PRODUCTION OF 1,25-DIHYDROXYVITAMIN-D-3 BY PERFUSED PIG SKIN

Keratinocytes produce large amounts of 1,25 dihydroxyvitamin D-3 (1,25 (OH)(2)D-3) in vitro. 1,25(OH)(2)D-3 is detectable in anephric humans and pigs and can be increased to near-normal levels by vitamin D or 25 0HD, indicating an extrarenal source. To determine whether the skin is one of these extrarenal sources, we perfused isolated flaps of porcin e skin for 8 h with 250HD(3) in serum-free medium at 1 ml/min, collect ing the venous effluent as 15-min samples. The samples were extracted and the vitamin D metabolites purified by high-performance liquid chro matography and assayed by competitive protein-binding techniques. Prod uction of 1,25(OH)(2)D-3 continued for the duration of the perfusion, tending to increase in the last 2 hours. The amount of 1,25(OH)(2)D-3 produced varied both with time in the same pig skin and between pig sk ins; maximum production of 1,25(OH)(2)D-3 in these experiments was 8 p g/min. 24,25(OH)(2)D-3 production was higher than 1,25(OH)(2)D-3 produ ction, reaching a maximum rate of 180 pg/min. Considering that the pro duction rate of 1,25(OH)(2)D-3 in humans is 1.25 ng/min and that a 48- cm(2) skin flap represents 1/350 the surface area of a human, and assu ming that human and pig skin make 1,25(OH)(2)D-3 at comparable rates, one can determine that the skin has the potential to maintain near-nor mal levels of 1,25(OH)(2)D-3 in the absence of kidneys when provided w ith adequate substrate.