The main function of the lower urinary tract (LUT) is to store urine ,passed down from the kidneys through the ureters, and to expel it through the outflow tract to the exterior. The outflow tract is composed of the bladder neck and urethra. Reciprocal function between the bladder and urethra allows for micturition to occur. The common pattern associated with micturition is an initial drop in urethral pressure followed by an increase in pressure in the bladder, whilst the reduction in urethral pressure is maintained. The process of micturition is co-ordinated by somatic and autonomic neurons. Both the bladder and urethra are innervated by the parasympathetic and sympathetic nervous systems, as well as somatic control of the external urethral sphincter. The smooth muscle within the bladder and urethra develops spontaneous tones that are altered by the innervation. Relaxation of the urethra is non-adrenergic non-cholinergic (NANC) mediated, mainly through action of nitric oxide (NO). Other non-nitrergic, NANC mechanisms ,such as ATP and CO, have been identified in urethral smooth muscle relaxation. It is believed that NO mediates its relaxant effects through increasing intracellular concentration of cGMP.
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The lower urinary tract (LUT) is composed of the urinary bladder and the outflow tract (bladder neck and urethra). Reciprocal function of the bladder and urethra allows for the storage and elimination of urine. The urinary bladder is a hollow muscular, dome-like vesicle which lies in the pelvic cavity (posterior to the symphis pubis). The main function of the LUT is to store urine, which is passed down from the kidneys through the ureters, without leakage in the bladder and to expel it down through the urethra to the exterior (Micturition). The common pattern associated with micturition is an initial drop in urethral pressure followed by an increase in pressure in the bladder, whilst the reduction in urethral pressure is maintained. This is a complex function that requires neuronal control of the musculature in the LUT. Positioning of the bladder can differ between genders, in males the bladder sits anterior to the rectum whereas in females the bladder is situated inferior and anterior to the uterus and vagina. The ureters extend from the medial side of the kidneys descending towards the bladder where they enter at the posterolateral surface of the bladder. The area within the bladder where the ureters endings meet and urethral opening join is known as the trigone, this area differs physiologically from the detrusor wall (Seeley et al., 2003).
The urethra, whose main function in both genders is to transport urine from the bladder to the exterior, extends from the anterior and inferior aspect of the bladder. The urethra maintains continence in the LUT by relaxing when voiding and remaining contracted when not. This continence is governed mainly by neuronal stimulation from the autonomic and somatic nervous system (Canda et al., 2008).
Figure 1: The above diagram depicts the overall anatomy of the lower urinary tract in both genders. Anatomical differences between genders are represented clearly in this diagram, the most notable difference being the difference in length of the urethra between genders. The male LUT also has the prostate, which has no direct influence over the function of the outflow tract in a healthy male, but may cause outflow obstruction if enlarged (Fry et al., 2009).
Musculature of the Lower Urinary Tract
The bladder can be separated into two areas: (1) the bladder dome and (2) the bladder base (which consists of the trigone and bladder neck) (Fry et al., 2009). The dome of the bladder can be divided into separate tissue layers; from inside out;
Detrusor muscle (smooth muscle layer) (Fry et al., 2009).
The detrusor muscle is structurally and functionally different from the smooth muscle in the trigone and the bladder. Detrusor smooth muscle is ordered longitudinally and circularly, the muscles bundle together in the detrusor and are surrounded in connective tissue. Smooth muscle cells are spindle-shaped cells with a central nucleus. They are very small in size, when relaxed they can reach to be 5-6 Âµm in diameter. The bundles that are formed between these smooth muscle cells act as functional units known as fascicles, their orientation within the bladder wall has a functional impact on the bladder, in its shape and intraluminal pressure (Andersson and Arner, 2004).
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In males, the urethra can be divided into 4 sections (only the first 3 sections mentioned have a contribution towards urinary continence)
Bladder neck (pre-prostatic).
Bulbar and penile sections (Brading, 1999).
The urethra is composed mainly of muscle tissue (both smooth and skeletal), there are 3 layers;
A thick inner longitudinal smooth muscle layer,
A thin, circular, smooth muscle layer in the middle,
An outer striated muscle layer (known as the rhabdosphincter) (Canda et al., 2008).
The urethra is certainly different between genders, in males the urethra extends from the base of the bladder neck all the way to the external orifice of the penis, with an average length of approximately 20cm (8 inches) whereas that of a female has an average length of approximately 4cm (1.5 inches). (Thibodeau and Patton, 2004, Chen and Brading, 1992).
In both genders, an internal urethral sphincter is formed at the exit point of the urethra from the bladder; it is formed of both elastic tissue and smooth muscle. This internal sphincter is not under voluntary control, this sphincter is richly innervated by autonomic nerves (sympathetic and parasympathetic). The main function of this internal sphincter is to prevent urinary leakage until the pressure from within the bladder is great enough to force the urine out from it (Thibodeau and Patton, 2004).
On the outside of the urethra, and just below the bladder neck, there is an almost circular surrounding of skeletal muscle. This cross-section of skeletal muscle is known as the external urinary sphincter (rhabdosphincter). This sphincter acts almost like a valve, controlling the flow of urine out from the bladder and down through the urethra.
This external sphincter is under somatic control which normally contracted, keeping the urethral tract closed and thus preventing passage of urine. When the somatic excitatory effect is inhibited it causes relaxation of the rhabdosphincter which will allow the passage of urine (Thibodeau and Patton, 2004, Seeley et al., 2003). The skeletal muscle has been shown to not just differ, anatomically, between genders but also between species. In human male, the skeletal muscle extends from the base of the bladder, in front of the prostate, along the full length of the membranous urethra. In human female, the skeletal muscle extends from the urethra distally, which is similar to that of the guinea pig (Brading, 1999).
The skeletal muscle can also differ with age, in young people the rhabdosphincter may encircle the urethra, however, in adults it is almost horse-shoe shaped in appearance. Two different types of skeletal muscle have been identified in the rhabdosphincter; slow and fast twitch skeletal fibres (Brading, 1999).
In male rhabdosphincters, approximately 35% slow twitch and 65% fast twitch fibres as compared to 13% fast twitch fibres in females (Brading, 1999). These fibres are believed to contribute greatly to the resting urethral pressure, in humans. It is thought that the slow twitch fibres are key in regulating the resting urethral pressure and that the fast twitch fibres play a key role in reflex contractions upon increase of intrabdominal pressure (Brading, 1999).
Urethral smooth muscle exhibits spontaneous electrical activity which can contribute to the overall myogenic tone. This myogenic tone seems to be dependent on ligand-gated (L-type) and transient voltage gated ion channels (T-type) Ca2+ channels, which allows for continuous entry of Ca2+. This is demonstrated by blockade of the T-type channels, resulting in decreased amounts of action potentials being fired thus decreasing the spontaneous activity of the cells (Bradley et al., 2004, Fry et al., 2009) and by removal of extracellular Ca2+ which results in the abolishment of the myogenic activity (Brading, 1999). Smooth muscle spontaneous activity may also be modified in the presence of interstitial cells (ICs), ICs have been shown to be closely connected with nerves that contain nitric oxide synthase (NOS) and smooth muscle cells in the urethra (Lyons et al., 2007, Fry et al., 2009).
Innervation of the Lower Urinary Tract
Before continuing to describe the innervation of the LUT, an overview of the nervous system will be briefly discussed. The nervous system of the body can be broken down into 2 main sections (1) Central Nervous System (CNS) and (2) Peripheral Nervous System (PNS).
The PNS is then split into the somatic nervous system (SNS) and the autonomic nervous system (ANS). The ANS is divided into the sympathetic and the parasymapthetic nervous system.
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The sympathetic nervous sytem and parasympathetic nervous system are quite different from one another,both anatomically and physiologically, even though both originate in the CNS and emerge from the spinal cord.(Howland and Mycek, 2006).
Figure 2: Division of the nervous system in humans ,as described above, in diagram format (Benoit, 2004).
Autonomic Innervation of the Urethra
The sympathetic nervous system emerges from the thoracic and lumbar regions of the spinal cord and can extend into both the hypogastric and pelvic nerves. The pre-ganglionic chains of the sympathetic are short compared to the longer post-ganglionic chains. The post-ganglionic chains of the sympathetic division commonly secrete noradrenaline (NA)which normally acts on adrenergic receptors (alpha (Î±) and beta (Î²))(Howland and Mycek, 2006).
The sympathetic innervation of the urethral smooth muscle originates in the intermediolateral nuclei in the thoraco-lumbar region of the spinal cord (T10-L2). These fibres can run through both the hypogastric nerve and the pelvic nerves. The sympathetic post-ganglionic fibres secrete NA which acts on Î±1 receptors to induce contraction of the smooth muscle (not just within the urethral tract but also at the bladder neck) (Andersson and Wein, 2004, Yoshimura et al., 2007).
The parasymapthetic nervous system emerges from the cranium and the scaral region of the spinal cord. The pre-ganglionic chains of the parasympathetic system are long and have shorter post-ganglionic chains, which act mainly on muscarininc receptors located on the effector organs/tissue. These nerves are also known as cholinergic nerves due to the neurotransmitter acetylcholine (Ach) (Howland and Mycek, 2006). Parasympathetic fibres that emerge from the sacral region in the spinal column are conducted to the urethral smooth muscle through the pelvic nerve and result in relaxation of the urethral smooth muscle (Fry et al., 2009, Yoshimura et al., 2007).
Both adrenergic and cholinergic nerves have been shown to contain other transmitters and transmitter generating enzymes than NA and acetylcholine. These other transmitters are known as non-cholinergic non-adrenergic (NANC) transmitters and do play a role in urethral smooth muscle. An example of a NANC transmitter is nitric oxide (NO), which has been shown to exert an inhibitory (relaxant) effect on urethral smooth muscle (Andersson and Wein, 2004).
Figure 3: The above diagram depicts the general innervation of the LUT. Sympathetic fibres extend from the thoracic-lumbar region of the spinal cord into the hyogastric nerve ,through the inferior mesenteric ganglion, from where stimulation of the urethral smooth muscle occurs through NA.
Parasympathetic fibres extend from the sacral regions of the spinal cord into the pelvic ganglion, post-ganglionic fibres then inhibit urethral smooth muscle through action of NO. Somatic nerves also extend from the sacral region of the spinal cord through the pudendal nerve from where they stimulate the rhabdosphincter with Ach (Yoshimura et al., 2007).
Autonomic Innervation of the Bladder
Contraction of the smooth muscle in the bladder (Detrusor muscle) is mainly mediated by parasympathetic stimulation, as determined by blocking with both TTX (tetrodotoxin - Na+ channel blocker) and muscarinic receptor antagonist atropine. In the bladder, purinergic transmission plays an important role in the contraction of the smooth muscle. ATP is involved in purinergic signalling within the bladder, by acting on P2X and P2Y receptors. In a normal bladder, ATP is broken down by ATPases in the neuromuscular junction so to avoid over activity in the detrusor muscle. In humans displaying overactive bladders, it was shown that it could be purinergic receptor mediated due to excess ATP (Fry et al., 2009). This was demonstrated through a non-hydrolysable ATP analogue agonist ABMA, which at first activates the receptors but leads to de-sensitisation of the receptors showing that both ATP and acetylcholine (Ach) are involved in an overactive detrusor muscle contraction (Harvey et al., 2002, Fry et al., 2009)
As previously highlighted above, sympathetic nerves act on the detrusor muscle through the hypogastric nerve. Sympathetic post-ganglionic fibres emit NA which acts on the beta-adrenergic receptors that are present in the detrusor muscle, both the Î²2 and Î²3 adrenoceptors (more predominant action from the latter) (Yoshimura et al., 2007).
Somatic Innervation of the Urethra
In the LUT, somatic nerves offer excitatory stimulation to the striated muscle of the rhabdosphincter (and of the pelvic floor) through the pudendal nerve. These neurons extend from an area known as Onuf's nucleus, located in the anterior horn of the sacral segments (S2-S4) of the spinal cord. These fibres emit Ach which acts on nicotinic receptors present on the striated muscle of the rhabdosphincter maintaining contraction (closure) during storage (Yoshimura et al., 2007).
Afferent (Sensory) Innervation
The stretch receptors in the bladder wall sense distension, and upon stimulation emit afferent nervous impulses to the lumobosacral spinal cord (S2-S4 sacral segments and T11-L2 thoracolumbar segments) through the pelvic, hypogastric and pudendal nerves (Yoshimura et al., 2007). In response to these impulses, autonomic nerve fibres emit action potentials towards the urinary bladder resulting in contraction of the bladder wall. Preceding this contraction, a relaxation of the external urethral sphincter occurs through somatic inhibition. This relaxation is mediated through both autonomic and somatic neurons.
The micturition reflex is controlled in the brain, in the pons and cerebrum. Afferent neurons send action potentials from the sacral regions involved to these higher areas in the brain, resulting efferent action potentials result in either initiation or inhibition of the micturition reflex. As the volume of the bladder increases, the number of afferent action potentials emitted through sensory fibres becomes more frequent thus increasing the need for micturition. (Andersson and Hedlund, 2002). Voluntary initiation of micturition can cause an increase in the frequency of action potentials travelling through efferent fibres from the cerebrum thus resulting in relaxation of the external urethral sphincter (Seeley et al., 2003, Thibodeau and Patton, 2004, Yoshimura et al., 2007).
Pharmacology of the Urethra
In most species, contraction of the musculature within the urethra helps maintain urinary continence. These contractions are controlled by neural stimulation, through adrenergic, cholinergic and NANC mechanisms, of the urethral smooth muscle tone and rhabdosphincter.
To a certain degree, contraction of the urethral smooth muscle (particularly the proximal region) depends on adrenergic receptor (AR) stimulation from both the Î± and Î² type ARs, mainly the Î± type AR in the urethra as opposed to the Î² type which mainly governs sympathetic mediation in the bladder (Chen and Brading, 1992). There are 2 main types of Î±-ARs: Î±1- and Î±2-ARs present in the urethral smooth muscle. In human urethral smooth muscle the Î±1-AR subtype has been shown to be the most predominant type (Andersson and Wein, 2004). Studies have shown that, in humans, the Î±-AR stimulation accounts for up to 50% of the intraurethral pressure through usage of specific antagonists and anaesthesia (Appell et al., 1980, Furuya et al., 1982, Andersson and Wein, 2004).
Contractile responses within the human female urethra where shown to differ depending on region by Taki et al. (1999), using NA and clonidine as contractile agents. NA was shown to produce the highest concentration-dependent contractions in all regions, with the highest value to be shown in the proximal region of the urethra (Taki et al., 1999).
The contractile functionality of the Î±-ARs was demonstrated by Chen and Brading (1992) in rabbit urethral smooth muscle, through usage of selective Î±1 and Î±2 blockers, prazosin and yohimibine, respectively and selective Î±1 and Î±2 agonists, phenylephrine and clonidine, respectively. It was shown that both blockers caused dose-dependent inhibition of the selective agonist induced contractions demonstrating the presence and function of Î±-ARs in the rabbit urethral smooth muscle (Figures 4 & 5) (Chen and Brading, 1992).
Figure 4: White squares indicate control. Varying Prazosin concentrations (White triangle: 10-8 M, Black square: 10-7 M, Black triangle: 10-6 M) The above diagram shows the effect of varying prazosin, selective Î±1 blocker, concentrations have on rabbit urethral smooth muscle with phenylephrine (5x10-5 M) induced contractions. A dose-dependent inhibition of phenylephrine occurred in the presence of this blocker, shifting the dose-response curve to the right (Chen and Brading, 1992).
Figure 5: White squares; Control. Yohimibine; 10-7 M (white triangle) and 10-6 M (Black triangle). The above diagram depicts the effect of Yohimibine, a selective Î±2 blocker, has on rabbit urethral smooth muscle contractile response to clonidine (5x10-5 M). The selective antagonist shows a significant inhibition of the clonidine induced contraction at 10-6M (Chen and Brading, 1992).
Î²-ARs have been detected in urethral smooth muscle of animals and humans. However, the roles of Î²-ARs in the urethra have not been fully established. They are more commonly found in the bladder neck rather than urethra, where the sub-type Î²2Â-AR is commonly found. Administration of clenbuterol, a selective Î²2Â-AR agonist, was shown to increase intraurethral pressure in humans (Yasuda et al., 1993). However, it was also shown that Î²2Â-AR agonists can reduce the intraurethral pressure in humans (Thind et al., 1993). Blockage of the Î²2-ARs may provide a useful therapy for stress urinary incontinence (SUI) by enhancing the effects of NA on Î±-ARs (Abrams et al., 2002).
Whereas treatment with clenbuterol has been suggested to act on the rhabdosphincter (Canda et al., 2008, Andersson and Wein, 2004, Morita et al., 1995).
Urethral smooth muscle is densely innervated by parasympathetic (cholinergic) fibres, these cholinergic fibres (named so because they release Ach as a neurotransmitter at both the pre- and post-ganglionic junctions) act on muscarinic receptors. These muscarinic receptors also can cause contraction of the urethral smooth muscle and can vary in predominance in species. There are four subtypes of muscarinic receptors (M1-M4), as demonstrated in rat bladder (Canda et al., 2008). In the rabbit, only the first 3 types (M1-M3) have been shown to cause contraction (Mutoh et al., 1997, Nagahama et al., 1998). In the pig, contraction appears to be mediated predominantly by the M2 and M3 receptors. However, in the pig the two aforementioned receptors cause contraction mainly in the circular smooth muscle layer whereas the M2 receptor has been shown to cause contraction in the longitudinal layer (Yamanishi et al., 2002). In humans, urethral contraction through muscarinic receptors was demonstrated mainly in the longitudinal layer of the urethral smooth muscle (Anderson, 1993).
For micturition to occur, first the rhabdosphincter and urethral smooth muscle must relax. This is achieved through inhibition of the somatic innervation of the rhabdosphincter striated muscle and through NANC mediated relaxation of the urethral smooth muscle, which is believed to be mediated mainly by nitric oxide (NO) and possibly purinergic elements such as ATP. After initial relaxation of the outflow tract, decreasing the intraurethral pressure and outflow resistance, the bladder contracts thus increasing the intravesicular pressure and expelling any urine that may be present in the bladder.
There are different factors contributing to urethral relaxation
Inhibition of NA on through pre-synaptic muscarinic receptor stimulation.
Muscarinic stimulation of longitudinal smooth muscle causes contraction resulting in widening and shortening of the muscle, thus decreasing intra-urethral pressure.
NANC mediated relaxation of the smooth muscle layers (Canda et al., 2008).
NO has been demonstrated to be an important inhibitory neurotransmitter in the LUT. Thornbury et al (1992) demonstrated the presence of NO mediated relaxation in the bladder neck muscle in sheep (Thornbury et al., 1992). Relaxations were observed in tissue during electrical field stimulation (EFS), with clear cut relaxations becoming most apparent at lower frequency stimulations (0.5 - 1 Hz). Higher frequency stimulations (>1 Hz) did not produce as clear cut relaxations due to contraction during the stimulus. After termination of each stimulus a contraction followed which is referred to as a "rebound contraction" (Thornbury et al., 1992). These relaxations were further revealed after administration of atropine and guanethidine, cholinergic and adrenergic antagonists, reduced the contractions thus showing that the relaxations were NANC in character (Thornbury et al., 1992). The NO theory was tested by using a competitive inhibitor (L-NAME) of NO-synthase (NOS), the enzyme that produces NO from L-arginine. Addition of L-NAME into the organ bath, in the presence of atropine and guanethidine, greatly reduced the relaxation at all stimulus frequencies, suggesting that NO is involved in the relaxation (Fig 6, middle trace) (Thornbury et al., 1992). Addition of L-arginine into solution competitively reversed the effects of the NOS blocker, L-NAME, further strengthening the NO theory (Fig 6, bottom trace) (Thornbury et al., 1992).
Figure 6: (Top trace) Relaxations and "rebound contractions" in response to EFS (0.2 - 4 Hz) in the presence of atropine (10-6M) and guanethidine (10-6M). (Middle trace) Addition of L-NAME (10-4M), relaxations are severely reduced but not abolished.(Bottom trace) Addition of L-Arginine (10-3M), the increase in relaxant effect suggests the involvement of NO (Thornbury et al., 1992).
Waldeck et al (1998) performed a similar experiments on rabbit urethral smooth muscle. Their aim was to investigate the association of hyperpolarisation with NO mediated relaxation (Waldeck et al., 1998). Strips of rabbit urethral smooth muscle were pre-contracted with NA and were exposed to varying concentrations of NO. It was found that the strips would produce brief concentration dependent relaxations in response to NO administration, supporting the view of NO as a potential inhibitory neurotransmitter and that NO did not mediate its relaxant effects through membrane hyperpolarisation (Fig 7 &8) (Waldeck et al., 1998).
Figure 7: Strip of rabbit urethral smooth muscle, pre-contracted with NA (3 ÂµM) exposed to varying concentrations of NO (1-100ÂµM). Dose dependent relaxation is produced (Waldeck et al., 1998).
Figure 8: The resting membrane potential (39Â±1 mV) remains unaltered by the addition of NO (30ÂµM) as compared to the bottom trace which was treated with levcromakalim (100ÂµM, a K+ channel opener). Addition of levcromakalim resulted in a hyperpolarisation (Waldeck et al., 1998).
NO donor S-nitroso-L-cysteine was used in an attempt to mimic the relaxant effect of NO in sheep bladder neck muscle. The procedure was successful in producing a relaxant effect in a strip that was pre-contracted with NA. The NO donor produced rapid relaxation of the pre-contracted tissue, along with a following "rebound contraction", suggesting it can mimic the relaxant effects of NO (Fig 9) (Thornbury et al., 1992).
Figure 9: (A) Bladder neck strip pre-contracted with NA (10-6M) exposed to s-nitroso-l-cysteine (Cys-NO, 1.6 x 10-6 M), relaxant effect observed followed by "rebound contraction". (B) Strip pre-contracted with NA (10-5 M) and then exposed to cumulative concentrations of Cys-NO ( 10-8 - 10-3 M), relaxant effect observed with increasing dosage. (C) -log graph expression of [Cys-NO] dose response (Thornbury et al., 1992).
In smooth muscle, NO-mediated responses are often linked to an increase in cGMP production. It is believed that NO exerts a relaxant effect, in rabbit urethra, through activating guanylate cyclase which produces cyclic guanosine monophosphate (cGMP) (Waldeck et al., 1998, Morita et al., 1992). Although the full mechanism of NO mediated relaxation is not understood, there are some ideas as to what mechanisms lay behind the relaxation. As summarised in fig 10 below (Canda et al., 2008), cGMP is believed to activate protein kinase 1 (PK1) in smooth muscle (Persson et al., 2000). The function of PK1 in the urethral smooth muscle of was demonstrated by Persson et al (2000) in mice lacking the gene for PK1. Urethral strips taken from mice with the gene for PK1 elicited frequency dependent relaxations during EFS (Persson et al., 2000). The relaxations were abolished by L-NG-nitro arginine (L-NOARG) during stimulation (Persson et al., 2000). When the same procedure was carried out on urethral strips taken from mice lacking the gene for PK1, relaxant responses to EFS were attenuated with small relaxations occurring at higher stimulus frequencies. Attempts to inhibit the relaxant response with L-NOARG did not work, suggesting the involvement of another relaxant transmitter (Persson et al., 2000).
Figure 10: The above diagram depicts the mechanisms involved in smooth muscle relaxation through the L-arginine/NO pathway increasing cGMP. (1) cGMP dependent protein kinase activates Ca2+-K channels causing hyperpolarisation of the cell membrane (Robertson et al., 1993). (2) Activation of Ca2+-K+ Channels by NO (Peng et al., 1996). (3) Reduced intracellular levels of Ca2+ due to sequestration of Ca2+ (Bolotina et al., 1994). or (4) reduced Ca2+ sensitivity (Koh et al., 1995). Diagram (Canda et al., 2008).
Carbon monoxide (CO) has been shown to have a similar effect to NO in the pig urethra by Werkström et al.( 1997). They showed that there is a distribution of nerves containing CO producing enzymes haem oxygenase 1 and 2 (HO-1, HO-2) through immunohistochemistry techniques (Werkström et al., 1997). Studies in the female pig urethral smooth muscle shows that CO has potential as a relaxant mediator in this tissue, as demonstrated by (Schroder et al., 2002). Addition of YC-1, a soluble guanylyl cyclase activator, increased the maximal relaxant effect of CO in the tissue to a similar level of relaxant effect obtained using NO (Schroder et al., 2002).
Inhibitory innervation of the guinea pig urethral smooth muscle was also investigated for CO by Werkström et al.(1998). HO-2 was found to be present in nerves in the detrusor, bladder neck and urethra using immunohistochemical methods (Werkstrom et al., 1998). EFS evoked extensive, frequency dependent relaxations that could not be inhibited by L-NOARG or ODQ ( guanylate cyclase inhibitor) and application of exogenous CO evoked a minor relaxation, suggesting that CO may not be involved in NANC inhibitory control of this tissue (Werkstrom et al., 1998). Exogenous application of CO to rabbit urethral smooth muscle tissue did not produce a relaxant effect, nor was there any immunoreactivity for HO-1 or HO-2 nerves in this tissue either, suggesting that CO plays no role in the inhibitory NANC transmission in this tissue (Werkström et al., 1997).
ATP ,as already discussed, can cause contraction in the bladder via stimulation of purinergic receptors. These purinergic receptors are classified as ligand-gated receptors (P2X) and G-protein coupled receptors (P2Y), both receptors exerting different biological functions when stimulated. The P2X receptors have 7 sub-types and the P2Y have 8 subtypes (Hernandez et al., 2009). Figure 11 depicts a breakdown of the purinergic receptors and their sub-types.
Figure 11: Breakdown of the purinergic receptors (types and sub-types) and their agonists. ATP acts on the P2X/Y receptors and adenosine acts on the P1 receptors (Canda et al., 2008).
ATP is believed to cause smooth muscle relaxation via the G-protein coupled P2Y receptors, after breakdown to ADP, and the A2a receptors, after breakdown to adenosine (Hernandez et al., 2009). Hernandez et al. (2009) treated pig bladder neck tissue (urothelium stripped and pre-contracted with phenylephrine) with non-selective P2 antagonist, suramin and PPADS. The result obtained was a slightly reduced relaxant effect, suggesting that ATP does mediate some part in the relaxant effect (Hernandez et al., 2009). Evidence supporting the theory that ATP mediated relaxation is a non-neuronal mechanism was demonstrated. Administration of exogenous ATP induced relaxations in the presence of TTX. Suggesting the possibility of ATP inducing its relaxation on P2 receptors located on the smooth muscle (Hernandez et al., 2009).
The innervation of musculature in the LUT play a vital role in urinary continence and elimination through changes in the intravesicular and intraurethral pressure. Autonomic and somatic innervation help maintain a sustained tone in the LUT until inhibited by neural and endogenous mechanisms. NO has been shown to play a major role in mediating the relaxation of the urethral smooth muscle in several species including sheep, rabbit, pig and human (Andersson and Persson, 1994), and is believed to exert it's relaxant effect through increasing intracellular cGMP (Waldeck et al., 1998, Morita et al., 1992). Both ATP and CO were also shown to be involved in the relaxation of urethral smooth muscle in a non-nitrergic NANC fashion. CO exerts its effects in a similar manner to NO, whereas ATP was shown to be a non-neural, endogenous mechanism of relaxation in pig bladder neck tissue (Hernandez et al., 2009). Relaxation in the urethral smooth muscle is mainly NANC mediated with other non-neural mechanisms also exerting an effect.