Studies of the rapid effects of parathyroid hormone and prostaglandins on 45Ca uptake into chick and rat bone in vivo

The rapid effects of parathyroid hormones and a variety of prostaglandins on net uptake of 45Ca into the skeleton have been investigated in chicks and, in a limited parallel study, in immature rats. Intravenous injection of bovine (b) parathyroid hormone(1-34) (bPTH(1-34)) or 16,16-dimethyl prostaglandin E2 (16,16-dimethyl PGE2) in a 45Ca-labelled vehicle, combined with subsequent microwave fixation of tissue isotope levels, resulted in rapid (3-15 min) net inhibition of 45Ca uptake into endochondral bone (femur) in chicks (12 days old) and rats (4 weeks old). Use of 125I-labelled albumin and [14C]mannitol indicated that these responses were not a reflection of gross changes in tissue vascular or extracellular space. In rats, bPTH(1-84) also caused significant net inhibition of 45Ca uptake into femur at 10 min. Both bPTH(1-34) and 16,16-dimethyl PGE2 produced generally smaller decreases in 45Ca uptake into chick dermal bone (calvarium) at 3-15 min. In rat calvarium, however, these agents stimulated net uptake of 45Ca at these times. When microwave fixation was omitted, inhibitory responses were reduced or disappeared, while the stimulatory response in rat calvarium was enhanced. Responses to natural prostaglandins (PGE1, PGE2, PGF2 alpha and PGI2) in chicks at 3 min were similar but less marked than those to 16,16-dimethyl PGE2; 45Ca uptake into femur and, to a lesser extent in calvarium, being inhibited. In rats, PGE1, PGE2 and PGF2 alpha showed a tendency to decrease 45Ca uptake into femur while PGE1 and PGE2 both increased 45Ca uptake into calvarium.