Relationship between Bioassay and Radioimmunoassay Estimates of Prolactin in Rat Serum*

Abstract

RIA [radioimmunoassay] and bioassay (BA) estimates of the concentration of PRL [prolactin] in rat serum were compared using homologous assay systems. A rat mammary gland organ culture BA was developed which allowed objective and quantitative measurement of serum PRL. The serum samples were tested at 2 doses against 3 doses of the RP[rat pituitary]-1 rat PRL standard and the responses were quantified by a histometric procedure. The index of precision of the BA was 0.1-0.4 and the secretory response was not affected by physiological concentrations of rat GH [growth hormone], T3 [triiodothyronine] or by gonadal steroids. Serum samples taken from female rats during the estrous cycle, from suckled lactators, from rats bearing a PRL-secreting pituitary tumor [MtTW15] and from females treated with perphenazine were assayed. A highly significant correlation (r [correlation coefficient] = 0.99) was obtained between the 2 assay methods with 21 samples. The regression coefficient was 4.20 .+-. 0.55. Although the 2 assays are highly correlated for measuring serum PRL, the RIA measures only about 25% of the hormone that is detected by the mammary gland organ culture BA.