Action of rat prolactin on plasma somatomedin levels in the rat and on somatomedin release from perfused rat liver.

Rat prolactin at a concentration of 50 ng/ml perfusion medium stimulated the production of somatomedin-like activity (SLA) from the perfused liver of normal rats. The effect was demonstrable in perfusions performed at 11.00 h in which rat prolactin caused a mean (+/- S.E.M.) increase in the uptake of [35S]sulphate into rat costal cartilage in vitro of 64 +/- 14% in comparison with controls, but at 15.00 h no effect was observed. No effect or rat prolactin on hypophysectomized rat liver was detectable at 11.00 h. Hypophysectomized and sham-operated rats were given five intravenous injections of 50 microgram rat prolactin or a similar volume of hormone solvent at 12 h intervals. Plasma somatomedin activity (SMA) and cartilage metabolism, measured by the uptake of radioactive sulphate and thymidine by costal cartilage in vitro, were similar in hypophysectomized animals given rat prolactin or hormone solvent. Sham-operated rats given rat prolactin showed a significant increase of plasma SMA and cartilage metabolism compared with control animals. The production of SLA by rat liver in response to rat prolactin may be related to the density of specific hepatic lactogenic receptors, since these are absent or present only in low numbers in hypophysectomized animals.