REDUCTION OF OVARIAN OXYTOCIN CONTENT FROM EARLY LUTEAL-PHASE DOES NOT AFFECT THE CORPUS-LUTEUM SECRETORY FUNCTION IN CATTLE

It was found previously that the reduction of ovarian oxytocin (OT) content by up to 80% on d 12 and 16 of the cycle affects neither luteolysis nor estrous cycle duration in cattle. Because ovarian OT is suggested to be involved in progesterone synthesis/secretion, in the present study we wanted to investigate whether ovary depletion of OT in the early stages of the estrous cycle will influence the corpus luteum (CL) secretory function. In experiment 1, heifers (n = 15) had cannulae inserted into the aorta abdominalis through the coccygeal artery. The tip of each cannula was placed cranial to the origin of the ovarian artery. Noradrenaline (NA; 4 mg) was infused on d 5-10 for 30 min daily, evoking release of OT and lower, but evident, increases in the progesterone levels. This did not affect the length of the estrous cycle compared with control heifers infused with saline only. Because ovarian OT cannot be restored once discharged, NA served as a tool to deplete the CL of OT in experiment 2. The same dose of NA was given on d 5-10 to 4 heifers. The corpora lutea used to determine OT tissue concentrations were obtained by surgery under local anaesthesia 10-15 min after the last NA infusion. This dose of NA, depleted CL of OT by about 51% compared with control heifers. We conclude that significant reduction of ovarian OT content on d 5-10 does not affect the CL function. Thus, if ovarian OT is involved in ovarian steroidogeneis, it plays a modulatory rather than a mandatory role.