The urinary bladder generates spontaneous contractions that are triggered by action potentials. Pharmacological evidence suggests that the after-hyperpolarization phase of these action potentials is mediated by apamin-sensitive small conductance KCa (SK) channels. The goal of this study was to determine the role of one of the known SK channel subtypes, SK3, in controlling the function of urinary bladder smooth muscle (UBSM).
We utilized transgenic mice targeted with a tetracycline-based genetic switch to control the expression of the SK3 gene. Homozygous targeted animals (SK3 T/T) over-express SK3 channels, while dietary treatment with doxycycline (DOX) suppresses SK3 expression below the levels of wild-type animals. We performed immunofluorescence microscopy to visualize the location of SK3 protein in the bladder wall of wild-type, SK3 T/T, and DOX-treated animals. To obtain the baldders, the animals were humanely killed.
SK3 channel expression was found in the urothelium and the UBSM layers of the bladder wall but no SK3 protein was seen in the intrinsic nerve plexus of the bladder. SK3 immunoreactivity was greatly increased in SK3 T/T animals, consistent with SK3-overexpression. DOX-treatment suppressed the level of SK3 immunoreactivity, consistent with suppression of SK3 expression. Electrophysiological recordings of SK currents in isolated UBSM cells revealed a threefold increase in SK current density in SK3 T/T myocytes. We characterized urodynamic performance in conscious mice instrumented with bladder catheters. The catheters were implanted under general anaesthesia with isoflurane (1-3%, inhaled). SK3 overexpressors had a dramatic increase in bladder capacity compared to wild-type animals (302 ± 41 µl vs. 140 ± 23 µl, mean ± S.E.M., n = 7, P < 0.05, Student’s unpaired t test). Suppression of SK3 channel expression with DOX did not reverse this increase in bladder capacity (393 ± 41 µl), suggesting long-term structural remodelling of the bladder associated with SK3 over-expression. Suppression of SK3 channels with DOX did, however, induce bladder overactivity as indicated by a substantial (7-fold) increase in the number of non-voiding contractions per cystometrogram. In isolated strips of UBSM, spontaneous contraction frequency was elevated in strips from SK3 suppressed animals (DOX-treated). The SK channel blocker apamin increased contraction frequency and amplitude in strips from wild-type, SK3 T/T, and DOX-treated animals, but the effect was most pronounced in the SK3 overexpressors.
All together, these data suggest that SK3 channels inhibit urinary bladder contractility, and that both urothelial and smooth muscle SK channels may regulate bladder function.
This work was supported by NIH DK53832.