Active potassium absorption in rat distal colon.

Abstract

Active potassium (K+) absorption in rat distal colon was investigated by measuring mucosal-to-serosal (JK,ms) and serosal-to-mucosal (JK,sm) 42K+ fluxes (.mu.equiv h-1 cm-2) across isolated stripped mucosa under short-circuit conditions in normal and dietary Na-depleted animals. As previously demonstrated, removal of Na+ from both mucosal and serosal solutions bathing the normal colon slightly increased net K+ absorption as a result of inhibition of JK,sm without affecting Jk,ms, while in the Na-depleted group net K+ secretion (-0.54 .+-. 0.11) was converted to a marked net K+ absorption (1.68 .+-. 0.30, P < 0.001). In both groups of animals in Na+-free Ringer solution, JK,ms exhibited saturable and linear components, while JK,sm was a linear function of [K+]. Estimated affinity constants (mM) for saturable net K+ absorption were similar in normal (0.52 .+-. 0.12) and Na-depleted (0.67 .+-. 0.11) animals; however, there was a greater than 3-fold increase in the saturable flux (Jmax) from 0.54 .+-. 0.04 in the normal colon to 1.78 .+-. 0.08 .mu.equiv h-1 cm-2 in Na-depleted animals. Mucosal orthovanadate (100 .mu.M) inhibited JK,ms in both normal (control, 0.66 .+-. 0.05 vs. orthovanadate, 0.36 .+-. 0.03 .mu.equiv h-1 cm-2, P < 0.001) and Na-depleted animals (control 1.20 .+-. 0.13 vs. orthovanadate 0.77 .+-. 0.07 .mu.equiv h-1 cm-2, P < 0.01) without affecting Jk,sm or the short-circuit current. In the Na-depleted group mucosal omeprazole or SCH28080 (100 .mu.M), inhibitors of gastric K+-H+-ATPase, insignificantly or slightly reduced (by 10%) JK,ms respectively; in contrast, mucosal ouabain (1 mM) markedly inhibited JK,ms (control, 1.61 .+-. 0.16 vs. ouabain, 0.83 .+-. 0.98 .mu.equiv h-1 cm-2, P < 0.001). Mucosal Na+ appeared to be a competitor of K+ uptake across the apical membrane. These results indicate that dietary Na-depletion increases electroneutral K+ absorption by increasing its transport capacity and suggest that the mechanism of this active K+ absorption process may involve an apical K+-ATPase with properties that are unlike the gastric K+-H+-ATPase but similar, in part, to Na+-K+-ATPase.