EVIDENCE FOR A NOVEL INHIBITION OF CALCIUM UPTAKE INTO CHICK BONE IN RESPONSE TO BOVINE PARATHYROID HORMONE (1-34) OR 16, 16-DIMETHYL PROSTAGLANDIN E2 IN VIVO

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Abstract
Mechanisms of initial hypercalcaemic responses to parathyroid hormone (PTH) and 16, 16-dimethyl prostaglandin (PG) E2 have been investigated in 10-to 12-day-old chicks in vivo using a combination of acute 45Ca injection and microwave fixation to stabilize tissue isotope levels. Single i.v. injection of 16, 16-dimethyl PGE2 (20 μg/100 g body wt) caused an approximately 100% increase in soft tissue 45Ca levels compared with vehicle control injected chicks at 30 min. 45Ca levels were lowered in calvarium by 26% and in femur by 60% with this treatment. Bovine PTH (1-34) (3.3 μg/100 g body wt) had no effect on soft tissue 45Ca levels, but in calvarium it had a similar effect to the PG. In femur this dose of PTH lowered 45Ca by 19%. When expressed on an absolute basis (c.p.m./ 100 mg tissue wt), responses to the PG in soft tissue were only 3 and 10% respectively of those in femur and calvarium. The duration of inhibitory responses in bone were examined and those to PTH found to be transient (< 45 min) compared with the responses to the PG (> 135 min). Dose-response curves for PTH- and PG-induced inhibition of 45Ca uptake into femur at 15 min were essentially parallel and indicated that the lowest doses of PTH and PG used (0.74 μg and 1.1 μg/100 body wt respectively) produced significant responses. In a separate experiment it was found that inhibition of 45Ca uptake into femur was evident as early as 3 min following PTH or PG injection. The experiments described in this paper collectively indicate a higherto unrecognized action of PTH and PGE in the regulation of Ca metabolism in chicks which is to inhibit entry of Ca into bone.

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