Effect of Oxytocin, Prostaglandin F2α and Reproductive Tract Manipulations on Uterine Contractility in Holstein Cows on Days 0 and 7 of the Estrous Cycle

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Abstract
The influence of various treatments on cattle at the time of natural estrus vs estrus mduced by prostaglandin F2α (PGF2α) or at d 7 of the estrous cycle were studied; the latter is when embryo transfer often is performed. Eight lactating and 25 nonlactating, normal cycling cows were tested many times while in estrus and 7 d after estrus. A balloon was positioned in the body of the uterus to record changes in intrauterine pressure following clitoral massage, cervical massage. vaginal distention, electrical stimulation of the cervix and vagina, tailhead rubbing, odder massage and the injection of oxytocin or PGF2α. Blood oxytocin and intramammary pressure were measured. There were no differences between cows in estrus spontaneously or those induced, so these groups were combined. Intravenous oxytocin injections of .5, 1, 2, 4 and 15 IU increased blood levels of oxytocin. Intramammary pressure was increased by all oxytocin doses, but ≥ 2 IU were required to cause substantial changes in uterine contractions. As expected, the peak contractions during control periods for cows in estrus were high, averaging 31 mm hg vs 11 mm Hg on d 7. None of the manipulations of the reproductive organs caused detectable oxytocin release or increases in intramammary pressure, contrasting to responses to massage of the udder. Clitoral massage increased peak uterine pressure by 32 to 60% in four experiments. It did not induce luteinizing hormone release. The contraction was immediate, was not sustained and could be obtained repeatedly, suggesting a reflex response. Treatment with PGF2α increased intramammary pressure. It increased uterine pressure on d 7, but had no effect at estrus. This contrasts with injected oxytocin, which resulted in the highest amplitude for cows in estrus, although the response on d 7 was greater in proportion to the low activity in controls at that time. It is concluded that manual manipulation of the reproductive tract (other than clitoral massage) has little effect on uterine contractility.