In order to develop better therapeutics to prevent or treat problems in labour, a greater understanding of the mechanisms that generate and regulate myometrial contraction is urgently needed. Microtubules are filamentous intracellular structures and integral components of the cytoskeleton. They mediate several cellular functions, including cell division and intracellular protein transport, as well as modulating signal transduction arising from extracellular ligand binding. Microtubule stability has been shown to affect signalling from several G protein-coupled receptors (GPCRs) in several cell types. Their disruption using depolymerization agents such as colchicine and nocodazole enhances signalling by cAMP-linked GPCRs such as β-adrenoreceptors (β-AR) in some cells. In renal and mesenteric arteries, microtubule disruption increases vasorelaxation via the β-AR agonist, isoprenaline, which is brought about by increased microtubule delivery and insertion of Kv7 channels in the membrane, facilitating hyperpolarisation (Lindman, 2018).   In cerebral artery myocytes, microtubules have also shown to be important in supporting coupling between the SR and the plasma membrane, with roles in facilitating contraction and vascular tone (Pritchard, 2017). However, little is known about the contribution of microtubules towards ion channel and membrane receptor dynamics, and hence excitation, in other smooth muscles, including myometrium. 

Here, we investigated the effect of microtubule depolymerization on spontaneous, β2-AR-mediated relaxation and PGE₂-mediated contraction in pregnant and non-pregnant rodent uterus.  

Isometric tension recordings were made on strips of longitudinal myometrium (8 mm x 5mm x 2mm) from time-mated pregnant (d18) c57/BL6 mice (n=5) or non-pregnant Wistar rats (n=5) following humane sacrifice.  Firstly, the effect of increasing concentrations of colchicine (1µM-500µM) was investigated on force of spontaneous contraction (mN) and integral of force (area under the curve, AUC) (n=4). Next, the effect of increasing concentrations (1nM-1µM) of ritodrine (a β2-AR-selective agonist) or PGE₂ (1nM-100µM) was investigated on paired strips with and without pre-treatment (40 mins) with 500µM colchicine (n=5). Significance was taken as P<0.05 by one-way ANOVA and Dunnett’s post-hoc test. IC50s and EC50s were compared by F test.

Application of increasing concentrations of colchicine alone (up to 500µM) had no significant effect on force of spontaneous contraction (P=0.2247) or force integral (P=0.8464). Pre-incubation with colchicine however, enhanced the relaxatory effect of ritodrine in pregnant myometrium: Following colchicine pre-treatment, IC50s for ritodrine on force and AUC reduced from 570nM (95% CI 51.5nM–6.0µM) to 67.5nM (95% CI 29.9nM–153.0nM) and from 63.9nM (95% CI 45.3nM–90.2nM) to 37.8nM (95% CI 26.8nM–53.0nM) respectfully, (P=0.0295), but did not change in non-pregnant myometrium (P=0.0693 and P=0.21).  Colchicine pre-treatment also tended to increase the potency of PGE₂ in non-pregnant myometrium: the EC50s for PGE₂ on AUC reduced from 2.09µM (95% CI 122.0nM–42.8µM) to 85.5nM (332pM–22µM) but were not significantly different (P=0.460).

Like other smooth muscles, data suggest that microtubule integrity plays an important role in mediating cellular signalling from some GPCRs in myometrium.  Differences observed between pregnant and non-pregnant tissues indicate that there may be gestation-dependent effects on the activity of the microtubule network and warrants further investigation, as does investigating the effect of microtubule disruptors on agonist-mediated contraction and relaxation in human myometrium.