Mechanism of water transport across the upper portion of the descending thin limb of long-looped nephron of hamsters

Abstract

The mechanisms of water transport across the upper portion of the descending limb of long-looped nephron (LDL_u) were examined by microperfusion of segments isolated from hamster kidneys. Because of cation permselectivity a streaming voltage was generated when a transmural osmotic gradient was imposed. When 100 mmol/l urea was added to the bath, the streaming voltage was −4.9±0.4 mV. Addition of 10⁻⁴mol/l parachloromercuribenzene sulfonate (PCMBS) decreased the voltage to −2.4±0.7 mV. This effect was associated with changes in osmotic water permeability (P_f, 10⁻³ cm/s) from 243±42 to 47±15. PCMBS also decreased the transmural diffusional water permeability (P_dw, 10⁻³ cm/s) from 9.4±0.6 to 7.2±0.6. The inhibitory effect of PCMBS was prevented by pretreatment with dithiothreitol.N-Butanol permeability was measured as an index of cellular resistance for diffusion. Large differences between P_f and P_dw can be explained both by cellular resistance to diffusion and by resistance through a water channel with a single file mechanism. The apparent activation energy for water transport, 13.3×10³ joule/mol (3.16 kcal/mol), was low. These findings are compatible with the hypothesis that a water channel exists in this segment. PCMBS also inhibited the NaCl diffusion voltage, a parameter indicating cation permselectivity, in parallel with suppression of the streaming voltage, suggesting that the water channel is in part associated with cation permselectivity. The possibility that the PCMB-sensitive cation-permselective pathway exists in parallel cannot be ruled out.