Large conductance calcium-dependent potassium (BK_Ca) channels are thought to play a key role in maintaining uterine quiescence during pregnancy. In uterine myocytes isolated from pregnant human myometrium BK_Ca channels appear to lose their calcium and voltage dependence with the onset of labour (Khan et al. 1993, 1997). Furthermore, changes in protein expression of the BK_Ca channel subunits in myometrium of the mouse (Benkusky et al. 2000), human (Zhou et al. 2000) and rat (Song et al. 1999) are altered during gestation. The mechanism(s) whereby calcium sensitivity of the BK_Ca channel (and hence contractility) is dramatically altered at parturition remain to be determined. The β-subunit of the BK_Ca channel is an accessory protein known to modulate BK_Ca channel calcium sensitivity of the BK_Ca channel. We sought to investigate, by western blotting, whether expression of the α- and β-subunit of the human myometrial BK_Ca channel is linked to the loss of calcium sensitivity and hence the altered uterine excitability observed with parturition.

Following written informed patient consent, human myometrial samples were taken from term (38-40 weeks gestation) elective (non-labour), emergency (labour) Caesarean sections or pre-menopausal non-pregnant women undergoing hysterectomy. Approval for this study was obtained from Southern Derbyshire Ethics Committee. After tissue homogenisation, membrane proteins were separated by polyacrylamide gel electrophoresis, the gel blotted and probed with either polyclonal anti-BK_Ca α-subunit antibody (raised against residues at position 913-926) or anti-BK_Ca β-subunit antibody (raised against residues at position 118-132) followed by alkaline phosphatase-linked secondary swine anti-rabbit IgG antibody. BK_Ca channel proteins were detected using enhanced chemiluminescence (ECL; Immunstar, Bio-Rad) and results expressed semi-quantitatively converted to means ± S.D. Significance was accepted as P < 0.05 using Student’s unpaired two-tailed t test.

Western blot analysis revealed a band corresponding to the 120 kDa hslo α-protein detected with anti-α_913-926 antibody. The level of the α-subunit of the BK_Ca channel in the human myometrium remained constant in non-labour, labour and non-pregnant tissue. The β-subunit was observed as a 35 kDa band which showed a 74 ± 23 % decrease in labour (n = 15) compared with non-pregnant tissue (n = 8; P < 0.001), a 56 ± 22 % decrease in labour (n = 15) compared with non-labour tissue (n = 16, P < 0.05) and a 38 ± 7 % decrease in non-labour (n = 16) compared with non-pregnant tissue (n = 8).

Our findings demonstrate significant reduction of BK_Ca channel β-subunit protein expression in labour compared with non-labour and non-pregnant tissue with little change in the α-subunit. This downregulation of the β-subunit may underlie the diminished Ca²⁺ sensitivity of the BK_Ca channel seen in the human myometrium during labour.

This work is supported by Tommy’s Campaign. We would like to thank Merck Research Laboratories, Rahway, NJ for the kind gift of the α-bSlo and β-bSlo antibodies.

Benkusky, N.A., Fergus, D.J., Zucchero, T.M. & England, S.K. (2000). J. Biol. Chem. 275, 27712-27719.

Khan, R.N., Smith, S.K., Morrison, J.J. & Ashford, M.L.J. (1993). Proc. Royal Soc. Lond. B 251, 9-13.

Khan, R.N., Smith, S.K., Morrison, J.J. & Ashford, M.L.J. (1997). Am. J. Physiol. 273, C1721-1731.

Song, M., Zhu, N., Olcese, R., Barila, B., Toro, L. & Stefani, E. (1999). FEBS Lett. 260, 427-432.

Zhou, X.-B., Wang, G.-X., Ruth, P., Huneke, B. & Korth, M. (2000). Am. J. Physiol. 279, C1751-1759.