Water permeability in rat distal tubules.

Abstract

AbstractOsmotic and diffusional water permeability was studied in rat distal tubules in antidiuresis and water diuresis with a microperfusion technique. In antidiuresis the osmotic water permeability was 6.1 ± 1.2. 10‐‐8 em3 cm‐2 sec‐1 (cm H2O)‐l and the diffusional water permeability 327 ± 132 10‐‐5 cm/sec. In water diuresis obtained by hypophysectomy and water loading the osmotic water permeability was 1.6±1.6 10‐‐8 cm3 cm‐2 sec‐1 (cmH2O)‐1 and the diffusional water permeability was 157±32 10‐‐5 cm/sec, both values being significantly lower than in antidiuresis. The osmotic water permeability is much larger than the diffusional water permeability; this can only be partly accounted for by the existence of an unstirred layer and is supporting evidence for the concept of aqueous pores, through which the water moves down osmotic gradients mainly as bulk flow and not as a purely diffusional flow. The comparatively larger increase in osmotic water permeability than in diffusional water permeability in antidiuresis suggests that ADH can change the pore area‐pore diameter relation so that the bulk flow increase is larger than the diffusional water flow increase.