Serum Levels of Prolactin in Sheep As Measured By Radioimmunoassy1

Abstract

A specific and sensitive double-antibody radioimmunoassay for ovine prolactin has been developed utilizing rabbit antisera to ovine prolactin (NIH-P-S8) and purified ovine prolactin for radioiodination. The assay will quantitate prolactin in pituitary extracts or serum and will detect as little as 100 pg of prolactin. The activity in serum and pituitary extracts which inhibited the radioimmunoassay was demonstrated to behave in a similar manner to purified prolactin-¹³¹I by both Sephadex gel filtration and polyacrylamide gel electrophoresis. Ovine LH, TSH, FSH, and GH do not influence the estimation of prolactin in the radioimmunoassay. Prolactin activity disappeared from the serum after hypophysectomy of ewes. Exogenous prolactin could be completely recovered when varying amounts were added to a constant volume of serum. Serum levels of prolactin appear to increase every 3–4 days in the cycle, and during the first 19 days of pregnancy. There were no differences in circulating levels of prolactin noted between cyclic and pregnant ewes except for the increased levels in cyclic ewes at the subsequent estrus. A peak prolactin level was noted on the day of estrus very near the time of the ovulatory release of luteinizing hormone. In pregnant ewes prolactin levels in serum decreased during the first 2 months, remained very low during the third and fourth month of gestation (2.0–6.0 ng NIH-P-S8/ml) then began to increase gradually 3-5 weeks prior to parturition. A dramatic rise in prolactin was noted 3 days before parturition.