The effects of chronic dosing on the β1 and β2-adrenoceptor antagonism of betaxolol and atenolol

Summary Six normal subjects were given once daily treatment for 15 days with placebo (PL), betaxolol 10 mg (B10), 40 mg (B40); atenolol 100 mg (A100); and nadolol 40 mg (N40). Measurements of ₁-adrenoceptorblockade (reduction of exercise heart rate) and of ₂-adrenoceptor-blockade (attenuation of isoprenaline induced finger tremor) were made after the first, eighth and fifteenth doses of each drug.Plasma concentrations showed dose related increases between 10 mg and 40 mg doses of betaxolol, and there was significant drug accumulation at steady state compared with after single dosing. The reduction in exercise heart rate (EHR) with B10 was less in comparison with all other treatments.There were no significant differences in effects between single and chronic-dosing for any of the treatments (% reduction EHR compared with placebo, on days 1 and 15): B10 (18.2, 19.0), B40 (28.6, 26.5); A100 (22.7, 23.1); N40 (26.6, 23.8). Dose-ratios for attenuation of isoprenaline-induced finger tremor (IT₁₀₀) were significantly greater with B40 compared with B10 or A100 (no dose-ratio for finger tremor could be calculated for N40).There were no differences between single and chronic-dosing (IT₁₀₀ dose-ratios on days 1 and 15): B10 (3.0, 2.5), B40 (4.4, 5.3); A100 (3.0, 3.0). The attenuation of isoprenaline-induced chronotropic response (IH₂₅) by N40 was significantly greater in comparison with all other treatments. IH₂₅ dose-ratios (on days 1 and 15) were as follows: B10 (2.8, 3.6), B40 (5.1, 5.8); A100 (3.6, 3.6); N40 (19.0, 17.4).Thus, despite drug accumulation after chronic-dosing, there was no evidence of any increase in either ₁ or ₂-adrenoceptor antagonism at steady-state in comparison with after single-dosing. The apparent dissociation between plasma concentration and -adrenoceptor antagonism after chronic-dosing my be a consequence of -adrenoceptor up-regulation, resulting in partial attenuation of -blockade.     

β-Adrenoceptor regulation and functional responses in the guinea-pig following chronic administration of the long-acting β2-adrenoceptor agonist formoterol

Formoterol is a long acting ₂-adrenoceptor agonist designed for the alleviation of the symptoms of asthma. This study examined the effects of 14 day administration of formoterol (200 g/kg/day i.p.) on ₁- and 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 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). 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 M) 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 pK_B 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 ₁- and 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 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 pK_B 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 chrome formoterol administration produces selective down-regulation of 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 -agonist stimulation with no change in the maximum response.     

Comparative effects of β-carbolines on platelet aggregation and lipid membranes

The effects of 14 β-carbolines on human platelet aggregability were comparatively studied, and the effects on lipid membranes were determined. Several β-carbolines inhibited platelet aggregation induced by collagen, epinephrine, adenosine 5'-diphosphate, platelet-activating factor and thrombin. This activity was structure-dependent. Of all the compounds examined, 1-methyl-1,2,3,4-tetrahydro-β-carboline was the most potent. Treatment with 15-177 μM 1-methyl-1,2,3,4-tetrahydro-β-carboline inhibited the aggregation responses to different stimulants by up to 50%. Its potency was comparable to or greater than that of the antiplatelet reference, aspirin. The next most effective compound was 1-methyl-3,4-dihydro-β-carboline. The structure-antiplatelet activity relationship indicated that this activity is reduced by oxidation to 1-methyl-β-carboline, by demethylation to 1,2,3,4-tetrahydro-β-carboline and by 6-hydroxylation, 7-hydroxylation and 3-carboxylation. Active 1-methyl-1,2,3,4-tetrahydro-β-carboline fluidized biomimetic membranes at 25-250 μM which corresponded to the antiaggregatory concentrations, although relatively inactive 6-hydroxy-1-methyl-1,2,3,4-tetrahydro-β-carboline showed no significant effects on the membranes. β-Carbolines are considered to be effective antiplatelet agents that inhibit human platelet aggregation by interacting with lipid membranes to modify fluidity.     

Comparison of parameters of in vitro lymphocyte β2-adrenoceptor function in normal and asthmatic subjects

There is conflicting data in the literature as to whether subsensitivity of in-vivo ₂-adrenoceptor (₂-AR) responses in patients with asthma is due to an endogenous defect of ₂-AR or an effect of exogenous ₂-agonist therapy. The purpose of the study was to compare in-vitro parameters of lymphocyte ₂-AR function in eight age and sex matched normal [FEV₁, 98 (2) % predicted] volunteers and asthmatic [FEV₁, 60 (5) % predicted] subjects. The asthmatic group were washed out for 4 weeks by substituting inhaled ₂-agonist therapy with ipratropium bromide, in order to exclude possible exogenous effects of ₂-agonist exposure. Receptor binding affinity (K_d) and density (B_max) were evaluated using (-)¹²⁵I-iodocyanopindolol and maximal cAMP response (E_max) was assayed following stimulation with isoprenaline (10^–4M).No significant differences were found between the normal and asthmatic group for K_d (pmol·l^–1): 9.65 vs 10.2, B_max (fmol/10⁶ cells): 1.9 vs 1.6, or E_max (pmol/10⁶ cells): 4.24 vs 4.85.     

Differentiation of cardiac chronotropic and inotropic effects of β-adrenoceptor agonists

Summary The relative inotropic and chronotropic activity of -adrenoceptor agonists was studied in the noradrenaline-depleted, anaesthetized cat. Terbutaline, a selective ₂-adrenoceptor agonist, gave at a certain dose a more pronounced chronotropic than inotropic response, while a new ₁-selective adrenoceptor agonist (–)-H 80/62 produced the same degree of chronotropic and inotropic stimulation. The results indicate that there is some difference between the -adrenoceptors in the sinus node mediating chronotropic stimulation and -adrenoceptors in the ventricular myocardium mediating stimulation of the contractile force. It has been shown that there are both ₁- and ₂-adrenoceptors in the heart (Carlsson et al., 1972). In the light of this finding it is hypothetized that there are differences in the relative distribution of ₁- and ₂-adrenoceptors in the sinus node and in the myocardium. Although ₁ is the predominant type of -adrenoceptor in both regions, the ₁: ₂ concentration ratio seems to be higher in the myo-cardium, than in the sinus node.     

Molecular mechanisms for the persistent bronchodilatory effect of the β2-adrenoceptor agonist salmeterol

Background:

β₂-adrenoceptor agonists are effective bronchodilators. In vitro studies demonstrated long-lasting airway smooth muscle relaxation by salmeterol after washout, the quick disappearance of this effect in presence of antagonists and its recovery after antagonist removal. Current explanations invoke salmeterol accumulation in the membrane (‘diffusion microkinetic’ model) or the existence of salmeterol-binding ‘exosites’. An alternative model based on ‘rebinding’ of a dissociated ligand to the receptor molecules also produces an apparent decrease in the ligand''s dissociation rate in the absence of competing ligands.

Purpose and approach:

Computer-assisted simulations were performed to follow the receptor-occupation by a salmeterol-like ligand and a competing ligand as a function of time. The aptness of the models to describe the above in vitro findings was evaluated.

Key results:

The ‘diffusion microkinetic’ model is sufficient to explain a long-lasting β₂-adrenoceptor stimulation and reassertion as long as the membrane harbors a high concentration of the agonist. At lower concentration, ‘rebinding’ and, in second place, ‘exosite’ binding are likely to become operational.

Conclusions and implications:

The ‘rebinding’ and ‘exosite’ binding mechanisms take place at a sub-cellular/molecular scale. Pending their demonstration by experiments on appropriate, simple models such as intact cells or membranes thereof, these mechanisms remain hypothetical in the case of salmeterol. Airway smooth muscle contraction could also be governed by additional mechanisms that are particular to this macroscopic approach.     

Comparative β-adrenoceptor blocking effect and pharmacokinetics of bucindolol and propranolol in man

Summary The -adrenoceptor blocking properties and pharmacokinetics of bucindolol 150 mg were compared to those of propranolol 80 mg and a placebo in a double-blind trial in 6 healthy volunteers. Heart rate (HR), systolic (SBP) and diastolic (DBP) blood pressures and peak expiratory flow rate (PEFR) at rest and during vigorous exercise, and plasma renin activity (PRA) at rest, were measured before and at intervals up to 24 h after oral administration of the drugs. Bucindolol reduced exercise tachycardia and decreased exercise PEFR, thus behaving as a non-selective -adrenoceptor blocking drug. In contrast to propranolol, bucindolol did not reduce resting HR and PRA, probably because of its intrinsic sympathomimetic activity. It decreased resting DBP in relation to its peripheral vasodilator properties. The effects of bucindolol developed as early as 30 min after administration and lasted up to 24 h, whereas its T_max and T_1/2 were 1.6 and 3.6 h respectively. Comparison of the time courses of plasma bucindolol and the cardiac -adrenoceptor blockade strongly suggests that in man bucindolol undergoes an extensive first-pass effect, leading to the formation of one or more active metabolites.     

Quantifying the effects of diuretics and β-adrenoceptor blockers on glycaemic control in diabetes mellitus – a systematic review and meta-analysis

Aims

Although there are reports that β-adrenoceptor antagonists (beta-blockers) and diuretics can affect glycaemic control in people with diabetes mellitus, there is no clear information on how blood glucose concentrations may change and by how much. We report results from a systematic review to quantify the effects of these antihypertensive drugs on glycaemic control in adults with established diabetes.

Methods

We systematically reviewed the literature to identify randomized controlled trials in which glycaemic control was studied in adults with diabetes taking either beta-blockers or diuretics. We combined data on HbA_1c and fasting blood glucose using fixed effects meta-analysis.

Results

From 3864 papers retrieved, we found 10 studies of beta-blockers and 12 studies of diuretics to include in the meta-analysis. One study included both comparisons, totalling 21 included reports. Beta-blockers increased fasting blood glucose concentrations by 0.64 mmol l⁻¹ (95% CI 0.24, 1.03) and diuretics by 0.77 mmol l⁻¹ (95% CI 0.14, 1.39) compared with placebo. Effect sizes were largest in trials of non-selective beta-blockers (1.33, 95% CI 0.72, 1.95) and thiazide diuretics (1.69, 95% CI 0.60, 2.69). Beta-blockers increased HbA_1c concentrations by 0.75% (95% CI 0.30, 1.20) and diuretics by 0.24% (95% CI −0.17, 0.65) compared with placebo. There was no significant difference in the number of hypoglycaemic events between beta-blockers and placebo in three trials.

Conclusions

Randomized trials suggest that thiazide diuretics and non-selective beta-blockers increase fasting blood glucose and HbA_1c concentrations in patients with diabetes by moderate amounts. These data will inform prescribing and monitoring of beta-blockers and diuretics in patients with diabetes.     

Stereoselectivity at the β2-adrenoceptor on macrophages is a major determinant of the anti-inflammatory effects of β2-agonists

Previous research has shown that beta-adrenoceptor (beta-AR) agonists have potent anti-inflammatory capabilities, e.g. represented by suppression of release of the proinflammatory cytokines. Aim of this research was to determine whether the effects of beta-agonists on LPS-induced TNFalpha and IL-10 release are influenced by their different stereochemistry. In addition, the role of the beta-AR subtypes was studied. The effect of two stereoisomers of the selective beta2-AR agonist TA2005 [(R,R)- and (S,S)-] on the LPS-induced TNFalpha and IL-10 release by U937 macrophages was compared. The (R,R)-stereoisomer was 277 times more potent in inhibiting the TNFalpha release than the (S,S)-form. The (R,R)-stereoisomer also appeared to be more potent in increasing the IL-10 release. In radioligand binding studies the affinity of (R,R)-TA2005 for the beta-adrenoceptor was 755 times higher than the (S,S)-TA2005 stereoisomer. In addition, the elevation of intracellular cAMP in U937 cells appeared to be stereoselective: (R,R)-TA2005 was more potent in elevating intracellular cAMP. The effect of both stereoisomers on the LPS-induced TNFalpha release could almost completely be antagonized by preincubation with the selective beta2-AR-antagonist ICI-118551. Further evidence that the effect of the beta-agonists is mediated via the beta2-adrenoceptor subtype exclusively was acquired by incubation of U937 cells with selective beta1- and beta3-agonists. None of these receptor subtype agonists showed significant suppressive effect on TNFalpha release. This study provides additional proof that the anti-inflammatory effects of beta2-agonists are mediated via the beta2-adrenoceptor and indicates that these effects are highly dependent on the stereoselectivity of the ligand.     

Comparative effects ofβ 2-adrenoceptor agonists on intracranial self-stimulation,Sidman avoidance,and motor activity in rats

The effects of -adrenoceptor agonists were compared in various operant behavioral tasks, particularly intracranial self-stimulation (ICSS). Clenbuterol, salbutamol, and terbutaline all reduced responding by rats that lever-pressed for low stimulation intensities. The effects of clenbuterol in this test were completely reversed by propranolol, and those of salbutamol were partly reversed. Intermediate doses of clenbuterol and salbutamol slowed the initiation of rewarding brain stimulation in a shuttlebox but had little or no effect on the termination latencies. However, higher doses of both drugs lengthened the termination latencies. Motor activity was reduced at doses that attenuated ICSS responding. Complete tolerance occurred within 4 days to the effects of clenbuterol and salbutamol on leverpressing ICSS and to the effects of clenbuterol on motor activity. The apparent performance deficits induced by these drugs were overcome by more intense motivation. For example, even at high doses, clenbuterol reduced ICSS leverpressing only partially when animals bar-pressed for high rather than low stimulation intensities. Furthermore, all three drugs failed to alter Sidman avoidance responding at doses up to 100 times those that attenuated ICSS responding. It is concluded that although -adrenoceptor agonists cause apparent sedation in rats, this sedation is limited and shows rapid tolerance.     

Functional evidence for differential regulation of β-adrenoceptor subtypes

It is known that β-adrenoceptor function can be influenced by various physiological, pathological or pharmacological interventions, but only limited work has been done to determine whether the β₁ and β₂-adrenoceptor subtypes are regulated independently or together. Stella O'Donneii and Janet Wanstall present recent evidence from functional response data for β-adrenoceptor agonists that suggests that β₁- and β₂-adrenoceptors can be independently regulated, at least in circumstances in which noradrenaline levels are manipulated, or ageing occurs, or thyroid status is changed. This functional evidence supports conclusions that can be drawn from radioligand binding studies, although these have mainly been carried out on tissues different from those examined in the functional studies.     

Agonist effects of zinterol at the mouse and human β3-adrenoceptor

The present study investigates the action of zinterol at β₃-adrenoceptors. We used mouse primary brown adipocytes and Chinese hamster ovary (CHO-K1) cells expressing the mouse or human β₃-adrenoceptor. Zinterol was a full agonist at increasing cyclic AMP levels in primary brown adipocytes (which express β₁- and β₃-adrenoceptors but not β₂-adrenoceptors), and this effect was almost totally abolished in adipocytes derived from β₃-adrenoceptor knock-out (KO) mice. Zinterol was also a full agonist at increasing another biological end-point, glucose uptake in brown adipocytes. This effect was reduced in adipocytes derived from β₃-adrenoceptor KO mice, with the remaining response sensitive to β₁-adrenoceptor antagonism. To determine whether the effect of zinterol on β₃-adrenoceptors in primary brown adipocytes can be replicated in a recombinant system, we used CHO-K1 cells expressing the mouse or human β₃-adrenoceptor. Zinterol was a full agonist at mouse and human receptors with respect to increasing cyclic AMP levels, with pEC₅₀ values similar to that of the selective β₃-adrenoceptor agonist (R, R)-5-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]-amino]-propyl]1,3-benzodioxole-2,2-dicarboxylate (CL316243) at the mouse receptor. At the human receptor, zinterol was more potent at increasing cyclic AMP levels than CL316243. In cytosensor microphysiometer studies, zinterol was a full agonist for increases in extracellular acidification rates at the mouse and human β₃-adrenoceptor. Therefore, we have shown that zinterol is a potent, high-efficacy β₃-adrenoceptor agonist at the endogenous mouse β₃-adrenoceptor in primary brown adipocytes and at the cloned mouse and human β₃-adrenoceptor expressed in CHO-K1 cells. Zinterol is therefore one of few β-adrenoceptor agonists with high potency and efficacy at the human β₃-adrenoceptor.     

Comparative study of β-carotene and microencapsulated β-carotene: Evaluation of their genotoxic and antigenotoxic effects

β-Carotene (BC) is one of the natural pigments that is most commonly added to food; however, the utilization of BC is limited due to its instability. Microencapsulation techniques are commonly used because they can protect the microencapsulated material from oxidization. Nevertheless, the properties of the encapsulated compounds must be studied. We compared the antigenotoxic potential of pure and microencapsulated β-carotene (mBC) in Wistar rats. Two doses of BC or mBC (2.5 or 5.0 mg/kg) were administered by gavage over a period of 14 days. The final gavage was followed by an injection of doxorubicin (DXR). After 24 h the animals were euthanized. The micronucleus test results showed that when both mBC and DXR were given, only the higher dose was antigenotoxic. The results of the comet assay show that when given in association with DXR, mBC had protective effects in the liver. The differences between the results obtained with BC and mBC suggest that possibly the carotenoid biodisponibility was modified by the process of microencapsulation. In conclusion, mBC does not lose its protective properties, but higher doses must be used to observe antigenotoxic effects. This is the first time that the genotoxicity and antigenotoxicity of a microencapsulated compound was evaluated in vivo.     

Effects of β2-agonist- and dexamethasone-treatment on relaxation and regulation of β-adrenoceptors in human bronchi and lung tissue

1. Long-term treatment with β₂-adrenoceptor agonists can lead to a decreased therapeutic efficacy of bronchodilatation in patients with obstructive pulmonary disease. In order to examine whether or not this is due to β-adrenoceptor desensitization, human bronchial muscle relaxation was studied in isolated bronchial rings after pretreatment with β₂-adrenoceptor agonists. Additionally, the influence of pretreatment with dexamethasone on desensitization was studied.
2. The effect of β₂-agonist incubation alone and after coincubation with dexamethasone on density and affinity of β-adrenoceptors was investigated by radioligand binding experiments.
3. In human isolated bronchi, isoprenaline induces a time- and concentration-dependent β-adrenoceptor desensitization as judged from maximal reduction in potency by a factor of 7 and reduction of 73±4% in efficacy of isoprenaline to relax human bronchial smooth muscle.
4. After an incubation period of 60 min with 100 μmol l⁻¹ terbutaline, a significant decline in its relaxing efficacy (81±8%) and potency (by a factor 5.5) occurred.
5. Incubation with 30 μmol l⁻¹ isoprenaline for 60 min did not impair the maximal effect of a subsequent aminophylline response but led to an increase in potency (factor 4.4).
6. Coincubation of dexamethasone with isoprenaline (120 min; 30 μmol l⁻¹) preserved the effect of isoprenaline on relaxation (129±15%).
7. In radioligand binding experiments, pretreatment of lung tissue for 60 min with isoprenaline (30 μmol l⁻¹) resulted in a decrease in β-adrenoceptor binding sites (B_max) to 64±1.6% (P<0.05), while the antagonist affinity (K_D) for [³H]-CGP-12177 remained unchanged.
8. In contrast, radioligand binding studies on lung tissue pretreated with either dexamethasone (30 μmol l⁻¹) or isoprenaline (30 μmol l⁻¹) plus dexamethasone (30 μmol l⁻¹) for 120 min did not lead to a significant change of B_max (160±22.1% vs 142.3±28.7%) or K_D (5.0 nmol l⁻¹ vs 3.5 nmol l⁻¹) compared to the controls.
9. In conclusion, pretreatment of human bronchi with β-adrenoceptor agonists leads to functional desensitization and, in lung tissue, to down-regulation of β-adrenoceptors. This effect can be counteracted by additional administration of dexamethasone. Our model of desensitization has proved useful for the identification of mechanisms of β-adrenoceptor desensitization and could be relevant for the evaluation of therapeutic strategies to counteract undesirable effects of long-term β-adrenoceptor stimulation.

     

Formoterol and salmeterol induce a similar degree of β2-adrenoceptor tolerance in human small airways but via different mechanisms

BACKGROUND AND PURPOSE

Steroids prevent and reverse salbutamol-induced β₂-adrenoceptor tolerance in human small airways. This study examines the effects of the long-acting β₂ agonists (LABAs) formoterol and salmeterol, and the ability of budesonide to prevent desensitization.

EXPERIMENTAL APPROACH

Long-acting β₂ agonists in the presence and absence of budesonide were incubated with human precision-cut lung slices containing small airways. Tolerance was deduced from measurements of reduced bronchodilator responses to isoprenaline and correlated with β₂-adrenoceptor trafficking using a virally transduced, fluorescent-tagged receptor. The ability of the LABAs to protect airways against muscarinic-induced contraction was also assessed.

KEY RESULTS

Following a 12 h incubation, both formoterol and salmeterol attenuated isoprenaline-induced bronchodilatation to a similar degree and these effects were not reversible by washing. Pre-incubation with budesonide prevented the desensitization induced by formoterol, but not that induced by salmeterol. Formoterol also protected the airways from carbachol-induced bronchoconstriction to a greater extent than salmeterol. In the epithelial cells of small airways, incubation with formoterol promoted receptor internalization but this did not appear to occur following incubation with salmeterol. Budesonide inhibited the formoterol-induced reduction in plasma membrane β₂-adrenoceptor fluorescence.

CONCLUSIONS AND IMPLICATIONS

Although both formoterol and salmeterol attenuate isoprenaline-induced bronchodilatation, they appear to induce β₂-adrenoceptor tolerance via different mechanisms; formoterol, but not salmeterol, enhances receptor internalization. Budesonide protection against β₂-adrenoceptor tolerance was correlated with the retention of receptor fluorescence on the plasma membrane, thereby suggesting a mechanism by which steroids alter β₂-adrenoceptor function.     

α2-adrenoceptor regulation of blood glucose homeostasis

The α(2A)-adrenoceptor has been identified as an important regulator of blood glucose homeostasis. α(2A)-Adrenoceptors on pancreatic β-cells inhibit insulin secretion, and α(2A)-adrenoceptors on sympathetic nerves and on adrenomedullary chromaffin cells limit sympathoadrenal output. Recently, human α(2A)-adrenoceptor gene polymorphisms that influence α(2A)-adrenoceptor expression and function have been described. Increased α(2A)-adrenoceptor expression has been associated with impaired glucose-stimulated insulin secretion, elevated fasting blood glucose levels and an increased risk of type 2 diabetes. Accordingly, administration of α(2)-adrenoceptor agonists generally increases blood glucose levels, in spite of the ensuing sympatholysis that would be expected to lower blood glucose as a result of diminished α(1)- and β-adrenoceptor activation. α(2)-Adrenoceptor antagonists increase insulin secretion and reduce blood glucose levels by inhibiting tonically active α(2A)-adrenoceptors on pancreatic β-cells, but may also enhance sympathoadrenal output. In addition, α(2)-adrenoceptor antagonists potentiate the insulinotropic effect of sulphonylurea drugs, pointing to a potentially serious adverse drug interaction when the two classes of drugs are combined. The α(2)-adrenoceptor antagonist atipamezole is widely used in veterinary medicine, and sulphonylureas are prescribed for the treatment of type 2 diabetes in cats and dogs. Even if no dedicated α(2)-adrenoceptor antagonists are in clinical use in humans, some antipsychotic and antidepressant drugs are relatively potent α(2)-adrenoceptor antagonists. In the treatment of type 2 diabetes, α(2)-adrenoceptor agonists could possibly protect against sulphonylurea-induced hypoglycaemia, and α(2)-adrenoceptor antagonist drugs could improve insulin secretion. The potential usefulness of such drugs may vary between individuals, depending on α(2A)-adrenoceptor genetics, sympathetic tone and concomitant pathological conditions, such as cardiovascular disease and obesity.     

The duration of action of non-β2-adrenoceptor mediated responses to salmeterol

1. To investigate further the mechanism of the long duration of action of the selective β₂-adrenoceptor agonist, salmeterol, we have determined the duration of action of some responses to salmeterol which are not mediated throughβ₂-adrenoceptors.
2. In the presence of propranolol (1 μM), salmeterol (1–30 μM) caused concentration-related relaxation of superfused, pre-contracted strips of guinea-pig gastric fundus. On washing the tissues, these relaxant responses were rapidly lost, the time to 50% recovery being approximately 30 min.
3. In human neutrophils, salmeterol (1–100 μM) caused concentration-related inhibition of FMLP-induced O₂⁻ release. Propranolol (1 μM) had little or no effect on the inhibitory activity of salmeterol. Washing the cells twice over a 40 min period caused a marked reduction of the inhibitory activity of salmeterol.
4. In guinea-pig superfused trachea, in the absence of propranolol, infusions of (RS)-salmeterol (10–30 nM) and the less potent (S)-enantiomer of salmeterol (300–3000 nM) inhibited electrically-induced contractile responses. When the infusion was stopped, there was no recovery from the inhibitory responses within 200 min. In the presence of propranolol (1 μM), infusions of (RS)-salmeterol (10 μM) and (S)-salmeterol (10–100 μM) also inhibited the contractile responses, but, in contrast, on stopping the infusions differences were observed in recovery times. Thus no appreciable recovery was observed from the responses to (RS)-salmeterol, whereas a rapid loss of inhibition was observed on stopping the infusion of (S)-salmeterol, the time to 50% recovery being 30–35 min.
5. These relatively short-lasting effects of salmeterol which are not mediated through β₂-adrenoceptors, contrast with the persistence of the responses which are mediated through β₂-adrenoceptors seen in a variety of tissues, but are similar to the rate of dissociation of salmeterol observed from artificial membranes. These observations suggest that the sustained agonist activity of salmeterol is peculiar to responses mediated by β₂-adrenoceptors.