Biphasic effect of PMSG on lactation in the rat

A single injection of PMSG to rats on Day 10 of lactation produced a biphasic inhibitory effect on litter growth. The initial effect (24-72 h) occurred in conjunction with a rise in the concentration of oestradiol-17 beta and LH and a decline in serum prolactin, whereas the secondary effect (96 h onward) occurred in conjunction with an increase in oestradiol-17 beta and prolactin concentrations. Progesterone implants prevented the initial, but not the secondary, effect of PMSG on lactation without producing any effect on serum oestradiol-17 beta concentrations. The results suggest that the initial effect of PMSG on litter growth may be produced by changes in pituitary prolactin and gonadotrophin secretion rather than by an effect of oestrogens acting on the mammary gland.