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Microelectrodes with 2-micronm-tip diameters arranged to record differentially between this ascending limbs (ALH) and ascending vasa recta (AVR), gave values of 1.95 +/- 0.17 mV, ALH positive, in hydropenia with mineral oil bathing the kidney. Average values remained in the range of 1-2 mV when the ALH and AVR values were obtained sequentially, when the kidney was bathed in Ringer solution, when 5-8 Momega Ling-Gerard microelectrodes were used in the ALH, or when the hamsters were in saline diuresis. These results contradict reports of earlier studies with high impedance Ling-Gerard electrodes that the PD was -9 mV, ALH negative. Perfusion of ALH in saline diuresis with solutions of various compositions provided estimates of ionic transport numbers: tNa+ = 0.33 +/- 0.01, tK+ = 0.00 +/- 0.02, tC1- = 0.67 +/- 0.02. When the perfusion solution was designed to have the same Na+, K+, and C1- concentrations as AVR plasma, the PD was 1.36 +/- .20 mV; when ouabain or furosemide were included (10(-5) M), the PD declined 1.35 +/- 0.21 mV and 1.41 +/- 0.28 mV, respectively. The results suggest that active C1- transport is mainly responsible for the PD, but that diffusion potentials can contribute.

Exogenous choline was administered into the wing vein of chickens until steady-state plasma choline levels were achieved. Both plasma and urine were analyzed for free choline by a choline kinase, radiochemical microassay that did not require prior extraction of choline from the biological fluids. Choline was infused at rates from 0.5 to 20.0 mumol/kg-min. Total and urinary clearance were assessed at the steady state reached in each experiment. At the endogenous level of plasma choline of 0.019 mM, total clearance of choline from the plasma was about 50 ml/kg-min and urinary clearance was 0.06 ml/kg-min. There was no significant increase in urinary clearance of choline produced by infusion loads from 0.5 to 1.0 mumol/kg-min. However, as the infusion of choline was increased further, extraurinary clearance decreased while the contribution of the kidneys to total clearance of choline from the plasma increased. At the choline infusion rate of 12.5 mumol/kg-min, plasma choline was 0.5 mM and urinary choline clearance had reached a maximum value of 18.5 ml/kg-min for two kidneys, removing more than 70% of the infused choline.