Generation of transepithelial potentials by isolated perfused reptilian distal tubules

Transepithelial potentials were measured in the most distal segments of garter snake (Thamnophis spp.) distal tubules perfused in vitro. The segments generated high lumen-negative potentials when sodium was in the lumen. The size of the potentials was a saturable function of luminal sodium concentrations between 0 and 32 mM. The potentials were stable with time only when sodium concentrations in the lumen were less than 30 mM. Perfusion with high sodium concentrations resulted in transient potentials. Stable potentials changed markedly when the lumen sodium concentration or the bath potassium concentration was altered suddenly, but they were independent of lumen potassium concentrations and bath sodium concentrations. Amiloride stimulated or inhibited the potentials, ouabain partially depressed them, and ethacrynic acid and cyanide inhibited them slowly and often reversibly. We conclude that distal transepithelial potentials reflect sodium transport from lumen to bath across a tight asymmetric epithelium which differs from other sodium-transporting epithelia in that stable transepithelial potentials are maintained only with luminal sodium concentrations less than 30 mM.