Secretion of placental lactogen, growth hormone, and prolactin in late pregnant rats.

The secretory patterns of placental lactogen (PL), GH, and PRL were determined in conscious unrestrained late pregnant rats by measurement of the plasma concentrations of each hormone by RIA. Blood samples were collected over a 4-h period at 15-min intervals on days 18 and 19 of gestation. From days 18-19, GH levels increased 2-fold, no change was observed in PRL levels, while PL concentrations increased slightly. There was an ultradian rhythm present in GH and PRL secretion, with a frequency of two to three secretory spikes per 4-h collection period. The variable concentrations of PL in the peripheral circulation were suggestive of an episodic or ultradian secretory pattern. PL levels increased 100% or more within 30 min (i.e. from 485 to 1132 ng/ml) or decreased as much as 65% within 60 min (i.e. from 792 to 247 ng/ml). The relative magnitude of these changes were similar to, but not coincident or correlated with, those for GH.