ACTIVE UREA TRANSPORT INDEPENDENT OF H+ AND NA+ TRANSPORT IN FROG-SKIN EPITHELIUM

We investigated the relationship between H+ secretion (J(H)), Na+ absorption (J(Na)), and urea transport (J(u)) in skin of frogs (Rana esculenta) adapted to running tap water, NaCl (100 mM), and KCl (100 mM). In addition, cell morphological changes, particularly in the mitochondria-rich cells (MRC), were followed. NaCl adaptation stimulated an active J(u), reduced J(Na) and J(H), and caused a decrease in the apical surface of MRC. After KCl adaptation, J(Na) and J(H) were increased and highly correlated, with a twofold increase in J(u), whereas the numerous MRC developed infoldings on their apical membranes. No correlation was found between J(H) and J(u). Clamping the skins in a range of +/- 50 mV or changing the external pH from 7.4 to 5.4 (at high cellular buffering power) had no effect on J(u). Depolarization of the basolateral membranes (serosal KCl-Ringer) had no effect on J(u). J(u) was reversibly blocked by acidification of the cells by oxygen-free solution and sulfhydryl reagents (Hg2+, p-chloromercuribenzenesulfonic acid, and N-ethylmaleimide). Diethylstilbestrol, a proton transport blocker, had no effect on J(u). Apical addition of amiloride and derivatives (phenamil and ethylisopropyl amiloride) reversibly blocked J(u), whereas ouabain had no effect. We conclude that a cation (Na+ or H+)-dependent process is unlikely to exist in R. esculenta skin. A primary active transport in a two-step process is the simplest hypothesis to account for the energy-dependent J(u) that develops in NaCl-adapted frogs.