Effects of acute and long-term administration of prolactin on bone <SUP>45</SUP>Ca 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.

High-calcium diet modulates effects of long-term prolactin exposure on the cortical bone calcium content in ovariectomized rats

“…Two models have been widely used to study the effects of high physiological prolactin in vivo, i.e., daily prolactin injection and AP transplantation (28,45). Our laboratory has reported that both techniques produced high physiological prolactin levels of 70 -100 ng/ml (39), which were comparable to the levels during pregnancy in rats (10).…”

“…Their results indicated an age-dependent integrative response of the small intestine to prolactin and high-calcium diet. As for bones, the stimulatory effects of prolactin on bone calcium deposition were greater in young rats than in adult rats (28), but the interaction of high-calcium diet and prolactin had not been reported. It was possible that high-calcium diet might differentially affect the actions of prolactin on the bone calcium content in young and adult ovariectomized rats.…”

“…1). The protocol also allowed a shorter experimental period (28,40), thus minimizing radioactive use and contamination.…”

“…Besides Ovx, a number of factors, such as high-calcium diet and age of animals, could modulate actions of prolactin (4,28,45). High-calcium diet has been a standard regimen to alleviate negative calcium balance after Ovx (16).…”

High Physiological Prolactin Induced by Pituitary Transplantation Decreases BMD and BMC in the Femoral Metaphysis, but Not in the Diaphysis of Adult Female Rats

“…However, the direct effect of PRL on osteoblasts to enhance bone turnover was complicated by its indirect actions through hyperprolactinemiainduced hypogonadism, which in turn induced chronic estrogen deficiency [8,9]. Moreover, the PRL-enhanced calcium absorption may indirectly contribute to the increase in bone calcium deposition, as previously suggested by the 45 Ca kinetic and histomorphometric techniques [10][11][12]. Nevertheless, the effects of PRL on bone were mostly confined to the trabecular sites [13], e.g., sternum, vertebrae, and trochanter, because a 2-week exposure to high physiological PRL altered calcium deposition or the total calcium content in these bones, but not in the tibia and femur of adult rats [6,14].…”