Variations of liver prolactin receptors during the estrous cycle in normal rats and in the genetically hypoprolactinemic IPL nude rat

Abstract

Ovine prolactin (oPRL) binding to liver membranes was studied during the estrous cycle in normal and in genetically hypoprolactinemic rats. Serum levels of hormones were measured by radioimmunoassay and prolactin (PRL) binding was determined using 125I-ovine PRL in the 100000 × g pellet. Scatchard plots obtained were curvilinear throughout the estrous cycle in the normal rat. They were analyzed in reference to the co-operativity model and to the Hill model which give the factor δ and Hill's coefficient ( nH), respectively. During the estrous cycle, 8 values varied from 3.77 ± 0.66 on the day of estrus to 13.48 ± 1.34 on the day of proestrus at 16.00 h. At the same time, nH were 0.97 ± 0.033 on the day of estrus and 0.72 ± 0.025 on the day of proestrus at 16.00 h. On the other hand, the number of PRL receptors did not change significantly throughout the estrous cycle. Moreover, the dissociation of 125I-oPRL from its receptor was accelerated by the presence of native ovine oPRL. These results suggest the presence of a negative co-operativity which reached a maximum on the day of proestrus in the normal rat. This co-operativity during the estrous cycle was not found in liver from genetically hypoprolactinemic (IPL nude) rats, which present a total absence of lactation. The 8 values did not vary significantly and were 6.52 ± 1.30 on the day of estrus and 4.41 ± 0.52 on the day of proestrus at 16.00 h. The difference between the two rat strains was statistically significant on the day of proestrus at 16.00 h for both 8 and nH values. PRL itself could be involved in the negative co-operativity observed: the δ value was at a maximum when the serum PRL level was highest. Moreover, only PRL was significantly decreased in the IPL nude rat. This study shows that liver PRL-binding characteristics (mainly the affinity) vary during the estrous cycle. These variations are not found in the genetically hypoprolactinemic rat, suggesting that the liver may play a physiological role in mammary gland preparation for lactation.