Bioactivity of serum and pituitary prolactin during the rat estrous cycle

The availability of a mitogenic bioassay for PRL in which multiple, small samples could be analyzed led us to examine circulating and pituitary PRL bioactivity during the estrous cycle of the rat. Bio- (B) and immuno (I)-activities were compared in individual samples using rat PRL RP-1 standard. B:I ratios ranged between 2.3 and 6.8 for serum and 1.4 and 3.2 for pituitaries. Serum bioactivity increased 24-fold during the surge of PRL on the afternoon of proestrus, whereas pituitary bioactivity decreased almost 5-fold during this time period. Serum B:I ratios were significantly (P less than 0.01) elevated on the day of proestrus (0900-1800 h) compared to those on diestrous day 2. Pituitary B:I ratios ranged from a high of 2.3 +/- 0.3 on diestrous day 2 and 1500 h on proestrus to 1.8 +/- 0.1 on diestrous day 1. Differences between mean B:I ratios of pituitaries obtained at different cycle stages were not statistically significant. Interestingly, serum B and I were not correlated (r = 0.55, 0.68) at the peak of the proestrous PRL surge when pituitary extracts exhibited the highest correlation (r = 0.94). The augmented bioactivity of serum compared to that in pituitaries was not due to a synergistic serum effect. It is likely that the proestrous PRL surge, similar to the preovulatory LH surge, results from secretion of a pool of high biological activity. Since no evidence was found for a lactogen synergist in the circulation, the difference between serum and pituitary bioactivity could reflect an averaging of releasable and nonreleasable pools of PRL with varying degrees of biopotency. That these differences result from postsecretion enhancement of PRL biopotency, possibly due to changes in molecular structure and/or conformation, also cannot be ruled out. PRL biopotency in the circulation of the rat is much higher than that reported for women, perhaps reflecting the physiological relevance of PRL to luteal function in the rat.