Production of 1,25 dihydroxyvitamin D3 by perfused pig skin

Keratinocytes produce large amounts of 1,25 dihydroxyvitamin D3 (1,25(OH)2D3) in vitro. 1,25(OH)2D3 is detectable in anephric humans and pigs and can be increased to near-normal levels by vitamin D or 250HD, indicating an extrarenal source. To determine whether the skin is one of these extrarenal sources, we perfused isolated flaps of porcine skin for 8 h with 250HD3 in serum-free medium at 1 ml/min, collecting the venous effluent as 15-min samples. The samples were extracted and the vitamin D metabolites purified by high-performance liquid chromatography and assayed by competitive protein-binding techniques. Production of 1,25(OH)2D3 continued for the duration of the perfusion, tending to increase in the last 2 hours. The amount of 1,25(OH)2D3 produced varied both with time in the same pig skin and between pig skins; maximum production of 1,25(OH)2D3 in these experiments was 8 pg/min. 24,25(OH)2D3 production was higher than 1,25(OH)2D3 production, reaching a maximum rate of 180 pg/min. Considering that the production rate of 1,25(OH)2D3 in humans is 1.25 ng/min and that a 48-cm2 skin flap represents 1/350 the surface area of a human, and assuming that human and pig skin make 1,25(OH)2D3 at comparable rates, one can determine that the skin has the potential to maintain near-normal levels of 1,25(OH)2D3 in the absence of kidneys when provided with adequate substrate.