650 656
Words of Wisdom
Re: Inhibition of Cholinergic Neurotransmission by
b
3
-adrenoceptors Depends on Adenosine Release and
A
1
Receptors Activation in Human and Rat Urinary
Bladders
Silva I, Costa AF, Moreira S, et al
Am J Physiol Renal Physiol.
http://dx.doi.org/10.1152/ajprenal.00392.2016Expert’s summary:
The direct detrusor relaxant effect of
b
3
-AR stimulation as a
primary mechanism to improve overactive bladder (OAB)
symptoms has been questioned, and various other mecha-
nisms have been advanced. Silva et al hypothesized that aden-
osine formed from the catabolism of cAMP in the detrusor may
act as a retrograde messenger via prejunctional A
1
receptors
and inhibit the cholinergic activity exerted by
b
3
-AR stimula-
tion. They showed, using several well-characterized antibo-
dies, that
b
3
-ARs could be demonstrated on detrusor muscle
cells, urothelium, and suburothelial structures; however, they
found no
b
3
-ARs on cholinergic nerves. Functionally, Silva et al
showed that isoprenaline (1
m
M) decreased [
3
H]-ACh (acetyl-
choline) release from stimulated human and rat detrusor
strips. Mirabegron (0.1
m
M) and the selective
b
3
-AR agonist
CL316 243 (1
m
M) mimicked isoprenaline (1
m
M) inhibition.
Their effects were prevented by blocking
b
3
-ARs with the
selective antagonists L748 337 (30 nM) and SR59230A
(100 nM) in human and rat detrusor, respectively. Mirabegron
and isoprenaline increased extracellular adenosine in the
detrusor. Blocking A
1
receptors with 1,3-dipropyl-8-cyclopen-
tylxanthine (DPCPX, 100 nM) or blocking the equilibrative
nucleoside transporter (ENT) with dipyridamole (0.5
m
M) pre-
vented the inhibitory effects of mirabegron and isoprenaline.
Dipyridamole prevented isoprenaline-induced adeno-
sine outflow from rat detrusor and this effect was mimicked
by the ENT1 inhibitor
S
-(4-nitrobenzyl)-6-thioinosine
(NBTI, 30
m
M). Cystometry recordings in anaesthetized rats
demonstrated that SR59230A, DPCPX, dipyridamole, and
NBTI reversed the decrease in voiding frequency caused by
isoprenaline (0.1–1000 nM).
Expert’s comments:
It has been suggested that
b
-AR agonists, including mirabe-
gron, relieve OAB symptoms through various mechanisms,
such as direct relaxation of detrusor muscle by stimulation of
cAMP generation, opening of potassium channels, inhibition
of spontaneous contractile activity in the bladder, and reduc-
tion in bladder afferent activity. However, the site(s) and
mechanism(s) of action of
b
3
-AR agonists have not been estab-
lished and have been the subject of several recent studies
[1].
Coelho et al
[2]demonstrated that
b
3
-ARs are abundantly
located in ACh-containing nerve fibres in the mucosa and
muscular layers of the human bladder. No
b
3
-AR immuno-
reactivity was detected on urothelial cells, suburothelial
interstitial cells, or smooth muscle cells
[2] ,in contrast to
previous findings and the results of Silva et al. This difference
in results is difficult to explain, considering that the same
antibody (SC-1472) was used. However, functional data
strongly suggest that
b
3
-ARs are located on both detrusor
and urothelial cells, and the main pathways for activation of
bladder afferent activity are myogenic and urothelial
[3].
b
3
-AR stimulation inhibits afferent function and reduces
urgency, and a relevant question is how afferent activity is
initiated. Detrusor muscle cells exhibit a spontaneous
myogenic activity that seems to be modulated by an input
from efferent nerves (eg, ACh from cholinergic nerves) or by
local factors (eg, extraneuronal ACh, prostanoids). It cannot
be excluded that this contractile activity during filling
generates afferent signalling involving both A
d
and C fibres.
The data reported by Silva et al suggest that inhibition of
cholinergic neurotransmission by
b
3
-AR activation results
from adenosine release from stimulated detrusor smooth
muscle cells via ENT1, leading to retrograde activation of
prejunctional A
1
receptors. Prejunctional receptors, both
b
3
-ARs and A
1
, may be located in the pelvic ganglion or in the
bladder wall. However,
b
3
-AR agonists may also, directly or
indirectly, modulate signalling in suburothelial nerves. Silva
et al suggested that urothelium-derived adenosine release
frommechanically sensitive umbrella cells may be involved.
According to the hypothesis of Silva et al,
b
3
-AR–induced
adenylyl cyclase activation and cAMP generation in detrusor
smooth muscle fibres and urothelial cells are required for
generation of adenosine. It has been shown that
b
3
-AR
agonists (in high concentrations) increase cAMP in detrusor
cells
[4] ,but a main question is whether such an increase is
necessary to decrease afferent signalling. However, the
assumption that
b
3
-AR activation during bladder filling
inhibits ACh release fromparasympathetic neurons by direct
stimulation of prejunctional
b
3
-ARs
[2]and/or indirectly by
generation of adenosine may not necessarily be mutually
exclusive, and bladder micromotions that generate afferent
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