Inducible knockout mutagenesis reveals compensatory mechanisms elicited by constitutive BK channel deficiency in overactive murine bladder

The large-conductance, voltage-dependent and Ca2+-dependent K+ (BK) channel links membrane depolarization and local increases in cytosolic free Ca2+ to hyperpolarizing K+ outward currents, thereby controlling smooth muscle contractility. Constitutive deletion of the BK channel in mice (BK-/-) leads to an overactive bladder associated with increased intravesical pressure and frequent micturition, which has been revealed to be a result of detrusor muscle hyperexcitability. Interestingly, time-dependent and smooth muscle-specific deletion of the BK channel (SM-BK)(-/-)) caused a more severe phenotype than displayed by constitutive BK-/- mice, suggesting that compensatory pathways are active in the latter. In detrusor muscle of BK-/- but not SM- BK-/- mice, we found reduced L-type Ca2+ current density and increased expression of cAMP kinase (protein kinase A; PKA), as compared with control mice. Increased expression of PKA in BK-/- mice was accompanied by enhanced beta-adrenoceptor/cAMP-mediated suppression of contractions by isoproterenol. This effect was attenuated by about 60-70% in SM- BK-/- mice. However, the Rp isomer of adenosine-3',5'-cyclic monophosphorothioate, a blocker of PKA, only partially inhibited enhanced cAMP signaling in BK-/- detrusor muscle, suggesting the existence of additional compensatory pathways. To this end, proteome analysis of BK-/- urinary bladder tissue was performed, and revealed additional compensatory regulated proteins. Thus, constitutive and inducible deletion of BK channel activity unmasks compensatory mechanisms that are relevant for urinary bladder relaxation.