Analysis of standing droplets in rat proximal tubules.

Open Access
Publisher Website
Google Scholar
Abstract
Volume, osmolality, and concentrations for Na, Cl and raffinose were measured as a function of time in standing droplets within rat intermediate and late proximal tubules. Standing droplet reabsorption proceeds without the development of a measurable osmotic difference across the epithelium. After 140 s of tubular exposure, droplet-to-plasma concentration differences are observed for raffinose, Na and Cl with the observed Na concentration difference, usually referred to as limiting gradient, being .apprx. 9 mM. Smaller or even no limiting differences would probably be attained with longer exposure times. Previous values measured for the limiting Na concentration in the rat proximal tubule were determined before the attainment of constant concentrations. Assuming that the Na concentration measured is the limiting value, active NaCl transport accounted for a very small fraction, < 6%, of the volume reabsorption; using an alternative approach of fitting a theoretical model to the experimental data, active NaCl transport is again estimated to account for only 6% of the total reabsorbate. The previous interpretation that a limiting Na concentration gradient constitutes the most direct evidence for active Na transport may be in error; the gradient measured can be modeled without incorporating active NaCl transport.