Tracer kinetics and actions of oral and intraperitoneal 1,25-dihydroxyvitamin D3 administration in rats

Tracer kinetic parameters of [3H]-1,25(OH)2D3 were calculated from data obtained following its acute oral (p.o.) or intraperitoneal (i.p.) administration. In normal rats studied after the tracer had distributed into the body, the slope and intercept of the log-serum [3H]-1,25(OH)2D3 versus time relationship were not significantly influenced by the route of administration. Pretreatment with 1,25(OH)2D3 (0.2 micrograms/100 g/day) by the same route as the tracer resulted in the following changes: in p.o. rats the serum [3H]-1,25(OH)2D3 intercept was much lower but the slope was not changed; in i.p. rats the intercept was not changed but the slope was increased. Both p.o. and i.p. treatment with 1,25(OH)2D3 lowered the weight gain and diet consumption, and increased serum calcium, kidney tissue calcium and urinary excretion of orally administered 45Ca. All the measures of bioactivity were greater in the i.p. dosed rats than in the p.o. dosed rats. We conclude that the p.o. 1,25(OH)2D3 was less potent because of diminished bioavailability due to self induction of its presystemic metabolism and inactivation.