Single proximal tubules of Necturus kidney IV. Dependence of H₂O movement on osmotic gradients

¹ Biophysical Laboratory, Harvard Medical School, Boston, Massachusetts

The relationship between net water flux and osmotic gradients across the Necturus kidney proximal tubule was studied with stopped flow microperfusion technique. To minimize water movement induced by Na transport, perfusion solutions contained 62.5 mm/l. NaCl; at this concentration no net water movement is observed with isosmolar solutions. Mannitol was added to prepare perfusion solutions with gradients ±70 mOs/l. relative to plasma. With these gradients, water movement was symmetrical in both directions across the membrane: tubular volume decreased 46% with hypoosmolar perfusion fluid, and increased 51% with hyperosmolar fluid. The membrane water permeability coefficient, P_f, is 0.15 x 10^–8 ml/(cm² sec. cm H₂O). This coefficient permits calculation of net water movement ascribable to plasma protein osmotic pressure. The calculated value is approximately 1% of the fluid normally absorbed by the tubule. This conclusion was confirmed in separate experiments in which albumin was added to perfusion fluid to produce a protein concentration 67% higher than in plasma. Under these conditions, water was still absorbed from the tubule, indicating that proteins do not play a major role in water absorption.