Inhibition of short-circuit current by triaminopyrimidine in isolated toad urinary bladder

The organic cation 2,4,6-triaminopyrimidine (TAP) produced inhibition of short-circuit current (SCC) when added to either the mucosal or serosal surface of the isolated urinary bladder of the toad. Fifty percent inhibition was produced by 10(-3) M TAP in the mucosal solution at pH 6.8 when the mucosal [Na+] was 113 mM. The actions of TAP resemble those produced by amiloride in several ways: a) inhibition of SCC by mucosal application is rapid; b) the mucosal inhibition is fully reversible; c) high concentrations in the serosal solutions produce irreversible inhibition; and d) the concentration required to produce 50% inhibition from the mucosal side is reduced when mucosal [Na+] is reduced. It is postulated that mucosal application of TAP and amiloride inhibit short-circuit current in high-resistance epithelia via action at a common locus.