Restoration of lactation in bromocriptine-treated rats by prolactin replacement: comparison of constant versus pulsatile infusion and intrahepatic versus intrajugular routes of delivery.

The effectiveness of pulsed vs constant infusion of ovine(o) prolactin (PRL), given by different schedules, at restoring lactation in PRL-suppressed rats was compared, and the possibility that the liver participates in the restorative effects of the infused hormone was investigated. Lactating dams were given subcutaneous injections of bromocriptine (BC) between days 7 and 12 postpartum to suppress endogenous PRL secretion. Osmotic minipumps were used to infuse the oPRL into either the jugular vein or the hepatic portal vein. The latter route would expose the liver to higher concentrations of PRL than would intrajugular infusion. Constant infusion of oPRL in different doses was, overall, more effective at restoring lactation (i.e. litter weight gain) than was giving pulses, regardless of the site of delivery. Infusion of the PRL at 100 micrograms/rat/day in pulses of 1h duration was ineffective at frequencies of either 4 or 8/day, whereas pulses of 2h duration were effective at both of these frequencies. Infusing that dose of oPRL was equally effective whether it was given in 4 or 8 pulses/day of 2 h duration. Intrahepatic infusion of oPRL was not more effective than intrajugular delivery regardless of the schedule of administration. These results indicate that pulse duration is a more important determinant of the effectiveness of the galactopoietic action of PRL in the lactating rat than is pulse frequency. No evidence was obtained that the liver participates in the galactopoietic effects of PRL.