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

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Abstract
The mechanisms of water transport across the upper portion of the descending limb of long-looped nephron (LDLu) 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 (Pf, 10−3 cm/s) from 243±42 to 47±15. PCMBS also decreased the transmural diffusional water permeability (Pdw, 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 Pf and Pdw 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.

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