Bidirectional renal tubular transport of free choline: a micropuncture study

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Abstract
Choline concentration was measured in plasma, tubular fluid and urine from rats anesthetized with Inactin and infused with choline to steady-state levels. Choline was measured by an enzymatic, radiochemical microassay. At endogenous plasma choline concentration of 24 .+-. 1 .mu.M (SE), extensive choline reabsorption occurred in proximal convoluted tubules (PCT). The fraction of filtered choline remaining in PCT ((TF/P)choline/(TF/P)In .times. 100) was 17 .+-. 2%. More reabsorption took place distally, since fractional excretion of choline in urine was only 1%. When the plasma choline concentration was increased to 116 .+-. 4 .mu.M, mean end proximal and early distal (TF/P)choline/(TF/P)In .times. 100 values were, respectively, 68 .+-. 5 and 8 .+-. 1%. At a still higher plasma choline concentration of 581 .+-. 44 .mu.M, net secretion occurred, since (TF/P)choline/(TF/P)In .times. 100 was 109 .+-. 4 and 122 .+-. 6% in PCT and distal tubules, respectively. Fractional excretion in urine was 152 .+-. 16%. At low plasma choline concentration levels choline was reabsorbed from PCT as well as from the pars recta and/or loop of Henle. At high plasma choline concentration there was net secretion into tubule fluid by PCT, as well as by the pars recta and/or loop of Henle. These micropuncture data support earlier evidence from in vivo experiments showing bidirectional tubular transport of choline.

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