Mechanisms of K+ transport in isolated turtle urinary bladder. Induction of active K+ secretion in a K+-absorbing epithelium.

Transepithelial K(+) movement was studied in vitro in the short-circuited turtle bladder by increasing luminal K(+) permeability and by inhibiting the basolateral Na/K pump. Luminal addition of amphotericin B caused net K(+) secretion (180+/-52 nmol/h) compared with net K(+) absorption (42+/-6 nmol/h) in control bladders. Serosal ouabain and luminal amiloride abolished K(+) secretion in amphotericin-treated bladders; ouabain restored net absorption (45+/-16 nmol/h). The direction and rate of net K(+) transport are controlled by the relative K(+) permeabilities and the Na/K pump sites at the two cell membranes of the epithelium.