The kidney in regulation of plasma choline in the chicken.

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.