Differences in the effects of vasopressin and oxytocin on rabbit myometrial activity and a possible mediation of prostaglandins

Abstract

Uterine responses to vasopressin and oxytocin were monitored in non-pregnant and 3- or 6-8-day-pregnant rabbits by recording the intrauterine pressure. Oxytocin stimulated uterine activity in all groups, but the effect of vasopressin was stimulatory in non-pregnant animals, inhibitory in those 3 days post coitum and weakly stimulatory in those later in pregnancy. Inhibition of prostaglandin (PG) synthesis, by the administration of indomethacin, reduced the spontaneous uterine activity as well as the responses to oxytocin and vasopressin in the non-pregnant rabbits, but had little effect in the pregnant animals. During infusion of PGF-2alpha, PGE-1 or PGE-2 in 6-8-day-pregnant rabbits, the stimulatory response to vasopressin, although slight before the infusion, was inhibited whereas the stimulatory response to oxytocin remained virtually unchanged. The results suggest that vasopressin and oxytocin under certain hormonal conditions, are able to activated the uterine contractions by mechanisms in which the involvement of PG is not obligatory.

REVIEW Possible actions of gonadal oxytocin and vasopressin

“…After copulation there is an initial rapid increase in activity followed by a gradual decline to quiescence by about Day 4 of pregnancy (Aref & Hafez, 1973). In the non-pregnant rabbit oxytocin and vasopressin both stimulate uterine activity (Harris, 1947;Laudanski, Âkerlund & Batra, 1977;Nissenson, Flouret & Hechter, 1978). The response to oxytocin is increased by oestrogen treatment and abolished by progesterone (Harris, 1947;Nissenson et al, 1978).…”

Prostaglandin-induced luteolysis in pregnant and pseudopregnant rabbits and the resultant effects on the myometrial activity

“…The difference between the pseudopregnant and pregnant rabbits in this respect was statistically significant (Table 1). The fact that there was no change in myometrial activity with PG infusion in the nonpregnant uterus, which is generally more sensitive than the pregnant uterus (Laudanski et al, 1977), indicates that the increased myometrial activity in the pseudopregnant does was not a result of a direct action of the infused PG on the myometrium. This is further supported by the observation that the uterine activity in the pseudopregnant rabbits did not change with the low amount of PG administered until the progesterone levels had started to decrease (see also Challis et al, 1974).…”

“…The spontaneous uterine activity in the non-pregnant rabbits before administration of PG (Text-fig. lc) was typical in pattern for this endocrine condition (lack of progesterone) with contractions of comparatively high amplitude (mean 16-4 mmHg), medium frequency (mean 10-4 pressure cycles/10 min) and long duration (see, for example, Porter, 1968;Laudanski et al, 1977). These recordings exhibited a regular type of myometrial activity and there was a distinct relaxation phase after each contraction (Text-fig.…”

The Action of Bradykinin and Oxytocin on the Isolated Whole Uterus and Myometrium of the Rat in Oestrus

“…More recently Chan (1977) using rat isolated uteri has suggested that the myometrial receptors for oxytocin and those for prostaglandin are functionally separate and distinct and that the uterotonic action of oxytocin is independent of prostaglandin-participation. However, although the involvement of prostaglandin in the uterotonic action of oxytocin does not appear to be obligatory, it seems to be dependent upon the hormonal environment as demonstrated by Laudanski, Akerlund & Batra (1977) using rabbit uterus in vivo.…”