Prolactin: The Hypophyseal Factor That Terminates Embryonic Diapause in Mink

The purpose of this experiment was to determine whether prolactin (PRL) is the factor that activates the quiescent corpus luteum (CL) and terminates the delay that precedes implantation in mink. Animals were hypophysectomized or sham-hypophysectomized 6 days after the second of two matings. Eight hypophysectomized mink received 0.5 mg ovine PRL (NIH-P-Si 3) daily through Days 21-24 of the experiment (Day 0 = day of surgery). Five sham-hypophysectomized and one hypophysectomized animal received no hormone therapy after surgery. All animals were bled at 3 day intervals until termination by exsanguination between Days 21 and 24. Uteri were observed by means of midventral laparotomy between Days 14 and 16. The hypophysectomized, untreated mink displayed neither luteal activation nor embryo implantation throughout the duration of the experiment. In hypophysectomized mink injected with PRL, luteal activation, as indicated by an increase in peripheral progesterone above pretreatment levels, had begun by Day 3 and persisted through Day 15 (P<0.05). Uterine swellings were present in six of eight PRL-treated mink at Days 14-16 and in seven of eight at Days 21-24. These swellings were found to contain implanted embryos at necropsy. Luteal activation occurred by Day 9 in sham.hypophysectomized mink, and progesterone continued to increase through Day 24. No evidence of implantation was present at Days 14-16 in this group but three of five had implanted by Days 2 1-24. The results demonstrate that PRL alone will induce tuteal activation and embryo implantation in hypophysectomized mink. However, PRL alone appeared not to be able to sustain luteal function. It is suggested that photoperiod acts through hypophyseal PRL secretion to terminate em