1. Introduction
Overactive bladder (OAB) is a common condition affecting millions of people worldwide. It has been defined by the International Continence Society as a syndrome characterized by urgency (with or without urge incontinence), usually with frequency, or nocturia [1, 2]. It has been proposed that OAB symptoms are closely associated with exaggerated spontaneous bladder contractions (SBCs), referred to as “detrusor overactivity” (DO)[3, 4] or “no-voiding contraction.” Consequently, DO is the target for medical treatment of the storage symptoms of OAB[5]. Thus, unraveling the underlying mechanisms of DO is valuable to identify novel drug targets for OAB treatments[3, 5].
OAB is prevalent in patients or animal models with bladder outlet obstruction (BOO)[6], injury to the spinal cord [7], diabetes mellitus[8], chronic ischemia[9], or aging[10]. These pathologic conditions are characterized by enhanced oxidative stress and excessive accumulation of reactive oxygen species (ROS) due to ischemia, ischemia–reperfusion, or inflammation[5, 11-13]. Thus, oxidative stress and the resulting increased levels of ROS has been proposed to be an important mechanism for DO development[14-16].
Hydrogen dioxide (H2O2) is the most likely ROS involved in signal transduction, and has been used commonly to investigate the effects of ROS[15, 17]. In support of the role of ROS in DO, anin vivo study in rats showed that intravesical instillation of H2O2 (0.003–0.3%) could induce OAB[15]. Intravesical injection of H2O2 has been utilized to establish an animal model of OAB[18]. In vitro studies in rats showed that H2O2increased the spontaneous contractions (SBCs) of isolated bladder strips in a concentration-dependent manner[17, 19]. However, the exact mechanisms underlying how excessive ROS leads to DO are not known. Furthermore, most of these types of studies have been from animal bladders, and studies on the human bladder are lacking.
Pathologic DO has been considered to be an amalgamation of small and localized asynchronous SBCs, which could be measured in vivoduring normal filling of the bladder and could also be recorded in vitro in isolated whole bladder or bladder strips from animal and human sources[20]. Studies on the origin or modulation of SBCs may help to unravel the underlying mechanisms of pathologic DO[20]. β3-adrenoceptor agonists which have been used to treat OAB were shown to suppress SBCs in human-bladder strips[21]. In vitro recording of SBCs has indicated that SBCs could be increased by nerve stimulation, muscarinic agonists[22], nicotinic ligands, adenosine triphosphate (ATP), and substance P (SP)[23], and decreased by noradrenaline and calcitonin gene-related peptide (CGRP)[24, 25]. The modulatory effects of SP and CGRP (the two neuropeptide transmitters released from bladder sensory afferents) implicate a role of the axon reflex or the “efferent” role of sensory afferents in SBCs regulation[25, 26].
Transient receptor potential (TRP)A1 channels are expressed on C-fiber bladder afferents [27, 28]. TRPA1 activation by intravesical instillation of TRPA1 activators such as allyl isothiocyanate (AITC) can initiate DO[27]. In vitro recordings have indicated that AITC enhanced the SBCs of isolated bladder strips from rats or guinea pigs by activating TRPA1 on bladder sensory afferents[29-31]. Those studies suggest a close correlation between activation of TRPA1 on sensory afferents and DO. Furthermore, Andrade et al[31]found that the release of SP and prostaglandin (PG)E2 contributed to AITC-mediated enhancement of SBCs, which suggests that TRPA1 activation-induced DO may result from the efferent role of bladder sensory afferents.
TRPA1 channels expressed on sensory afferents have been shown to be the main molecular target for H2O2[28, 32]. In rats or mice, H2O2 administration has been demonstrated to evoke increases in inward current or the intracellular calcium (Ca2+) concentration in a subset of dorsal-root ganglia neurons that were also responsive to TRPA1 agonists[32-34]. H2O2 application evoked a long-lasting firing of most capsaicin-sensitive high-threshold afferents via TRPA1 activation in the guinea-pig bladder[28].
Given the excitatory role of H2O2 on bladder sensory afferents and the modulatory role of bladder sensory afferents on SBCs, we hypothesized that TRPA1 activation by ROS induced the release of neurotransmitters from peripheral sensory nerve terminals of the bladder contributed to DO. There are pronounced differences in SBCs patterns between humans and animals[35]. Also, studies examining the effects of ROS on the contractile activity of the bladder have been conducted almost exclusively on laboratory animals. Hence, in the present study we investigated H2O2-induced effects on bladder contractile activity and the mediating mechanisms using the human bladder tissue.