LACK OF EFFECT OF OXYTOCIN GIVEN INTO THE AORTA-ABDOMINALIS ON CORPUS-LUTEUM FUNCTION IN CATTLE

The aim of present study was to infuse moderate doses of oxytocin into the aorta abdominalis at different stages of the oestrous cycle in ca ttle to investigate its effect on the secretory function of the corpus luteum (CL). Mature heifers (n=12) were synchronized with a luteolyti c dose of Oestrophan. One day before the experiments the animals had c annulae inserted into the aorta abdominalis through the coccygeal arte ry, with the tip placed cranial to the origin of the ovarian artery. T his allowed direct application of the drug into the reproductive tract . In a preliminary experiment (n=4) we found that 40 IU of oxytocin gi ven for 5 h would be a more suitable dose to enhance the peripheral co ncentration of this peptide slightly over its physiological pulses. Th is dose of oxytocin was infused in Experiment 1 (n=4) once daily for 5 h on days 10, 11 and 12 of the oestrous cycle and in the same way on days 4, 8, 12 and 16 in Experiment 2. In all of these trials oxytocin affected progesterone or testosterone secretion measured in the periph eral blood. Oxytocin given into the aorta abdominalis acts not only on the CL but also on the uterus, oviduct, blood vessels in the reproduc tive tract etc. Hence the response of CL can be masked by the effect o f oxytocin on these tissues. It is, therefore, assumed that oxytocin a cts on the bovine CL inside the ovary but not as a systemic hormone.