THE MECHANISM OF THE EXCRETION OF VITAMIN C BY THE HUMAN KIDNEY AT LOW AND NORMAL PLASMA LEVELS OF ASCORBIC ACID 1

Abstract

In a previous study (1) the excretion and renal tubular reabsorption of vitamin C at normal and artificially elevated plasma concentrations of the vitamin were reported. Refinements in the chemical determination of vitamin C in plasma and urine (2, 3) have made it possible to further supplement our previous data with observations at the lower ranges of the plasma concentration. On the basis of the previous evidence it was concluded that the excretion of vitamin C at any plasma concentration was determined by the plasma concentration, by the rate of glomerular filtration and by the rate of tubular reabsorption. It was further observed that the reabsorptive mechanism was limited by a maximal rate and that, when the vitamin was presented to the tubules by the glomerular filtrate at a rate exceeding this maximum, the excess was excreted in the urine. The observations we are reporting in this study are concerned with the nature of the reabsorptive process at the lower plasma levels of vitamin C. In the previous studies the plasma concentrations of the vitamin varied from 1.5 to 25 mgm. per cent. In the present study the plasma concentrations varied from 0.03 to 2.05 mgm. per cent. PROCEDURE Nineteen normal individuals were used in this study. Clearances were determined at constant plasma levels of the vitamin, and only one level was studied on any day. When necessary, vitamin C was given orally the night prior to and the morning of the experiment in order to obtain the desired plasma concentration. Fluids were forced on the day prior to the test, and on the morning of the experiment the subject drank 1000 cc. of normal saline. The first clearance period started 1ll to 2 hours after the last dose of water. The bladder was cath-eterized and washed out with saline. This specimen of urine was discarded. The catheter was left in situ and 1 This research was aided by a grant from the Josiah Macy Jr. Foundation. clamped. At 30-or 60-minute intervals urine specimens were collected and the bladder washed out with saline and air. In a previous study it was found (16) that no destruction of ascorbic acid occurred in bladder urine for periods as long as 5 hours. It seemed therefore perfectly safe to allow the urine to collect in the bladder for 30-to 60-minute intervals. Bloods were taken at frequent intervals during the clearance periods. The urines as collected were immediately acidified with glacial acetic acid and kept in the refrigerator until analyzed. No longer than 2 hours elapsed before analysis. Vitamin C in urine was determined in the photoelectric colorimeter by the method described by Evelyn et al (2). With the titrimetric method it was difficult to determine the vitamin C clearance at low plasma concentrations due to the interference of relatively large amounts of non-vitamin C-reducing substances in the urine. Since vitamin C is the only known substance which will react with dichlorophenolindophenol immediately, the Evelyn technique provides a satisfactory method for distinguishing between vitamin C and any non-vitamin C-reducing substances. Other substances reacting with the dye do so at a fairly slow rate. Where very small amounts of non-vitamin C-reducing substances are present, or where large concentrations of ascorbic acid necessitate dilutions, then the error in the determination of urinary ascorbic acid is of the magnitude of +2 per cent. Where the concentration of reducing substances other than vitamin C is fairly large, or where there are very small amounts of ascorbic acid present, then the percentage error is increased. In pure solutions of ascorbic acid, when the concentration ranges up to 2.00 mgm. per cent, duplicate analyses agree within 0.03 mgm. per cent, so that, if the absolute concentration is 2.00 mgm. per cent, there is a 1.5 per cent error; if 1.00 mgm. per cent, a 3 per cent error; and if 0.50 mgm. per cent, a 6 per cent error. When non-vitamin C-reducing substances are present, the method of extrapolation required increases this error. Plasma vitamin C was determined by the method of Mindlin and Butler (3). We have found it possible to consistently reproduce plasma figures within 0.04 mgm. per cent. The absolute magnitude of the error is constant over the entire range of plasma values. Simultaneous inulin clearances were not done in this study because the magnitude of the vitamin C clearances at these low plasma levels was such that the variations in the normal inulin clearance could not appreciably affect the vitamin C/inulin clearance ratio. In calculating the 685