Kinetics of intravenously administered 25-hydroxyvitamin D3 in sheep and the effect of exposure to ultraviolet radiation.

This report describes studies of 25-hydroxyvitamin D3 (25OHD3) kinetics in 10 yearling crossbred wethers (40 to 52 kg body weight) administered [3H] 25-hydroxyvitamin D3 [( 3H] 250HD3). The wethers were divided into two groups of five before dosing with the labeled compound: Group 1 wethers were exposed 2 h daily for 8 to 10 wk to ultraviolet radiation from fluorescent sunlamps. Plasma 250HD3 increased from 13 to 31 ng/ml by d 25 and then remained at a steady-state concentration until d 70. In the non-ultraviolet-exposed wethers (Group 2), plasma 250HD3 averaged 13.2 ng/ml and did not change over the course of the experiment. Six weeks after the start of the experiment three wethers from each group were injected intravenously with 50 microCi of [3H]250HD3, and the other two wethers from each group were similarly dosed 2 wk later. Blood was drawn at suitable time intervals for up to 240 h post-dosing. The kinetic data were satisfied by curve-fitting the data to a two-compartment, pharmacokinetic mathematical model. Individual and mean values for these kinetic analyses are presented with their statistical analyses. Physiological half-life of the tritiated 2500 HD3 in wethers exposed to ultraviolet irradiation was 388 +/- 26.4 h (means +/- SE); in the control it was 393 +/- 29.6 h (P greater than .05). In the wethers exposed to ultraviolet the elimination and clearance rates of 250HD3 were significantly greater than in the control wethers. A two-compartment model provided an acceptable description of 250HD3 disposition kinetics in wethers. Turnover rate constants did not seem to change, but changes were due to increased pool size in the irradiated wethers.