Studies of the Measurement of Bovine and Porcine Prolactin by Radioimmunoassay and by Systemic Pigeon Crop-Sac Bioassay1

Abstract

Problems in assigning a physiological role for prolactin may stem from difficulty in its quantification. We have evaluated the specificity of the systemic crop-sac bioassay and correlations between this bioassay and a specific radioimmunoassay. The systemic pigeon crop-sac assay gave slopes for impure bovine pituitary extracts that were 2.5 times greater than those for purified bovine prolactin. Qualitatively, the responses were also found to be different when the gross appearances of the crop-sac epithelia were compared after administration of each of the hormone preparations. A specific radioimmunoassay for bovine prolactin, showing no cross reaction with a variety of highly purified preparations of anterior pituitary hormones, was utilized for comparative purposes. Crude bovine and porcine pituitary extracts and purified bovine prolactin gave parallel responses by immunoassay, and potency estimates could be assigned to all materials. When purified bovine prolactin was measured, potency estimates by bioassay were in agreement with those by radioimmunoassay. Nonparallelism in the bioassay prevented such comparisons for impure materials, but in all cases the bioassay gave a higher prolactin potency than the immunoassay when impure materials were studied. Fractionation of pooled bovine pituitary glands into several fractions suggested that the major part of the apparent nonspecificity of the bioassay was due to materials chemically localized in fraction also containing ACTH. Subsequent bioassay of porcine ACTH preparations of 2 degrees of purity suggested that ACTH itself and some other unidentified nonprolactin material may be capable of stimulating the crop-sac epithelium. These ACTH preparations contained no prolactin as measured by immunoassay. The histological changes produced by these ACTH preparations were identical with that produced by purified prolactin. It is not known whether the effect of ACTH was a direct one, or whether it released other substances which produced the effect. However, corticosterone and hydrocortisone did not have crop-sac stimulating properties. (Endocrinology88: 991, 1971)