Water Absorption in the Proximal Tubule: Effect of Bicarbonate, Chloride Gradient, and Organic Solutes

Simultaneous in vivo capillary and luminal microperfusion studies were performed in the superficial proximal convoluted tubule of the rat to determine the effect of intraluminal bicarbonate, the imposition of a transepithelial chloride gradient, and the addition of organic solutes to the luminal perfusion solution on the rates of water absorption (J_v). The capillary perfusion solution contained NaCl, NaHCO₃, and KCl. Perfusion of both capillary and lumen with the same solution resulted in a J_v of 3.01 ± 0.24 nl/min/mm. Imposition of a transepithelial chloride gradient (equimolar substitution of NaCl for NaHCO₃ in the luminal solution) resulted in a J_v of 3.18 #pL 0.21 nl/min/mm (P = NS). Addition of cyanide to both solutions in the presence of a chloride gradient resulted in a significantly lower J_v of 2.21 ± 0.19 nl/min/mm. Luminal substitution of Na cyclamate for NaHCO₃ resulted in a solution containing no bicarbonate and no chloride gradient. J_v averaged 0.34 ± 0.08 nl/min/mm. Addition of cyanide to the solution totally inhibited J_v. The addition of D-glucose, L-alanine, or both, to luminal solutions containing bicarbonate or in the presence of a chloride gradient did not significantly affect J_v. Addition of both organic solutes to the NaCl-Na cyclamate luminal solution resulted in a significantly higher J_v of 0.77 ± 0.14 nl/min/mm. These studies indicate that J_v in the rat superficial proximal tubule is influenced by active sodium transport, by the presence of bicarbonate in the lumen, and/or by the imposition of a transepithelial chloride gradient. The organic solutes D-glucose and L-alanine also influence water absorption, but this effect could only be demonstrated under some experimental conditions.