muscarinic receptor stimulation. However, it

smooth

muscle

preparations

seems

was not clear whether this factor came from the

to vary between species, and is

probably

detrusor muscle or from both the bladder and

also dependent on experimental factors (see,

the urothelium. Hawthorn et al.27 presented

Andersson and Arner1).30 Characteristically,

data suggesting the presence of a diffusable,

thesecontractionsareresistanttotheNa-channel

urothelium-derived inhibitory factor, which

blocker (TTX) and cannot be blocked by hex-

could not be identified, but appeared to be nei-

amethonium, atropine, a-AR blockers, b-AR

ther nitric oxide, a cyclooxygenase product, a

blockers, or suramin, apparently excluding

catecholamine, adenosine, g-aminobutyric acid

direct involvement of nerves and nerve –

(GABA) nor any substance sensitive to apamin.

released transmitters.1 Contractions can be

The identity and possible physiologic role of the

effectively inhibited by L-type Ca2+ channel

unknown factor remains to be established and

blockers and K+ channel openers, supporting

should offer an interesting field for further

the important role of L-type Ca2+ channels for

research. These mechanisms can be involved in

the activity. It cannot be excluded that these

the pathophysiology of the overactive bladder

spontaneous contractions generates part of the

syndrome (OAB) and detrusor overactivity

afferent activity (“afferent noise”) during filling

(DO) and thus seem to be interesting targets for

of the bladder.40

pharmacologic intervention.

Myocytes

Cholinergic Receptors

Muscarinic Receptors

Myogenic activity can be defined as the ability

Muscarinic receptors comprise five subtypes,

of a smooth muscle cell to generate mechanical

activity independent of external stimuli.1 In the

encoded by five distinct genes.41 The five gene

individual myocyte, contractile activity is pre-

products

correspond to

pharmacologically

ceded and initiated by an action potential, which

defined receptors,and M1 – M5 is used to describe

is calcium driven.28 It has been suggested that

both the molecular and pharmacological sub-

the detrusor muscle is arranged into units (mod-

types. In the human bladder, the mRNAs for all

ules),which are circumscribed areas of muscle.29

muscarinic receptor subtypes have been dem-

These modules show contractile activity during

onstrated,42,43 with a predominance of mRNAs

the filling phase of the micturition cycle and

encoding M

2

and M

receptors.42,44 These recep-

might be controlled by several factors including

3

tors are also functionally coupled to G-proteins,

a peripheral myovesical plexus, consisting of

but the signal transduction systems vary.45–48

intramural ganglia and ICC30. Intercellular con-

Detrusor smooth muscle contains muscarinic

nections may contribute to module control, but

receptors of mainly the M

2

and M subtypes.45–49

The M3

3

also locally generated mediators. Kinder and

receptors in the human bladder are

Mundy31 found that spontaneous contractile

believed to be the most important for detrusor

activity developed more often in muscle strips

contraction. Jezior et al.50 suggested that musca-

from overactive than normal bladders, a finding

rinic receptor activation of detrusor muscle

underlined by Brading,32 and confirmed by Mills

includes both non-selective cation channels and

et al.33 Turner and Brading34 discussed the

activation of Rho-kinase. Supporting a role of

occurrence of “patchy denervation” in cases of

Rho-kinase in the regulation of rat detrusor con-

DO with subsequent changes of the smooth

traction and tone, Wibberley et al.51 found that

muscle cells, e.g., supersensitivity to ACh. Such

Rho-kinase inhibitors (Y-27632, HA 1077) inhib-

an increased sensitivity has been demonstrated

ited contractions evoked by carbachol without

in smooth muscle preparations from patients

affecting the contraction response to KCl. They

with idiopathic and neurogenic DO.35 It has

also demonstrated high levels of Rho-kinase

been reported that suburothelial ICC respond to

isoforms (I and II) in the bladder. Schneider

purinergic stimulation by firing Ca2+ tran-

et al.49 concluded that carbachol-induced con-

sients.36 Interestingly, these suburothelial ICC

traction of human urinary bladder is mediated

may be able to affect the activity of the detrusor

via M receptors and largely depends on Ca2+

myocytes.37–39 The frequency of the spontane-

3

entry through nifedipine-sensitive channels and

ous rhythmic contractions of isolated detrusor

activation of the Rho-kinase pathway Thus, the

128

Practical Urology: EssEntial PrinciPlEs and PracticE

main pathway for muscarinic receptor activation

in the rat and human urothelium, the receptor

of the detrusor via M3 receptors may be calcium

proteins and mRNAs, respectively, for all musca-

influx via L-type calcium channels,and increased

rinic receptor subtypes (M1–M5) were demon-

sensitivity to calcium of the contractile machin-

strated.65 However, the expression pattern of the

ery produced via inhibition of myosin light

different subtypes in the human urothelium was

chain phosphatase through activation of Rho-

reported to differ: the M1 receptors on basal

kinase.52

cells, M2 on umbrella cells, M3 and M4 homoge-

The functional role for the M2

receptors has

nously, and M5 with a decreasing gradient from

not been clarified, but it has been suggested that

luminal to basal cells.43 Mansfield et al.66 found,

M2 receptors may oppose sympathetically medi-

using RT-PCR analysis, an abundant expression

ated smooth muscle relaxation, mediated by

of muscarinic M2 receptors in the human blad-

b-ARs.53 M receptor stimulation may also acti-

der mucosa. These receptors may occur at other

2

locations than the urothelium, e.g., on subu-

vate non-specific cation channels54 and inhibit

K

ATP

channels through activation of protein

rothelial interstitial cells.67,68

kinase C55,56. In certain disease states, M2 recep-

The suburothelial nerve plexus is close to the

tors may contribute to contraction of the blad-

urothelium.69–71 The urothelium, as mentioned

der. Thus, in the denervated rat bladder, M2

previously, has been suggested to work as a

receptors, or a combination of M2

and M3, medi-

mechanosensory conductor, and in response to,

ated contractile responses and the two types of

e.g., distension, it releases ATP affecting under-

receptor seemed to act in a facilitatory manner

lying afferent nerve fibers via purinoceptors.25,72

to mediate contraction.57–59 In obstructed,hyper-

This may consequently modify the afferent

trophied rat bladders, there was an increase in

response of the bladder.73,74 ACh is produced in

total and M receptor density, whereas there was

the urothelium, but the mechanism behind its

a reduction2in M receptor density.60 The func-

release does not seem to involve vesicular exo-

3

cytosis.75,76 The organic cation transporter 3

tional significance of this change for voiding

function has not been established. Pontari et al.61

subtype has been demonstrated in and sug-

analyzed bladder muscle specimens from

gested to be involved in the non-neuronal release

patients with neurogenic bladder dysfunction

from rat urothelium.76

to determine whether the muscarinic receptor

subtype mediating contraction shifts from M3 to

Nicotinic Receptors

the M2 receptor subtype, as found in the dener-

Although nicotinic ACh receptors in both the

vated,hypertrophied rat bladder.They concluded

that whereas normal detrusor contractions are

central and peripheral nervous systems play a

mediated by the M3 receptor subtype, in patients

prominent role in the control of urinary bladder

with neurogenic bladder dysfunction, contrac-

function, little is known regarding expression or

tions can be mediated by the M2 receptors.

function of nicotinic receptors in the bladder

Muscarinic receptors may also be located on

urothelium. Beckel et al.77 examined the expres-

the pre-synaptic nerve terminals and partici-

sion and functionality of nicotinic receptors in

pate in the regulation of transmitter release. The

the urothelium, as well as the effects of stimula-

inhibitory pre-junctional muscarinic receptors

tion of nicotinic receptors on the micturition

have been classified as M4 in the human blad-

reflex. mRNA for the a3, a5, a7, b3, and b4 nico-

der.62 Pre-junctional facilitatory muscarinic

tinic subunits was identified in rat urothelial

receptors appear to be of the M1.63 The muscar-

cells using RT-PCR. Western blotting also con-

inic facilitatory mechanism seems to be upregu-

firmed urothelial expression of the a3- and a7-

lated in hyperactive bladders from chronic

subunits. Application of nicotine to cultured rat

spinal cord transected rats. The facilitation in

urothelial cells elicited an increase in intracel-

these preparations is primarily mediated by M

3

lular Ca2+ concentration, indicating that at least

muscarinic receptors.63,64

some of the subunits form functional channels.

Muscarinic receptors have also been demon-

These effects were blocked by the application of

strated on the urothelium/suburothelium. The

the nicotinic antagonist hexamethonium.

porcine urothelium was found to expresses a

More investigations are needed to establish

high density of muscarinic receptors, even

the functional role of urothelial nicotinic recep-

higher than the bladder smooth muscle,27 and,

tors, normally and in bladder disorders.