Potassium absorptive pump at the luminal membrane of turtle urinary bladder.

In the absence of transepithelial electrochemical gradients, the direction of net K transport across the turtle urinary bladder is from the mucosal (M) to the serosal (S) solution. Under control conditions, M leads to S flux was 101 +/- 5 nmol . h-1 . (8 cm2)-1, S leads to M flux was 59 +/- 4, and the net absorptive flux was 42 +/- 6. The K absorptive pump was characterized by examining its dependence on voltage and ambient sodium and its sensitivity to mucosal ouabain. Lumen-negative voltages caused an increase rather than the expected decrease in active K absorption. Thus, the active K flux appeared to be coupled to a flow of positive charge in the opposite direction, possibly representing Na secretion in excess of K absorption. Net K absorption was abolished by removal of Na from the medium and by mucosal, but not serosal, addition of ouabain. The reverse electrogenicity, Na dependence, and ouabain sensitivity of K absorption indicate that the K pump of the mucosal membrane has characteristics of a Na-K-ATPase.