CHARACTERIZATION OF THE TRANSLOCATION PROCESS OF VITAMIN-D-3 FROM THE SKIN INTO THE CIRCULATION

The cutaneous synthesis of vitamin D-3 and the subsequent translocation of vitamin D-3 into the circulation are two key steps in the vitamin D endocrine system. To study the kinetic aspects of cutaneous synthesis and translocation of vitamin D-3, both in vitro and in, vivo chicken models have been developed. To assess the capacity of chicken skin to generate vitamin D-3, the concentrations of 7-dehydrocholesterol (7-DHC) in different skin areas were determined It was found that the highest concentration of 7-DHC was in the leg skin (3524 +/- 937 ng cm(-2)), which was about 30 times greater than that in the back (120 +/- 62 ng cm(-1)). Whole body exposure of chickens to UV-B radiation (0.5 J cm(-2)) resulted in the production of previtamin D-3 (preD(3)) in the skin of the legs and feet (43 +/- 7 and 54 +/- 17 ng cm(-2), respectively), whereas (k(1)) and reverse (k(2)) rate constants of the preD(3) reversible arrow vitamin D-3 reaction in the leg skin were greatly increased compared to those in n-hexane (k(1), 0.367 vs. 0.0369 h(-1); k(3), 0.042 US, 0.0059 h(-1)). In vivo, the determined rate constants k(1), k(2), and k(3) for the consecutive reactions preD(3) reversible arrow vitamin D-3-->vitamin D-3 were 0.257, 0.034, and 0.114 h(-1) respectively. To evaluate the circulating concentration of vitamin D-3 in response to W-B radiation, chicken legs were irradiated. The time course revealed a 4-fold increase in the circulating concentration of vitamin D-3, with a peak about 30 h postradiation. No appreciable amount of preD(3) could be detected in the circulation in the early hours after W-B radiation, suggesting the existence of a process responsible for the specific translocation of vitamin D-3 from the skin into the circulation.