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

Abstract

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, PGF_2α 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 PGF_2α showed a tendency to decrease ⁴⁵Ca uptake into femur while PGE₁ and PGE₂ both increased ⁴⁵Ca uptake into calvarium. J. Endocr. (1987) 115, 369–377