The effects of oxytocin and progestagens on myometrial contractility in vitro during equine pregnancy.

Progesterone or progestagens are thought to maintain myometrial quiescence in pregnant mares, although this has not been proven. In the present study, the contractility of the equine myometrium was tested in vitro using samples collected from pregnant mares (n=33) between day 68 and day 340 of gestation. Myometrial samples were equilibrated in aerated Krebs buffer and subjected to one or more of these treatments: (i) oxytocin only; (ii) initial oxytocin treatment followed by combined oxytocin and progesterone or another progestagen; and (iii) initial oxytocin treatment followed by Krebs buffer followed by progestagen; (iv) initial progesterone treatment followed by progesterone and oxytocin. Spontaneous contractile activity did not occur within 180 min. The oxytocin-only treatment resulted in a significant (P < 0.01) dose-dependent increase in myometrial resting tension. Myometrial contraction amplitude and frequency tended to increase and decrease, respectively, with increasing concentrations of oxytocin, but the effect was not significant. However, there was no correlation between the amplitude and frequency of myometrial contractions and gestational age. Treatment with progestagens did not alter the amplitude or frequency of oxytocin-stimulated myometrial contractions, regardless of whether progestagens were given with oxytocin (treatment (ii)), without oxytocin (treatment (iii)) or before oxytocin treatment (treatment (iv)). These data support the results of in vivo studies on the stimulatory effect of oxytocin on equine myometrium and indicate that progestagens are ineffective at controlling myometrial contractility in vitro. It is hypothesized that other hormones may be involved in this process.