Pregnancy, parturition, and lactation in hypophyseal stalk-transected beefheifers

Progesterone secretion is crucial for maintaining pregnancy to parturition in mammalian species, and in cattle the corpus luteum is the primary source of this hormone. This study determined the roles of prolactin (PRL), growt h hormone (GH) and luteinizing hormone (LH) in the luteotropic process in b eef heifers hypophyseal stalk-transected (HST, n=7) or sham operated (sham operated controls, SOC, n=9) during midgestation. The main finding was that endogenous PRL and GH maintained progesterone secretion in HST heifers in a similar manner to that in SOC throughout pregnancy. Serum PRL averaged 37 vs 187 and GH 2 vs 4 ng/ml in HST heifers compared with SOC, whereas LH ab ruptly decreased to undetectable levels alter HST compared with a modest 0. 4 ng/ml in SOC heifers. The second finding was that parturition and lactati on occurred in HST heifers with calf delivery induced to occur at the same time as SOC. Milk production in HST animals was severely limited, and postp artum estrus obliterated compared with SOC. The suckling stimulus sustained milk ejection in HST heifers in spite of diminished PRL, GH, thyroid stimu lating hormone, thyroxine and eri-iodothyronine secretion. The results sugg est that PRL, GH and possibly placental lactogen are luteotropic during pre gnancy in cattle.