PARENT SESSION
Pregnancy and Parturition

UTERINE DISTENSION IS A KEY FACTOR IN THE REGULATION OF OXYTOCIN RECEPTORS IN THE TAMMAR WALLABY.

Siebel, Andrew^1,2, Bathgate, Ross¹, Parry, Laura^1,2, ^{1 2}

ABSTRACT- Oxytocin (OT) has a wide range of physiological actions in mammals, including contraction of uterine myometrial cells to expel the fetus during parturition. Female marsupials have two separate uteri and in monovular species, such as the tammar wallaby, one uterus is gravid with the single fetus whereas the contralateral uterus is nongravid. In the pregnant tammar, there is a marked increase in myometrial OT receptors (OTRs) on Day 23 of the 26-day gestation but in the gravid uterus only. The aim of this study was to investigate whether or not distension of the gravid uterus is responsible for the upregulation in OTRs. In the first experiment, surgical removal of the fetus was performed on Day 20 of gestation. There were three surgical groups: i) fetectomy, including placenta and uterine fluid; n=4 ii) fetectomy and placenta only; n=4 iii) sham operated controls; n=3. Three days after surgery, the animals were euthanized, myometrial tissues were collected and OTR concentrations measured in both uteri using a radioreceptor assay with ¹²⁵I-OTA as the labeled ligand. A significant (p<0.05: paired t-test) increase in OTRs was observed in the gravid uterus compared with the nongravid in all surgical groups. Removal of the feto-placental unit and yolk sac fluid (YSF) reduced distension in the gravid uterus and resulted in a significant (p=0.03: Mann-Whitney U test) decrease in myometrial OTRs (334.6 ± 60.3 fmol/mg protein) compared with controls (584.6 ± 28.5 fmol/mg protein). In contrast, there was no significant difference in myometrial OTR concentrations in the gravid uterus of sham operated controls and animals in which uterine distension was maintained without the feto-placental unit (454.6 ± 112.6 fmol/mg protein). There was also no significant effect of surgery on OTRs in the nongravid uterus. In experiment 2, we examined OTR concentrations in the myometrium of animals that appeared pregnant with a large, distended uterus (n=6). However, these uteri lacked a normal fetus or vascular placenta. In all animals, OTR concentrations in the distended uterus (479 ± 103.3 fmol/mg protein) were significantly (p=0.04: paired t-test) higher than the non-distended uterus (171.4 ± 52.2 fmol/mg protein). These data demonstrate that distension of the gravid uterus may affect OTR upregulation in the myometrium of pregnant tammars.

KEY WORDS: oxytocin receptor, uterus, parturition, marsupial