Solid-Phase Radioimmunoassay of Ovine Prolactin in Antibody-Coated Tubes. Prolactin Secretion during Estradiol Treatment, at Parturition, and during Milking1

Abstract

The antibody-coated tube method of solid-phase radioimmunoassay has been applied to the measurement of prolactin in ovine plasma. The sensitivity of the assay was 2 ng prolactin per ml of plasma, and the coefficient of variation over the working range of 4–120 ng/ml plasma was 8–30%. No significant cross-reaction with ovine luteinizing hormone or growth hormone was observed. Non-specific binding of prolactin tracer to plastic tubes was minimized by incubation at 4 C; plasma samples were diluted 1:20, and prolactin standards were incubated in 5% hypophysectomized sheep plasma or horse serum. In anestrous ewes, plasma prolactin showed episodic fluctuations which were unrelated to periods of luteinizing hormone (LH) secretion; administration of estradiol by injection or continuous infusion stimulated the release of both prolactin and LH. At parturition, plasma prolactin levels were extremely high (>400 ng/ml) and remained elevated for 1–2 days after delivery. During lactation, prolactin secretion rose at the time of preparation for milking, and remained high during and after milking, again with extremely large fluctuations in plasma prolactin levels. In these studies, the coated-tube assay method proved to be particularly useful for measurement of serial prolactin levels in large numbers of plasma samples. (Endocrinology91: 1329, 1972)