Relationship between bioassay and radioimmunoassay measurement of prolactin in the IPL rat, a hypoprolactinemic rat strain

IPL (Institut Pasteur, Lyon) nude, hypoprolactinemic rats exhibit delayed puberty and a complete lack of lactation. To characterize the secretion of circulating forms of prolactin (PRL) of these rats, PRL concentrations were measured in serum and pituitaries of males and females under various physiological conditions. Two assay methods, a radioimmunoassay (RIA) and a sensitive bioassay (NB2BA) were employed. Normal rats of the Sprague-Dawley strain were tested simultaneously, as controls. The pituitary content of PRL, estimated either by RIA or by NB2BA, in IPL nude males and females was similar to that of normal male and female rats. On the contrary, serum PRL levels of IPL male rats, measured by RIA or NB2BA, were significantly reduced when compared to normal rats. In both groups, there was a close correlation between the results obtained by the two methods, the NB2BA estimates being higher. However, the NB2BA/RIA ratio was significantly decreased in serum from IPL nude rats compared to controls, indicating that the circulating form of PRL was less bioactive in this group. Castrated male rats injected with estradiol showed sharply increased PRL values as estimated by RIA or NB2BA. The increase was greater (35-fold) in IPL nude rats then in normal rats (9-fold), but these increases resulted in serum PRL levels being similar in the two groups. However, the NB2BA/RIA ratio remained significantly reduced in IPL nude rats. In female rats, PRL was measured during different physiological states: estrus, diestrus, proestrus at 1000, 1200, and 1600 h and Days 1 and 21 of gestation and 2 days postpartum.(ABSTRACT TRUNCATED AT 250 WORDS)