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 (NB2 BA) 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 NB2 BA, 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 NB2 BA, 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 NB2 BA estimates being higher. However, the NB2 BA/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 NB2 BA. 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 NB2 BA/RIA ratio remained significantly reduced in IPL nude rats. In female rats, PRL was measured during different physiological states: estrus, diestrus, pro estrus at 1000, 1200, and 1600 h and Days I and 21 of gestation and 2 days postpartum. PRL estimates by NB2BA or RIA in 1PL nude rats were decreased in all conditions, although the differences were not significant. However, the NB2 BA/RIA ratio was significantly decreased on the day of diestrus (1 ± 0.2 and 3.09 ± 0.52) for IPL nude and normal rats, respectively, during the day of estrus (2.06 ± 0.34 and 3.78 ± 0.54) and in the post -partum period (1.08 ± 0.14 and 2.17 ± 0.24) for IPL nude and normal rats, respectively. Serum rat placental lactogen II (rPL2) values were found to be similar in both groups of rats, perhaps accounting for normal mammary gland development during gestation in this new rat strain. We conclude that circulating PRL biopotency in IPL nude rats is sharply decreased. The relative bioactivity, as representated by NB2 BA/RJA ratio, was not corrected by estradiol injection, at least in the male rats. These findings could contribute to an understanding of the endocrine characteristics of this rat strain.