Beta3 adrenergic stimulation inhibits the opossum lower esophageal sphincter

The internal anal sphincter: regulation of smooth muscle tone and relaxation

Basal tone in the internal anal sphincter (IAS) is primarily myogenic. Neurohumoral substances like angiotensin II may partially provide external signal for the basal tone in the IAS. The sphincteric relaxation on the contrary is neurogenic by activation of non-adrenergic non-cholinergic (NANC) nerves that release nitric oxide (NO), vasoactive intestinal polypeptide (VIP) and perhaps carbon monoxide. Because of the presence of spontaneous tone, the IAS offers an excellent model to investigate the nature of the inhibitory neurotransmission for NANC relaxation. Work from different laboratories in different species concludes that NO is the major contributor in the NANC relaxation. This may invoke the role of other inhibitory neurotransmitters such as VIP, working partly via NO. An understanding of the basic regulation of basal tone in the IAS and nature of inhibitory neurotransmission are critical in the pathophysiology and therapeutic potentials in the anorectal motility disorders.

Role of adenylate and guanylate cyclases in beta1-, beta2-, and beta3-adrenoceptor-mediated relaxation of internal anal sphincter smooth muscle

The purpose of the present study was to ascertain the role of adenylate (AC) versus guanylate cyclase (GC) signaling pathways in the internal anal sphincter (IAS) smooth muscle relaxation by beta(1)-, beta(2)-, and beta(3)-adrenoceptor (AR) activation by xamoterol, procaterol, and disodium 5-[(2R)-2-(3-chlorophenyl)-2-hydroxy-ethyl]amino)propyl]-1,3-benzodioxole-2,2-dicarboxylate (CL 316243), respectively. The above-mentioned agonists produced concentration-dependent relaxation of the smooth muscle strips. Both the selective G(i/o)alpha and G(s)alpha antagonists 8,8'-(carbonylbis(imino-3,1-phenylene))bis-(1,3,5-naphthalene trisulfonic acid) (NF 023) and 4,4',4",4"'-(carbonylbis(imino-5,1,3-benzenetriylbis(carbonylimino)))tetrakis-benzene-1,3-disulfonic acid (NF 449), respectively, inhibited the relaxation induced by procaterol. However, only NF 023 inhibited the relaxation induced by xamoterol and CL 316243. 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one, a soluble GC inhibitor, significantly inhibited the relaxation induced by different agonists. In contrast, the selective AC inhibitor [9-(tetrahydro-2'-furyl)adenine] (SQ 22536) inhibited only the relaxation induced by procaterol. (9R,10S,12S)-2,3,9,10,11,12-Hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg: 3',2',1'-kl]pyrrolo[3,4-l][1,6]benzodiazocine-10-carboxylic acid, hexyl ester (KT 5720), a cAMP-dependent protein kinase inhibitor, attenuated the relaxation by procaterol, whereas (9S,10R,12R)-2,3,9,10,11,12, hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9.12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-I][1,6]benzodiazocine-10-carboxylic acid methyl ester (KT 5823), a selective cGMP-dependent protein kinase (PKG) inhibitor, attenuated the relaxation induced by xamoterol and CL 316243. Xamoterol produced significant increase in cGMP levels, whereas only procaterol enhanced the cAMP levels. Western blot analysis confirmed the presence of beta(1), beta(2), and beta(3)-AR subtypes in the IAS. In summary, beta(2)-AR activates both G(s)alpha and G(i/o)alpha-protein subunits and induces relaxation in the rat IAS via both cAMP/cGMP pathways. In contrast, the beta(1)/beta(3)-ARs activation causes the smooth muscle relaxation via G(i/o)alpha-protein subunit/GC/GMP/PKG pathway. These studies are important for the understanding of intracellular mechanisms underlying IAS smooth muscle relaxation and in turn the pathophysiology of certain anorectal motility disorders.

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

Effects of activation of beta3-adrenoceptor (beta3-AR) have not been determined in the spontaneously tonic smooth muscle of the internal anal sphincter (IAS). The effects of disodium (R,R)-5-[2-[2-3-chlorophenyl)-2-hydroxyethyl]-amino]propyl]-1,3-benzodioxole-2,2-dicarboxylate (CL 316243), a selective beta3-AR agonist, on the basal smooth muscle tone and direct release of nitric oxide (NO) by circular smooth muscle strips of the opossum IAS were determined. We also examined the presence of endothelial nitric oxide synthase (eNOS) protein by Western blot studies. CL 316243 produced a concentration-dependent relaxation of the smooth muscle that remained unmodified by different neurohumoral antagonists. The smooth muscle relaxation by CL 316243 was selectively antagonized by L 748337, a beta3-AR antagonist. Such relaxation was several times longer than by isoproterenol. The effect of CL 316243 was significantly attenuated by a nonselective NOS inhibitor N(omega)-nitro-l-arginine (l-NNA) and by putative inhibitor of eNOS l-N5-(1-iminoethyl)-ornithine dihydrochloride (l-NIO). Inhibitors of iNOS [N-(3-aminomethyl)benzyl acetamide 2HCl] and nNOS [1-[2-(trifluoromethylphenyl)imidazole]] had no effect on this relaxation. Relaxation of the IAS smooth muscle induced by CL 316243 was accompanied by an increased release of NO; this was attenuated by l-NNA and l-NIO. In addition, Western blot studies revealed the presence of eNOS in the circular smooth muscle of the IAS. These data demonstrate potent and protracted IAS smooth muscle relaxation by beta3-AR activation, which is partly transduced via NOS, possibly smooth muscle eNOS. Multiple signal-transduction pathways including NOS activation may explain the characteristic IAS relaxation by beta3-AR activation. The studies may have therapeutic implications in anorectal motility disorders.

Achalasia: a comprehensive review

Achalasia is a rare neurologic deficit of the esophagus, producing a syndrome of impaired relaxation of the lower esophageal sphincter and decreased motility of the esophageal body for which the cause is unknown. The resultant chronic esophageal stasis produces discomforting symptoms that can be managed with medication, chemical paralysis of the lower esophageal sphincter, mechanical dilation, or surgical esophagomyotomy. Chemical paralysis by injection of the esophagus with botulinum toxin and dilation with an inflatable balloon offers good short-term relief of symptoms; however, the best long-term results are produced by surgery, and advancing minimally invasive techniques continually reduce the morbidity of these operations. The type of surgical procedure, the necessity for fundoplication, and the order of treatment continue to be unresolved issues, but prospective evaluation with objective followup should allow us to provide the optimal treatment regimen to our patients.

Functional and molecular characterization of beta-adrenoceptors in the internal anal sphincter

The purpose of the present study was to characterize different beta-adrenoceptors (beta-ARs) and determine their role in the spontaneously tonic smooth muscle of the internal anal sphincter (IAS). The beta-AR subtypes in the opossum IAS were investigated by functional in vitro, radioligand binding, Western blot, and reverse transcription-polymerase chain reaction (RT-PCR) studies. ZD 7114 [(S)-4-[2-hydroxy-3-phenoxypropylaminoethoxy]-N-(2-methoxyethyl)phenoxyacetamide], a selective beta(3)-AR agonist, caused a potent and concentration-dependent relaxation of the IAS smooth muscle that was antagonized by the beta(3)-AR antagonist SR 59230A [1-(2-ethylphenoxy)-3-[[(1S)-1,2,3,4-tetrahydro-1-naphthalenyl]amino]-(2S)-2-propanol hydrochloride]. Conversely, the IAS smooth muscle relaxation caused by beta(1)- and beta(2)-AR agonists (xamoterol and procaterol, respectively) was selectively antagonized by their respective antagonists CGP 20712 [(+/-)-2-hydroxy-5-[2-[[2-hydroxy-3-[4-[1-methyl-4-(trifluoromethyl)-1H-imidazol-2-yl]phenoxy]propyl]amino]ethoxy]-benzamide methanesulfonate salt] and ICI 118551. Saturation binding of [(125)I]iodocyanopindolol to beta-AR subtypes revealed the presence of a high-affinity site (K(d1) = 96.4 +/- 8.7 pM; B(max1) = 12.5 +/- 0.6 fmol/mg protein) and a low-affinity site (K(d2) = 1.96 +/- 1.7 nM; B(max2) = 58.7 +/- 4.3 fmol/mg protein). Competition binding with selective beta-AR antagonists revealed that the high-affinity site correspond to beta(1)/beta(2)-AR and the low affinity site to beta(3)-AR. Receptor binding data suggest the predominant presence of beta(3)-AR over beta(1)/beta(2)-AR. Western blot studies identified beta(1)-, beta(2)-, and beta(3)-AR subtypes. The presence of beta(1)-, beta(2)-, and beta(3)-ARs was further demonstrated by mRNA analysis using RT-PCR. The studies demonstrate a comprehensive functional and molecular characterization of beta(1)-, beta(2)-, and beta(3)-ARs in IAS smooth muscle. These studies may have important implications in anorectal and other gastrointestinal motility disorders.