Pharmacological analysis of atypical beta-adrenoceptors in the guinea pig gastric fundus using the beta(3)-adrenoceptor antagonist bupranolol

The role of l-cysteine/Hydrogen sulfide pathway on β3-Adrenoceptor- induced relaxation in mouse gastric fundus

In this study, we investigated the possible role of the l-cysteine/hydrogen sulfide pathway in β₃-adrenoceptors-mediated relaxation in isolated mouse gastric fundus tissue. l-cysteine (endogenous H₂S; 10^-6-10^-2 M), sodium hydrogen sulfide (NaHS; exogenous H₂S; 10^-6-10^-3 M), selective β₃-adrenoceptors agonist BRL 37344 (10^-9-10^-4 M) and non-selective β-adrenoceptor agonist isoprenaline (10^-9-10^-4 M) produced concentration-dependent relaxation in mouse gastric fundus. The non-selective β-adrenoceptors antagonist propranolol (10^-6 M) inhibited the relaxant response to isoprenaline but not to BRL 37344. On the other hand, the selective β₃-adrenoceptors antagonist SR 59230A (10^-5 M) inhibited the relaxant responses to BRL 37344. In addition, cystathionine-gamma-lyase (CSE) inhibitor D,L-propargylglycine (PAG, 10^-2 M), cystathionine-beta-synthase inhibitor (CBS) aminooxyacetic acid (AOAA, 10^-2 M), and the combination of these inhibitors significantly reduced the relaxant responses induced by l-cysteine and BRL 37344. Pre-incubation of gastric fundal strips with propranolol (10^-6 M) and SR 59230A (10^-5 M) did not affect relaxations to l-cysteine and NaHS. Also, the existence of CSE, CBS, 3-mercaptopurivate sulfur transferase (3-MST) enzymes and β₃-adrenoceptors were detected in gastric fundal tissue. Furthermore, basal H₂S release was detected in the measurements. H₂S level increased in the presence of l-cysteine, NaHS, and BRL 37344. The increase in H₂S level by l-cysteine and BRL 37344 decreased significantly with PAG and AOAA enzyme inhibitors. These results suggest that endogenous H₂S is synthesized from l-cysteine at least by CBS and CSE enzymes. Also, β₃-adrenoceptors are found in the mouse stomach fundus and mediate BRL 37344-induced relaxations, and l-cysteine/hydrogen sulfide pathway plays a partial role in β₃-adrenoceptors-mediated relaxation in mouse gastric fundus tissue.

Pharmacological properties of β-adrenoceptors mediating rat superior mesenteric artery relaxation and the effects of chemical sympathetic denervation

AIMS

β-Adrenoceptors (β-ADRs) mediating the relaxation of rat superior mesenteric arteries (SMAs) were pharmacologically identified, and the effects of chemical sympathetic denervation on β-ADR-mediated relaxation were examined.

MAIN METHODS

The tension changes of endothelium-denuded SMAs were isometrically recorded and the mRNA of endothelium-denuded SMA β-ADR was detected using RT-PCR.

KEY FINDINGS

In endothelium-denuded SMAs contracted with ≥10^-7 M phenylephrine (an α₁-ADR agonist), isoprenaline (a β-ADR agonist)-induced relaxation was competitively inhibited by 3 × 10^-9-10^-8 M propranolol (a β_1,2-ADR antagonist), but not further affected by ≥10^-8 M propranolol. Although isoprenaline-induced relaxation was not affected by ICI-118,551 (10^-9-10^-8 M; a β₂-ADR antagonist), it was competitively inhibited by atenolol (10^-7-3 × 10^-7 M; a β₁-ADR antagonist) in the presence of ICI-118,551. In the presence of 10^-7 M propranolol, isoprenaline- and CGP-12177A (a β₃-ADR partial agonist)-induced relaxation was competitively inhibited by high concentrations of bupranolol (a β_1,2,3-ADR antagonist), with pA₂ values of 6.49 and 5.76, respectively. We detected the mRNA of β₁- and β₃-ADRs in endothelium-denuded SMAs. Treatment with 6-hydroxydopamine (a catecholaminergic neurotoxin) reduced maximal isoprenaline-induced relaxation in the presence and absence of 10^-7 M propranolol, but not CGP-12177A-induced relaxation.

SIGNIFICANCE

Isoprenaline-induced relaxation of rat SMAs is mediated by β₁- and β₃-ADRs. β-ADR-mediated relaxation of rat SMAs is shown to be attenuated by chemical sympathetic denervation. The differences in the effects of bupranolol and chemical sympathetic denervation on the responses to isoprenaline and CGP-12177A in rat SMAs might be explained by the possible presence of multiple β₃-ADRs with different pharmacological properties.

β-Adrenoceptor subtypes and cAMP role in adrenaline- and noradrenaline-induced relaxation in the rat thoracic aorta

Object We identified the β-adrenoceptor (β-AR) subtypes responsible for the relaxant responses to adrenaline (AD) and noradrenaline (NA) in the rat thoracic aorta and examined the role of cAMP which is involved in these relaxant responses. Methods The effects of β-AR antagonists or the adenylyl cyclase inhibitor SQ 22,536 on AD- and NA-induced relaxant responses in phenylephrine-induced contraction and increases in cAMP levels were examined in isolated, endothelium-denuded rat thoracic aorta segments. Results AD-induced relaxation was completely suppressed by propranolol (10⁻⁷ M) or by ICI-118,551 (10⁻⁸ M) plus atenolol (10⁻⁶ M), and was also very strongly inhibited by ICI-118,551 (10⁻⁸ M) alone. AD (10⁻⁵ M) increased tissue cAMP levels by approximately 1.9-fold compared with that in non-stimulated aortic tissue, but did not significantly increase cAMP levels in the presence of ICI-118,551 (10⁻⁸ M) or SQ 22,536 (10⁻⁴ M). AD-induced relaxation was strongly suppressed by SQ 22,536 (10⁻⁴ M). NA-induced relaxation was almost completely suppressed by atenolol (10⁻⁶ M) plus ICI-118,551 (10⁻⁸ M) although it was hardly affected by ICI-118,551 (10⁻⁸ M) alone. NA (10⁻⁵ M) increased tissue cAMP levels by approximately 2.2-fold compared with that in non-stimulated aortic tissue, but did not significantly increase cAMP levels in the presence of atenolol (10⁻⁶ M) or SQ 22,536 (10⁻⁴ M). NA-induced relaxation was strongly suppressed by SQ 22,536 (10⁻⁴ M). Conclusion In rat thoracic aorta, AD- and NA-induced relaxations, which are both strongly dependent on increased tissue cAMP levels, are mainly mediated through β₂- and β₁-adrenoceptors respectively.

Role of Beta-3-Adrenoceptor in Catecholamine-Induced Relaxations in Gastric Fundus from Control and Diabetic Rats

The contribution of beta-adrenoceptor subtypes to the catecholamine-mediated relaxations in gastric fundus from control and streptozotocin (STZ)-induced diabetic rats were investigated. Isolated organ bath studies and molecular techniques were used to characterize the beta-adrenoceptor subtypes mediating relaxation of rat gastric fundus. Isoprenaline-mediated relaxation was not significantly changed by nadolol (beta(1)-/beta(2)-adrenoceptor antagonist; 1 micromol/l) but only shifted to the right by SR59230A (3-(2-ethylphenoxy)-1-[[(1S)-1,2,3,4-tetrahydronaphth-1-yl]amino]-(2S)-2-propanol oxalate salt, 0.1-1 micromol/l), a selective beta(3)-adrenoceptor antagonist, in a competitive manner. Relaxant responses to noradrenaline were antagonized in a concentration-dependent manner by SR59230A (0.1-1 micromol/l), but not by metoprolol (selective beta(1)-adrenoceptor antagonist; 0.1-1 micromol/l) and ICI-118551 (1-[2,3-(dihydro-7-methyl-1Hinden- 4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol hydrochloride, selective beta(2)-adrenoceptor antagonist; 0.1-1 micromol/l). SR59230A (1 micromol/l) also caused a significant rightward shift in fenoterol-induced relaxation while ICI-118551 (1 micromol/l) did not have any effect. Selective beta(3)-adrenoceptor agonist, BRL37344 ([4-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]amino]propyl]phenoxy]acetic acid), caused biphasic relaxation which was not affected by nadolol (1 micromol/l). SR59230A (1 micromol/l) abolished only the first phase of BRL37344 response. beta(1)-, beta(2)- and beta(3)-adrenoceptor mRNA expressions have been detected in a similar intensity in gastric fundus from control rats. Experimental diabetes caused a significant decrease in E(max) and pD(2) values of isoprenaline and noradrenaline. Diabetes also reduced E(max) but not pD(2) value of the first component of BRL37344-induced relaxation response. The band intensity of mRNA transcript of beta(3)-adrenoceptor was reduced in diabetics while no alteration has been found for beta(1)- and beta(2)-adrenoceptor mRNA transcripts between groups. These results show that functional beta-adrenoceptor subtype involved in catecholamine-mediated relaxations is beta(3)-adrenoceptor, and its function and mRNA expression are decreased in diabetes.

New insights into beta-adrenoceptors in smooth muscle: distribution of receptor subtypes and molecular mechanisms triggering muscle relaxation

1. The beta-adrenoceptor is currently classified into beta(1), beta(2) and beta(3) subtypes and all three subtypes are expressed in smooth muscle. Each beta-adrenoceptor subtype exhibits tissue-specific distribution patterns, which may be a determinant controlling the mechanical functions of corresponding smooth muscle. Airway and uterine smooth muscles abundantly express the beta(2)-adrenoceptor, the physiological significance of which is established as a fundamental regulator of the mechanical activities of these muscles. Recent pharmacomechanical and molecular approaches have revealed roles for the beta(3)-adrenoceptor in the gastrointestinal tract and urinary bladder smooth muscle. 2. The beta-adrenoceptor is a G(s)-protein-coupled receptor and its activation elevates smooth muscle cAMP. A substantial role for a cAMP-dependent mechanism(s) is generally believed to be the key trigger for eliciting beta-adrenoceptor-mediated relaxation of smooth muscle. Downstream effectors activated via a cAMP-dependent mechanism(s) include plasma membrane K(+) channels, such as the large-conductance, Ca(2+)-activated K(+) (MaxiK) channel. 3. Beta-Adrenoceptor-mediated relaxant mechanisms also include cAMP-independent signalling pathways. This view is supported by numerous pharmacological and electrophysiological lines of evidence. In airway smooth muscle, direct activation of the MaxiK channel by G(s)alpha is a mechanism by which stimulation of beta(2)-adrenoceptors elicits muscle relaxation independently of the elevation of cAMP. 4. The cAMP-independent mechanism(s) is also substantial in beta(3)-adrenoceptor-mediated relaxation of gastrointestinal tract smooth muscle. However, in the case of the beta(3)-adrenoceptor, a delayed rectified K(+) channel rather than the MaxiK channel seems to mediate, in part, cAMP-independent relaxant mechanisms. 5. In the present article, we review the distribution of beta-adrenoceptor subtypes in smooth muscle tissues and discuss the molecular mechanisms by which each subtype elicits muscle relaxation, focusing on the roles of cAMP and plasma membrane K(+) channels.

Evidence for the primary role for 4-aminopyridine-sensitive K(v) channels in beta(3)-adrenoceptor-mediated, cyclic AMP-independent relaxations of guinea-pig gastrointestinal smooth muscles

Gastrointestinal smooth muscles exhibit relaxation in response to the stimulation of beta-adrenoceptors with catecholamines. Subtypes of beta-adrenoceptors which mediate catecholamine-elicited relaxations in gastrointestinal smooth muscles are predominantly atypical beta-adrenoceptors including beta(3)-adrenoceptors. Gastrointestinal smooth muscle relaxations mediated via beta(3)-adrenoceptors can occur independently of intracellular cyclic adenosine monophosphate (AMP) elevation. One of the mechanisms responsible for cyclic AMP-independent smooth muscle relaxation following activation of G(s) protein-coupled receptors could be activation of voltage-gated K(+) channels. In the present study, possible contribution of two types of K(+) (large-conductance, Ca(2+)-sensitive and voltage-gated K(+), BK(Ca); voltage-gated, K(v)) channels to beta(3)-adrenoceptor-mediated, cyclic AMP-independent relaxations was compared in gastric fundus and duodenum smooth muscles isolated from the guinea-pig. In these gastrointestinal smooth muscles, three catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline) and two beta(3)-adrenoceptor agonists ((R(*), R(*))-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid sodium (BRL37344) and (+/-)-[4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy] -1,3-dihydro-2H-benzimidazol-2-one] hydrochloride ((+/-)-CGP12177A)) elicited a concentration-dependent relaxation in the presence of beta(1)- and beta(2)-adrenoceptor antagonists. The relaxations were unaffected by an adenylyl cyclase inhibitor, SQ-22536 (100 microM), which indicates their characteristic of cyclic AMP-independency. On the other hand, the SQ-22536-resistant, beta(3)-adrenoceptor-mediated relaxant components were potently attenuated when the tone was raised using high-KCl (80 mM) or in the presence of a K(v) channel blocker, 4-aminopyridine (4-AP, 1-3 mM). Iberiotoxin (100 nM), a selective blocker of BK(Ca) channels which significantly contribute to cyclic AMP-independent vascular smooth muscle relaxations induced through activation of G(s) protein-coupled receptors, did not apparently show any inhibitory effects on SQ-22536-resistant, beta(3)-adrenoceptor-mediated relaxations in these gastrointestinal smooth muscles. The present results indicate that 4-AP-sensitive K(v) channels play a primary role in beta(3)-adrenoceptor-mediated, cyclic AMP-independent relaxations of guinea-pig gastrointestinal smooth muscles. In these smooth muscles, BK(Ca) channels seem to apparently contribute insignificantly to cyclic AMP-independent relaxations following stimulation of beta(3)-type of adrenoceptors.

The .BETA.3-Adrenoceptor-Mediated Relaxation Induced by Dopamine in Guinea Pig Taenia Caecum

The mechanisms of the beta-adrenoceptor-mediated relaxation induced by dopamine in guinea pig taenia caecum were examined. The relaxant response to dopamine was unaffected by propranolol (10(-8)-10(-5) M) or phentolamine (10(-8)-10(-5) M). Atenolol (3 x 10(-7)-3 x 10(-4) M), butoxamine (10(-7)-10(-4) M), prazosin (10(-8)-10(-5) M), yohimbine (10(-8)-10(-5) M), SCH 23390 (10(-8)-10(-5) M) and haloperidol (10(-8)-10(-5) M) had no effect on the potency of dopamine. The response to dopamine was antagonized in a concentration-dependent manner by bupranolol (3 x 10(-6)-3 x 10(-5) M), and Schild plot of the data revealed the pA2 value of 5.55 and the slope of the regression line was 1.13. These results suggest that the relaxant response to dopamine in the guinea pig taenia caecum is mainly mediated by the beta3-adrenoceptors.

Cyclic AMP-independent relaxation mediated by beta3-adrenoceptors on guinea pig gastrointestine

In this study, we investigated the signal transduction pathway involved in beta(3)-adrenoceptor-mediated relaxations of guinea pig gastric fundus and duodenum. In the presence of beta1- and beta2-adrenoceptor blockade, the potency (pD2 value) of catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline) and beta(3)-adrenoceptor agonists ((R*, R*)-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid sodium (BRL37344) and (+/-)-[4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one] hydrochloride ((+/-)-CGP12177A)) to induce relaxation was not affected by the adenylate cyclase inhibitor, 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ-22,536, 100 microM). Catecholamines induced an elevation of cyclic AMP and SQ-22,536 significantly abolished the responses of gastric fundus. However, cyclic AMP levels were unaltered by the beta3-adrenoceptor agonists in gastric fundus and by the five agonists in duodenum. Furthermore, the relaxant responses to catecholamines and to beta3-adrenoceptor agonists were unaffected by the cyclic AMP-dependent protein kinase inhibitor, N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide (H-89, 10 microM) in gastric fundus. These results suggest that beta3-adrenoceptor-induced relaxation is mediated through both cyclic AMP-dependent and cyclic AMP-independent pathways in gastric fundus and through a cyclic AMP-independent pathway in duodenum.

Partial agonistic activity of labetalol, the arylethanolamine, on beta 3-adrenoceptors in the guinea-pig gastric fundus

1. The agonistic and antagonistic effects of labetalol, the alpha1- and beta-adrenoceptor antagonist, were studied on beta3-adrenoceptors in the guinea-pig gastric fundus. 2. Labetalol caused a concentration-dependent relaxation with a pD2 value of 5.58 +/- 0.09 and an intrinsic activity of 0.64 +/- 0.06, which was not affected by pretreatment with both the selective beta1-adrenoceptor antagonist, (+/-)-atenolol (100 microM), and the selective beta2-adrenoceptor antagonist, (+/-)-butoxamine (100 microM). 3. However, the non-selective beta1-, beta2- and beta3-adrenoceptor antagonist, (+/-)-bupranolol (3-30 microM), shifted the concentration-response curve of labetalol to the right (pA2 value=5.97 +/- 0.08). 4. In the presence of (+/-)-atenolol (100 microM) and (+/-)-butoxamine (100 microM), relaxations to catecholamines [(-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline], to the selective beta3-adrenoceptor agonist, BRL37344, and to the non-conventional partial beta3-adrenoceptor agonist, (+/-)-CGP12177A, were weakly antagonized by labetalol (10 microM). 5. These results indicate that labetalol, the arylethanolamine, acts as a partial agonist on beta3-adrenoceptors in the guinea-pig gastric fundus.

Partial agonistic properties of (+/-)-pindolol at atypical beta-adrenoceptors in the guinea pig gastric fundus

(+/-)-Pindolol ([1-(1H-indol-4-yloxy)-3-[(1-methylethyl)- amino]-2-propanol)]) is a partial agonist at atypical beta-adrenoceptors in the guinea pig gastric fundus. (+/-)-Pindolol induced concentration-dependent relaxation in this tissue. However, the relaxant responses of (+/-)-pindolol were not antagonized by a combination of the selective beta(1)-adrenoceptor antagonist atenolol (10(-4) mol/l) and the selective beta(2)-adrenoceptor antagonist butoxamine (10(-4) mol/l). In the presence of both atenolol and butoxamine, the nonselective beta(1)-, beta(2)- and beta(3)-adrenoceptor antagonist (+/-)-bupranolol (10(-5)-10(-4) mol/l) caused a concentration-dependent rightward shift of the concentration-response curves for (+/-)-pindolol. Schild plot analyses of (+/-)-bupranolol against (+/-)-pindolol gave the pA(2) value of 5.46 +/- 0.03 and Schild slope was not significantly different from unity. Furthermore, (+/-)-pindolol (10(-5) mol/l) weakly but significantly antagonized the relaxant responses to catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline), a selective beta(3)-adrenoceptor agonist BRL37344 ((R*,R*)-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid sodium salt) and a nonconventional partial beta(3)-adrenoceptor agonist (+/-)-CGP12177A ([4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one] hydrochloride). These results suggest that (+/-)-pindolol acts as a partial agonist at atypical beta-adrenoceptors in the guinea pig gastric fundus.

Further characterization of β3-adrenoceptors in the guinea pig gastric fundus: stereoselectivity, β-adrenoceptor alkylation, and structure-activity relationship

The stereoselectivity of β3-adrenoceptors, the effect of a β-adrenoceptor alkylating agent, and the structure–activity relationship at β3-adrenoceptors were investigated on the guinea pig gastric fundus. Isomeric activity ratios ((+)/(–)) for isomers of isoprenaline and noradrenaline were 20.9-fold and 43.7-fold, respectively, and were less than those obtained for activation of β1- and β2-adrenoceptors in the guinea pig atria and trachea, respectively. The concentration–response curves to the catecholamines ((–)-isoprenaline, (–)-noradrenaline, and (–)-adrenaline), the selective β3-adrenoceptor agonist BRL37344 ((R*,R*)-(±)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid sodium), and the nonconventional partial β3-adrenoceptor agonist (±)-CGP12177A ((±)-[4-[3-[(1,1-dimethylethyl) amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one] hydrochloride) were resistant to blockade by (±)-pindobind (10 µM), the β-adrenoceptor alkylating agent. Furthermore, (±)-nadolol, which belongs to the aryloxy propanolamine class and has β1- and β2-adrenoceptor antagonistic characteristics, displays agonistic activity at β3-adrenoceptors. These results indicate that pharmacological characteristics of the β3-adrenoceptors of guinea pig gastric fundus differ from those of β1- and β2-adrenoceptors. (–)-Noradrenaline and (–)-adrenaline were more potent than dopamine and (–)-phenylephrine, respectively. In addition, dobutamine was 22-fold more potent than dopamine. These results suggest that the 4-hydroxyl group at the catechol ring and the β-hydroxyl group and the large moiety on the alkylamine chain characterized efficacy at β3-adrenoceptors.Key words: β3-adrenoceptor, stereoselectivity, β-adrenoceptor alkylating, structure–activity relationship, guinea pig gastric fundus.

Structure-activity relationship studies of (+/-)-terbutaline and (+/-)-fenoterol on beta3-adrenoceptors in the guinea pig gastric fundus

(+/-)-Terbutaline and (+/-)-fenoterol are both arylethanolamine analogs that have tertbutyl and aryliso-propyl substituents respectively at the a position on the nitrogen of the ethanolamine side chain. In the present study, we have investigated the structure-activity relationships of (+/-)-terbutaline and (+/-)-fenoterol as beta3-adrenoceptor agonists in the guinea pig gastric fundus. (+/-)-Terbutaline and (+/-)-fenoterol induced concentration-dependent relaxation of the precontracted gastric fundus with pD2 values of 4.45+/-0.10 and 5.90+/-0.09, and intrinsic activities of 1.00+/-0.03 and 0.99+/-0.01 respectively. The combination of the selective beta1-adrenoceptor antagonist (+/-)-atenolol (100 microM), and the selective beta2-adrenoceptor antagonist (+/-)-butoxamine (100 microM), produced a 2 and 6 fold rightward shift of the concentration-response curves for (+/-)-terbutaline and (+/-)-fenoterol respectively, without depressing the maximal responses. The order of potency of these agonists was (pD2 value): (+/-)-fenoterol (5.09+/-0.10) > (+/-)-terbutaline (4.13+/-0.08). In the presence of (+/-)-atenolol and (+/-)-butoxamine, however, the non-selective beta1, beta2- and beta3-adrenoceptor antagonist (+/-)-bupranolol caused a concentration-dependent rightward shift of the concentration-response curves for (+/-)-terbutaline and (+/-)-fenoterol. Schild plot analyses of the effects of (+/-)-bupranolol against these agonists gave pA2 values of 6.21+/-0.07 ((+/-)-terbutaline) and 6.37+/-0.06 ((+/-)-fenoterol) respectively, and the slopes of the Schild plot were not significantly different from unity (p>0.05). These results suggest that the relaxant responses to (+/-)-terbutaline and (+/-)-fenoterol are mainly mediated through beta3-adrenoceptors in the guinea pig gastric fundus. The beta3-adrenoceptor agonist potencies of arylethanolamine analogs depend on the size of the end of the alkylamine side chain.

Functional properties of atypical beta-adrenoceptors on the guinea pig duodenum

In this study, we attempted to further characterize atypical beta-adrenoceptors on the guinea pig duodenum. (-)-Enantiomers of isoprenaline and noradrenaline were more potent than its (+)-enantiomers. The isomeric activity ratios ((+)/(-)) were less than those obtained in the guinea pig atria and trachea. The concentration-response curves to catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline), to the selective beta(3)-adrenoceptor agonist, BRL37344 ((R*, R*)-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid sodium), and to the non-conventional partial beta(3)-adrenoceptor agonist, (+/-)-CGP12177A ((+/-)-[4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one] hydrochloride), were resistant to blockade by (+/-)-pindobind, the beta-adrenoceptor alkylating agent. (-)-Noradrenaline and (-)-adrenaline were more potent than dopamine and (-)-phenylephrine, respectively. Selective beta(2)-adrenoceptor agonists possess agonistic activities at atypical beta-adrenoceptors. (+/-)-Propranolol and (+/-)-bupranolol had no agonistic effect, whereas (+/-)-alprenolol, (+/-)-pindolol, (+/-)-nadolol, (+/-)-CGP12177A and (+/-)-carteolol exhibited agonistic activities at atypical beta-adrenoceptors. These results suggest that pharmacological properties of atypical beta-adrenoceptors differ from those of conventional beta(1)- and beta(2)-adrenoceptors on the guinea pig.

Agonistic activity of SR59230A at atypical beta-adrenoceptors in guinea pig gastric fundus and duodenum

We have recently suggested that atypical beta-adrenoceptors are present in guinea pig gastric fundus and duodenum. In the present study, we have shown that SR59230A (3-(2-ethylphenoxy)-1-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-(2S)-2-propanol oxalate), a selective beta(3)-adrenoceptor antagonist, possesses agonistic activities at atypical beta-adrenoceptors in these tissues. SR59230A caused concentration-dependent relaxations. However, (+/-)-propranolol (1 microM) did not affect SR59230A-induced relaxations. Pretreatment of with a combination of (+/-)-propranolol (1 microM) and the non-selective beta(1)-, beta(2)-, beta(3)- and beta(4)-adrenoceptor antagonist, (+/-)-bupranolol (30 microM), significantly antagonized the relaxant effects induced by SR59230A. The results clearly indicate that SR59230A acts as an atypical beta-adrenoceptor agonist on guinea pig gastric fundus and duodenum.

Effect of SR59230A on atypical beta-adrenoceptor mediating relaxation in the Guinea Pig gastric fundus

The purpose of the present study was to clarify whether atypical beta-adrenoceptors which presented in the guinea pig gastric fundus are beta(3)-adrenoceptors or putative beta(4)-adrenoceptors. In the presence of both the selective beta(1)-adrenoceptor antagonist atenolol (10(-4) mol/l) and the selective beta(2)-adrenoceptor antagonist butoxamine (10(-4) mol/l), the selective beta(3)-adrenoceptor antagonist SR59230A caused a concentration-dependent rightward shift of the concentration-response curve to catecholamines (isoprenaline, noradrenaline and adrenaline) and beta(3)-adrenoceptor agonists (BRL37344 and CGP12177A) in the guinea pig gastric fundus. Schild plot analyses of SR59230A against these agonists gave pA(2) values of 7.35 +/- 0.03 (isoprenaline), 7.26 +/- 0.04 (noradrenaline), 7.26 +/- 0.05 (adrenaline), 7.79 +/- 0.03 (BRL37344) and 6.74 +/- 0.03 (CGP12177A), respectively, and all Schild slopes were not significantly different from unity. These results suggest that atypical beta-adrenoceptors mediating relaxant responses of these agonists in the guinea pig gastric fundus are beta(3)-adrenoceptors rather than putative beta(4)-adrenoceptors.

Partial agonistic effects of carteolol on atypical beta-adrenoceptors in the guinea pig gastric fundus

The properties of the beta1-/beta2-adrenoceptor partial agonist carteolol were investigated in atypical beta-adrenoceptors on the guinea pig gastric fundus. Carteolol induced concentration-dependent relaxation in this tissue (pD2 = 5.55, intrinsic activity = 0.94). However, a combination of the selective beta1-adrenoceptor antagonist atenolol (100 microM) and the selective beta2-adrenoceptor antagonist butoxamine (100 microM) produced only small rightward shifts in the concentration-response curves of carteolol in the gastric fundus (pD2 = 4.91, intrinsic activity = 0.94). In the presence of both atenolol (100 microM) and butoxamine (100 microM), the non-selective beta1-, beta2- and beta3-adrenoceptor antagonist (+/-)-bupranolol (10-100 microM) caused a concentration-dependent right-ward shift of the concentration-response curves for carteolol in the guinea pig gastric fundus. Schild plot analyses of the effects of (+/-)-bupranolol against carteolol gave the pA2 value of 5.29 and the Schild slope was not significantly different from unity. Furthermore, carteolol (10 microM) weakly but significantly antagonized the relaxant responses to catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline), a selective beta3-adrenoceptor agonist BRL37344 ((R*,R*)-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxy-acetic acid sodium salt) and a non-conventional partial beta3-adrenoceptor agonist (+/-)-CGP12177A ([4-[3-[(1,1dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one] hydrochloride) in the guinea pig gastric fundus. These results suggest that the partial agonistic effects of carteolol are mediated by atypical beta-adrenoceptors in the guinea pig gastric fundus.

Further evidence that (+/-)-carteolol-induced relaxation is mediated by beta2-adrenoceptors but not by beta3-adrenoceptors in the guinea pig taenia caecum

The properties of the beta1- and beta2-adrenoceptor partial agonist (+/-)-carteolol were investigated against the beta2- and beta3-adrenoceptors of the taenia caecum of the guinea pig. (--)-Isoprenaline and (+/-)-carteolol induced concentration-dependent relaxation in this tissue. The non-selective beta1- and beta2-adrenoceptor antagonist (+/-)-propranolol (10-100 nM), the selective beta2-adrenoceptor antagonist ICI 118,551 (10-100 nM) and the non-selective beta1-, beta2- and beta3-adrenoceptor antagonist (+/-)-bupranolol (10-100nM), caused a concentration-dependent rightward shift of the concentration-response curves for (--)-isoprenaline and (+/-)-carteolol. Schild regression plot analyses carried out for (+/-)-propranolol against (--)-isoprenaline and (+/-)-carteolol gave pA2 values of 8.35 and 8.24, respectively. Schild plot analyses of ICI 118,551 against (--)-isoprenaline and (+/-)-carteolol gave pA2 values of 8.47 and 8.41, respectively. Schild plot analyses of (+/-)-bupranolol against (--)-isoprenaline and (+/-)-carteolol gave pA2 values of 8.47 and 8.53, respectively. Slopes of the Schild plots were not significantly different from unity. These results suggest that the relaxant effects of (+/-)-carteolol in the guinea pig taenia caecum are mediated by beta2-adrenoceptors but not by beta3-adrenoceptors.

Partial agonist activity of carteolol on atypical beta-adrenoceptors in the guinea pig duodenum

The partial agonist activities of carteolol were investigated on atypical beta-adrenoceptors of duodenum on the guinea pig. Carteolol produced a concentration-dependent relaxation of the guinea pig duodenum (pD(2)=4.85), which was not significantly affected by propranolol (1 microM). In the presence of propranolol (1 microM), however, the non-selective beta(1)-, beta(2)- and beta(3)-adrenoceptor antagonist, bupranolol (30 microM), caused a rightward shift of the concentration-response curves for carteolol (apparent pA(2)=5.31). Moreover, carteolol (10 microM) weakly, but significantly, antagonized the relaxations in response to catecholamines (isoprenaline, noradrenaline and adrenaline), to a selective beta(3)-adrenoceptor agonist, (R*, R*)-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]pheno xyacetic acid sodium salt (BRL37344), and to a non-conventional partial beta(3)-adrenoceptor agonist, [4-[3-[(1, 1-dimethylethyl)amino]-2-hydroxypropoxy]-1, 3-dihydro-2H-benzimidazol-2-one] hydrochloride (CGP12177A), also in the guinea pig duodenum (apparent pA(2)=5.77, 5.92, 6.05, 6.56 and 5. 58, respectively). These results suggest that the partial agonist effects of carteolol are mediated by atypical beta-adrenoceptors in the guinea pig duodenum.

Functional identification of beta3-adrenoceptors in the guinea-pig ileum using the non-selective beta-adrenoceptor antagonist (+/-)-bupranolol

1. To clarify whether there is a species difference or a tissue difference in beta3-adrenoceptors, the beta3-adrenoceptors mediating relaxations to catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline), a selective beta3-adrenoceptor agonist BRL37344 and a non-conventional partial beta3-adrenoceptor agonist (+/-)-CGP12177A (a potent beta1- and beta2-adrenoceptor antagonist with a partial beta3-adrenoceptor agonist property) were investigated in the guinea-pig ileum. 2. Catecholamines and beta3-adrenoceptor agonists induced concentration-dependent relaxations of pre-contracted strips of the guinea-pig ileum. The rank order for their relaxing potency was (-)-isoprenaline (pD2: 7.60) > BRL37344 (7.05) > (-)-noradrenaline (6.38) > (+/-)-CGP12177A (6.25) > (-)-adrenaline (6.07). 3. In the presence of the non-selective beta1- and beta2-adrenoceptor antagonist (+/-)-propranolol (1 microM), only small rightward shifts of the concentration-response curves (CRCs) to these agonists were observed and the rank order of potency of agonists was BRL37344 (pD2: 7.00) > (+/-)-CGP12177A (6.17) > (-)-isoprenaline (6.01) > (-)-noradrenaline (5.69) > (-)-adrenaline (5.41). 4. In the presence of (+/-)-propranolol (1 microM), the additional presence of (+/-)-bupranolol (3-30 microM), a non-selective beta1-, beta2- and beta3-adrenoceptor antagonist, caused a concentration-dependent rightward shift of the CRCs to catecholamines and beta3-adrenoceptor agonists. Schild plot analyses of (+/-)-bupranolol against these agonists gave pA2 values of 6.02 ((-)-isoprenaline), 6.03 ((-)-noradrenaline), 6.01 ((-)-adrenaline), 6.56 (BRL37344) and 5.74 ((+/-)-CGP12177A), respectively. All Schild plot slopes were not significantly different from unity. The pA2 values of (+/-)-bupranolol obtained for the guinea-pig beta3-adrenoceptors were about one log unit less than the values obtained for the rat beta3-adrenoceptors and about two log units less than the values obtained for dog beta3-adrenoceptors. 5. These results confirm that functional beta3-adrenoceptors are present in the guinea-pig ileum and that the relaxations of these agonists are mainly mediated via beta3-adrenoceptors in this tissue. The differential antagonistic potency of (+/-)-bupranolol may suggest that there is a species difference between the three species (guinea-pig, dog and rat) in their beta3-adrenoceptors.

The beta3-adrenoceptor-mediated relaxation induced by epinephrine in guinea pig taenia caecum

The mechanisms of the beta-adrenoceptor mediated relaxation induced by epinephrine in guinea pig taenia caecum were examined. The relaxant response to epinephrine was unaffected by propranolol (approximately 10(-5) M) or phentolamine (approximately 10(-5) M). The response to epinephrine was antagonized in a concentration dependent manner by bupranolol, and Schild plot of the data revealed the pA2 value of 5.87. Epinephrine significantly increased cyclic AMP level in this preparation. Bupranolol (10(-4) M) significantly decreased the cyclic AMP level that was elicited by epinephrine, whereas propranolol (10(-5) M) produced no effect. These results suggest that the relaxant response to epinephrine in the guinea pig taenai caecum is mainly mediated by beta3-adrenoceptors.