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

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 micrograms/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 micrograms/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 (less than 45 min) compared with the responses to the PG (greater than 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 micrograms and 1.1 micrograms/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.(ABSTRACT TRUNCATED AT 250 WORDS)