Effects of acute and long-term administration of prolactin on bone ⁴⁵Ca uptake, calcium deposit, and calcium resorption in weaned, young, and mature rats

Abstract

The acute effect of prolactin on bone 45Ca uptake and the long-term effect on calcium turnover in femur, tibia, sternum, and lumbar vertebrae 5 and 6 were evaluated in weaned, young, and mature female Wistar rats. A dose-dependent increase in 45Ca uptake at 60 min after intraperitoneal administration of 0.01 and 0.02 mg prolactin/100 g body weight was seen in femur of mature rats and in femur, tibia, and vertebrae of weaned rats. In contrast, bones of young rats were less responsive and responded only to the higher dose of prolactin. Daily subcutaneous injection of 0.25 mg prolactin/100 g body weight for 2 weeks in weaned rats enhanced calcium deposit in femur while increasing calcium resorption in all four bones. Young rats responded to prolactin only by enhancing calcium deposit in femur, tibia, and sternum. Mature rats, on the other hand, responded by increasing calcium resorption from all four bones while enhancing calcium deposit in femur, tibia, and sternum. It could be concluded that effects of prolactin varied with bone type and age of animals. Prolactin, in general, enhanced calcium turnover in both compact and trabecular bones of mature rats. Young rats responded to prolactin by increasing the rate of calcium deposit, whereas weaned rats increased calcium release. Nevertheless, all three age groups exhibited a net gain in calcium after 2 weeks of prolactin administration.

Rapid inactivation and apoptosis of osteoclasts in the maternal skeleton during the bone remodeling reversal at the end of lactation

“…Short-term increases in PRL are believed to promote bone turnover (resorption and formation) while prolonged increases, e.g., hyperprolactinemia, are associated with uncoupling of remodeling with an excess bone resorption (Abraham et al, 2003). PRL also promotes the passive intestinal absorption of calcium (Krishnamra and Seemoung, 1996), perhaps contributing to the calcium economy during lactation (Quan-Sheng and . Thus, the rapid inactivation of osteoclasts observed after weaning is also consistent with the expected and observed decreases in PRL.…”

Reproductive Hormones and Bone

“…In vivo 45 Ca uptake studies in the rat demonstrated that PRL enhanced calcium turnover in compact and trabecular bones of mature rats. Young rats responded to PRL by increasing the rate of calcium deposition leading to net bone gain, whereas weaned rats increased calcium release, leading to net bone loss in adult animals [49]. Fig.…”

“…These PRL effects appeared to be mediated by activation of the phosphatidylinositol 3-kinase pathway. Thus, unlike osteoblasts derived from adult humans, PRLexposed hFOB cells exhibited indices suggestive of increased osteoblastogenesis and decreased osteoclastogenesis [50•], which could explain the in vivo stimulatory effects of PRL on bone in young, but not adult, rats [49]. The age-dependent effects of PRL suggest that the stage of maturation of the HPG axis may influence the directional effects of PRL action on the skeleton.…”

Pituitary-bone connection in skeletal regulation

“…Osteoclasts themselves do not possess PRLRs [93]. In contrast, in infant rats, PRL causes net bone gain [96], increased osteocalcin expression. Likewise, in human fetal osteoblast cells, PRL decreases the RANKL/ OPG ratio [95].…”