Studies of acute effect of prolactin on distribution of absorbed calcium and long-term effect on calcium balance in weaned, young, and sexually mature rats.

An acute effect of a single dose of 0.02 mg prolactin/100 g body weight administered intraperitoneally on distribution of absorbed calcium and the effect of daily subcutaneous injections of 0.25 mg prolactin/100 g body weight for 13 days on calcium balance were assessed in weaned, young, and mature female rats. The acute administration of prolactin failed to affect distribution of absorbed calcium at 2 h after instillation of test solution. In contrast, the daily injection of a lower dose of prolactin over 13 days significantly decreased fecal and urinary excretion of 45Ca, an index of absorbed calcium, in mature rats, while having no effect on muscle and tibial 45Ca contents. In young rats, in addition to a reduction in the urinary 45Ca excretion, prolactin decreased the gastrocnemius muscle total calcium content from 7.28 +/- 0.37 to 5.58 +/- 0.37 mumol/g dry weight (p < 0.01) while increasing tibial calcium content from 6.65 +/- 0.18 to 7.35 +/- 0.15 mmol/g dry weight (p < 0.01). Bone formation (represented by serum alkaline phosphatase) in weaned rats was significantly elevated by prolactin, but bone resorption (represented by urinary hydroxyproline) was not altered. It could be concluded that prolonged administration of prolactin decreased calcium excretion in mature rats and increased bone formation and tibial calcium content in growing rats.