Changes in serum prolactin concentrations and ovarian prolactin receptors during embryonic diapause in mink

Experiments were conducted to determine if prolactin receptors were present in the mink ovary, and to examine the relationship between receptor numbers and serum levels of prolactin (PRL) during embryonic diapause and blastocyst reactivation. For analysis of the physicochemical properties of prolactin receptors, ovaries were obtained from anestrous mink. All binding determinations were made using 125I-ovine prolactin (125I-oPRL), and 20 micrograms of tissue protein from the 100,000 X g particulate fraction. To quantify prolactin receptors during gestation, 20 primiparous mink were mated twice on consecutive days between 4 and 10 March and assigned randomly to one of two groups. Mink in Group 1 (N=8) were killed on 13 March when blastocysts were completing their migration into the uterus and entering a state of diapause. Animals in Group 2 (N=10) were killed on 26 March during the period of blastocyst reactivation, just prior to implantation. To determine serum levels of prolactin during gestation, an additional 20 primiparous mink were similarly mated and bled every 4 days from 15 March to 8 April, and then every 7 days until 23 April. Prolactin concentrations were determined by a heterologous double antibody radioimmunoassay using porcine PRL for both tracer and standards. Optimum conditions for binding 125I-oPRL to ovarian membranes were attained at 25 degrees C after 12 h. Scatchard analysis revealed a single class of high affinity binding sites with a Kd of 6.14 X 10(-11) M. The total concentration of receptors during anestrus was 85 fmol/mg protein, which increased significantly during embryonic diapause to 484 fmol/mg protein, then declined to 16 fmol/mg during blastocyst reactivation.