Acute effect of prolactin on bone 45Ca accumulation in rats

In an attempt to evaluate the acute effect of PRL on bone turnover, experiments were performed in sexually mature female Wistar rats which were given an intraperitoneal (i.p.) administration of 1.25 mM CaCl2 solution containing 6 microCi 45Ca. Dose response study showed a two fold higher rate of 45Ca uptake in 30 min by control trabecular bones (sternum and vertebrae 5-6) compared with control compact bones (femur and tibia). Femur exhibited a dose dependent increase in 45Ca uptake in response to pharmacological doses of 0.01 and 0.02 mg PRL/100 g body weight but was less responsive to 0.04 mg PRL/100 g body weight. Tibia only responded with increased 45Ca uptake to 0.02 and 0.04 mg PRL/100 g body weight while trabecular bones were unresponsive. By varying the intervals between administration of 45Ca (0 min) and PRL (0, 30 or 60 min) and bone harvesting (30, 60 or 90 min), it was found that 0.01 mg PRL/100 g body weight had a biphasic action on 45Ca movement. It initially enhanced 45Ca influx into femur by 30 min followed by an accelerated 45Ca efflux from femur back to the circulation. It could be concluded that PRL acutely stimulated calcium turnover in compact bone.