Heterogeneity of prolaction in rats : increase in neutral fraction and its role in luteal function during early pregnancy

Molecular heterogeneity of prolaction (PRL) was studied by chromatofocusing method in pregnant rats. The pituitary and serum samples were obtained from S-D rats at 4 : 00 on Day-8 of pregnancy. Chromatofocusing showed two peaks of PRL, with different isoelectric points of lesss than 4.8 (peak 1) and 4.8 or greater (peak 2). Peak 1 and 2 showed molecular weight of approximately 23,000 and 25,000, respenctively. These two peaks of PRL were also found in the pituitary and serum samples of cyclis rats (proestrous, 17 : 00) and Day-5 lactating rats (15 minutes after the initiation of suckling). However, the percentage of the peak 2 in the total PRL activity was significantly (p＜0.05) greater in the pregnant rats than the proestrous or lactating rats. In the receptor binding assay using the menbrane fractions of the luteal cells from Day-8 pregnant rats, the peak 2 showedgreater binding activity than the peak 1. There was no significant difference in the reactivity with the menbrance fractions of mammary gland between peak 1 and peak 2 fractions. These results indicated that some particular fractions of PRL with greater reactivity with the CL cells (peak 2) would be increased during early pregnancy in rats and would play an important role in the maintenance of the luteal function.