Unstable bladder contractions may contribute to the symptoms of the overactive bladder and it is debated if they originate in the smooth muscle or from neuronal influences. A myogenic cause implies fundamental changes to detrusor smooth muscle and one possible factor is protrusion junctions between cells (Elbadawi et al. 1993), leading to speculation that enhanced intercellular coupling co-ordinates electrical activity and generates greater spontaneous activity. However, only gap junctions mediate electrical coupling and there is sparse evidence for them in detrusor. This study measured the intracellular resistivity of human detrusor from normal and unstable bladders and how this related to the presence of gap junction proteins.
Bladder biopsies were obtained at open surgery, placed in Ca-free Hepes buffer and used immediately. Patients had urodynamically defined stable or overactive bladders. Local Ethical Committee approval and patient consent were obtained for each specimen. Intracellular resistivity, and in particular junctional impedance was measured from preparation impedance (20 HzÐ300 kHz; Fry et al. 1999). Immunoconfocal detection of Cx43, Cx40 and Cx45 used established monoclonal or polyclonal antibodies (Coppen et al. 1998). Data are medians (25 %, 75 % interquartiles); differences between data sets used Mann-Whitney tests.
Intracellular resistivity, Ri, after correction for extracellular resistance was 817 (572, 1173; n = 15) Ω cm, and 1426 (1264, 1787; n = 10) Ω cm in samples from stable and unstable bladders and was significantly greater (P < 0.05) in the latter group. Ri values were divided into cytoplasmic (Rc) and junctional (Rj) resistivities. Rc values were not significantly different in the two groups (294 (253, 411) vs. 374 (306, 412) Ω cm, P > 0.05), whereas Rj values were significantly greater in the unstable group (511 (368, 778) vs. 1052 (895, 1368) Ω cm, P = 0.01). Punctate connexin43 labelling was intense in a suburothelial band of cells, but unequivocal detrusor labelling was not observed in the muscle layer. Connexin40 labelling was prominent in endothelial cells of larger arterioles but not elsewhere. Connexin45 labelling showed as small, sharply defined areas specifically in the detrusor and localized to boundaries between smooth muscle cells. Northern blots of the separated mucosal and detrusor layers mirrored that of the corresponding confocal microscopy images. Mean connexin45 transcript levels were reduced to 65 % in the unstable compared with the stable bladders (P < 0.05).
Connexin45 is expressed in detrusor smooth muscle. The decrease in connexin45 transcript quantity, coupled with a raised junctional resistivity implies that intercellular coupling is reduced in the unstable bladder. How this contributes to aberrant contractile function remains to be determined.
We thank the MRC for financial support.
All procedures accord with current local guidelines and the Declaration of Helsinki.