β₂-Agonists have been shown to modulate K⁺ currents and stimulate the activity of large conductance calcium-activated potassium (BK_Ca) channels in myometrium (Anwer et al. 1992; Hamada et al. 1994). However, it is not known whether the mechanism of this activation involves a direct or indirect interaction between the β₂-adrenergic receptor (AR) and BK_Ca channels. The aim of this study was to investigate the nature of this association using confocal immunofluorescence and co-immunoprecipitation studies.

This study was approved by the Southern Derbyshire Ethics Committee and written informed consent was obtained from all tissue donors. Myometrial biopsies were obtained from singleton term pregnant women (gestational age >37 weeks) undergoing either (1) elective Caesarean section before the onset of labour (n = 6) or (2) emergency Caesarean section after spontaneous labour (cervical dilatation > 3 cm; n = 6). Tissue was divided into two parts. One half for confocal microscopy was processed to yield cytospins. Thus, isolated myometrial cells were obtained following enzymatic dispersal, and fixed in 2 % (w/v) paraformaldehyde for double staining immunofluorescence with the avidin-biotin complex technique. The remaining tissue was stimulated with 1 mM ritodrine (a β₂-sympathomimetic) for immunoprecipitation with either β₂-AR or BK_Ca channel antibody. Proteins were resolved by 10 % SDS-PAGE then blotted with either β₂-AR or BK_Ca antibody as appropriate.

Confocal microscopic visualization demonstrated the colocalization of β₂-AR and BK_Ca channel proteins mainly at the plasma membrane of human myometrium from both labouring and non-labouring women. Co-immunoprecipitation experiments revealed that β₂-AR antibody was able to immunoprecipitate BK_Ca channel protein and that BK_Ca antibody was able to immunoprecipitate β₂-AR protein in both groups of women.

This study demonstrated that β₂-AR and BK_Ca channel are colocalized in the human myometrium. They are also apparently linked by a direct protein-protein interaction. This close association suggests the synergistic role of these two membrane proteins in myometrial relaxation. Further studies are needed to examine the functional correlation between the β₂-AR and BK_Ca channel in pregnant human myometrium particularly at the onset of labour.

We would like to thank the staff and patients in the Department of Obstetrics and Gynaecology, Derby City General Hospital who assisted with this study.