Water permeability of brush border membrane vesicles from kidney proximal tubule.

Brush border membrane vesicles (BBMV) maintain an initial hydrostatic pressure difference between the intra- and extravesicular medium, which causes membrane strain and surface area expansion (Soveral, Macey & Moura, 1997). This has not been taken into account in prior osmotic water permeability Pf evaluations. In this paper, we find further evidence for the pressure in the variation of stopped-flow light scattering traces with different vesicle preparations. Response to osmotic shock is used to estimate water permeability in BBMV prepared with buffers of different osmolarities (18 and 85 mosM). Data analysis includes the dissipation of both osmotic and hydrostatic pressure gradients. Pf values were of the order of 4 x 10(-3) cm sec-1 independent of the osmolarity of the preparation buffer. Arrhenius plots of Pf vs. 1/T were linear, showing a single activation energy of 4.6 kcal mol-1. The initial osmotic response which is significantly retarded is correlated with the period of elevated hydrostatic pressure. We interpret this as an inhibition of Pf caused by membrane strain and suggest how this inhibition may play a role in cell volume regulation in the proximal tubule.