THE EFFECTS OF OXYTOCIN AND VASOPRESSIN DURING THE OESTROUS CYCLE AND PREGNANCY ON THE UTERUS OF A MARSUPIAL SPECIES, THE QUOKKA (SETONIX BRACHYURUS)

Abstract

SUMMARY Uteri of the marsupial species Setonix brachyurus suspended in a solution with a cation concentration similar to that of Setonix plasma ultrafiltrate, responded to oxytocin, arginine-vasopressin (AVP) and arginine-vasotocin at all stages of the oestrous cycle (28 days) and gestation (27 days). In cyclic intact non-pregnant animals the potency of AVP in evoking contractile responses of the uterus was greater than that of oxytocin when the myometrium was oestrogen-dominated, i.e. from about day 19 to about 2 days after oestrus. In contrast, during the luteal phase (day 7–18) the uterine muscle was much more responsive to oxytocin than to AVP, although the threshold concentration for oxytocin was raised. The uteri of bilaterally ovariectomized quokkas responded more markedly to AVP than to oxytocin after treatment of the animal in vivo with an oestrogen. The responsiveness of uteri of progesterone-treated ovariectomized quokkas to vasopressin was equal to or less than their responsiveness to oxytocin. The response of the pregnant Setonix uterus to oxytocin was always greater than that to AVP, even late in pregnancy when at the corresponding stage of the oestrous cycle in the non-pregnant quokka (day 19–23) the myometrium was more responsive to AVP than to oxytocin. At earlier stages of pregnancy there were indications of a similar degree of progesterone block to oxytocin as during the luteal phase in non-pregnant Setonix. It has frequently been assumed that the endocrine milieu in marsupials is the same during the oestrous cycle and in pregnancy. The present results suggest that there are differences.