Prolactin receptors in the hypoprolactinemic male rat (IPL nude rat): Measurement in testis and induction in liver

Abstract

In order to evaluate the importance of PRL in the regulation of its own receptors, characteristics of specific binding for PRL were studied in membrane preparations from liver and testis of a new hypoprolactinemic male rat, the IPL nude male rat, and this was compared to those found for normal male rats. Under basal conditions, hepatic specific binding of PRL in IPL nude rats, as in normal rats was not detectable. Following castration, it became detectable in both groups, and was 6.99 ± 0.78% and 6.34 ± 0.87% for IPL nude and normal rats respectively. Under such conditions, the apparent affinity constant (K a) and the binding capacity ( N max ) obtained were also similar for both groups ( K a) = 1.36 ± 0.14.10 9M −1, N max = 102 ± 14 fmol/mg protein in IPL and K a = 1.34 ± 0.28.10 9 M N max = 97 ± 11 fmol/mg protein in normal rats) although a decrease in serum levels of PRL was observed in both groups. This decrease was greater for IPL nude rats. As already reported, estradiol injection following castration was able to further increase the percentage of PRL hepatic specific binding (4 times). Furthermore, our results demonstrated that the affinity constant was significantly increased by estradiol injection in both groups. On the other hand, for testicular PRL binding characteristics, a statistically significant difference was found between IPL nude and normal rats. The PRL specific binding percentage was 7.01 ± 0.85% for the IPL nude rat and 10.07 ± 0.64% for the normal rat. By Scatchard analysis, the K a of testicular membranes for labelled oPRL was similar in both groups, while the capacity differed ( N max = 9.82 ± 1.25 fmol/mg protein for IPL nude rat and N max = 26.06 ± 4.39 fmol/mg protein for normal rats). These data established the fact that IPL nude male rats presented characteristics of hepatic PRL receptors similar to those of normal rats, while their testicular oPRL binding significantly differed. These findings therefore suggest that in genetic hypoprolactinemic rats (IPL nude rats), PRL might be more involved in the regulation of testicular PRL receptors than in that of hepatic receptors.