In vivo mobilization of bone calcium in thyroparathyroidectomized rats by 1 alpha, 25-dihydroxycholecalciferol...

Calcitriol, 0.5 microgram/rat, given subcutaneously (sc), did not mobilize 45Ca from bone in intact, pre-labeled rats, but caused significant elevation of serum radioactivity in thyroparathyroidectomized (TPTX) rats. The response to calcitriol in TPTX rats was dose-dependent with significant activity at sc doses of 40 ng/rat and above when measured at 18 hours after dosing. Parathyroid hormone, 1 alpha-hydroxycholecalciferol, 1 alpha, 24R,25-trihydroxycholecalciferol and 5,6-trans-25-hydroxycholecalciferol were also effective in TPTX rats. The latter two compounds required microgram doses to mobilize 45Ca from bone. Calcitriol, 25-hydroxycholecalciferol and 24R,25-dihydroxycholecalciferol were inactive in TPTX rats even with doses of 40 microgram/rat and measurement of serum radioactivity up to 72 hours after dosing.