Serum Prolactin Levels in Rats During Different Reproductive States

Abstract

Rat serum prolactin as determined by radioimmunoassay during the estrous cycle was found to be highest during estrus (68.5 ±7.4 ng/ml) and lowest in diestrus (27.6 ± 5.0 ng/ml). Serum prolactin levels during proestrus, estrus and metestrus were not significantly different. There were no cyclic changes in serum prolactin levels in mature male rats. Estrogen injections increased both pituitary and serum prolactin, whereas ovariectomy decreased both. During the first 3 days of pregnancy, serum prolactin was higher (19.1 ±7.4 to 30.2 ±4.5 ng/ml) than during the following 18 days (8.3 ± 1.1 to 10.9 ±1.2 ng/ml). On the 22nd day of pregnancy serum prolactin levels rose to 29.2 ±13.8 ng/ml. On the 1st day post partum, serum prolactin increased to 65.5 ±19.0 ng/ml, and a high serum concentration was maintained by the suckling stimulus for at least 8 days. On the 15th and 23rd days post partum, serum prolactin fell to about half (25.7 ±5.5 ng/ml) of the level on the first day post partum. Removal of litters from mother rats on the 4th day post partum resulted in a rapid decline in serum prolactin, reaching pregnancy levels 3 hr later (8.3 ±0.1 ng/ml). When litters of 10 pups each were returned to their mothers for 1/2 or 3 hr of suckling after 12 hr of nonsuckling, serum prolactin increased precipitously to 130.3 ±19.6 and 108.3 ±7.8 ng/ml, respectively, and pituitary content fell sharply. Regular suckling by litters constantly with their mothers maintained high pituitary and serum prolactin levels. (Endocrinology86: 506, 1970)