Kallikrein inhibitors decrease short-circuit current by inhibiting sodium uptake.

We have reported that aprotlnin, a reversible inhibitor, and D-Pbe-Phe-Arg chlorometbyl ketone (DPPA), an irreversible inhibitor of kallikrein, decrease short-circuit current (SCC) in the urinary bladder of the toad Bufo marinus in a dose-dependent fashion. Both agents act more rapidly and potently when added to the mucosai side of the bladder, exhibiting similar potency and effectiveness. To further define their site(s) of action we have used amphoteridn B, a polyene antibiotic which interacts with the mucosai cell membrane and increases its cation permeability, presumably by altering a barrier at the apical border that limits the rate of Na+ uptake by epithelial cells. Thus, the SCC-inhibiting effect of amiioride, a diuretic that blocks the mucosai Na+ uptake site, is reversed by mucosai amphoteridn B treatment. We found that the effects of either aprotinin or DPPA were reversed after the addition of amphotericin B (2.0 X 10"* M) to the mucosai side of the bladder. Also, when Initial SCC was completely blocked by amiioride (1.0 X 10* 4 M) the addition of amphotericin B (2.0 X 10~* M) to the mucosai bath induced an amiloride-insensitive SCC, which was not affected by high concentrations of either aprotinin or DPPA, but was inhibited by serosal addition of ouabain (1.0 X 10 M). We conclude that these kallikrein inhibitors act at a site along the active transport pathway that is proximal to the ouabain-sensitive site, and that removal of the apical barrier by amphotericin B prevents their effect on SCC. Thus, the apical barrier appears to be a likely site of the action of these agents on SCC, suggesting a close relationship between this structure and a serine proteinase involved in the uptake of Na+ by the toad bladder. (Hypertension 3 (suppl II): II-92-II-95, 1981) KEY WORDS toad bladder amphibian membranes sodium transport