The Renal Excretion of Folic Acid.

The urinary excretion of tritiated folic acid after a single intravenous injection in man has been measured by Johns, Sperti, and Burgen (1). Plasma concentrations of the labeled compound fell sharply initially, and then more slowly. Urinary clearance in terms of the later, slowly declining plasma levels was shown, at plasma concentrations above 10 jug per L, to be independent of plasma level and to average 51 ml per minute. The finding of an upper fixed limit lower than the glomerular filtration rate (130 to 140 ml per minute) was correlated with binding of folic acid to plasma proteins. Ultrafiltration and equilibrium dialysis demonstrated a practically uniform binding over the concentration range of 5 to 3,000 ug per L, with an average of 64%o. The renal excretion of folic acid at these high plasma levels could be accounted for simply by glomerular filtration and urinary excretion of unbound folic acid. At plasma levels lower than 10 ug per L, the urinary clearance appeared to decline, suggesting some renal tubular reabsorption of folic acid, but no threshold plasma concentration was defined, suggesting a relatively ineffective renal mechanism for the conservation of folic acid. At these lower plasma concentrations, the accuracy of the data was not great because of the low levels of radioactivity involved. Tritium-labeled folic acid of much higher specific activity has now become available. This material has been used in the present study to investigate more completely the renal excretion of folic acid by the single renal intra-arterial injection technique (2). /