Involvement of the adrenergic system on the release of prolactin and lactogenesis at the end of pregnancy in the rat

Abstract

ABSTRACT The part played by the adrenergic system on the release of prolactin and lactogenesis induced by prostaglandin F2α and the antiprogesterone RU 486 was studied in pregnant rats. Two doses of prostaglandin F2α (150 μg) administered at 08.00 and 12.00 h on day 19 of pregnancy induced, at 12.00 h on day 20 (24 h after administration), a significant increase in the serum concentration of prolactin, with a significant decrease in serum progesterone levels. These hormonal changes significantly augmented casein and lactose levels in the mammary gland. Treatment with RU 486 (2 mg/kg) at 08.00 h on day 19 augmented casein and lactose concentrations in the mammary gland at 12.00 h on day 20 without modifying serum concentrations of prolactin and progesterone. The adrenergic antagonists, propranolol (3 mg/kg), metoprolol (10 mg/kg), ICI 118 551 (200 μg/kg), idazoxan (100 μg/kg) and prazosin (10 mg/kg), were administered s.c. at 12.00 and 20.00 h on day 19 and 08.00 h on day 20 of pregnancy to intact rats or to rats previously treated with RU 486 or prostaglandin F2α. These adrenergic antagonists did not modify serum prolactin or progesterone levels in intact or RU 486-treated rats, but serum prolactin levels in the prostaglandin F2α-treated group were significantly reduced by treatment with propranolol, metoprolol or prazosin. In addition, propranolol and ICI 118 551 also decreased the casein and lactose concentrations in the mammary glands of RU 486- and prostaglandin F2α-treated rats, while the other compounds had no effect. We also studied the effect of adrenergic antagonists on the release of prolactin and lactogenesis induced by the physiological decrease in progesterone at the end of pregnancy. On day 21 of pregnancy at 18.00 h, serum progesterone levels in intact rats were lower than 40 nmol/l, while serum prolactin and casein and lactose concentrations in the mammary gland were higher compared with values measured at 12.00 h on day 20. Treatments with propranolol, metoprolol or prazosin administered at 20.00 h on day 20 and 08.00 and 14.00 h on day 21 of pregnancy were capable of significantly reducing serum prolactin concentrations while only propranolol decreased mammary casein and lactose. The effect of propranolol was not mediated through a reduction in serum placental lactogen measured by Nb2 lymphoma cell bioassay. These results show that the adrenergic system participates, through α1 and β1 receptors, in the regulation of prolactin release induced by the decrease in progesterone in pregnant rats. They also show that β2-adrenergic receptors play a role in the induction of casein and lactose synthesis in the mammary gland. Journal of Endocrinology (1991) 129, 343–350