High K⁺ effects on PAH transport and permeabilities in isolated snake renal tubules

Effects of high potassium concentrations on para aminohippurate (PAH) transport by isolated, perfused snake (Thamnophis spp.) distal proximal renal tubules were studied. Increasing the potassium concentration in bath from 3 mM to 10 mM or 40 mM led to about 50% decrease in net PAH transport from bath to lumen in less than 10 min, but transport still occurred against conce tration gradient. Cell water PAH concentration was not significantly depressed in 10 mM potassium and was nearly double control level in 40 mM potassium. Apparent luminal membrane permeability to PAH, calculated from perfusion studies, averaged about 3.5 x 10^-5 cm sec^-1 in 3 mM potassium, 1.15 x 10^-5 cm sec^-1 in 10 mM potassium and 0.48 x 10^-5 cm sec^-1 in 40 mM potassium. Apparent peritubular membrane permeability, determined from PAH efflux from tubules with oil filled lumens averaged about 0.5 x 10^-5 cm sec^-1 in 3 mM potassium, 0.29 x 10^-5 cm sec^-1 in 10 mM potassium and 0.17 x 10^-5 cm sec^-1 in 40 mM potassium. These data suggest that high potassium concentrations depress transepithelial PAH transport primarily by reducing luminal and peritubular membrane permeabilities. Effect of high potassium on PAH transport was reversed within 20 min after restoration of control potassium concentration.