Oxytocin-prostagalandin interrelationships in the cow with pyometra

Abstract

Ovarian steroids, luteal oxytocin and uterine prostaglandin F 2 alpha (PGF 2 alpha) interact to regulate luteolysis in the cow. Since persistent corpora lutea are characteristic in cows with pyometra, the positive feedback relationship between oxytocin and PGF 2 alpha was examined in normal animals and in cows with clinical cases of pyometra. In Experiment 1, 10 nonmilking Holstein cows were injected with oxytocin (0.33 IU/kg bodyweight, i.m.) on Days 5, 10 and 15 of the estrous cycle and the PGF 2 alpha response, as reflected in plasma levels of 13,14-dihydro-15-keto-PGF 2 alpha, was compared with that of 10 similarly treated Holstein cows with pyometra. Estrous cycle length and estradiol and progesterone concentrations did not appear to be affected by single injections of oxytocin on the three days in cyclic cows. Progesterone and estradiol values in cows with pyometra were similar to those of normal cows, but the peak value and the area under the curve of the pulse of PGFM resulting from oxytocin injection were significantly greater in cows with pyometra. In Experiment 2, conversely, five cyclic, lactating Holstein cows at Day 15 and seven postparturient Holstein cows with pyometra were injected with a synthetic prostaglandin (cloprostenol, 500 μg, i.m.) to compare oxytocin values in frequently collected blood samples. Cyclic cows treated with prostaglandin on Day 15 secreted 70.3 ± 27.7 pg/ml/h of oxytocin, while cows with pyometra secreted 4.8 ± 5.4 pg/ml/h (P < 0.01). The persistence of corpora lutea in cows with pyometra is not due to an insufficiency in the synthesis of uterine PGF 2 alpha, but may be associated with inadequate secretion of ovarian oxytocin.