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In Vivo Mobilization of Bone Calcium in Thyroparathyroidectomized Rats by 1,25-Dihydroxycholecalciferol (Calcitriol) and Related Compounds

Department of Cell Biology,Roche Research Center, Nutley, NJ 07110

Calcitriol, 0.5 µg/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-hydroxycholecalciferol, 1,24R,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 µg/rat and measurement of serum radioactivity up to 72 hours after dosing.

KEY WORDS: • thyroparathyroidectomy • calcium mobilization • calcitriol • vitamin D metabolites.

Manuscript received 7 April 1980.