Recent advances in structure, binding sites with ligands and pharmacological function of beta-adrenoceptors obtained by molecular biology and molecular modeling

Ephedrine and phenylephrine induce opposite changes in cerebral and paraspinal tissue oxygen saturation, measured with near-infrared spectroscopy: a randomized controlled trial

While the effects of phenylephrine (PE) and ephedrine (E) on cerebral oxygen saturation (rS_cO₂) already has been studied, the effect on paraspinal oxygen saturation (rS_psO₂) is still unexplored. This study aims to assess the effect of PE and E on rS_cO₂ and rS_psO₂, measured with near-infrared spectroscopy. A randomized 4-treatment cross-over trial was designed in 28 patients under BIS-titrated anaesthesia with sevoflurane. If MAP decreased more than 20% from baseline, incremental doses of PE and/or E were given according to the randomization (group I: E-PE-E, group II: PE-E-PE, group III: E-E-E, group IV: PE-PE-PE). rS_cO₂ and rS_psO₂ on T₃-T₄, T₉-T₁₀ and L₁-L₂ were recorded. Differences in rSO₂ (post-pretreatment) within each group were analyzed with paired Student's t test. Differences in effects of PE and E on rS_cO₂ and rS_psO₂ were analyzed with linear mixed-modelling. Following PE administration, rS_cO₂ decreased significantly (- 2.7% ± 3.5), while it remained stable following E (- 0.6% ± 3.6). Contrastingly, rS_psO₂ at T₃-T₄, T₉-T₁₀ and L₁-L₂ slightly increased following PE (0.4% ± 2.5, 0.7% ± 2.0 and - 0.1% ± 1.4, respectively), while it decreased after E administration (- 1.3% ± 3.4%, - 0.7% ± 2.6% and - 1.3% ± 2.7%, respectively). Compared to E, PE administration was associated with a significant decrease in rS_cO₂ (- 2.1%, 95% CI [- 3.1%, - 1.2%], p < 0.001). In contrast, compared to PE, E was associated with a significant decrease in rS_psO₂ at T₃-T₄, T₉-T₁₀ and L₁-L₂ (- 2.0%, 95% CI [- 2.8, - 1.1], p < 0.001; - 1.4%, 95% CI [- 2.4%, - 0.4%], p = 0.006; and - 1.5%, 95% CI [- 2.3%, - 0.8%], p < 0.001, respectively). An opposite effect on rS_cO₂ and rS_psO₂ was observed after bolus administration of PE and E.

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.

Survey of Pharmacological Activity and Pharmacokinetics of Selected β-Adrenergic Blockers in Regard to Their Stereochemistry

The present survey concentrates on pharmacodynamics and pharmacokinetics of selected β-adrenergic blockers from the point of view of their stereochemistry. It could be shown that the activity in the arylaminoethanol and aryloxyaminopropanol group of β-blockers is higher in their (–)-enantiomers as compared with the (+)-enantiomers. The stereoisomers differ also in other types of bioactivity as well as in toxicity. The particular pharmacokinetic stages such as resorption, distribution, and metabolism are discussed in regard to their stereochemistry.

Alpha 1 adrenergic receptor-mediated inflammatory responses in human testicular peritubular cells

Stress activates the sympathetic nervous system and is linked to impaired fertility in man. We hypothesized that catecholamines by acting on testicular cells have a role in these events, possibly by fostering an inflammatory environment. The cells of the wall of seminiferous tubules, human testicular peritubular cells (HTPCs), express adrenergic receptors (ADRs) α1B, α1D, β1 and β2. A selective α1-ADR agonist, phenylephrine, increased intracellular Ca²⁺-levels in cultured HTPCs and induced COX-2, IL-6 and MCP-1 mRNA expression without affecting IL-1β mRNA. These changes were paralleled by a significant increase in the secretion of IL-6 and MCP-1. Epinephrine was also effective, but salbutamol, a selective β2-ADR agonist was not. Our results suggest that stress-associated elevation of catecholamines may be able to promote inflammatory events by targeting peritubular cells in the human testis. Blockage of α1-ADRs may therefore be a novel way to interfere with stress-related impairment of male reproductive functions.

Effects of norepinephrine and acetylcholine on the development of cultured Leydig cells in mice

Few data have suggested how norepinephrine (NE) and acetylcholine (Ach) regulate the development of Leydig cells in mice at prepuberty, except for data indicating endocrine effects. The present study aims to elucidate the roles of NE and Ach on the differentiation and proliferation of Leydig cells. Firstly, the expression of adrenergic receptors and muscarinic acetylcholine receptors in Leydig cells was investigated. It was found that adrenergic receptors (β1AR, β2AR, and α1D) and muscarinic acetylcholine receptors (M1 and M3) mRNA are expressed in adult Leydig cells. Then, the effects of NE and Ach on the differentiation and proliferation of Leydig cells were analyzed. The results showed that NE and Ach at 10 μM significantly increased the number of 3β-hydroxysteroid-dehydrogenase- (3β-HSD-) positive Leydig cells and improved the expression of proliferating cell nuclear antigen (PCNA) in Leydig cells on postnatal day (PD) 15 (P < 0.05). NE and Ach at 10 μM had no impact on the expression of PCNA mRNA (P > 0.05), but reduced the expression of 3β-HSD mRNA in adult Leydig cells and a murine Leydig tumor cell line (MLTC-1) (P < 0.05). Therefore, a conclusion may be reached that NE and Ach participated in stimulating the development of Leydig cells in mice from prepuberty to adult stage.

The adrenergic receptor subtypes present in frog (Rana esculenta) skin

Frog skin transports ions and water under hormonal control. In spite of the fundamental role played by adrenergic stimulation in maintaining the water balance of the organism, the receptor subtype(s) present in the skin have not been identified yet. We measured the increase in short-circuit current (ISC, an estimate of ion transport) induced by cirazoline, clonidine, xamoterol, formoterol, or BRL 37344, in order to verify the presence of alpha1, alpha2, beta1, beta2, or beta3 receptor subtypes, respectively. Only after treatment with formoterol, BRL 37344 and, to a lesser extent, cirazoline was measured a significant increase in ISC (57%, 33.2%, and 4.7%, respectively). The formoterol and BRL 37344 concentrations producing half-maximal effect (EC50) were 1.12 and 70.1 nM, respectively. Moreover, the formoterol effect was inhibited by treatment with ICI 118551 (antagonist of beta2 receptors) while SR 59230A (antagonist of beta3 receptors) had no effect; opposite findings were obtained when the BRL 37344 stimulation was investigated. Finally, by measuring the transepithelial fluxes of 22Na+ and 36Cl-, we demonstrated that Na+ absorption is increased by activation of beta2 and beta3 and is cAMP-sensitive, whereas the Cl- secretion is only increased by activation of beta2 receptors and is cAMP- and calmodulin-sensitive.

Studies on the mechanisms underlying beta-adrenoceptor-mediated relaxation of rat abdominal aorta

Mechanisms underlying beta-adrenoceptor (beta-AR)-mediated vascular relaxation were studied in the isolated rat abdominal aorta. In the endothelium-denuded helical preparations, a non-selective beta-AR agonist isoprenaline elicited a concentration-dependent relaxation. In the absence of beta-AR antagonists, isoprenaline-induced relaxation was not practically affected by an adenylyl cyclase inhibitor SQ 22,536 (300 microM), but was strongly diminished by high-KCl (80 mM). Isoprenaline-induced relaxation in the presence of SQ 22,536 was significantly diminished by iberiotoxin (IbTx, 0.1 microM), but was not affected by 4-aminopyridine (4-AP, 3 mM). Isoprenaline-induced relaxation was not also affected by SQ 22,536 (300 microM) even in the presence of CGP20712A (a beta(1)-selective antagonist) and ICI-118,551 (a beta(2)-selective antagonist) (0.1 microM for each), but was strongly diminished by high-KCl. By contrast, SQ 22,536-resistant, isoprenaline-induced relaxation in the presence of CGP20712A plus ICI-118,551 was not affected by IbTx (0.1 microM), but was inhibited significantly by 4-AP (3 mM). These results suggest that in rat abdominal aortic smooth muscle: 1) both beta(1)-/beta(2)-AR- and beta(3)-AR-mediated relaxations substantially involve cAMP-independent mechanisms; 2) beta(1)-/beta(2)-AR-mediated, cAMP-independent relaxant mechanisms are partly attributed to the large-conductance, Ca (2+)-sensitive K(+) (MaxiK, BK) channel whereas beta(3)-AR-mediated relaxant mechanisms are attributed to K(v) channel.

Pharmacological characterization of the β-adrenoceptor that mediates the relaxant response to noradrenaline in guinea-pig tracheal smooth muscle

Pharmacological characteristics of beta-adrenoceptors (beta-ARs) mediating noradrenaline-induced relaxation were investigated in guinea-pig tracheal smooth muscle. The inhibitory effects of several types of beta-AR antagonists on noradrenaline-induced relaxation against histamine contraction were scrutinized with Schild plot analysis. The concentration-response curve for noradrenaline obtained in the absence of phentolamine and uptake inhibitors was competitively antagonized by all of the beta-AR antagonists used in this study (propranolol, bupranolol, atenolol, butoxamine and ICI-118,551). However, their pA2 values were markedly less than the expected values for beta1-AR and beta2-AR. On the other hand, pA2 values of ICI-118,551 (6.85) determined in the presence of phentolamine suggested a contribution of a beta1 -AR rather than beta2 -AR. In the presence of phentolamine and uptake inhibitors (desipramine and deoxycorticosterone), the Schild plot for atenolol was a better fit, with two distinct straight lines. The pA2 values of atenolol provided by the regression were: approximately 7.0, which corresponds to the expected beta1-AR value, and approximately 6.5, which was 3 times less than the expected value for beta1 -AR, and thus the possible presence of two classes of beta1 -AR (beta1(Low) and beta1(High)) was suggested. This view was also supported by Schild plot analysis for propranolol, which fit two straight lines each with a slope of 1.0. The present findings indicate that beta1 -ARs contributing to noradrenaline-elicited relaxation in guinea-pig tracheal smooth muscle exhibit diverse pharmacological characteristics and may be subdivided into at least two classes with distinct affinities for atenolol. Under physiological conditions, beta1(Low) rather than beta1(High) seems to play a more significant role in noradrenaline-regulated airway smooth muscle tone.

Norepinephrine acts as D1-dopaminergic agonist in the embryonic avian retina: late expression of beta1-adrenergic receptor shifts norepinephrine specificity in the adult tissue

Dopamine is the main catecholamine found in the chick retina whereas norepinephrine is only found in trace amounts. We compared the effectiveness of dopamine and norepinephrine in promoting cyclic AMP accumulation in retinas at embryonic day 13 (E13) and from post-hatched chicken (P15). Dopamine (EC(50)=10microM) and norepinephrine (EC(50)=30microM), but not the beta(1)-adrenergic agonist isoproterenol, stimulated over seven-fold the production of cyclic AMP in E13 retina. The cyclic AMP accumulation induced by both catecholamines in embryonic tissue was entirely blocked by 2microM SCH23390, a D(1) receptor antagonist, but not by alprenolol (beta-adrenoceptor antagonist). In P15 retinas, 100microM isoproterenol stimulated five-fold the accumulation of cAMP. This effect was blocked by propanolol (10microM), but not by 2microM SCH23390. Embryonic and adult retina display beta(1) adrenergic receptor mRNA as detected by RT-PCR, but the beta(1) adrenergic receptor protein was detected only in post-hatched tissue. We conclude that norepinephrine cross-reacts with D(1) dopaminergic receptor with affinity similar to that of dopamine in the embryonic retina. In the mature retina, however, D(1) receptors become restricted to activation by dopamine. Moreover, as opposed to the embryonic tissue, norepinephrine seems to stimulate cAMP accumulation via beta(1)-like adrenergic receptors in the mature tissue.

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.

Adrenaline produces the relaxation of guinea-pig airway smooth muscle primarily through the mediation of beta(2)-adrenoceptors

The beta-adrenoceptor subtype that mediates adrenaline-induced relaxation was pharmacologically identified in smooth muscle cells of the isolated guinea-pig trachea. Adrenaline produced a concentration-dependent relaxation with a pD(2) value of 7.1. The concentration-response curve for adrenaline was shifted rightwards in a competitive fashion by the beta(1)-/beta(2)-nonselective antagonists propranolol and bupranolol, with pA(2) values of 8.85 and 8.97, respectively. Adrenaline-induced relaxation was not affected by the beta(1)-selective antagonists atenolol and CGP-20, 712A within the concentration ranges supposed to antagonize the beta(1)-subtype (atenolol, <or=10(-6) M; CGP-20, 712A, <or=10(-8) M). By contrast, the concentration-response curve for adrenaline was shifted rightwards in a competitive fashion by atenolol at concentrations >or=3x10(-6) M with a pA(2) value of 5.77. The concentration-response curve for adrenaline was also competitively antagonized by the beta(2)-selective antagonists butoxamine and ICI-118,551 with pA(2) values of 6.86 and 8.73, respectively. The pA(2) values of beta-adrenoceptor antagonists (propranolol, bupranolol, atenolol, butoxamine and ICI-118,551) tested against adrenaline were consistent with the values when tested against salbutamol, a beta(2)-selective adrenoceptor agonist. The present findings provide evidence that the relaxant response of the smooth muscle of the guinea-pig trachea to the adrenal medulla hormone, adrenaline, is mainly mediated through beta(2)-adrenoceptors.

Noradrenergic receptor mRNA expression in adult rat superficial dorsal horn and dorsal root ganglion neurons

Noradrenaline (NAdr) has well documented analgesic actions at the level of the spinal cord. Released from bulbospinal projections onto superficial dorsal horn (SDH) neurons, NAdr modulates the excitability of these neurons through the activation of alpha1, alpha2 or beta adrenoceptors. This study utilised in situ hybridisation to determine the specific expression of adrenoceptors within adult rat lumbar SDH and dorsal root ganglion (DRG) neurons, and reports the presence of alpha1A, alpha1B, alpha2B, beta1 and beta2 adrenoceptor mRNA within SDH neurons, and the presence of alpha1A, alpha1B and alpha2C adrenoceptor mRNA within DRG neurons. The present study provides an insight into the modulation of sensory processing at the level of the spinal cord following adrenoceptor activation.

Characterization of β3-adrenergic receptor: determination of pharmacophore and 3D QSAR model for β3 adrenergic receptor agonism

The beta3-adrenoreceptor (beta3-AR) has been shown to mediate various pharmacological and physiological effects such as lipolysis, thermogenesis, and intestinal smooth muscle relaxation. It also plays an important role in glucose homeostasis and energy balance. Molecular modeling studies were undertaken to develop predictive pharmacophoric hypothesis and 3D-QSAR model, which may explain variations in beta3-AR agonistic activity in terms of chemical features and physicochemical properties. The two softwares, CATALYST for pharmacophoric alignment and APEX-3D for 3D-QSAR modeling were used to establish the structure activity relationships for beta3-AR agonistic activity. Among the several statistically significant models, the selection of the best pharmacophore and 3D-QSAR model was based on its ability to estimate the activity of external test sets of similar and different structural types along with the reasonable consistency of the model with the limited information of the active site of beta3-AR. The final 3D-QSAR model was derived using the pharmacophoric alignments from the hypothesis which consisted of four chemical features: basic or positive ionizable feature on the nitrogen of the aryloxypropylamino group, two ring aromatic features corresponding to the phenyl ring of the phenoxide and the benzenesulphonamido groups and a hydrogen-bond donor (HBD) in the vicinity of the nitrogen atom of the benzenesulphonamido group with the most active molecule mapping in an energetically favorable extended conformation. This hypothesis was in agreement with the site directed mutagenesis studies on human beta3-AR and correlated well the observed and estimated activity both in, training and both the external test sets. It also mapped reasonably well to six beta3-AR agonists of different structural classes under clinical development and thus this hypothesis may have a universal applicability in providing a powerful template for virtual screening and also for designing new chemical entities (NCEs) as beta3-AR agonists.

cAMP-independent mechanism is significantly involved in beta2-adrenoceptor-mediated tracheal relaxation

The role of cAMP in the beta2-adrenoceptor-mediated relaxation in response to salbutamol was examined in guinea pig tracheal smooth muscle. The concentration-dependent salbutamol-induced relaxation was antagonized in a competitive fashion by a beta2-selective adrenoceptor antagonist, butoxamine, with a pA2 value of 6.90. Salbutamol (10 microM) elevated the tracheal smooth muscle cAMP content by about fivefold, a response which was significantly inhibited by an adenylyl cyclase inhibitor, 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22,536, 100 microM). However, the salbutamol-elicited relaxation was not diminished by SQ 22,536 (100 microM). These results provide evidence for the first time that a cAMP-independent mechanism(s) is involved in beta2-adrenoceptor-mediated tracheal smooth muscle relaxation in the guinea pig.

MaxiK channel mediates beta2-adrenoceptor-activated relaxation to isoprenaline through cAMP-dependent and -independent mechanisms in guinea-pig tracheal smooth muscle

We examined the contribution of large-conductance, Ca(2+)-sensitive K+ (MaxiK) channel to beta2-adrenoceptor-activated relaxation to isoprenaline in guinea-pig tracheal smooth muscle focusing on the role for cAMP in the coupling between beta2-adrenoceptor and MaxiK channel. Isoprenaline-elicited relaxation was confirmed to be mediated through beta2-type of adrenoceptor since the response was antagonized in a competitive fashion by a beta2-selective adrenoceptor antagonist butoxamine with a pA2 value of 6.56. Isoprenaline-induced relaxation was significantly potentiated by a selective inhibitor of cyclic AMP-specific phosphodiesterase, Ro-20-1724 (0.1-1 microM). cAMP-dependent mediation of MaxiK channel in the relaxant response to isoprenaline was evidenced since the potentiated response to isoprenaline by the presence of Ro-20-1724 (1 microM) was inhibited by the channel selective blocker, iberiotoxin (IbTx, 100 nM). This concept was supported by the finding that the relaxation to a membrane permeable cAMP analogue, 8-bromo-cAMP (1 mM), was susceptible to the inhibition by IbTx. On the other hand, isoprenaline-induced relaxation was not practically diminished by an adenylyl cyclase inhibitor SQ 22,536 (100 microM). However, isoprenaline-induced relaxation in the presence of SQ 22,536 was suppressed by IbTx. Characteristics of isoprenaline-induced relaxant response, i.e., impervious to SQ 22,536 but susceptible to IbTx, were practically mimicked by cholera toxin (CTX, 5 microg/ml), an activator of adenylyl cyclase coupled-heterotrimeric guanine nucleotide-binding regulatory protein Gs. These findings indicate that in guinea-pig tracheal smooth muscle: 1) MaxiK channel substantially mediates beta2-adrenoceptor-activated relaxation; 2) both cAMP-dependent and -independent mechanisms underlie the functional coupling between beta2-adrenoceptor and MaxiK channel to induce muscle relaxation; and 3) direct regulation of MaxiK channel by Gs operates in cAMP-independent coupling between beta2-adrenoceptor and this ion channel.

Evidence showing that beta-adrenoceptor subtype responsible for the relaxation induced by isoprenaline is principally beta2 but not beta1 in guinea-pig tracheal smooth muscle

1. The present study was carried out to pharmacologically identify the beta-adrenoceptor subtype that mediates isoprenaline-elicited relaxation in the isolated guinea-pig tracheal smooth muscle, to answer the question whether it is beta(1)- or beta(2)-subtype? 2. Isoprenaline as well as salbutamol, a well-known beta(2)-selective adrenoceptor agonist, produced a concentration-dependent relaxation with a pD(2) value of 8.12 vs. 7.54 for salbutamol. 3. Isoprenaline-elicited relaxation was not affected by beta(1)-selective antagonists, atenolol and CGP-20,712A, within the concentration ranges supposed to antagonize beta(1)-subtype: atenolol, < or =10(-6) M; CGP-20,712A, < or =10(-8) M. 4. By contrast, the concentration-response curves for isoprenaline as well as salbutamol were shifted rightwards in a competitive fashion by atenolol at the concentrations > or =3 x 10(-6) M. However, pA(2) values of atenolol against isoprenaline (5.86) and salbutamol (5.71) were consistent with the value corresponding to beta(2)- but not to beta(1)-subtype (around 7.00), and these values were not significantly different from each other. 5. Competitive antagonism of the relaxations to isoprenaline and salbutamol were also obtained with beta(2)-selective antagonists, butoxamine and ICI-118,551. Against isoprenaline and salbutamol, the pA(2) values of butoxamine (6.51 vs. 6.81) and ICI-118,551 (8.83 vs. 8.90) were substantially identical. Thus the primary mediation of beta(2)-receptor in the relaxations was strongly supported. 6. The present findings provide evidence that the beta-adrenoceptor which mediates isoprenaline-elicited relaxation of guinea-pig tracheal smooth muscle is essentially beta(2)- but not beta(1)-subtype. The present study also indicates the importance of using multiple receptor antagonists with different pA(2) values to pharmacologically identify the responsible receptor subtype in smooth muscle mechanical responses.

Bopindolol: pharmacological basis and clinical implications

Bopindolol, a non-selective antagonist of beta 1- and beta 2-adrenoceptors (ARs), has been found by pharmacological, molecular biological techniques and molecular modeling to have several unique properties. Bopindolol produces sustained blockade of beta 1- and beta 2-ARs, has intrinsic sympathomimetic as well as membrane stabilizing actions, inhibits renin secretion, and interacts with 5-HT receptors. Also, our recent molecular modeling studies identified possible interaction sites between bopindolol and beta-AR subtypes. The reviewed studies support our findings that bopindolol is non-selective for beta 1- and beta 2-ARs, has low affinity for beta 3-AR subtype and has pharmacological properties that are likely to be beneficial in the treatment of cardiovascular diseases.