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.