Evidence for a role of the liver in the luteotropic action of prolactin in rats.

Recent studies indicate that the liver may participate in growth-promoting and lactogenic activity of prolactin (PRL). Accordingly, our study was designed to determine whether the liver participates in the luteotropic activity of the hormone in rats. Normally cycling Long-Evans rats received infusions of solvent or 50 micrograms ovine (o) PRL per day into either the external jugular vein (EJV) or the hepatic portal vein (HPV) via osmotic minipumps. The modes of delivery were continuous, 2 pulses of 8 h each per day (2-8), or 4 pulses of 2 h each per day (4-2). Observation of daily vaginal smears was used to classify rats as either cyclic or predominantly diestrus (PD). In a second study, rats fitted with osmotic minipumps received oPRL or vehicle as above, and serial blood samples were obtained for measurement of serum oPRL and rat (r) PRL concentrations. Estrous cycles of normal length were observed in 9 of 11 rats (82%) receiving solvent infusions. All 8 animals receiving continuous oPRL infusions were classified as PD, regardless of site of infusion. In addition, in 4 of 5 (80%) HPV-infused and 5 of 6 (83%) EJV-infused rats on the 2-8 pulse schedule, the smear patterns were classified as PD. However, when rats received 4-2 EJV, 9 of 13 animals (69%) were classified as cyclic, whereas 9 of 11 rats (82%) were classified as PD when oPRL was similarly pulsed into the HPV. Differences in serum concentrations of oPRL between oPRL-treated and vehicle-infused rats were significant only for continuously infused animals.(ABSTRACT TRUNCATED AT 250 WORDS)