AUTORADIOGRAPHIC STUDIES ON THE DISTRIBUTION OF C14‐LABELLED ASCORBIC ACID AND DEHYDROASCORBIC ACID

SUMMARY Ascorbic acid-1-C14 and dehydroascorbic acid-1-C14 have been administered intravenously to adult male mice, pregnant female mice and 16 days old rats, and intraperitoneally to 10 days old rats. The distribution of the injected substances at various intervals (2 minutes—6 days) after injection has been studied by whole body autoradiography, microautoradiography and impulse counting. The chemical identity of the redioactivity in the tissues at various intervals (5 minutes-4 days) after injection has been investigated by thin-layer chromatography of extracts of some selected tissues, which showed a high concentration of radioactivity. The chromatographic investigation indicated that the radioactivity in the adrenals, liver, spleen, kidney, salivary glands and brain almost exclusively represented ascorbic acid, regardless of whether ascorbic acid-1-G14 or dehydroascorbic acid-1-C14 had been injected. In spite of the rapid reduction of injected C14-dehydroascorbic acid the distribution of the two vitamin forms were very different after intravenous injection. The distribution patterns gradually became similar but as long as 3–4 days after injection some differences were noticeable. The distribution patterns have been described and discussed in detail. The results which functionally seem to be most interesting have been summarized below. After an intravenous injection of C14-ascorbic acid the accumulation in the central nervous system proceeded very slowly. There seemed to be a complete blood-brain barrier and the radioactivity penetrated from the choroid plexa and from the periphery possibly together with simultaneously secreted cerebrospinal fluid. At 6 days after injection the central nervous system showed the highest concentration in the body. After an intravenous injection of C14-dehydroascorbic acid the concentration in the central nervous system rapidly increased and as early as 2 minutes after injection radioactivity was found in the whole brain and spinal cord and the concentration markedly exceeded that of the blood. The striking difference in rate of penetration was also demonstrated by the impulse counting investigation. After as long as 4 days ‘he concentrztion in the brain was higher after injection of C“-dehydroascorbic acid than after C14-ascorbic acid. Injection of dehydroascorbic acid has been found to cause an intense hyper-activity of short duration (Patterson and Mastin, 1951) and also a rise in the higher nervous activity lasting for serveral days (Lyhs and Tegeler, 1963). This may be put in relation to the rapid and persistant accumulation of C14-dehydroascorbic acid, and possibly also to the demonstrated function of ascorbic acid and dehydroascorbic acid in the synthesis of norepinephrine and serotonin respectively (Cooper and Melcer, 1961; Kaufman. 1966). Both the reduced and oxidized forms were taken up in the sympathetic ganglia, adrenal medulla, pancreatic islets and in some scattered cells in the thyroid, possibly the parafollicular cells, which are all probable sites of synthesis of norepinephrine and serotonin. Dehydroascorbic acid was rapidlv accumulated in the majority of cells of the pancreatic islets. The uptake of dehydroascorbic acid as well as its demonstrated diabetogenic properties may possibly be put in relation to the localization of serotonin in the pancreatic B-cells. Young animals which are resistant to the diabetogenic action of alloxan (Ferner, 1952) did not specifically accumulate dehydroascorbic acid nor ascorbic acid in the pancreatic islets. Dehydroascorbic acid was not accumulated in the adrenal cortex, whereas ascorbic acid was rapidly taken up there. Stimulation of the adrenal cortex with ACTH causes a rapid decrease in the concentration of ascorbic acid (Sayers et al., 1944). The present results may indicate that ascorbic acid is oxidized at the reactions induced by ACTH in the adrenal cortex and then leaves the cells as dehydroascorbic acid. The transfer across the placenta proceeded with the same slow rate regardless of whether C14-ascorbic acid or C14-dehydroascorbic acid had been injected. The highest concentration in the fetuses was found in the central nervous system and in the adrenals. Like the uptake of C14-ascorbic acid in the central nervous system of the adult animals the uptake in the fetal central nervous system proceeded slowly, possibly indicating that injected dehydroascorbic acid had been reduced before or during the placental transfer. In the skeletal tissues of the young animals an uptake was noted in those parts where mineralization was going on, which suggests a function for vitamin C in the mineralization process. Neither C14-ascorbic acid nor C“-dehydroascorbic acid seemed to be incorporated into the formed hard tissues. In the developing teeth the highest concentration was noted in the enamel in those parts where matrix formation had just ceased as indicated by the reduced height of the ameloblasts. In those parts there was a massive accumulation of C14-ascorbic acid through the whole thickness of the enamel. After long survival periods the radioactivity was redistributed towards the cervical parts of the teeth. C14-dehydroascorbic acid was not accumulated in the enamel. The massive uptake of C14-ascorbic acid in the enamel at this stage of development may be put in relation to changes in the composition of the organic component during the mineralization of the enamel. In the metabolism of collagen ascorbic acid is considered to be a co-factor at the hvdroxylation of proline and lysine (Robertson, 1964). Ascorbic acid may have this function also in the developing enamel.