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

Abstract

Ovine prolactin (oPRL) binding to liver membranes was studied during pregnancy in normal and in genetically hypoprolactinemic rats. Prolactin (PRL) binding was determined using 125I-oPRL in the 100 000 × g pellet. Scatchard plots obtained changed throughout the pregnancy in the normal rat, being almost linear from days 2 to 10, becoming curvilinear (convex) on day 16, and linear again at the end of pregnancy. They were analyzed with reference to the co-operativity and Hill models, which give δ and nH, respectively. During pregnancy, 8 values varied and were respectively 2.48 ± 0.66 1.84 ± 0.64 0.52 ± 0.06 and 1.69 ± 0.25 on days 3, 10, 16 and 22, and the 8 value on day 16 was significantly different from other days; the nH value estimated on day 16 was 1.10 ± 0.031. These results suggest the presence of a positive co-operativity on day 16 of pregnancy. Over the same period, a huge increase in the capacity occurred on day 10 and reached a maximum on day 14. It remained elevated until the day before parturition. In the IPL nude rat, the δ value (0.92 ± 0.45) on day 16 was significantly different from that of normal rats and indicated an absence of positive co-operativity on this day in the IPL nude rat liver. This finding was confirmed by an nH value (0.99 ± 0.39) close to 1. The PRL-binding capacity was similar to that of normal rats, except on day 14, where it was significantly decreased. These results are discussed in relation to hormonal variations during pregnancy, particularly with regard to serum PRL and placental lactogen values. These findings show that liver PRL-binding characteristics change during pregnancy, and suggest that liver PRL binding may play a major role in lactation.