Preterm and difficult labours remain major health issues. With a better understanding of the mechanisms involved in phasic myometrial contractions, it may be possible to better control and alter uterine function and thus help to prevent such labour. Previous work has indicated several regional differences in uterine cellular characteristics (Smith et al. 1998) and thus the aim of this work was to determine if there were any regional differences in contraction during spontaneous activity and if agonist/antagonist application had a regional effect upon contraction.

Pregnant rats (18-21 days gestation) were killed humanely with CO₂ and myometrium dissected into three regions (cervical, middle and ovarian), and in some experiments loaded with Indo-1 for Ca²⁺ measurements. Force and frequency of spontaneous and agonist-induced contractions were measured. Analysis was performed using Student’s t test for unpaired data. Significance was taken as P < 0.05; n = number of animals.

The main finding of the study was that the ovarian end of the uterus contracted with a significantly greater frequency than the other two regions, though with less force (n = 40). The Ca²⁺ responses mirrored those of force (n = 4). In terms of responses to stimulation with oxytocin (100 nM, n = 7) and carbachol (100 µM, n = 6) there were no significant differences in responses, as was also the case with the agonist Bay K 8644 (2 µM, n = 4), and the antagonists nifedipine (500 nM, n = 6), an inhibitor of calcium entry, and caffeine (10 mM, n = 6), an inhibitor of SR calcium release. However, in the case of niflumic acid, a blocker of Ca²⁺-activated Cl^– channels, it was found that contractility in the ovarian region was more sensitive than the other two regions in terms of decrease of contractility produced (n = 6).

From these data we conclude that there is little or no regional difference in term myometrium in its responses to stimulation and Ca²⁺ entry. There is evidence that the ovarian end may be involved in pacemaker activity as it contracts more frequently, and we suggest that Ca²⁺-activated Cl^– channels may be involved in this process and be more abundant there. This part of the uterus has also been shown to be more spontaneously active in a study of rats in oestrous (Crane & Martin, 1991).