The importance of choice of agonist in studies designed to predict beta 2 : beta 1 adrenoceptor selectivity of antagonists from pA2 values on guinea-pig trachea and atria

The negative chronotropic effects of (±)-propranolol and (±)-4-NO2-propranolol in the rat isolated right atrium are due to blockade of the 6-nitrodopamine receptor

The positive chronotropic action induced by 6-nitrodopamine (6-ND) is selectively blocked by β1-adrenoceptor antagonists at concentrations that do not affect the positive chronotropic effect induced by dopamine, noradrenaline, and adrenaline. Here, the effects of ( ±)-propranolol, ( ±)-4-NO2-propranolol, and ( ±)-7-NO2-propranolol were investigated in the rat isolated right atrium. The atrium was mounted in glass chambers containing gassed (95%O2:5%CO2) and warmed (37 °C) Krebs-Henseleit's solution, and the isometric tension registered (PowerLab system). ( ±)-Propranolol, ( ±)-4-NO2-propranolol, and ( ±)-7-NO2-propranolol caused concentration-dependent falls in the spontaneous atrial frequency (pIC50: 4.80 ± 0.10, 4.64 ± 0.10, and 4.95 ± 0.10, respectively). The calculated pA2 values for ( ±)-propranolol, ( ±)-4-NO2-propranolol, and ( ±)-7-NO2-propranol on noradrenaline-induced positive chronotropism were 8.44 ± 0.08, 6.41 ± 0.07, and 9.21 ± 0.29, respectively. The positive chronotropism induced by 6-ND (10 pM) was blocked by ( ±)-propranolol (1 µM) and ( ±)-4-NO2-propranolol (30 nM), whereas ( ±)-7-NO2-propranolol (1 µM) had no effect on 6-ND-induced responses. The pIC50 of ( ±)-propranolol, ( ±)-4-NO2-propranolol, and ( ±)-7-NO2-propranolol were significantly shifted to the right in L-NAME-treated atria. The discrepancy between pA2 values of ( ±)-propranolol and its respective pIC50 indicates that the falls in atrial rate induced by ( ±)-propranolol should not be attributed to b-adrenergic antagonism. The reduced chronotropism by ( ±)-propranolol was unaffected by the sodium channel inhibitors tetrodotoxin and lidocaine but that was abolished in atria pre-treated with ( ±)-4-NO2-propranolol. The finding that ( ±)-propranolol reduces spontaneous atrial rate only in concentrations that affect 6-ND-induced positive chronotropism confirms the role of this catecholamine as an endogenous modulator of heart chronotropism. ( ±)-4-NO2-Propranolol behaves as a selective antagonist of 6-ND in the rat isolated atrium.

Involvement of beta3-adrenergic receptors in relaxation mediated by nitric oxide in chicken basilar artery

The response of basilar arteries to noradrenaline varies among many animal species, but remains little studied in poultry. Accordingly, we aimed to characterize the adrenergic receptor (AR) subtypes that modulate vascular response in basilar arteries in the chicken, with isometric recording of arterial ring tension using an organ bath. We demonstrated the presence of both alpha and beta (α and β) receptor subtypes through evaluating the response to noradrenaline, with and without a range of β-AR and α-AR antagonists. The concentration-dependent relaxations then induced by a range of β-AR agonists indicated a potency ranking of isoproterenol > noradrenaline > adrenaline > procaterol. We then investigated the effects of β-AR antagonists that attenuate the effect of isoproterenol (propranolol for β_1,2,3-ARs, atenolol for β₁-ARs, butoxamine for β₂-ARs, and SR 59230A for β₃-ARs), with Schild regression analysis, ascertaining multiple β-AR subtypes, with neither the β₁-AR nor the β₂-AR as the dominant subtype. SR 59230A was the only antagonist to yield a pA₂ value (7.52) close to the reported equivalent for the relevant receptor subtype. Furthermore, treatment with SR 58611 (a β₃-AR agonist) induced relaxation, which was inhibited (P < 0.01) by L-NNA and SR 59230A. Additionally, treating basilar arterial strips (containing endothelium) with SR 58611 induced nitric oxide (NO) production, which was inhibited (P < 0.01) by L-NNA and SR 59230A. Based on this first characterization of AR subtypes in chicken basilar arteries (to our knowledge), we suggest that α- and β-ARs are involved in contraction and relaxation, and that β₃-ARs, especially those on the endothelium, may play an important role in vasodilation via NO release.

Pharmacological identification of β-adrenoceptor subtypes mediating isoprenaline-induced relaxation of guinea pig colonic longitudinal smooth muscle

Object We aimed to identify the β-adrenoceptor (β-AR) subtypes involved in isoprenaline-induced relaxation of guinea pig colonic longitudinal smooth muscle using pharmacological and biochemical approaches. Methods Longitudinal smooth muscle was prepared from the male guinea pig ascending colon and contracted with histamine prior to comparing the relaxant responses to three catecholamines (isoprenaline, adrenaline, and noradrenaline). The inhibitory effects of subtype-selective β-AR antagonists on isoprenaline-induced relaxation were then investigated. Results The relaxant potencies of the catecholamines were ranked as: isoprenaline > noradrenaline ≈ adrenaline, whereas the rank order was isoprenaline > noradrenaline > adrenaline in the presence of propranolol (a non-selective β-AR antagonist; 3 × 10^-7 M). Atenolol (a selective β₁-AR antagonist; 3 × 10^-7-10^-6 M) acted as a competitive antagonist of isoprenaline-induced relaxation, and the pA₂ value was calculated to be 6.49 (95% confidence interval: 6.34-6.83). The relaxation to isoprenaline was not affected by ICI-118,551 (a selective β₂-AR antagonist) at 10^-9-10^-8 M, but was competitively antagonized by 10^-7-3 × 10^-7 M, with a pA₂ value of 7.41 (95% confidence interval: 7.18-8.02). In the presence of propranolol (3 × 10^-7 M), the relaxant effect of isoprenaline was competitively antagonized by bupranolol (a non-selective β-AR antagonist), with a pA₂ value of 5.90 (95% confidence interval: 5.73-6.35). Conclusion These findings indicated that the β-AR subtypes involved in isoprenaline-induced relaxation of colonic longitudinal guinea pig muscles are β₁-AR and β₃-AR.

The design, synthesis and pharmacological characterization of novel β₂-adrenoceptor antagonists

BACKGROUND AND PURPOSE

Selective and potent antagonists for the β(2) -adrenoceptor are potentially interesting as experimental and clinical tools, and we sought to identify novel ligands with this pharmacology.

EXPERIMENTAL APPROACH

A range of pharmacological assays was used to assess potency, affinity, selectivity (β(2) -adrenoceptor vs. β(1) -adrenoceptor) and efficacy.

KEY RESULTS

Ten novel compounds were identified but none had as high affinity as the prototypical β(2) -adrenoceptor blocker ICI-118,551, although one of the novel compounds was more selective for β(2) -adrenoceptors. Most of the ligands were inverse agonists for β(2) -adrenoceptor-cAMP signalling, although one (5217377) was a partial agonist and another a neutral antagonist (7929193). None of the ligands were efficacious with regard to β(2) -adrenoceptor-β-arrestin signalling. The (2S,3S) enantiomers were identified as the most active, although unusually the racemates were the most selective for the β(2) -adrenoceptors. This was taken as evidence for some unusual enantiospecific behaviour.

CONCLUSIONS AND IMPLICATIONS

In terms of improving on the pharmacology of the ligand ICI-118,551, one of the compounds was more selective (racemic JB-175), while one was a neutral antagonist (7929193), although none had as high an affinity. The results substantiate the notion that β-blockers do more than simply inhibit receptor activation, and differences between the ligands could provide useful tools to investigate receptor biology.

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.

Beta-adrenoceptor responses of the airways: for better or worse?

Beta2-adrenoceptor agonists are the first-line treatment of asthma and chronic obstructive pulmonary disease (COPD), in which a short-acting beta2-adrenoceptor agonist is used as required for relief of bronchoconstriction. A long-acting beta2-adrenoceptor agonist may be added to an inhaled corticosteroid as step 3 in the management of chronic asthma. Long-acting beta2-adrenoceptor agonists may also be added in treatment of COPD. This review examines the beneficial and detrimental effects of beta2-adrenoceptor agonists. The beneficial effects of beta2-adrenoceptor agonists are mainly derived from their bronchodilator activity which relieves the bronchiolar narrowing and improves air flow. The potential anti-inflammatory actions of stabilizing mast cell degranulation and release of inflammatory and bronchoconstrictor mediators, is considered. Other potential beneficial responses include improvements in mucociliary clearance and inhibition of extravasation of plasma proteins that is involved in oedema formation in asthma. The side effects of beta2-adrenoceptor agonists are primarily related to beta2-adrenoceptor-mediated responses at sites outside the airways. Of major concern has been the development of tolerance and this is discussed in relation to incidence of increased morbidity and mortality to asthma over the past three decades. A clinical aspect of beta2-adrenoceptor pharmacology in recent years has been the recognition of genetic polymorphism of the receptor and how this affects responses to and tolerance to beta2-adrenoceptor agonists. A controversial feature of beta2-adrenoceptor agonists is their stereoisomerism and whether the inactive (S)-isomer of salbutamol had detrimental actions in the commercially used racemate. The consensus is that despite these adverse properties, beta2-adrenoceptor agonist remains the most useful pharmacological agents in the management of asthma and COPD.

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.

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.

Lipid solubility of vasodilatory vanilloid-type beta-blockers on the functional and binding activities of beta-adrenoceptor subtypes

Various vanilloid-type beta-adrenoceptor blockers were studied on guinea pig right atrium and trachea and rat colon. In addition, we also investigated their beta(1)-, beta(2)-, and beta(3)-adrenoceptor binding affinities. All these beta-adrenergic antagonists inhibited (-)isoproterenol-induced positive chronotropic effects of the right atrium and tracheal relaxation responses in a concentration-dependent manner. Some of these agents prevented the inhibition of rat colon spontaneous motility by (-)isoproterenol. Of the agents tested, we found that ferulidilol, eugenodilol, eugenolol, isoeugenolol, and ferulinolol, as well as propranolol and metoprolol, possessed beta(3)-adrenoceptor blocking activities, others were nearly without effectiveness. Furthermore, the binding characteristics of vanilloid-type beta-adrenergic antagonists were evaluated in [3H]CGP-12177, a beta(1)/beta(2)-adrenoceptor blocker and a beta(3)-adrenoceptor agonist, binding to beta(1)-, beta(2)-, and beta(3)-adrenoceptor sites in rat ventricle, lung, and interscapular brown adipose tissue (IBAT) membranes, respectively. Eugenodilol, eugenolol, metoprolol, isoeugenolol, and ferulinolol were less potent than both propranolol and ferulidilol in competing for the beta(3)-adrenoceptor binding sites. From the results of in vitro functional and binding studies, we suggested that propranolol, ferulidilol, eugenodilol, eugenolol, metoprolol, isoeugenolol, and ferulinolol all possessed beta(3)-adrenoceptor blocking activities. On the other hand, we also found that eugenodilol, eugenolol, metoprolol, isoeugenolol, and ferulinolol had a low lipid solubility in comparison with propranolol and ferulidilol. In conclusion, we proposed that beta(3)-adrenoceptor antagonistic actions of these vanilloid-type beta-blockers were positively correlated with their lipid solubility.

Vanidipinedilol: a vanilloid-based beta-adrenoceptor blocker displaying calcium entry blocking and vasorelaxant activities

Calcium channel and beta-adrenoceptor blockade have proved highly useful in antihypertensive therapy. Studies of the mechanisms of action of vanidipinedilol that combine these effects within a single molecule are described here. Intravenous injection of vanidipinedilol (0.1, 0.25, 0.5, 1.0, and 2.0 mg/kg) produced dose-dependent hypotensive and bradycardic responses, significantly different from nifedipine-induced (0.5 mg/kg, i.v.) hypotensive and reflex tachycardic effects in pentobarbital-anesthetized Wistar rats. A single oral administration of vanidipinedilol at doses of 10, 25, and 50 mg/kg dose-dependently reduced blood pressure with a decrease in heart rate in conscious spontaneously hypertensive rats (SHRs). In the isolated Wistar rat atrium, vanidipinedilol (10(-7), 10(-6), and 10(-5) M) competitively antagonized the (-)isoproterenol-induced positive chronotropic and inotropic effects and inhibited the increase in heart rate induced by Ca2+ (3.0-9.0 mM) in a concentration-dependent manner. The parallel shift to the right of the concentration-response curve of (-)isoproterenol and CaCl2 suggested that vanidipinedilol possessed beta-adrenoceptor-blocking and calcium entry-blocking activities. On tracheal strips of reserpinized guinea pig, cumulative doses of vanidipinedilol (10(-10) to 3x10(-6) M) produced dose-dependent relaxant responses. Preincubating the preparation with ICI 118,551 (10(-10), 10(-9), 10(-8) M), a beta2-adrenoceptor antagonist, shifted the vanidipinedilol concentration-relaxation curve significantly to a region of higher concentrations. These results implied that vanidipinedilol had a partial beta2-agonist activity. In the isolated thoracic aorta of rat, vanidipinedilol had a potent effect inhibiting high-K+-induced contractions. KCI-induced intracellular calcium changes of blood vessel smooth muscle cell (A7r5 cell lines) determined by laser cytometry also was decreased after administration of vanidipinedilol (10(-8), 10(-7), 10(-6) M). Furthermore, the binding characteristics of vanidipinedilol and various antagonists were evaluated in [3H]CGP-12177 binding to ventricle and lung and [3H]nitrendipine binding to cerebral cortex membranes in rats. The order of potency of beta1- and beta2-adrenoceptor antagonist activity against [3H]CGP-12177 binding was (-)propranolol (pKi, 8.59 for beta1 and 8.09 for beta2) > vanidipinedilol (pKi, 7.09 for beta1 and 6.64 for beta2) > atenolol (pKi, 6.58 for beta1 and 5.12 for beta2). The order of potency of calcium channel antagonist activity against [3H]nitrendipine binding was nifedipine (pKi, 9.36) > vanidipinedilol (pKi, 8.07). The ratio of beta1-adrenergic-blocking/calcium entry-blocking selectivity is 0.1 and indicated that vanidipinedilol revealed more in calcium entry-blocking than in beta-adrenergic-blocking activities. It has been suggested that vanidipinedilol-induced smooth muscle relaxation may involve decreased entry of Ca2+ and partial beta2-agonist activities. In conclusion, vanidipinedilol is a nonselective beta-adrenoceptor antagonist with calcium channel blocking and partial beta2-agonist associated vasorelaxant and tracheal relaxant activities. Particularly, the vasodilator effects of vanidipinedilol are attributed to a synergism of its calcium entry blocking and partial beta2-agonist activities in the blood vessel. A sustained bradycardic effect results from beta-adrenoceptor blocking and calcium entry blocking, which blunts the sympathetic activation-associated reflex tachycardia in the heart.

Pharmacological characterization of EK112, a new combined angiotensin II and thromboxane A(2) receptor antagonist

The pharmacological characterization of EK112, a new combined angiotensin II and thromboxane A(2) receptor blocking agent, was examined in this study. EK112 was found to be a angiotensin II receptor antagonist, as revealed by its competitive antagonism of angiotensin II-induced smooth muscle contraction (pA(2) value of 7. 63 +/- 0.14) in rabbit aorta. It also had an angiotensin II blocking action in guinea pig ileum (pA(2) value of 7.87 +/- 0.67). Additionally, EK112 also possessed thromboxane A(2) receptor blocking activity, since it competitively antagonized aortic contractile responses elicited by U46619 and PGF(2alpha)(pK(B) values of 6.67 +/- 0.09 and 6.24 +/- 0.09, respectively) in rat. In contrast, EK112 did not affect the contractile responses to many other receptor agonists. EK112 did not mimic that of the angiotensin-converting enzyme (ACE) inhibitor, captopril, to enhance the muscle contraction elicited by bradykinin in guinea pig ileum, suggesting that EK112 did not inhibit ACE. Neither cyclic AMP nor cyclic GMP content in rat aortic rings was changed by EK112. These data demonstrate that EK112 is a selective antagonist of angiotensin II > thromboxane A(2) thromboxane A(2) receptor.

Isoeugenolol: a selective beta1-adrenergic antagonist with tracheal and vascular smooth muscle relaxant properties

Isoeugenolol (1.0, 3.0, 5.0 mg/kg, i.v.) produced a dose-dependent bradycardia and a decrease in blood pressure in anesthetized Wistar rats. Isoeugenolol inhibited the tachycardia effects induced by (-)isoproterenol, but had no blocking effect on the arterial pressor responses induced by (-)phenylephrine. In isolated guinea pig tissues, isoeugenolol antagonized (-)isoproterenol-induced positive inotropic and chronotropic effects on the atria and tracheal relaxations in a concentration-dependent manner. The apparent pA2 values for isoeugenolol on right atria, left atria and trachea were 7.63+/-0.03, 7.89+/-0.12 and 6.12+/-0.05, respectively, indicating that isoeugenolol was a highly selective beta1-adrenoceptor blocker. On the other hand, isoeugenolol produced a mild direct cardiac depression at high concentration and was without intrinsic sympathomimetic activity (ISA). In isolated rat thoracic aorta, isoeugenolol relaxed more potently the contractions induced by (-)phenylephrine (10 microM) and 5-HT (10 microM) than those by high K+ (75 mM). In isolated guinea pig trachea, isoeugenolol attenuated the carbachol (1 microM)-con-tracted trachea more significantly than those contracted with high K+. Furthermore, the binding characteristics of isoeugenolol and various beta-adrenoceptor antagonists were evaluated in [3H]CGP-12177 binding to rat ventricle, lung and interscapular brown adipose tissue (IBAT) membranes. The -log IC50 values of isoeugenolol for predominate beta1-, beta2- and beta3-adrenergic receptor sites were 5.82+/-0.09, 4.74+/-0.05 and 4.73+/-0.12, respectively. In conclusion, isoeugenolol was found to be a highly selective beta1-adrenoceptor antagonist with tracheal and vascular smooth muscle relaxant activities, but was devoid of alpha-adrenoceptor-blocking action.

beta-Adrenergic receptor subtypes in melanophores of the marine gobies Tridentiger trigonocephalus and Chasmichthys gulosus

The subtype of beta-adrenergic receptors in melanophores of the marine gobies Tridentiger trigonocephalus and Chasmichthys gulosus was studied. Pigment of denervated melanophores in isolated, split caudal fins was preliminarily aggregated by incubating the specimens in a physiological saline containing 10 microM phentolamine and 30-100 microM verapamil or 2-10 nM melatonin, and the responses of the melanophores to a beta-adrenergic agonist added to the incubating medium were recorded photoelectrically. The beta-adrenergic agonists noradrenaline, adrenaline, isoproterenol, salbutamol and, dobutamine were all effective in evoking a dispersion of melanophore pigment in the presence of phentolamine and verapamil or melatonin. The pigment-dispersing effect of noradrenaline (beta 1-selective agonist) was inhibited by metoprolol (beta 1-selective antagonist), propranolol,- and butoxamine. Whereas, the effect of salbutamol (beta 2-selective agonist) was hardly inhibited by metoprolol, though it was considerably inhibited by propranolol and ICI-118551. It was estimated that beta 1- and beta 2-adrenergic receptors coexist at ratios of 8.6:91.4, in the melanophore of Tridentiger trigonocephalus, and 25:75, in the melanophore of Chasmichthys gulosus, through the analyses of Hofstee plots of the effects of the beta-adrenergic drugs. It was suggested that the relation between the pigment-dispersing effect of a beta-adrenergic agonist on the melanophores and the concentration of the drug follows mass action kinetics, when the effect is mainly caused by the activation of beta 2-adrenergic receptors of the melanophores. However, when it is mainly caused by the activation of beta 1-adrenergic receptors of the melanophores, the relation does not follow mass action kinetics.

The effects of diabetes on beta-adrenoceptor mediated responsiveness of human and rat atria

The literature on the influence of diabetes on cardiac beta-adrenoceptors is still a matter of controversy. Hence, in the present study, the responsiveness of spontaneously beating right atria from streptozotocin (STZ)-diabetic rats to beta-adrenoceptor agonists were compared with those from non-diabetic controls. The responsiveness of right atria from 8-week diabetic rats to the chronotropic effects of isoprenaline, noradrenaline and fenoterol was found to be unchanged. As the disease progressed, on the other hand, the diabetic atria were found to have decreased responsiveness to the chronotropic effects of noradrenaline. The pD2 value and maximum chronotropic effect of noradrenaline were decreased in 14-week diabetic right atria when compared with those from age-matched controls. A significant decrease in the maximum chronotropic response to isoprenaline with no change in pD2 value was also observed in 14-week diabetes. These results suggest that beta 1--but not beta 2-adrenoceptor mediated chronotropic responses were reduced in the right atria due to the increase in the duration of diabetes. On the other hand, the inotropic responses to beta-adrenoceptor agonists were also assessed on diabetic and nondiabetic human atrial tissue. There were no significant differences in the inotropic responses to each agonists in either of the diabetic and nondiabetic human atrial tissues. The full agonist potency order was isoprenaline > or = fenoterol > noradrenaline.

Capsinolol: the first beta-adrenoceptor blocker with an associated calcitonin gene-related peptide releasing activity in the heart

1. The beta-adrenoceptor blocking and calcitonin gene-related peptide (CGRP)-releasing properties of capsinolol (N-[4-(2-hydroxy-3 (isopropylamino) propoxy)-3-methoxybenzyl]-nonanamide), derived from nonivamide, were investigated under in vivo and in vitro conditions. 2. Capsinolol (0.1, 0.5, 1.0 mg kg-1, i.v.), as well as (+/-)-propranolol, produced a dose-dependent bradycardia response and a temporary pressor action in urethane-anaesthetized normotensive Wistar rats. These cardiovascular effects were different from the vagus reflex and parasympathetic efferent effects shown by capsaicin (0.1 mg kg-1, i.v.) in the rat. 3. Capsinolol (1.0 mg kg-1) inhibited the tachycardia effects induced by (-)-isoprenaline, but had no blocking effect on the arterial pressor responses induced by (-)-phenylephrine. The findings suggest that capsinolol possesses beta-adrenoceptor blocking activity, but it has no alpha-adrenoceptor blocking activity. 4. In guinea-pig isolated tissues, capsinolol (10(-8) to 10(-6) M) antagonized (-)-isoprenaline-induced positive chronotropic and inotropic effects of the atria and tracheal relaxation responses in a concentration-dependent manner. The parallel shift to the right of the concentration-response curve of (-)-isoprenaline suggests capsinolol is a beta-adrenoceptor competitive antagonist. 5. Capsinolol (10(-5) to 10(-4) M) exhibited a positive cardiotonic effect that was not inhibited by (+/-)-propranolol and reserpine, but was inhibited by capsazepine (10(-6) M) and CGRP8-37 (10(-6) M). This effect was independent of intrinsic sympathomimetic effects. 6. An immunoassay of released CGRP from guinea-pig isolated perfused heart indicated that capsinolol increases the release of CGRP and thus produces positive cardiotonic effects. 7. In conclusion, capsinolol is a non-selective beta-adrenoceptor antagonist with capsaicin-like cardiotonic properties unrelated to traditional intrinsic sympathomimetic effects. It is suggested that capsinolol causes CGRP release from cardiac sensory neurones via a non-adrenergic mechanism and then activates CGRP receptors on cardiac muscle.

Vasomolol: an ultra short-acting and vasorelaxant vanilloid type beta 1-adrenoceptor antagonist

The ultra-short-acting and vasorelaxant beta 1-adrenoceptor blocking activities of vasomolol, a guaiacoxypropanolamine derivative of vanillic acid ethyl ester, were studied. Vasomolol (0.5, 1.0, 3.0 mg/kg intravenously, i.v.) produced a dose-dependent bradycardia response and demonstrated particularly a hypotensive action with an ultra-short-acting property in pentobarbital-anesthetized normotensive rats. Vasomolol's steady state of beta-blockade was attained < or = 10 min after initial infusion, and a rapid recovery from blockade occurred after discontinuation of the infusion, although intravenous infusion of vasomolol (300 micrograms/kg/min) could not inhibit pressor responses induced by (-)phenylephrine (10 micrograms/kg i.v.). In isolated rat thoracic aorta, vasomolol (1-10 microM) inhibited vascular smooth muscle contractions induced by both (-)phenylephrine (10(-5) M) and high K+ (75 mM) concentration dependently. This inhibitory effect of vasomolol was more sensitive on K(+)-induced than on (-)phenylephrine-induced contractions, suggesting that the block of Ca2+ influx may involve the major mechanism of vasorelaxation. In isolated guinea pig tissues, vasomolol (0.01-10 microM) antagonized the (-)isoproterenol (ISO)-induced positive inotropic and chronotropic effects of the atria and tracheal relaxation responses in a concentration-dependent manner. The parallel shift to the right of the concentration-response curve of (-)ISO suggested that vasomolol was a beta-adrenoceptor competitive antagonist. The effect of vasomolol was more potent on atria than on tracheal tissues, indicating that it possesses beta 1-adrenoceptor selectivity. In addition, vasomolol did not show intrinsic sympathomimetic activity (ISA). Moreover, the binding characteristics of vasomolol were evaluated in [3H]dihydroalprenolol ([3H]DHA) binding to porcine ventricular membranes. Vasomolol was an ultra-short-acting and highly selective beta 1-adrenoceptor antagonist with vasorelaxant activity and is devoid of ISA.

Effects of acute and short-term repeated application of fullerene C60 on agonist-induced responses in various tissues of guinea pig and rat

1. Effects of fullerene C60 in trachea, right atria, ileum and stomach (fundus) of guinea pig and vas deferens and uterus of rat were studied pharmacologically. 2. C60 (4 microM) had no direct effect in all tissues. In guinea pig trachea and heart, relaxation and positive inotropic and chronotropic actions of isoprenaline and in isolated rat vas deferens and uterus the responses on norepinephrine and oxytocin were not affected by the short-term repeated application of C60 30 mg/kg i.p. for 4 wk. 3. The pD2 values (potencies) of acetylcholine in ileum and its longitudinal muscle from guinea pig after the short-term repeated application of C60 were significantly smaller than those obtained without the application. The value of 5-hydroxytryptamine in rat stomach (fundus) also tended to be smaller than obtained without the application. 4. Atropine inhibited competitively the contractions for acetylcholine in the longitudinal muscles prepared from C60-treated and non-treated guinea pigs, and the pA2 values for atropine were not significantly different with each other. 5. These results suggest that C60 has no direct effects or antagonistic properties toward drug receptors, but sub-chronic exposure of C60 decreased responsiveness. This may be due to a change in post-receptor processes.

Beta-adrenoceptor regulation and functional responses in the guinea-pig following chronic administration of the long-acting beta 2-adrenoceptor agonist formoterol

Formoterol is a long acting beta 2-adrenoceptor agonist designed for the alleviation of the symptoms of asthma. This study examined the effects of 14 day administration of formoterol (200 micrograms/kg/day i.p.) on beta 1- and beta 2-adrenoceptors in guinea-pig cardiac and lung tissue. Quantitative autoradiography was used to measure changes in receptor density and organ bath studies determined alterations in functional response. Formoterol treatment produced marked reductions of between 43% and 77% in beta 2-adrenoceptor density in all regions of the heart (atrioventricular node, bundle of His, right and left bundle branches, interventricular and interatrial septa, right and left atria, ventricles and apex) and lung (bronchial and vascular smooth muscle and parenchyma) (P < 0.01, n = 6). beta 1-Adrenoceptor density remained unchanged in all cardiac and lung regions. In functional studies (-)-isoprenaline was 4 fold less potent at causing relaxation of carbachol (1 microM) precontracted tracheal smooth muscle (pD2: control 8.49 +/- 0.03, formoterol 7.91 +/- 0.10, P < 0.001, n = 4), but formoterol treatment did not change the ability of (-)-isoprenaline to elicit a maximum response. The pKB values for ICI 118,551, 7.33 +/- 0.08 in the control and 7.20 +/- 0.01 in formoterol treated animals, were between those expected for beta 1- and beta 2-adrenoceptors suggesting involvement of both subtypes in the response. In spontaneously beating right atria and electrically paced left atria, tissues in which responses are largely mediated by beta 1-adrenoceptors, there was no significant change in responses to (-)-isoprenaline (right atria pD2: control 8.45 +/- 0.02; formoterol 8.42 +/- 0.11; P = 0.77, n = 4) (left atria pD2: control 8.25 +/- 0.03; formoterol 8.47 +/- 0.08; P = 0.09, n = 4). In the presence of CGP 20712A (100 nM) the pKB values did not change with formoterol treatment (left atria: control 9.59 +/- 0.12, formoterol 9.66 +/- 0.12; P = 0.70, n = 4) (right atria: control 8.93 +/- 0.11, formoterol 9.11 +/- 0.07; P = 0.25, n = 4). The doses and route of administration of formoterol used in this study differs from those used clinically. However, this study demonstrates that chronic formoterol administration produces selective down-regulation of beta 2-adrenoceptors in the lung and heart. The changes in the lung are accompanied by a shift to the right in the concentration-response curve to beta-agonist stimulation with no change in the maximum response.

Forced swim stress: supersensitivity of the isolated rat pacemaker to the chronotropic effect of isoprenaline and the role of corticosterone

1. Forced swim (three daily sessions) resulted in an increased plasma corticosterone level and supersensitivity of the isolated rat pacemaker to the chronotropic effect of isoprenaline. 2. Bilateral adrenalectomy, performed 2 days before forced swim, abolished the development of pacemaker supersensitivity to isoprenaline. 3. Administration to rats of the antiglucorticoid compound RU-38486 prevented the development of pacemaker supersensitivity to isoprenaline. Pretreatment of rats not submitted to forced swim with the synthetic glucocorticoid RU-28362 causes pacemaker supersensitivity to isoprenaline. 4. Pretreatment of rats with diazepam or imipramine which block the forced swim-induced increase in the plasma level of corticosterone prevented the development of pacemaker supersensitivity to isoprenaline. 5. It is concluded that corticosterone plays a critical role in the modulation of the sensitivity to catecholamines of the pacemaker beta-adrenoceptors during adaptation to repeated stress.

A new beta-adrenoceptor blocking agent derived from dehydrozingerone

1. Dehydrozingeronolol (DZPN; 0.1, 0.5, 1.0 mg/kg, i.v.) produced a dose-dependent bradycardia response and a sustained pressor action in urethane-anesthetized normotensive rats. DZPN inhibited the tachycardia effects by (-)isoproterenol, but had no blocking effect on the arterial pressor responses induced by phenylephrine. 2. In in vitro study, DZPN antagonized (-)isoproterenol-induced positive chronotropic effects in guinea-pig isolated right atria and relaxation responses in rat isolated uterus horns. 3. DZPN causes mild direct cardiac depression at high concentrations without intrinsic sympathomimetic activity (ISA). 4. The order of potency of beta-adrenoceptor antagonists in competing for the [3H]dihydroalprenolol binding sites was (-)propranolol >> DZPN > or = atenolol.

Effect of salmeterol on human nasal epithelial cell ciliary beating: inhibition of the ciliotoxin, pyocyanin

1. Patients with airway infection by Pseudomonas aeruginosa have impaired mucociliary clearance. Pyocyanin is a phenazine pigment produced by P. aeruginosa which is present in the sputum of colonized patients, slows human ciliary beat frequency (CBF) in vitro and slows mucociliary transport in vivo in the guinea-pig. 2. We have investigated the effect of salmeterol, a long-acting beta 2-adrenoceptor agonist, on pyocyanin-induced slowing of human CBF in vitro. Salmeterol (2 x 10(-7) M) was found to reduce pyocycanin (20 micrograms ml-1)-induced slowing of CBF by 53% and the fall in intracellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) by 26% and ATP by 29%. 3. Another beta 2-adrenoceptor agonist, isoprenaline (2 x 10(-7) M), also inhibited pyocyanin-induced slowing of CBF by 39%. 4. The effects of salmeterol (30 min preincubation) persisted after washing the cells. 5. Propranolol (10(-7) M) and the beta 2-specific antagonist, ICI 118551 (10(-6) M) blocked the protective effects of salmeterol completely, but atenolol (10(-6) M) was less effective. These results suggested that the effects of salmeterol on pyocyanin-induced effects were mediated primarily via the stimulation of beta 2-adrenoceptors. 6. Pyocyanin-induced ciliary slowing is associated with a substantial fall in intracellular cyclic AMP and ATP. Salmeterol reversed the effects of pyocyanin on cyclic AMP and ATP. 7. Mucociliary clearance is an important defence mechanism of the airways against bacterial infection. Salmeterol may benefit patients colonized by P. aeruginosa, not only by its bronchodilator action, but also by protecting epithelial cells from pyocyanin-induced slowing of CBF.

Pharmacological characterization and anatomical localization of prejunctional beta-adrenoceptors in the rat kidney

1. The subtype and anatomical localization of beta-adrenoceptors mediating facilitation of stimulus-induced overflow of noradrenaline ('prejunctional beta-adrenoceptors') are not conclusively known to date. The present study was undertaken to characterize these receptors by use of pharmacological methods as well as to define their localization (prejunctional or postjunctional) with radio-ligand binding and autoradiography techniques combined with surgical denervation of the sympathetic innervation to the rat kidney. 2. Exposure of the kidney to (-)-isoprenaline, the nonselective beta-adrenoceptor agonist, resulted in a dose-dependent facilitation of stimulus-induced neurotransmitter overflow. This response was inhibited by propranolol, the beta 1- and beta 2-adrenoceptor antagonist, with a pA2 of 9.20 suggesting that the prejunctional beta-adrenoceptors are not of the beta 3-subtype. 3. The rank order of potency and potency ratios of beta-adrenoceptor agonists at renal prejunctional beta-adrenoceptors (EC50 for agonist/EC50 for (-)-isoprenaline) were: (-)-isoprenaline (1) > procaterol (2) > salbutamol (3) > adrenaline (10) > (+)-isoprenaline (25). However, dobutamine, the beta 1-adrenoceptor agonist, failed to enhance stimulus-induced overflow of noradrenaline. These results are indicative of the presence of beta 2-adrenoceptors as prejunctional beta-adrenoceptors. 4. Facilitation elicited by (-)-isoprenaline and procaterol, the selective beta 2-adrenoceptor agonist, was inhibited by ICI 118,551, the selective beta 2-adrenoceptor antagonist, with pKb values of 9.20 and 9.35, respectively at renal prejunctional beta-adrenoceptors. Similarly, the pKb values of metoprolol, the selective beta 1-adrenoceptor antagonist, at renal prejunctional beta-adrenoceptors were determined to be 6.25 and 6.18 against (-)-isoprenaline and procaterol, respectively. These results suggest the presence of a homogeneous population of beta 2-adrenoceptors as prejunctional beta-adrenoceptors. 5. Radio-ligand binding analysis of renal beta-adrenoceptors revealed the prevalence of the beta 1-subtype as compared to the beta 2-subtype (63% vs 37%). However, surgical denervation of the rat kidney, resulting in more than 90% reduction in renal noradrenaline content, selectively reduced the beta 2-adrenoceptor population by 80%, implying the presence of beta 2-adrenoceptors on renal sympathetic nerve terminals. 6. Autoradiographic analysis demonstrated the presence of beta 1-adrenoceptors on cortical structures such as glomeruli and tubules. beta-Adrenoceptors were found to be present on tubules (minor population), collecting tubules in outer medulla and the adventitia and adventitial-medial border of intraparenchymal branches of the renal artery. Surgical denervation of the rat kidney resulted in the disappearance of Beta2-adrenoceptors associated with the intraparenchymal branches, without affecting the Beta-adrenoceptor populations at other sites. These results support the notion that the Beta2-subtype is present on renal sympathetic nerve terminals and demonstrate that these prejunctional Beta2-adrenoceptors are associated with the renal vasculature and not with renal tubules.7. The results of the present investigation demonstrate that renal prejunctional Beta-adrenoceptors are of the Beta2-subtype in nature. These receptors are present on sympathetic nerve terminals which are associated with the renal vasculature.

Investigations into factors determining the duration of action of the beta 2-adrenoceptor agonist, salmeterol

1. This study has explored the mechanism underlying the long duration of action of the beta 2-adrenoceptor agonist, salmeterol. 2. Salmeterol, salbutamol and isoprenaline caused a concentration-related inhibition of electrically-induced contractile responses of the guinea-pig superfused trachea preparation. The effects of both isoprenaline and salbutamol were rapid in onset and rapidly reversed upon removal of the agonist. In contrast, the effects of salmeterol were slower in onset and could not be reversed by superfusion of the tissue with agonist-free Krebs solution even for periods of up to 10 h. 3. The effects of salmeterol were, however, readily reversed by a number of beta-adrenoceptor blocking drugs, as was the effect of a continuous infusion of isoprenaline. Upon removal of the antagonist, however, the effects of salmeterol and of the isoprenaline infusion were reasserted at a rate which was inversely related to the lipophilicity of a beta-adrenoceptor blocking drugs. 4. Salmeterol inhibited the binding of [125I]-(-)-iodopindolol (100 pM) to rat lung membranes (pIC50 7.1), with isoprenaline (pIC50 6.2) and salbutamol (pIC50 5.1) having lower potencies. The inhibition of binding by salmeterol was apparently non-competitive, whereas that produced by salbutamol and isoprenaline was competitive in nature. 5. Isoprenaline and salbutamol rapidly dissociated from their binding sites, whereas in marked contrast, the binding of salmeterol showed no dissociation for periods of up to 1 h. 6. These data are consistent with the mechanism in which salmeterol binds adjacent to the active site of the beta 2-adrenoceptor, such that the drug cannot be washed out of the tissue, yet can interact with and activate the receptor. This latter property is susceptible to antagonism by beta-adrenoceptor blocking drugs but is reassured when the antagonists are removed.

Neuronal control of airways smooth muscle

Sensory afferent nerves relay impulses from the airways to the central nervous system so that appropriate changes in bronchomotor tone and breathing patterns may occur. The dominant efferent control of airways smooth muscle is exerted via bronchoconstrictor parasympathetic cholinergic nerves. In some species this is opposed by bronchodilator sympathetic noradrenergic nerves. In addition, there exist both excitatory bronchoconstrictor and inhibitory bronchodilator non-adrenergic, non-cholinergic pathways. This review examines the role of the different branches of the autonomic nervous system in the control of airways smooth muscle tone with particular reference to modulation of these branches and the interactions which may exist between them.

Footshock stress-induced supersensitivity to isoprenaline in the isolated pacemaker of the rat: effects of the compounds RU-38486 and RU-28362

1. Three daily sessions of inescapable footshock stress of 30 min duration each increased the sensitivity of the isolated pacemaker of the rat to the chronotropic effect of isoprenaline. 2. The effect of inescapable footshock stress on the sensitivity of the isolated rat pacemaker to the chronotropic effect of isoprenaline was prevented by the daily administration of the compound RU-38486, a potent antiglucocorticoid which blocks the cytosolic receptor for corticosterone. 3. The administration of the compound RU-28362, a potent agonist of the cytosolic receptor for corticosterone, during 3 days to rats which were not submitted to footshock stress induces supersensitivity to the chronotropic effect of isoprenaline. 4. It is concluded that corticosterone plays an important role in the qualitative control of the rat pacemaker beta-adrenoceptor population during adaptation to repeated footshock stress.

Classification of the β-adrenoceptor subtype in the rat portal vein: effect of altered thyroid hormone levels

The potencies of the beta 1-adrenoceptor agonist, noradrenaline, and the beta 2-adrenoceptor agonist, fenoterol, at beta-adrenoceptors in portal vein were examined using preparations isolated from control, methimazole-treated or l-thyroxine-treated rats. Tissues were preincubated with phenoxybenzamine (1 mumol/l) to block alpha-adrenoceptors and neuronal and extraneuronal uptake. Fenoterol was approximately 400 times more potent than noradrenaline (-log IC50 7.85 vs. 5.26) in inhibiting the spontaneous contractions of the portal vein. The beta 2-adrenoceptor antagonist, ICI 118,551, was approximately 3000 fold more potent than the beta 1-adrenoceptor antagonist, atenolol, in blocking the effects of fenoterol (pA2 9.32 and 5.88 respectively) and 400 times more potent in antagonising noradrenaline (pA2 8.96 vs. 6.23). Treatment of rats with methimazole led to decreased myogenic tone, and treatment with thyroxine to increased tone. beta-Adrenoceptor binding densities and the relative potencies of the agonists and antagonists used were unaffected by methimazole treatment. Thyroxine administration was also without effect on the relative potencies of these compounds. Our data indicate that although the portal vein is a target tissue for thyroxine, as indicated by its influence on myogenic tone, the beta-adrenoceptor population in this preparation, confirmed to be of the beta 2-subtype, is unaffected.

Characterization of beta-adrenoceptor subtypes and indications for two cell populations in isolated bovine mesenteric lymphatic vessels

The pharmacological characterization of beta-adrenoceptor subtypes and the identification of two cell populations were investigated in isolated bovine mesenteric lymphatic vessels. The beta-adrenoceptor agonists isoprenaline, dobutamine and salbutamol concentration dependently decreased the amplitude and the frequency of spontaneous contractions and the amplitude of electrically induced contractions. The order of potency was isoprenaline greater than salbutamol greater than dobutamine. These effects were competitively antagonized by atenolol with pA2 values close to 7 with isoprenaline and dobutamine as agonists, and near 5.5 with salbutamol as the agonist. Noradrenaline concentration dependently reduced electrically induced contractions, an effect which was reversed to a slight enhancement after blockade of beta-adrenoceptors with propranolol (10(-6) M). These results confirmed the presence of postjunctional beta 1- and beta 2-adrenoceptor subtypes in lymphatic vessels and provide the first indication of the existence of two pharmacologically and functionally distinct cell populations, one of which exhibits pacemaker activity.

Mechanical responses to catecholamines in the longitudinal muscle of guinea-pig gastric fundus

1. In the longitudinal muscle of guinea-pig gastric fundus, adrenaline and phenylephrine (1-30 microM) both produced a slow contraction preceded by a relaxation. The slow contraction was strongly inhibited by prazosin (0.1 microM), but only weakly by yohimbine (1 microM), suggesting main contribution of alpha 1-adrenoceptors. 2. Most of the slow contraction was blocked by meclofenamate or indomethacin (0.1-0.3 microM). Both these drugs also inhibited spontaneously generated muscle tone. In some preparations, obtained from the apical fundus, a small contraction remained in the presence of meclofenamate. 3. During contraction induced by prostaglandin E2, adrenaline produced sustained relaxation and phenylephrine often transient relaxation, in the presence of meclofenamate. The transient relaxation, but not the sustained relaxation, was suppressed by prazosin. 4. In the presence of prostaglandin E2 (5 nM), after treating with phenoxybenzamine (30 microM) for 30 min, isoprenaline and adrenaline produced concentration-dependent relaxation, with IC50 s of 3.9 nM and 64 nM, respectively. Propranolol shifted these concentration-response curves to the right, with apparent pA2 s of 8.15 and 7.34, respectively. 5. It is suggested that in the fundic longitudinal muscle, adrenaline-induced contraction is mediated mainly by an increase in endogenous prostaglandin production through activation of alpha 1-adrenoceptors and that adrenaline produces transient relaxation through alpha 1-adrenoceptors and sustained relaxation through beta-adrenoceptors. The beta-adrenoceptors in the longitudinal muscle are more sensitive to adrenaline and isoprenaline than those in the circular muscle.

Negative chronotropic effect of a novel class III antiarrhythmic drug, UK-68,798, devoid of beta-blocking action on isolated guinea-pig atria

1. The chronotropic effects of a novel class III antiarrhythmic drug, UK-68,798, and the beta-adrenoceptor blocker, propranolol, for comparison, were studied on spontaneously beating right atria isolated from guinea-pigs in the absence and presence of increasing concentrations of isoprenaline (10(-10)-10(-4) M). 2. UK-68,798 (10(-9)-10(-5) M) decreased spontaneous atrial rate by 6-21%. Propranolol (10(-8) -10(-6) M) also had a negative but significantly smaller chronotropic effect. 3. UK-68,798 dose-dependently reduced the maximal positive chronotropic effect induced by isoprenaline, but without significantly shifting the concentration-response curve for isoprenaline in a parallel fashion. A pD'2 value of 5.88 was obtained. As expected, propranolol displayed a competitive inhibition with a pA2 value of 8.21. 4. The results demonstrate a negative chronotropic effect of UK-68,798, which is not associated with a beta-adrenoceptor blocking action. We suggest that the negative chronotropic effect is linked with potassium channel blockade and thereby the class III antiarrhythmic action of UK-68,798.

Subsensitivity to beta-adrenoceptor agonists in pacemakers isolated from senescent rats

1. The effects of aging on the responsiveness of the isolated pacemaker of the rat to the chronotropic effect of norepinephrine, isoproterenol and soterenol were studied. 2. Pacemakers isolated from senescent rats (22-24 months old) showed subsensitivity to norepinephrine, isoproterenol and soterenol, when compared with pacemakers isolated from young rats (3-4 months old). The maximum response to the partial agonist soterenol was reduced. 3. Determination of the pA2 value of metoprolol in pacemakers isolated from senescent and young rats showed that the chronotropic response is mediated by a homogeneous beta 1-adrenoceptor population. 4. Inhibition of extraneuronal uptake did not potentiate the chronotropic effect of isoproterenol in pacemakers isolated from senescent rats. Addition of cocaine shifted the concentration-effect curve for norepinephrine only 2.2-fold to the left in senescent rats. 5. It is concluded that, during aging, impairment of the extraneuronal and neuronal uptake mechanisms has an important role in the control of chronotropic responsiveness to catecholamines.

Coexistence of beta 1- and beta 2-adrenoceptors in the melanophore of the goby Tridentiger obscurus

The effects of beta-adrenergic agonists and antagonists on the pigmentary state of denervated melanophores in isolated, split, caudal fins of the goby Tridentiger obscurus were examined to investigate the function and the subtype of the beta-adrenoceptors of the melanophores. Salbutamol, terbutaline, and dobutamine partially inhibited the pigment-aggregating response of melanophores to norepinephrine. The effects of these beta-agonists were inhibited by propranolol. It was confirmed that the melanophores possess both alpha- and beta-adrenoceptors, and that the activation of the beta-adrenoceptors induces the dispersion of pigment in the melanophores. Norepinephrine, epinephrine, isoproterenol, dobutamine, salbutamol, and terbutaline evoked the dispersion of pigment in the melanophores in which pigment had previously been aggregated by treatment with verapamil in the presence of phentolamine. The pigment-dispersing effects of two beta 1-selective agonists, norepinephrine and dobutamine, were effectively inhibited by metoprolol, a selective antagonist of beta 1-receptors. By contrast, the pigment-dispersing effects of two beta 2-selective agonists, salbutamol and terbutaline, were not inhibited by metoprolol. Both the effects of nonselective agonists, epinephrine and isoproterenol, were partially inhibited by metoprolol. The actions of all of the beta-agonists used were effectively inhibited by propranolol, and they were partially inhibited by butoxamine. These results suggest co-existence of beta 1- and beta 2-adrenoceptors in the melanophores. The relative numbers of beta 1- and beta 2-adrenoreceptors as a percentage of the total population of beta-adrenoceptors were estimated to be 18.6% and 81.4%, respectively, from analyses of Hofstee plots of the effects of the beta-agonists on the melanophores in the presence of butoxamine or metoprolol.

In vitro inhibition of intestinal motility by phenylethanolaminotetralines: evidence of atypical beta-adrenoceptors in rat colon

1. The new compounds phenylethanolaminotetralines (PEAT), unlike the reference beta-adrenoceptor agonists isoprenaline (Iso), ritodrine (Ri) and salbutamol (Sal), produced half-maximal inhibition of spontaneous motility of rat isolated proximal colon at substantially lower concentrations (EC50 2.7-30 nM) than those inducing beta 2-adrenoceptor-mediated responses (relaxation of guinea-pig isolated trachea and rat uterus) and had virtually no chronotropic action (EC50 greater than 3 x 10(5) M) on the guinea-pig isolated atrium (a beta 1-adrenoceptor-mediated response). 2. The nonselective beta-adrenoceptor antagonists alprenolol and propranolol prevented the inhibition of rat colon motility by the PEAT with low and different potencies (pA2 values around 7.5 and 6.5 respectively). Conversely alprenolol and propranolol had a higher and similar potency (pA2 values around 9.0) in preventing typical beta 1 or beta 2-responses (increase in atrial frequency by Iso or tracheal relaxation by Ri or Sal). 3. The selective beta-adrenoceptor antagonists CGP 20712A (beta 1) and ICI 118,551 (beta 2) either alone or in combination, did not prevent rat colon motility inhibition by the representative PEAT SR 58611A, which was also fully resistant to alpha-adrenoceptor, acetylcholine, dopamine, histamine, opioid and 5-hydroxytryptamine antagonists. 4. These results indicate that the PEAT are a new class of beta-adrenoceptor agonists and suggest that their preferential intestinal action may be accounted for by selectivity for atypical beta-adrenoceptors, abundant in the rat colon and distinct from the currently recognized beta 1 and beta 2 subtypes.

Demonstration of both beta 1- and beta 2-adrenoceptors mediating negative chronotropic effects on spontaneous activity in isolated bovine mesenteric lymphatics

The pharmacological classification of beta-adrenoceptor subtypes in isolated bovine mesenteric lymphatics was studied by using various beta-adrenoceptor agonists and antagonists. Isoproterenol (ISP) (10(-10)-10(-8) M) produced dose-dependent negative chronotropic effects on the rhythm of spontaneous contractions in the lymphatic preparation. The ISP-induced effects were dose-dependently blocked by pretreatment with 10(-8)-10(-7) M metoprolol (a selective beta 1 antagonist) as well as 10(-8)-10(-7) M propranolol (a beta 1 and beta 2 antagonist). Dobutamine (a selective beta 1 agonist) caused a dose-dependent negative chronotropic effect, which was significantly blocked by pretreatment with 10(-8) M metoprolol. Salbutamol and procaterol (selective beta 2 agonists) also produced dose-dependent negative chronotropic effects, which were dose-dependently inhibited by pretreatment with 10(-8)-10(-7) M propranolol, but not by 10(-7) M metoprolol. The decreasing order of the relative potency was as follows: ISP greater than procaterol greater than salbutamol greater than dobutamine. These results suggest that bovine mesenteric lymphatics contain both beta 1- and beta 2-adrenoceptors and that both subtypes can produce a negative chronotropic effect on the rhythm of spontaneous contractions, when stimulated.

Beta 1-adrenoceptors mediate smooth muscle relaxation in mouse isolated trachea

1. The relaxant effects to the beta-adrenoceptor agonists isoprenaline, adrenaline, noradrenaline, RO363, procaterol and fenoterol were investigated in carbachol-contracted mouse isolated tracheal preparations. 2. The order of potencies for those beta-adrenoceptor agonists that induced full relaxation of carbachol-contracted mouse tracheal preparations was isoprenaline greater than RO363 greater than noradrenaline = adrenaline greater than fenoterol. The EC50 value of isoprenaline for relaxation was 46 nM. The beta 1-adrenoceptor-selective agonist, RO363 was ten times more potent than the beta 2-adrenoceptor-selective agonist, fenoterol. The highly beta 2-adrenoceptor-selective agonist procaterol was a partial relaxant and induced only 28 +/- 4% relaxation. 3. Relaxations induced by noradrenaline and isoprenaline were not significantly affected by the neuronal uptake inhibitor, cocaine (10 microM) or by the extraneuronal uptake inhibitor, deoxycorticosterone acetate (25 microM) respectively. The alpha-adrenoceptor agonist methoxamine induced no observable elevation of mouse tracheal smooth muscle tone. 4. Schild plots for the beta-adrenoceptor antagonists, atenolol and betaxolol (beta 1-adrenoceptor-selective) and ICI 118,551 (beta 2-adrenoceptor-selective) were linear, with slope values approaching unity. Mean pA2 values derived for atenolol, betaxolol and ICI 118,551 for antagonism of beta-adrenoceptor-mediated relaxation were 7.1, 8.4 and 7.2, respectively. These data were independent of the use of isoprenaline or noradrenaline as the agonist. 5. These findings indicate that beta-adrenoceptor-mediated relaxations of mouse isolated trachea occur predominantly through activation of beta 1-adrenoceptors.

Do both adrenaline and noradrenaline stimulate cardiac alpha-adrenoceptors to induce positive inotropy of rat atria?

1. The positive inotropic responses of rat paced left atria to adrenaline and noradrenaline were recorded. Desmethylimipramine (DMI, 1 microM) and metanephrine (10 microM) were initially present throughout. 2. The positive chronotropic responses of spontaneously beating right atria to adrenaline were used as a reference. In these, pindolol, in increasing concentrations, caused progressive shift of the concentration-response curves to the right, which yielded a pA2 value (8.15) compatible with antagonism of beta-adrenoceptors. 3. The left atrial tension responses to adrenaline showed an initial progressive displacement by pindolol (up to 3 microM) which gave an unexpectedly low pA2 value (6.48). However, with further increases in pindolol concentration there was no additional shift of the curve. In the presence of pindolol (3 microM), prazosin (0.1 microM) displaced the curve to the right but the pA2 value derived from this shift (7.75) was less than expected for alpha 1-adrenoceptor antagonism. 4. When the experiments in the presence of pindolol (3 microM) were repeated in the absence of DMI, prazosin displaced the concentration-response curves for adrenaline-induced left atrial tension to a greater extent and the pA2 value (8.76) was now compatible with adrenaline stimulating typical alpha 1-adrenoceptors. 5. The concentration-response curves for noradrenaline-induced left atrial tension were also progressively displaced to the right by pindolol (0.1, 0.3 and 1.0 microM). These concentrations yielded a Schild plot of unity slope and a pA2 value of 7.94 +/- 0.04. This was not significantly different from the pA2 value of 8.02 +/- 0.07 determined for pindolol against isoprenaline in the left atria, which indicates a normal interaction of noradrenaline with beta-adrenoceptors in the absence and presence of low concentrations of pindolol. 6. A further increase in the concentration of pindolol to 3 microM failed to induce an additional shift of the noradrenaline curves, whether a 'before and after' antagonist or a 'naïve tissue' design was adopted. Similarly, the rightwards shift of the concentration-response curves by timolol reached a limit as the concentration was increased. In all cases the limit of shift occurred at a noradrenaline EC50 value of 5-10 microM. 7. At the limit of beta-adrenoceptor antagonism, prazosin and dibenamine did not displace the noradrenaline curves further. The residual inotropic response to noradrenaline therefore appeared to be mediated via neither alpha- nor beta-adrenoceptors. 8. DMI, in the absence of beta-blockade, produced the potentiation of adrenaline and noradrenaline expected of a neuronal uptake inhibitor. However, in the presence of pindolol, there was no potentiation of the right atrial rate response to adrenaline while its left atrial tension responses were antagonized. This suggested that DMI was acting as an alpha-adrenoceptor antagonist. It also explained the less-than-expected shift by prazosin of the adrenaline responses in the presence of both pindolol and DMI, the latter drug already exerting some alpha-blocking activity. In contrast, the left atrial tension responses to noradrenaline in the presence of pindolol (1 microM) were neither potentiated nor antagonized by DMI. 9. When the effects of prazosin upon left atrial tension responses to noradrenaline in the presence of pindolol (10 microM) were examined in the presence of a lower concentration of DMI (O.1 microM) or cocaine (1O microM), again there was no further shift of the curve. However, when the effect of prazosin) The Macmillan Press Ltd 1989 598 K.L. WILLIAMSON & K.J. BROADLEY was examined in the absence of DMI, but in the presence of pindolol (1 and 1O microM) or timolol (3 microM), there was a small shift of the curves by prazosin (0.1 microM). This yielded pA2 values of 7.19, 7.34 +/- 0.1 and 7.66 +/- 0.09, which were at least one order of magnitude less than literature values and that obtained with adrenaline (8.76 +/- 0.18), and are not consistent with noradrenaline stimulating an alpha 1-adrenoreceptor in the presence of beta-adrenoceptor blockade, the increase in left atrial tension by noradrenaline does not appear to be mediated by beta l- or typical alpha-adrenoceptors. This is in contrast to adrenaline which in these conditions stimulates typical alpha 1-adrenoceptors.

A study of beta-adrenoceptors in rat lung parenchymal strip

The aim of the present study was to characterize the beta-adrenoceptor population in rat lung strip. For this purpose, Schild plots were obtained for the beta-adrenoceptor antagonists atenolol (beta 1-selective), butoxamine (beta 2-selective) and propranolol (non-selective), using three different agonists:isoprenaline (non-selective), salbutamol (beta 2-selective) and noradrenaline (beta 1-selective). The slopes of these Schild plots were close to the theoretical value of unity, and pA2 values determined with isoprenaline, salbutamol and noradrenaline as agonists were: for propranolol, 7.86 +/- 0.22, 7.72 +/- 0.15 and 7.89 +/- 0.23; for atenolol, 5.19 +/- 0.05, 5.33 +/- 0.07 and 5.47 +/- 0.22 and for butoxamine, 6.31 +/- 0.11, 6.34 +/- 0.03 and 5.99 +/- 0.23, respectively. These data suggest that pharmacological responses of rat isolated lung strip to beta-adrenoceptor agents are mediated by a homogeneous population of beta 2-adrenoceptors, although the presence of a minor population of beta 1-adrenoceptors undetected by the agonists used cannot be excluded.

In vitro desensitization of beta-adrenoceptors in guinea pig trachea: interactions between beta-adrenoceptor agonists and influence of adenosine and other drugs

The aim of this study was to investigate quantitatively the action of and the interaction between beta-adrenergic receptor agonists in desensitizing guinea pig isolated trachea. It was also to evaluate the influence of substances whose effects on desensitization are either disputed (theophylline, indomethacin, ketotifen, hydrocortisone) or unknown (nicardipine, Bay K 8644, fenspiride, adenosine). Tracheal strips were contracted with histamine (5 x 10(-5) M) or acetylcholine (5.10(-5) M) and concentration-response (C/R) curves for various beta-adrenoceptor agonists were determined before and after incubation (20 min to 4 h) with the same beta-adrenoceptor agonist (autodesensitization), with other beta-adrenoceptor agonists (cross-desensitization), or with a beta-adrenoceptor agonist and another substance. Our results show that the autodesensitization induced by isoprenaline is concentration dependent and that concentration dependence is more pronounced with salbutamol and fenoterol than with isoprenaline and adrenaline with respect to autodesensitization: shifts (log unit) of the C/R curves were 0.59 +/- 0.06 (N = 5) for salbutamol (10(-5) M), 0.78 +/- 0.09 (N = 5) for fenoterol (10(-6) M), 0.30 +/- 0.04 (N = 9) for isoprenaline (10(-5) M), and 0.33 +/- 0.05 (N = 5) for adrenaline (10(-5) M). Our studies of cross-desensitization (desensitization to isoprenaline, adrenaline, salbutamol, and fenoterol induced by incubation with isoprenaline 10(-5) M) showed a significantly greater shift in the C/R curves for fenoterol (0.56 +/- 0.08, N = 5) and salbutamol (0.62 +/- 0.05, N = 5) than for adrenaline (0.35 +/- 0.07, N = 5) and isoprenaline itself (0.30 +/- 0.05, N = 9). Of the substances we studied, none modified the desensitization induced by isoprenaline except hydrocortisone and adenosine. Hydrocortisone (10(-8) M) reduced it significantly, although to a negligible extent. Adenosine (3 x 10(-4) M) did not shift the C/R curve to isoprenaline by itself, but incubation of tracheal strips with adenosine and isoprenaline caused a significantly greater shift of C/R curves to isoprenaline (0.30 +/- 0.04) than incubation with isoprenaline alone (0.20 +/- 0.04) (P less than 0.05, N = 5). These experiments suggest that adenosine may have increased the uncoupling and/or down-regulation phenomena induced by isoprenaline, or modified adenylate cyclase-cAMP activity.

Beta-adrenoceptor desensitization in guinea-pig isolated trachea

Exposure to (-)-isoprenaline (25 microM, 1 h) caused a stereoselective, time and concentration-related decrease in smooth muscle beta 2-adrenoceptor function in guinea-pig trachea. Furthermore, tracheal relaxant responsiveness to the beta-adrenoceptor agonists (+/-)-fenoterol and (-)-noradrenaline was reduced, while that to theophylline and nitroprusside was unaffected. Responsiveness to forskolin was marginally but significantly reduced. Indomethacin, a cyclooxygenase inhibitor and mepacrine, an inhibitor of phospholipid turnover, had no significant effect on the extent of isoprenaline-induced desensitization. Conversely, cortisol (25 microM) significantly reduced desensitization and enhanced the rate of spontaneous recovery of responsiveness to isoprenaline. Desensitization was not accompanied by a reduction in the density of beta-adrenoceptors in the trachea, as assessed by binding and light microscopic autoradiography using [125I]iodocyanopindolol [( 125I]CYP). Thus, desensitization was probably caused primarily by beta-adrenoceptor/adenyl cyclase uncoupling. This model may be useful in investigations of the effect of glucocorticoids on the beta-adrenoceptor dysfunction recognized in severe asthma.

Agonist and antagonist characterization of a putative adrenoceptor with distinct pharmacological properties from the alpha- and beta-subtypes

1. Experiments were done to characterize a putative adrenoceptor which functions to inhibit longitudinal muscle tension development in the guinea-pig ileum. Several phenylethylamine based agonists were investigated: BRL 37344, (-)-isoprenaline, (+)-isoprenaline, noradrenaline, adrenaline, and fenoterol. Propranolol and nadolol were tested as antagonists. Agonist-induced inhibition of the contractile response to histamine was measured under equilibrium conditions with alpha-adrenoceptors and muscarinic cholinoceptors inhibited. 2. Inhibitory responses were obtained to (-)-isoprenaline and BRL 37344 that were resistant to beta-adrenoceptor blockage with propranolol (5 microM) and nadolol (10 microM). These resistant responses were antagonized by much higher concentrations of nadolol (30 to 1000 microM) yielding apparent pA2 values for nadolol of 4.31 with (-)-isoprenaline as the agonist, and 4.68 with BRL 37344 as the agonist. Similar apparent pA2 values for nadolol at the putative adrenoceptor were obtained with noradrenaline (4.79), adrenaline (4.68), and fenoterol (4.38). 3. The order and relative potency of agonists at the putative adrenoceptor was: BRL 37344 (20) greater than (-)-isoprenaline (8) greater than noradrenaline (1) greater than adrenaline (0.5) greater than fenoterol (0.35) greater than (+)-isoprenaline (0.27). 4. The resistance to blockade by propranolol (5 microM), the low affinity of nadolol, and the order and relative potency of agonists, suggest the presence of an adrenoceptor with distinct pharmacological characteristics from currently defined alpha- and beta-adrenoceptors.

Persistent beta-adrenoceptor blockade with alkylating pindolol (BIM) in guinea-pig left atria and trachea

The actions of alkylating pindolol (N8-bromoacetyl-N1-3'-(4-indolyloxy)-2'-hydroxypropyl[z]-1,8- diamino-p-menthane; BIM) have been examined on beta-adrenoceptors in guinea-pig left atria and trachea. In organ bath experiments, addition of BIM (greater than or equal to 0.1 microM), followed by washout, produced concentration-dependent rightward shifts of the dose-response curve to cumulative additions of (-)-isoprenaline and oxymethylene-isoprenaline and reductions in the maximal response. The "apparent" pA2 value for BIM was 9.23 +/- 0.20 (slope = 0.98 +/- 0.20). Changes in the maximal density of beta-adrenoceptor binding sites were determined in guinea-pig left atrial membranes using [125I]-cyanopindolol. BIM (0.1, 1.0 and 10 microM) produced 14, 23 and 41% reductions in Bmax with no change in KD. The binding of [125I]-BIM to guinea-pig left atrial membranes had a high non-specific binding component and a pseudo-Hill coefficient less than unity. The "apparent" KD value of [125I]-BIM at beta-adrenoceptor binding sites was 85.7 +/- 57.9 pM.

Positive chronotropic activity of angiotensin II in the pithed normotensive rat is primarily due to activation of cardiac beta 1-adrenoceptors

In the pithed normotensive rat the intravenous administration of increasing doses of angiotensin II caused an increase in heart rate by a maximum of 93 +/- 6 beats/min. This increase in heart rate was diminished dose-dependently by the concomitant infusion of the angiotensin II analogue saralasin. It was not affected by acute bilateral adrenalectomy but strongly reduced by pretreatment with reserpine. By using the selective antagonists of alpha 1-, alpha 2-, beta 1- or beta 2-adrenoceptors prazosin, yohimbine, atenolol and ICI 118,551, respectively, it was shown that this tachycardia was mediated by stimulation of cardiac beta 1-adrenoceptors. Moreover, the high endogenous angiotensin II level following pithing contributed to the basal heart rate in the pithed rat model. From our experiments it may be concluded that angiotensin II induces tachycardia in the pithed rat primarily by stimulating the sympathetic ganglia leading to the release of noradrenaline, which subsequently activates cardiac beta 1-adrenoceptors.

Inhibition of rat colon motility by stimulation of atypical beta-adrenoceptors with new gut-specific agents

The new putative beta-adrenergic agonists SR 58306A, 2-[(7-hydroxy-1,2,3,4-tetrahydronaphth-2-yl)amino]-1-phenylethanol hydrochloride and SR 58339A, 2-[(7-hydroxy-1,2,3,4-tetrahydronaphth-2-yl)amino]-1- (3-chlorphenyl) ethanol hydrochloride, were studied in vitro in comparison with reference compounds. SR 58306A and SR 58339A, unlike isoprenaline and the beta2 selective adrenergic agonists salbutamol and ritodrine, potently inhibited rat colon spontaneous contractions (EC50 5.9 and 1.1 x 10(-7) M) without increasing guinea-pig atrium frequency or relaxing guinea-pig trachea. The nonselective beta-adrenergic antagonists alprenolol, pindolol and propranolol competitively antagonized the action of SR 58306A on the colon, which was not prevented by either of the selective antagonists atenolol (beta 1) and ICI 118551 (beta 2). In the same preparation only alprenolol competitively antagonized isoprenaline; antagonism by either pindolol or propranolol was not competitive. These results suggest that in the rat colon isoprenaline interacts with different beta-receptor subclasses, whereas our new gut-specific compounds such as SR 58306A inhibit colonic motility by selectively stimulating atypical beta-adrenoceptors.

Repeated swimming stress and responsiveness of the isolated rat pacemaker to the chronotropic effect of noradrenaline and isoprenaline: role of adrenal corticosteroids

1. Repeated swimming stress (three daily sessions) resulted in an increased plasma corticosterone level and subsensitivity of the isolated rat pacemaker to noradrenaline and isoprenaline. 2. Repeated swimming stress was found to decrease the affinity of beta 1-adrenoreceptors for metoprolol. 3. Bilateral adrenalectomy performed 2 days before repeated swimming stress abolished the development of pacemaker subsensitivity to noradrenaline and isoprenaline and the decrease in beta 1-adrenoreceptors affinity for metoprolol. 4. It is concluded that adrenal corticosteroids, at least partially, mediate the swimming stress-induced subsensitivity of the isolated rat pacemaker to noradrenaline and isoprenaline.