Sodium pump function depends on the isoform composition of Na⁺ÐK⁺-ATPase, as isoforms show differential sensitivity to ions and inhibitors in a species- and tissue-dependant manner (Mobasheri et al. 2000). The control of uterine contraction is dependent on the sodium pump maintaining ionic gradients across the cell membrane and hence excitability. In order to better understand its role we have investigated the cellular distribution of Na⁺ÐK⁺-ATPase α and β isoforms and the effect of ouabain in the rat uterus.

Female pregnant and non-pregnant Wistar rats were humanely killed by cervical dislocation under CO₂ anaesthesia. Uterine horns were removed and transverse sections cut and mounted in OCT blocks. Cryostat sections (10 mm thick) were mounted on coated slides and fixed for 10 min in ice-cold methanol before blocking in 10 % normal goat serum in PBS. Expression of a panel of Na⁺ÐK⁺-ATPase isoforms was compared by indirect immunofluoresence using antibodies to the α and β subunit isoforms and goat anti-mouse or goat anti-rabbit IgG conjugated to FITC, respectively. Spontaneous contraction of strips of myometrial smooth muscle was measured in the presence or absence of oxytocin or in combination with the Na⁺ÐK⁺-ATPase inhibitor ouabain.

Abundant immunostaining of the α₁ and α₂ isoforms was observed in the basolateral membranes of uterine epithelia and in the myometrium. β₁ Subunit expression was positively correlated with areas shown to stain positively for the α₁ isoform. Diffuse α₂ but dense α₃ staining was visible throughout the endometrium of both pregnant and non-pregnant tissue. Relatively diffuse β₂ and β₃ staining was observed in all regions except for uterine glands, which produced higher levels of immunostaining for β₃. Application of 10 nM oxytocin increased force in non-pregnant tissue (n = 3). When oxytocin was applied following ouabain incubation (10 µM for 45 min), significantly more force was produced (P < 0.05, paired t test).

These results suggest that uterine Na⁺ÐK⁺-ATPase consists of α₁, α₂ and α₃ and β₁, β₂ and β₃ isoforms but the isoform combinations differ in the distinct anatomical regions of the uterus. The data with ouabain demonstrates the functional significance of the sodium pump in the myometrium, and suggests that elevation of intracellular Na⁺ leads to Ca²⁺ entry via Na⁺ÐCa²⁺ exchange and hence augmentation of force.

All procedures accord with current UK legislation.