Low Na⁺ effects on PAH transport and permeabilities in isolated snake renal tubules

Effects of low sodium concentrations on p aminohippurate (PAH) transport by isolated, perfused snake (Thamnophis spp.) distal proximal renal tubules were studied. Replacement of sodium in bath with choline led to significant depression of net PAH transport from bath to lumen in less than 10 min and to maximum depression (to 25-30% of control) in about 30 min, but transport still occurred against concentration gradient. In absence of sodium, PAH concentration in cell water was markedly depressed, but still slightly greater than that in bath or lumen. Apparent permeability of peritubular membrane, determined from PAH efflux from tubules with oil filled lumens, averaged about 0.5 x 10^-5 cm s^-1 in 150 mM sodium and about 1.1 x 10^-5 cm s^-1 in sodium free medium. Data suggest that both decreased rate of active transport and increased apparent permeability of peritubular membrane contribute to depression of net transepithelial PAH transport and cell water PAH concentration in sodium free medium. When sodium was restored to bath, net PAH transport nearly tripled in 15 min. Reduction of bath sodium concentration to one half control of perfusion with sodium free medium in lumen and control medium in bath had no effect on net PAH transport.