Role of nitric oxide in beta3-adrenoceptor activation on basal tone of internal anal sphincter

BDNF rescues aging-associated internal anal sphincter dysfunction

Aging can lead to rectoanal incontinence due to internal anal sphincter (IAS) dysfunction, which is characterized by a decrease in IAS tone and contractility and an increase in nonadrenergic noncholinergic (NANC) relaxation. We aimed to determine whether brain-derived neurotropic factor (BDNF) rescues this aging-associated IAS dysfunction (AAID). To do so, we studied the effects of BDNF on the basal and G protein-coupled receptors (GPCR)-stimulated IAS smooth muscle tone and on NANC relaxation in Fischer 344 rats representing different age groups [26-mo-old (aging) vs. 6-mo-old (young)], before and after tyrosine kinase receptor B (TrkB) antagonist K252a. We also used isolated smooth muscle cells (SMCs) to determine the effects of BDNF before and after different agonists. For some studies, we monitored NO release using smooth muscle perfusates. BDNF reversed AAID by rescuing the basal IAS tone and agonists [thromboxane A₂ analog (U46619) and angiotensin II (ANG II)]-induced contractility, and NANC relaxation. These rescue effects of BDNF were selective as K252a attenuated the changes in the IAS without modifying the effects of K⁺depolarization. Because of the direct association between the basal and GPCR-stimulated IAS tone and RhoA/ROCK activation, we speculate that this pathway in the rescue effects of BDNF. Conversely, our data suggest that aging-associated increased NANC relaxation is reversed by decreased release of NO and decrease in the sensitivity of the released inhibitory neurotransmitter. In summary, BDNF rescue of AAID involves RhoA/ROCK and inhibitory neurotransmission. These data have direct implications for the role of BDNF in the pathophysiology and therapeutic targeting of aging-associated rectoanal motility disorders.NEW & NOTEWORTHY These studies demonstrate that brain-derived neurotropic factor (BDNF) rescues the aging-associated internal anal sphincter (IAS) dysfunction, characterized by a decrease in IAS tone, and increase in non-adrenergic noncholinergic relaxation. We determined the effects of BDNF on the basal and GPCR (TXA₂ and ANG II)-stimulated IAS tone, and on NANC relaxation, before and after TrkB inhibitor K252a. BDNF may have an important role in the pathophysiology and therapeutic targeting of certain rectoanal motility disorders.

BDNF augments rat internal anal sphincter smooth muscle tone via RhoA/ROCK signaling and nonadrenergic noncholinergic relaxation via increased NO release

Presently, there are no studies examining the neuromodulatory effects of brain-derived neurotropic factor (BDNF) on the basal internal anal sphincter (IAS) tone and nonadrenergic noncholinergic (NANC) relaxation. To examine this, we determined the neuromuscular effects of BDNF on basal IAS smooth muscle tone and the smooth muscle cells (SMCs) and the effects of NANC nerve stimulation before and after high-affinity receptor tyrosine kinase receptor B (TrkB) antagonist K252a. We also investigated the mechanisms underlying BDNF-augmented increase in the IAS tone and NANC relaxation. We found that BDNF-increased IAS tone and SMC contractility were TTX resistant and attenuated by K252a. TrkB-specific agonist 7,8-dihydroxyflavone, similar to BDNF, also produced a concentration-dependent increase in the basal tone, whereas TrkB inhibitors K252a and ANA-12 produced a decrease in the tone. In addition, BDNF produced leftward shifts in the concentration-response curves with U46619 and ANG II (but not with bethanechol and K⁺ depolarization), and these shifts were reversed by K252a. Effects of Y27632 and Western blot data indicated that the BDNF-induced increase in IAS tone was mediated via RhoA/ROCK. BDNF-augmented NANC relaxation by electrical field stimulation was found to be mediated via the nitric oxide (NO)/soluble guanylate cyclase (sGC) pathway rather than via increased sensitivity to NO. In conclusion, the net effect of BDNF was that it caused an increase in the basal IAS tone via RhoA/ROCK signaling. BDNF also augmented NANC relaxation via NO/sGC. These findings may have relevance to the role of BDNF in the pathophysiology and therapeutic targeting of the IAS-associated rectoanal motility disorders.NEW & NOTEWORTHY These studies for the first time to our knowledge demonstrate that increased levels of brain-derived neurotrophic factor (BDNF; conceivably released from smooth muscle cells and/or the enteric neurons) has two major effects. First, BDNF augments the internal anal sphincter (IAS) tone via tyrosine kinase receptor B/thromboxane A2-receptor, angiotensin II receptor type 1/RhoA/ROCK signaling; and second, it increases nonadrenergic noncholinergic relaxation via nitric oxide/soluble guanylate cyclase. These studies may have relevance in therapeutic targeting in the anorectal motility disorders associated with the IAS.

Upregulation of β1-adrenoceptors is involved in the formation of gastric dysmotility in the 6-hydroxydopamine rat model of Parkinson's disease

Gastrointestinal dysmotility is one of the nonmotor symptoms of Parkinson's disease (PD). Gastroparesis and upregulated β-adrenoceptors (β-ARs) have been reported in rats with bilateral microinjection of 6-hydroxydopamine (6-OHDA) in the substantia nigra, but the underlying mechanism is unclear. The aim of the current study is to investigate the role of β-ARs in gastroparesis in 6-OHDA rats. Gastric motility was studied through strain gauge measurement. Immunofluorescence, real-time reverse transcription-polymerase chain reaction and Western blotting were performed to examine the expression of β-ARs. Norepinephrine (NE) inhibited gastric motility in a dose-dependent fashion in both control and 6-OHDA rats, but much stronger adrenergic reactivity was observed in the 6-OHDA rats. The inhibition of gastric motility by NE in both control and 6-OHDA rats was not affected by tetrodotoxin, a neural sodium channel blocker. Blocking β1-AR or β2-AR did not affect the inhibition of strip contraction by NE in control rats, but β1-AR blockage obviously enhanced the half maximal inhibitory concentration value of NE in 6-OHDA rats. Selective inhibition of β3-AR blocked the effect of NE significantly in both control and 6-OHDA rats. The protein expression of β1-AR, but not β2-AR and β3-AR in gastric muscularis externa was increased significantly in 6-OHDA rats. In conclusion, β3-AR involves the regulation of gastric motility in control rats, whereas the upregulation of β1-AR is responsible for enhanced NE reactivity in 6-OHDA rats and therefore is involved in the formation of gastroparesis. The effect of both β1-AR and β3-AR on gastric motility is independent of the enteric nervous system.

Internal anal sphincter parasympathetic-nitrergic and sympathetic-adrenergic innervation: a 3-dimensional morphological and functional analysis

BACKGROUND

Little detailed information is available concerning morphological and functional autonomic nerve supply to the internal anal sphincter. However, denervation of the sphincter potentially affects anal function after rectal surgery for cancer.

OBJECTIVE

The aim of this study was to identify the location and type (nitrergic, adrenergic, and cholinergic) of nerve fibers in the internal anal sphincter and to provide a 3-dimensional representation of their structural relationship in the human fetus.

MATERIALS AND METHODS

serial transverse sections were obtained from 14 human fetuses (7 male, 7 female, 15-31 weeks of gestation) and were studied histologically and immunohistochemically; digitized serial sections were used to construct a 3-dimensional representation of the pelvis.

MAIN OUTCOMES MEASURES

The location and type of internal anal sphincter nerves were assessed qualitatively.

RESULTS

Posteroinferior fibers originating from the inferior hypogastric plexus posteroinferior angle projected to the posterolateral and posterior rectal wall and internal anal sphincter, forming the inferior rectal plexus. The inferior rectal plexus contained vesicular acetylcholine transporter-positive (cholinergic), tyrosine hydroxylase-positive (adrenergic/sympathetic), and neural nitric oxide synthase-positive (nitrergic) fibers. The intrasphincteric vesicular acetylcholine transporter-positive fibers included both neural nitric oxide synthase-negative fibers and neural nitric oxide synthase-positive fibers (nitrergic-parasympathetic).

LIMITATIONS

The study focused on topographic and functional anatomy, so that quantitative data were not obtained. A small number of fetal specimens were available.

CONCLUSIONS

We report the precise location and distribution of the autonomic neural supply to the internal anal sphincter. This description contributes to the understanding of neurogenic postoperative sphincteric dysfunction. Three-dimensional cartography of pelvic-perineal neurotransmitters provides an anatomical and physiological basis for the selection and development of pharmacological agents to be used in the treatment of primary or postoperative continence and evacuation disorders.

Relaxation by beta 3-adrenoceptor agonists of the isolated human internal anal sphincter

AIMS

In this study, responses of beta(3)-adrenoceptor agonists were examined on human isolated internal anal sphincter (IAS) in order to explore their relaxant effects on hypertonicity of IAS.

MAIN METHODS

The relaxant efficacy (E(max)) and potency (-logIC(50)) of BRL37344 and SR58611A, beta(3)-adrenoceptor agonists, were examined in contracted IAS muscle strips. The presence of beta(3)-adrenoceptors, and changes in intracellular calcium and cyclic nucleotide levels in IAS muscle were tested by Western blotting, epifluorescence microscopy and enzyme immunoassay, respectively.

KEY FINDINGS

BRL37344 and SR58611A relaxed contracted IAS muscle (E(max)=27+/-3% and 35+/-3%; -logIC(50)=6.26+/-0.24 and 4.87+/-0.13; respectively). These relaxant responses were blocked by SR59230A, a selective beta(3)-antagonist but not by beta(1)/beta(2)-selective antagonists, neuronal inhibitor or inhibition of nitric oxide synthase. The E(max) of beta(3)-agonists was similar to that of beta(2)-selective agonists but smaller than that of isoprenaline (nonselective agonist) or beta(1)-selective agonists. BRL37344 (100 microM) increased cAMP (1.5-fold) without cGMP change, and depressed intracellular calcium signal. beta(3)-Adrenoceptor expression was smaller than that of beta(1)- and beta(2)-adrenoceptors.

SIGNIFICANCE

This is the first study demonstrating the presence of beta(3)-adrenoceptor in human IAS muscle and beta(3)-mediated relaxation of augmented sphincter tone. However, direct beta(3)-relaxation appears smaller than that obtained for nonselective agonists which may limit their potential use in the treatment of anorectal hypertonicity disorders.

Pharmacology of the internal anal sphincter and its relevance to faecal incontinence

1 The internal anal sphincter (IAS) has a spontaneous tone and is the main contributor to the maintenance of faecal continence. The spontaneous resting tone exhibited by the sphincter can be modified by neurotransmitters from the autonomic and enteric nervous systems. 2 In this review, the influence of the sympathetic and parasympathetic nervous systems on IAS tone are discussed and the putative roles of nitric oxide, carbon monoxide, vasoactive intestinal peptide and adenosine triphosphate in non-adrenergic non-cholinergic transmission are considered. 3 Faecal incontinence is a common condition that places a heavy financial burden on the health service and severely affects patients' quality of life. Resting anal pressure is reduced in patients with faecal incontinence and agents that increase sphincter tone tend to relieve symptoms. The results of clinical studies of the use of phenylephrine to treat faecal incontinence are reviewed. 4 It is concluded that the IAS is a potential target for drug development for the treatment of faecal incontinence.

Bioengineered internal anal sphincter derived from isolated human internal anal sphincter smooth muscle cells

BACKGROUND & AIMS

The internal anal sphincter (IAS) is a specialized circular smooth muscle that maintains rectoanal continence. In vitro models are needed to study the pathophysiology of human IAS disorders. We bioengineered sphincteric rings from human IAS smooth muscle cells (SMC) and investigated their response to cholinergic stimulation as well as investigated whether protein kinase C (PKC) and Rho kinase signaling pathways remain functional.

METHODS

3-Dimensional bioengineered ring (3DBR) model of the human IAS was constructed from isolated human IAS SMC obtained from surgery. Contractile properties and force generation in response to acetylcholine, PKC inhibitor calphostin-C, Rho/ROCK inhibitor Y-27632, permeable Rho/ROCK inhibitor c3-exoenzyme, and PKC activator PdBU was measured.

RESULTS

The human IAS 3DBR has the essential characteristics of physiologically functional IAS; it generated a spontaneous myogenic basal tone, and the constructs were able to relax in response to relaxants and contract in response to contractile agents. The constructs generated dose-dependent force in response to acetylcholine. Basal tone was significantly reduced by calphostin-C but not with Y-27632. Acetylcholine-induced force generation was also significantly reduced by calphostin-C but not with Y-27632. PdBU generated force that was equal in magnitude to acetylcholine. Thus, calphostin-C inhibited PdBU-induced force generation, whereas Y-27632 and c3 exoenzyme did not.

CONCLUSIONS

These data indicate that basal tone and acetylcholine-induced force generation depend on signaling through the PKC pathway in human IAS; PKC-mediated force generation is independent of the Rho/ROCK pathway. This human IAS 3DBR model can be used to study the pathophysiology associated with IAS malfunctions.

Diabetes mellitus and anal sphincter pressures: an experimental model in rats

PURPOSE

Symptoms of the gastrointestinal tract, frequent in patients with diabetes mellitus, which may be related to an increase in the production of free radicals, include alterations in the function of the sphincter anal musculature. Such alterations are characterized by a decrease of muscular tone associated with different degrees of fecal incontinence. This study was performed to show the alterations in the anal sphincter pressures of diabetic rats and to evaluate the role of nitric oxide and oxidative stress in this situation.

METHODS

Male Wistar rats weighing 250 to 400 g were used. The animals were divided in two groups: control and diabetic. Diabetes was induced through intraperitoneal injection of streptozotocin and the anal pressures were gauged by anorectal manometry. Nitric oxide was evaluated through measures of nitrites and nitrates, and oxidative stress through the technique of chemoluminescence.

RESULTS

There was a significant decrease in the sphincter anal pressure of diabetic animals 60 days after induction (P < 0.05). This pressure returned to basal values after administration of a nitric oxide synthase antagonist. The levels of nitrites and nitrates as well as of lipoperoxidation were significantly increased in the diabetic compared with the control group (P < 0.05).

CONCLUSIONS

In this study, hyperglycemia of diabetes mellitus caused an increase in the oxidative stress. Apparently the elevation of nitric oxide levels was one of the responsible factors for the decrease of anal sphincter pressures.

Tools to study beta3-adrenoceptors

Beta(3)-adrenoceptors mediate some of the effects of catecholamines on tissues such as blood vessels or the urinary bladder and are putative targets for the treatment of diseases such as the overactive bladder syndrome. Progress in the understanding of the presence, function, and regulation of beta(3)-adrenoceptors has been hampered by a lack of highly specific tools. "Classical" beta(3)-adrenoceptor agonists such as BRL 37,344 [(R*, R*)-(+/-)-4[2-[(3-chlorophenyl)-2-hydroxyethyl) amino] propyl] phenoxyacetic acid] and CGP 12,177 [(+/-)-4-(3-t-butylamino-2-hydroxypropoxy)benzimidazol-2-one] are only partial agonists in many settings, have limited selectivity over other beta-adrenoceptor subtypes, and may additionally act on receptors other than beta-adrenoceptors. More efficacious and more selective agonists have been reported and, in some cases, are in clinical development but are not widely available for experimental studies. The widely used antagonist SR 59,230 [3-(2-ethylphenoxy)-1-[(1,S)-1,2,3,4-tetrahydronapth-1-ylamino]-2S-2-propanoloxalate] is not selective for beta(3)-adrenoceptors, at least in humans, and may actually be a partial agonist. Radioligands, which are suitable either for the selective labeling of beta(3)-adrenoceptors or for the nonselective labeling of all beta-adrenoceptor subtypes, are also missing. beta(3)- and beta(1)/beta(2) double knockout mice have been reported, but their usefulness for extrapolations in humans is questionable based upon major differences between humans and rodents with regard to the ligand recognition and expression profiles of beta(3)-adrenoceptors. While the common availability of more selective agonists and antagonists at the beta(3)-adrenoceptor is urgently awaited, the limitations of the currently available tools need to be considered in studies of beta(3)-adrenoceptor for the time being.

Ischemic preconditioning attenuates postischemic leukocyte--endothelial cell interactions: role of nitric oxide and protein kinase C

BACKGROUND

Ischemic preconditioning (IPC) produces immediate tolerance to subsequent prolonged ischemia/reperfusion (I/R), although the underlying mechanism remains unknown. The purpose of this study was to examine the role of nitric oxide (NO) and protein kinase C (PKC) in IPC-attenuated post ischemic leukocyte-endothelium interactions.

METHODS AND RESULTS

Male Sprague-Dawley rats were randomized (n=8 per group) into 5 groups: sham-operated control group, IPC group, I/R group (4 h of pubic epigastric artery ischemia followed by 2 h of reperfusion), IPC+I/R group (30 min of ischemia followed by 30 min of reperfusion before I/R), and chelerythrine (PKC inhibitor)+IPC+I/R group. Intravital microscopy was used to observe leukocyte-endothelium interaction and to quantify functional capillaries in rat cremaster muscle flaps. The mRNA expressions of neuronal (n) NO synthase (NOS), inducible (i) NOS, and endothelial (e) NOS were determined by reverse transcription-polymerase chain reaction. The results showed that besides increasing functional capillary density, IPC also prevents I/R-induced increases in leukocyte rolling, adhesion, and migration. In the chelerythrine+IPC+I/R group, the IPC protective action was inhibited by the addition of chelerythrine. It was also observed that IPC upregulated nNOS, iNOS, and eNOS mRNA in I/R injured tissue, but this effect was not blocked by chelerythrine. Furthermore, specifically pretreated nNOS and iNOS inhibitors, along with a nonselective NOS inhibitor, were used in the IPC+I/R group to examine their possible antagonistic effects on leukocyte-endothelium interactions. Inhibition of the nNOS and iNOS activities did not block the beneficial effects of IPC. In contrast, pretreatment with the nonselective NOS inhibitor (NG-nitro-L-arginine methylester) in the IPC+I/R group almost completely blocked the protective effect of IPC.

CONCLUSIONS

Both NOS and PKC play a protective role during IPC, but probably in distinct ways. Furthermore, the results also indicate that eNOS, but not nNOS nor iNOS, is the key mediator of IPC-attenuated I/R-induced microcirculatory disturbance.

Mechanism of decreased response of jejunal smooth muscle to isoproterenol in aged rats

AIM: To investigate the response of jejunal smooth muscles (JSMs) to nonselective b-adrenoceptor agonist (Isoprot-erenol, Iso) in rats with different ages and its possible mechanism.

METHODS: The nonselective b-adrenoceptor agonist Iso and nonselective nitric oxide synthase (NOS) inhibitor Nw-nitro -L-arginine (L-NNA) were perfused into the baths of rat JSMs. Muscle force transducer (MFT) was used to measure the tension of JSMs.

RESULTS: The contractive frequency of JSMs was markedly lower in 20-mo rats than that in 3-mo rats (27.0±2.5 vs30±3.1, P<0.05). After perfusion with Iso (Final concentration: 1×10^-9-1×10^-6 mol/L), the variation rates of contractive amplitude of JSMs in 20-mo rats were lower than those in 3-mo rats (3×10^-8 mol/L: -41.4%, P<0.05; 1×10^-7 mol/L: -34.6%, P<0.05; 3×10^-7 mol/L: -25.0%, P<0.05; 1×10^-6 mol/L: -19.5%, P<0.05). After the administration of L-NNA, the relaxation responses of JSMs decreased in 3-mo rats, while there was no obvious variation in 20-mo rats. Before the administration of L-NNA, the pD₂ of b-adrenoceptor decreased in 20-mo rats as compared with that in 3-mo rats (6.52±0.54 vs 6.98±0.30, P<0.05).

CONCLUSION: The response of JSMs to Iso in aged rats decreases, and the mechanism may be related to the decreased effect of NOS or nitric oxide.

Development of a three-dimensional physiological model of the internal anal sphincter bioengineered in vitro from isolated smooth muscle cells

Fecal incontinence affects people of all ages and social backgrounds and can have devastating psychological and economic consequences. This disorder is largely attributed to decreased mechanical efficiency of the internal anal sphincter (IAS), yet little is known about the pathophysiological mechanisms responsible for the malfunction of sphincteric smooth muscle at the cellular level. The object of this study was to develop a three-dimensional (3-D) physiological model of the IAS bioengineered in vitro from isolated smooth muscle cells. Smooth muscle cells isolated from the IAS of rabbits were seeded in culture on top of a loose fibrin gel, where they migrated and self-assembled in circumferential alignment. As the cells proliferated, the fibrin gel contracted around a 5-mm-diameter SYLGARD mold, resulting in a 3-D cylindrical ring of sphincteric tissue. We found that 1) the bioengineered IAS rings generated a spontaneous basal tone, 2) stimulation with 8-bromo-cAMP (8-Br-cAMP) caused a sustained decrease in the basal tone (relaxation) that was calcium-independent, 3) upon stimulation with ACh, bioengineered IAS rings showed a calcium- and concentration-dependent peak contraction at 30 s that was sustained for 4 min, 4) addition of 8-Br-cAMP induced rapid relaxation of ACh-induced contraction and force generation of IAS rings, and 5) bioengineered sphincter rings show striking functional differences when compared with bioengineered rings made from isolated colonic smooth muscle cells. This is the first report of a 3-D in vitro model of a gastrointestinal smooth muscle IAS. Bioengineered IAS rings demonstrate physiological functionality and may be used in the elucidation of the mechanisms causing sphincter malfunction.

Neurochemistry within ventrolateral medulla and cardiovascular effects during static exercise following eNOS antagonism

Nitric oxide synthase (NOS), necessary for the production of nitric oxide from l-arginine, exists in three isoforms: neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS). We have previously demonstrated that blockade of nNOS within the rostral (RVLM) and caudal ventrolateral medulla (CVLM) differentially modulated cardiovascular responses to static exercise [Ishide, T., Nauli, S.M., Maher, T.J., Ally, A., 2003. Cardiovascular responses and neurotransmitter changes following blockade of nNOS within the ventrolateral medulla during static muscle contraction. Brain Res. 977, 80-89]. In this study, we have examined the effects of bilaterally microdialyzing a specific eNOS antagonist into the RVLM and CVLM on cardiovascular responses and glutamatergic/GABAergic neurotransmission during the exercise pressor reflex in anesthetized rats. Bilateral microdialysis of a selective eNOS antagonist, l-N(5)-(1-iminoethyl)ornithine (l-NIO; 10.0 microM) into the RVLM potentiated cardiovascular responses and increased extracellular fluid glutamate levels during a static muscle contraction. At the same time, levels of GABA within the RVLM were decreased. The cardiovascular responses and neurochemical changes to muscle contraction recovered after discontinuation of the drug. In contrast, bilateral application of the eNOS antagonist into the CVLM attenuated cardiovascular responses and glutamate concentrations during a static muscle contraction, but augmented levels of GABA. These results demonstrate that eNOS within the ventrolateral medulla plays an important role in modulating glutamate/GABAergic neurotransmission, that in turn regulates the exercise pressor reflex. The present study provides further evidence of simultaneous sympathoexcitatory and sympathoinhibitory effects of nitric oxide within the RVLM and CVLM involved in the neural control of circulation during static exercise.

The effects of N(omega)-propyl-L-arginine on reperfusion injury of skeletal muscle

N(omega)-Propyl-L-arginine (NPA) is reported to be a highly selective inhibitor of neuronal nitric oxide synthase (nNOS). This in vivo study observed its role in ischemia/reperfusion (I/R) injury in rat skeletal muscle. Our results showed that NPA infusion significantly increased vessel diameters and blood flow in reperfused cremaster muscle, and slightly increased contractile function in reperfused extensor digitorum longus (EDL) muscle. In addition, NPA treatment slightly increased I/R-mediated downregulation of nNOS and eNOS mRNA and protein levels. Although NPA showed a beneficial role in I/R injury, our in vivo data do not support NPA as a selective nNOS inhibitor. Also, our data do not provide any insight into the mechanism of NPA. Thus, the in vivo mechanism of action of NPA needs to be further identified, and the role of nNOS in skeletal muscle I/R still remains to be determined.