Studies on K+ permeability of rat gastric microsomes

A population of gastric membrane vesicles of high K+ permeability and of lower density than endoplasmic tubulovesicles containing (H+-K+)-ATPase was detected in gastric mucosal microsomes from the rat fasted overnight. The K+-transport activity as measured with 86RbCl uptake had a Km for Rb+ of 0.58 +/- 0.11 mM and a Vmax of 13.7 +/- 1.9 nmol/min X mg of protein. The 86Rb uptake was reduced by 40% upon substituting Cl- with SO2-4 and inhibited noncompetitively by ATP and vanadate with a Ki of 3 and 30 microM, respectively; vanadate also inhibited rat gastric (H+-K+)-ATPase but with a Ki of 0.03 microM. Carbachol or histamine stimulation decreased the population of the K+-permeable light membrane vesicles, at the same time increased K+-transport activity in the heavy, presumably apical membranes of gastric parietal cells, and enabled the heavy microsomes to accumulate H+ ions in the presence of ATP and KCl without valinomycin. The secretagogue-induced shift of K+ permeability was blocked by cimetidine, a H2-receptor antagonist. Four characteristics of the K+ permeability as measured with 86RbCl were common in the resting light and the carbachol-stimulated heavy microsomes; (a) Km for +Rb, (b) anion sensitivity (Cl- greater than SO2-4), (c) potency of various divalent cations (Hg2+, Cu2+, Cd2+, and Zn2+) to inhibit Rb+ uptake, and (d) inhibitory effect of ATP, although the nucleotide sensitivity was latent in the stimulated heavy microsomes. The Vmax for 86RbCl uptake was about 10 times greater in the resting light than the stimulated heavy microsomes. These observations led us to propose that secretagogue stimulation induces the insertion of not only the tubulovesicles containing (H+-K+)-ATPase, but also the light membrane vesicles containing KCl transporter into the heavy apical membranes of gastric parietal cells.