Active urea transport by the skin of Bufo viridis: amiloride- and phloretin-sensitive transport sites

Urea is actively transported inwardly (J(i)) across the skin of the green toad Bufo viridis. J(i) is markedly enhanced in toads adapted to hypertonic saline. We studied urea transport across the skin of Bufo viridis under a variety of experimental conditions, including treatment with amiloride and phloretin, agents that inhibit urea permeability in the bladder of Bufo marinus. Amiloride (10^-4 M) significantly inhibited J(i) in both adapted and unadapted animals and was unaffected by removal of sodium from the external medium. Phloretin (10^-4 M) significantly inhibited J(i) in adapted animals by 23-46%; there was also a reduction in J(i) in unadapted toads at 10^-4 and 5 x 10^-4 M phloretin. A dose-response study revealed that the concentration of phloretin causing half-maximal inhibition (K( 1/2 )) was 5 x 10^-4 M for adapted animals. J(i) was unaffected by the substitution of sucrose for Ringer solution or by ouabain. We conclude 1) the process of adaptation appears to involve an increase in the number of amiloride- and phloretin-inhibitable urea transport sites in the skin, with a possible increase in the affinity of the sites for phloretin; 2) the adapted skin resembles the Bufo marinus urinary bladder with respect to amiloride and phloretin-inhibitable sites; 3) we confirm earlier observations that J(i) is independent of sodium transport.