral osmolality of 260 mosmol/kg). In the epithelia of amphibians [17] and mammals [12] usually exposed to lower (220 mosmol/kg) and higher external osmolalities (300 mosmol/kg), the sum of cellular Na and K concentrations is also lower and higher by about 20 mmol/kg wet wt., respectively. This positive correlation between cellular cations and basolateral osmolalities suggests that different epithelial cells have very similar compositions, and that Na and K contribute substantially to total cellular osmolality.…”

Mechanism of action and toxicities of purgatives used for colonoscopy preparation

“…[6] Active K + absorption occurs in the distal colon where K + is taken up on the luminal side by two different H + -K + - ATPases. [9] Short chain fatty acids [SCFA] are absorbed by colonic epithelium in parallel with NaCl. The SCFA’s are produced during fermentation of dietary fibers by colonic bacteria.…”

“…In one study, 11% of patients could not tolerate this method. [8,9] Besides the large volume required, this method fell out of favor because it also induced weight gain and caused significant fluid and electrolyte imbalances. [20]…”