Volume absorption in the pars recta. I. "Simple" active Na+ transport

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Abstract
The role of simple active Na+ transport in driving net volume absorption (Jv, nl min-1 mm-1) in isolated rabbit superficial proximal straight tubules was studied. Tubule segments were perfused with a buffered NaCl solution containing no organic solutes; the bath contained 6% albumin, 5 mM L-alanine and 8.3 mM D-glucose; and the system contained no CO2. At 38.degree. C, Jv was 0.18 .+-. 0.02 (SE) and the observed transepithelial voltage (Veo, mV, lumen to bath) was -0.9 .+-. 0.1. With 0.1 mM ouabain, Jv and the observed Veo were 0, but 0.16 mM ethoxzolamide had no effect on Jv and Veo. After addition of glucose + alanine to the perfusate, Jv was 0.42 .+-. 0.05 and the observed Veo was -1.5 .+-. 0.1. After addition of 10 mM acetate alone to the perfusate, Jv was 0.43 .+-. 0.06. Observed Veo values were not actual transepithelial voltages (Vea), since the liquid junction potential between the bath and bath electrode was equal in magnitude but opposite in sign to the Donnan equilibrium voltage of 2.36 mV, lumen positive, between perfusate and the bath. In this nephron segment Na+ absorption may occur independently of the presence of organic solutes or HCO3- in the lumen, and the associated voltage displacement from Donnan equilibrium was a major driving force for passive Cl- absorption. In addition, the resulting NaCl absorption may serve as a driving force for approximately 50% of the Jv normally observed in these tubules, and organic co-factors such as glucose plus alanine, or acetate, enhanced the rate of active Na+ transport.