Effects of some α-adrenoceptor agonists and antagonists on the guinea-pig ileum

Summary The purpose of the present study was to further characterize the -adrenoceptors located on parasympathetic fibres. Segments of guinea-pig ileum were stimulated by transmural electrical pulses, and the ensuing contractions, which are due to the release of acetylcholine from postganglionic parasympathetic fibres, were monitored. Clonidine and tramazoline, which are thought to act preferentially on presynaptic -adrenoceptors, reduced the contractions, whereas phenylephrine and methoxamine, postsynaptic -adrenoceptor agonists, were ineffective. Contractions induced by acetylcholine were not changed by clonidine but were abolished by atropine. Yohimbine, piperoxan, phentolamine and thymoxamine reversed or prevented the inhibitory effect of clonidine. Prazosin and AR-C239 did not antagonize this effect. The inhibitory effect of tramazoline was antagonized by piperoxan but not by AR-C239 or by prazosin. Naloxone did not alter the action of clonidine, and piperoxan did not change the inhibitory effect of morphine.In conclusion, these experiments suggest the presence on cholinergic postganglionic fibres of both opiate receptors and -adrenoceptors. The latter appear to resemble more closely ₁-adrenoceptors than ₁-adrenoceptors.     

Synthesis and blocking activities of a new class of α₁-adrenoceptor antagonists

Finding effective chemotherapeutic agents for clinical use is a long-lasting goal in medicinal chemistry. In this study, we report a new class of α₁-adrenoceptor (α₁-AR) antagonists. Specifically, we describe the synthesis and the blocking activities toward α₁-AR of 7-(2-hydroxypropoxy)-3,4-dihydroisoquinolin-1(2H)-one 1 and its structurally perturbed analogs 2 – 11 that were designed according to the principle of bioisosterism. Their structures were identified with IR, ¹H NMR, MS, HRMS and elemental analysis. The blocking activities of compounds 1 – 11 were evaluated on isolated rat anococcygeus muscles. The results indicated that these compounds showed moderate to good activities. Among them, compound 1 exhibited the highest activity that was comparable to those of known α₁-AR antagonists tamsulosin and DDPH (1-(2,6-dimethylphenoxy)-2-(3,4-di- methoxyphenylethylamino)propane hydrochloride) and thus may be exploitable as a lead compound for the discovery of promising α₁-AR antagonists.     

Behavioural effects of the α2-adrenoceptor antagonists idazoxan and yohimbine in rats: comparisons with amphetamine

Although yohimbine has long been known to increase arousal, reactivity and anxiety in animals and humans, little is known about the behavioural effects of more selective ₂-adrenoceptor antagonists such as idazoxan. In a recent experiment, however, it was found that in rats both yohimbine and idazoxan increased low rates of lever pressing, an effect also produced by amphetamine. The purpose of the present study was to investigate further the effects of yohimbine and idazoxan in comparison with those of d-amphetamine on the operant behaviour of rats. In rats trained to press a lever on a FI 60s schedule to obtain food both yohimbine and idazoxan increased response rates, although the effect of yohimbine was considerably greater than that of idazoxan. Lower doses of d-amphetamine had no consistent effect on overall rates of responding whereas a higher dose suppressed responding. Characteristically, d-amphetamine increased responding during early portions of the intervals and decreased responding during the final portions. Idazoxan and yohimbine tended to increase responding throughout the intervals except immediately after reinforcement. When idazoxan was administered in combination with prazosin FI response rates were markedly decreased. Administration of DSP4 did not alter the response rate-increasing effects of either yohimbine or idazoxan. In rats trained to discriminate d-amphetamine from saline both idazoxan and yohimbine gave rise to responding on the saline associated lever. Combination of idazoxan with d-amphetamine did not antagonise the amphetamine cue but produced substantial reductions in response rates, probably due to toxicity. These results confirm previous findings that both idazoxan and yohimbine have behavioural stimulant effects but do not clarify the mechanisms involved. It is clear, however, that the behavioural actions of these ₂-adrenoceptor antagonists have little in common with those of the psychomotor stimulant amphetamine.     

Desirable properties of β3-adrenoceptor agonists: implications for the selection of drug development candidates

β3-adrenoceptor agonists are currently in clinical development for the treatment of overactive bladder and considered for several other indications. This Perspective discusses desirable properties of such drugs mainly based on the example of overactive bladder, but at least partly they should also be applicable to other indications of β(3)-adrenoceptor agonists or other drug classes and therapeutic areas. These include degree of selectivity for the molecular target in terms of affinity, intrinsic efficacy and ligand-directed signaling. The ability to cause agonist-induced desensitization and the potential impact of gene polymorphisms also need to be considered. Depending on intended indication, specific pharmacokinetic considerations may also apply. These findings challenge the usefulness of high-throughput screening assays based upon a single molecular response in an artificial system and emphasize the need for early use of in vivo testing in species considered to be predictive for the human situation.     

Discriminative stimulus effects of the α2-adrenoceptor antagonist idazoxan

Rats were trained to discriminate a dose of the ₂-adrenoceptor antagonist idazoxan (10 mg/kg IP) from saline. The discriminative stimulus produced by idazoxan was dose related and generalised to yohimbine. However, generalisation did not occur with a variety of compounds from other pharmacological categories including the ₁-adrenoceptor agonist cirazoline, the ₂-adrenoceptor antagonist prazosin, and the ₂-adrenoceptor agonist clonidine. The idazoxan stimulus was not antagonised by either prazosin or clonidine, although it was clear that idazoxan antagonised the reductions in response rate produced by clonidine. Dose-related responding on the idazoxan-associated lever was produced by the anxiolytics buspirone and ipsapirone and by their metabolite MJ 13653 (1-PP), which has previously been shown to be an ₂-adrenoceptor antagonist. In general, however, high levels of generalisation occurred with these three compounds only at doses which substantially reduced response rates. These results demonstrate that idazoxan can give rise to a discriminative stimulus which is probably mediated through antagonism at ₂-adrenoceptors although the failure of clonidine to block the idazoxan stimulus is difficult to explain.     

Stable expression and functional characterization of a human nicotinic acetylcholine receptor with α6β2 properties: discovery of selective antagonists

BACKGROUND AND PURPOSE

Despite growing evidence that inhibition of α6β2-containing (α6β2*) nicotinic acetylcholine receptors (nAChRs) may be beneficial for the therapy of tobacco addiction, the lack of good sources of α6β2*-nAChRs has delayed the discovery of α6β2-selective antagonists. Our aim was to generate a cell line stably expressing functional nAChRs with α6β2 properties, to enable pharmacological characterization and the identification of novel α6β2-selective antagonists.

EXPERIMENTAL APPROACH

Different combinations of the α6, β2, β3, chimeric α6/3 and mutant β3^V273S subunits were transfected in human embryonic kidney cells and tested for activity in a fluorescent imaging plate reader assay. The pharmacology of rat immune-immobilized α6β2*-nAChRs was determined with ¹²⁵I-epibatidine binding.

KEY RESULTS

Functional channels were detected after co-transfection of α6/3, β2 and β3^V273S subunits, while all other subunit combinations failed to produce agonist-induced responses. Stably expressed α6/3β2β3^V273S-nAChR pharmacology was unique, and clearly distinct from α4β2-, α3β4-, α7- and α1β1δε-nAChRs. Antagonist potencies in inhibiting α6/3β2β3^V273S-nAChRs was similar to their binding affinity for rat native α6β2*-nAChRs. Agonist affinities for α6β2*-nAChRs was higher than their potency in activating α6/3β2β3^V273S-nAChRs, but their relative activities were equivalent. Focussed set screening at α6/3β2β3^V273S-nAChRs, followed by cross-screening with the other nAChRs, led to the identification of novel α6β2-selective antagonists.

CONCLUSIONS AND IMPLICATIONS

We generated a mammalian cell line stably expressing nAChRs, with pharmacological properties similar to native α6β2*-nAChRs, and used it to identify novel non-peptide, low molecular weight, α6β2-selective antagonists. We also propose a pharmacophore model of α6β2 antagonists, which offers a starting point for the development of new smoking cessation agents.     

The α2-adrenoceptor antagonists idazoxan and yohimbine increase rates of DRL responding in rats

Previous studies have reported that antidepressant drugs exert specific effects on responding maintained by DRL schedules of reinforcement, giving rise to increased frequencies of reinforcement. In order to investigate whether the ₂-adrenoceptor antagonist idazoxan would produce similar effects, the actions of this compound were compared with those of yohimbine, imipramine, mianserin and d-amphetamine in rats trained to lever press for food reinforcement on a DRL 60-s schedule. Neither imipramine nor mianserin produced any effects on response rate or reinforcement frequency, except at the highest doses. In contrast, both idazoxan and yohimbine gave rise to dose-related increases in rates of responding and consequent decreases in reinforcement frequencies. Amphetamine also increased responding, but higher doses of this drug produced marked hyperactivity and stereotyped movements which were not observed after idazoxan and yohimbine. Although the present behavioural baseline was not sensitive to antidepressants, it demonstrated an unexpected activity of two ₂-adrenoceptor antagonists which deserves further investigation.     

Discriminative stimulus properties of β-carbolines characterized as agonists and inverse agonists at central benzodiazepine receptors

The discriminative stimulus properties of three -carboline derivatives were studied in three groups of rats trained, respectively, to discriminate diazepam (2.5 mg/kg IP), chlordiazepoxide (CDP, 5 mg/kg IP) or pentylenetetrazol (PTZ, 15 mg/kg IP) from saline in standard procedures employing two-lever operant chambers. Two -carbolines, ZK 91296 and ZK 93423, substituted for the benzodiazepines in both CDP- and diazepam-trained rats. The neutral benzodiazepine antagonist Ro 15-1788 blocked the diazepam discriminative stimulus and the ability of ZK 91296 to substitute for diazepam. A third -carboline, FG 7142, was not identified as benzodiazepine-like in generalization tests in either diazepam- or CDP-trained rats, but when administered together with CDP antagonized the benzodiazepine discriminative stimulus. In rats trained to discriminate PTZ from saline (a discrimination which is thought to depend on the anxiogenic properties of PTZ) the PTZ cue was antagonized by diazepam and ZK 93423, and partially antagonized by ZK 91296. The PTZ cue generalized to FG 7142 and this generalization was partially antagonized by Ro 15-1788. These results suggest that the three -carbolines provide more than one kind of discriminative stimulus, consistent with the classification of ZK 93423 as an agonist at central benzodiazepine receptors, with ZK 91 296 as a partial agonist, and with FG 7142 as an inverse agonist. Pharmacologically, ZK 93 423 and ZK 91 296 may exhibit anxiolytic qualities, whereas FG 7142 produces anxiogenic effects.     

Further study of prerequisites for the enhancement by α-adrenoceptor antagonists of the release of noradrenaline

Summary Segments of the rabbit ear artery were preincubated with (–)-³H-noradrenaline and then perfused/superfused and stimulated by transmural electrical pulses. The outflow of ³H-noradrenaline and total tritium was determined.In the first series of experiments, stimulation periods of approximately constant length (50 s) were used (cocaine 5 M present). Thirteen pulses (0.25 Hz) elicited an overflow of ³H-noradrenaline of 0.024% of tissue tritium; 26 pulses (0.5 Hz) elicited an overflow of 0.059%, and 52 pulses (1 Hz) of 0.166%. Rauwolscine 1 M did not change the overflow evoked by 13 pulses, increased that evoked by 26 pulses and increased most markedly that evoked by 52 pulses. Phentolamine 1 M decreased the overflow at 13, did not change the overflow at 26, and increased the overflow at 52 pulses. Corynanthine 1 M decreased the overflow at 13, and did not change the overflow at 26 and 52 pulses. The effect of tetraethylammonium (TEA) 100 M was opposite to that of rauwolscine; it increased most markedly the overflow evoked by 13 pulses, increased less that evoked by 26 pulses, and least the overflow at 52 pulses.In the second series of experiments, the frequency of stimulation was kept constant (2 Hz). In the absence of cocaine, 10 pulses elicited an overflow of ³H-noradrenaline of 0.023% of tissue tritium; 20 pulses elicited an overflow of 0.043%, and 40 pulses of 0.089%. Phentolamine 1 M did not change the overflow evoked by 10 pulses, increased that evoked by 20 pulses, and increased most markedly that evoked by 40 pulses. TEA 100 M increased the evoked overflow at all pulse numbers. Similar results were obtained in the presence of cocaine 5 M.The results demonstrate that the enhancement by -adrenoceptor antagonists of the release of noradrenaline depends on the biophase concentration of noradrenaline. Under the present conditions, graded increases in biophase noradrenaline concentration led to graded increases in the effect of the antagonists. A second prerequisite for the release-enhancing effect appears to be a sufficient length of the pulse train. Under the present conditions, graded increases in train length up to about 20s led to graded increases in the effect of the antagonists, even though the average biophase concentration of noradrenaline did not change with the pulse train length. This pattern of effects of the -antagonists is not shared by at least one other release-enhancing drug, namely TEA.     

Electrophysiological and electrochemical analysis of the secretion of ATP and noradrenaline from the sympathetic nerves in rat tail artery: effects of α2-adrenoceptor agonists and antagonists and noradrenaline reuptake blockers

Summary The aim of this study was to investigate whether or not nerve impulses release ATP and noradrenaline in parallel from the sympathetic nerve terminals of the rat tail artery. The extracellularly recorded excitatory junction current (EJC) was used to study, pulse by pulse, the release of ATP. An electrochemical method was used to study online the nerve stimulation-induced rise in the extracellular concentration of endogenous noradrenaline at the probe, a carbon fibre electrode (CF). This parameter, which does not directly represent noradrenaline release, but reflects release minus clearance, has been termed [NA]_CF. The effects of a number of pharmacological agents on the EJCs were examined both at 0.1 and 2 Hz, and the effects on the EJC response to 100 pulses at 2 Hz compared with that on the [NA]_CF response. Clonidine and xylazine were used as ₂-agonists, yohimbine and idazoxan as ₂-antagonists and desipramine and cocaine as blockers of noradrenaline reuptake. Most of these agents had unwanted side effects, especially at higher concentrations. However, clonidine and xylazine depressed at lower concentrations the EJC and [NA]_CF responses to about the same extent; these effects were partially or completely reversed by yohimbine. Yohimbine or idazoxan did not affect the EJCs at 0.1 Hz but enhanced the EJC and [NA]_CF responses to 100 pulses at 2 Hz to the same extent. All effects of desipramine (1 M) seemed explainable as a result of block of noradrenaline reuptake, while cocaine (10 M) in addition exerted an unspecific depressant (probably local anesthetic) effect. Under control conditions, both agents depressed the EJC but dramatically enhanced the [NA]_CF response to 100 pulses at 2 Hz. Addition of yohimbine prevented the depressant effect of desipramine on the EJCs completely and reduced that of cocaine, but increased their effects on the [NA]_CF response. These results are compatible with the view that ATP and noradrenaline are released in parallel from the sympathetic nerve terminals of this tissue. The different, and under some conditions even opposite, effects of desipramine or cocaine on the EJC and [NA]_CF responses are explainable in terms of the known post-secretory effects of these agents. Send offprint requests to M. Msghina at the above address     

Effect of α2-adrenoceptor agonists on the expression of morphine-withdrawal in rats

Summary Previous studies using clonidine indicate that ₂-adrenoceptors are involved in suppressing opiate-withdrawal symptoms. However, clonidine may act as a partial agonist at ₂-adrenoceptors and it also possesses significant ₁-receptor agonist activity.The aim of this study was to determine the role of ₂-adrenoceptors in the expression of opiate withdrawal signs using morphine-dependent rats. A range of agonists were selected for study on the basis of their differential preferences for -adrenoceptors.Hooded Wistar rats were made physically dependent on morphine (s.c. injection of an emulsion releasing a total of 250 mg/kg of morphine base over 48 h). Test drugs were injected s.c. followed by naloxone (10 mg/kg i.p.) 20 min later. The incidence of 5 selected withdrawal signs was recorded during the following 20 min. The ₂-adrenoceptor agonists displayed different profiles of activity. Azepexole (1–10 mg/kg) reduced all signs. Clonidine (80–800 g/kg) reduced all signs except paw shakes while guanfacine (25–250 g/kg) reduced all except jumping and diarrhoea. Talipexole (0.1–1 mg/kg) reduced all signs except diarrhoea which was not affected and jumping which was markedly enhanced. UK 14,304 (80–800 g/kg) reduced jumps, potentiated paw shakes but did not affect body shakes, teeth chattering or diarrhoea. The results suggest that there are subpopulations of ₂-adrenoceptors that modulate the expression of opiate withdrawal signs and/or that some of the drugs used affect receptors other than ₂-adrenoceptors.     

Antagonism of the hypothermic effect of clozapine in mice by centrally-active α2-adrenergic antagonists and α1-adrenergic agonists

Hypothermia induced by either clozapine or clonidine in mice was blocked by the ₂-adrenergic antagonists yohimbine, idazoxan, CH-38083, SKF 86466, and L-657,743. These effects were dose related, and the ID₅₀ values for inhibition of clozapine- or clonidine-induced hypothermia were fairly comparable. The order of potency for blocking clonidine-induced hypothermia was: L-657,743>CH-38083>yohimbine>idazoxan>SKF 86466. A very similar blockade hierarchy for clozapine-induced hypothermia was observed, with the order of the two most effective compounds being reversed. Hypothermia induced by either compound was not blocked by the peripherally-acting, selective ₂-adrenergic antagonist, L-659,066, indicating that blockade by the other compounds occurred centrally. The centrally-acting, ₁-adrenergic agonists St 587, cirazoline, and SKF 89748 were very effective in blocking the response to clozapine, but ineffective in antagonizing clonidine-induced hypothermia. The ED₅₀ values for the blockade of this response to clozapine, however, did not correlate with their reported potencies in stimulating either peripheral or central ₁-adrenergic receptors. This indicates that clozapine-induced hypothermia in mice is not a suitable model for evaluating the properties of central ₁-adrenergic compounds. Moreover, since the clonidine-induced hypothermia is not influenced by ₁-adrenergic agonists, this paradigm is preferable to clozapine-induced hypothermia in the assessment of ₂-adrenergic antagonism. The ability of ₂-adrenergic antagonists to block clozapine-induced hypothermia may result from the central overflow of norepinephrine, which is known to be brought about by this group of compounds. The neurochemical mechanism responsible for the anti-clozapine effect of the ₁-adrenergic agonists in mice is not clear. An as yet unknown property of these compounds, unrelated to ₁-agonism, may have to be considered. An interaction of these compounds at the high-affinity clozapine binding site is a possibility. Discovery of antagonists to clozapine may help to elucidate the mechanism of action of this atypical neuroleptic.     

Exploring the pharmacology of the pro-convulsant effects of α2-adrenoceptor antagonists in mice

The effects of selective and specific ₂-adrenoceptor antagonists on electroshock seizure threshold in mice were investigated. Idazoxan, at low doses, efaroxan, RX811059 and RX821002 significantly lowered seizure threshold. The ₁-agonist St 587 and the -agonist isoprenaline were also pro-convulsant. On the other hand the ₂-agonists clonidine and UK 14,304 produced small increases in seizure threshold. Anticonvulsant effects were also produced by low doses of the noradrenaline uptake inhibitor desipramine. This compound increases levels of noradrenaline in the synaptic cleft which could subsequently act at post-synaptic ₂-adrenoceptors. The pro-convulsant action of ₂-adrenoceptor antagonists could be explained in terms of two mechanisms: a) blockade of endogenous noradrenaline which may normally exert a tonic anti-convulsant influence on seizure threshold, through post-synaptic ₂-receptors and/or b) increased activation of ₁- and -adrenoceptors by elevated synaptic noradrenaline levels following blockade of pre-synaptic ₂-adrenoceptors. Of the ₂-antagonists tested, idazoxan was unusual in that high doses were not pro-convulsant; this difference may be explained by ₁-adrenoceptor mediated actions or be related to its recently described affinity at a non-adrenoceptor site — a function for which is currently unknown.     

Influence of neuronal uptake on pre- and postjunctional effects of α-adrenoceptor agonists in tissues with noradrenaline — ATP cotransmission

Summary This study was undertaken in an attempt to explain why in some of the tissues in which noradrenaline and ATP act as co-transmitters the noradrenergic component predominates, while in others the predominant component is purinergic. Four different tissues were used: the epididymal portion of the rat vas deferens and the rabbit ear artery, tissues where the noradrenergic component predominates, and the prostatic portion of the rat vas deferens and the rabbit jejunal artery, where the purinergic component predominates. The noradrenaline content as well as the electrically-evoked release of noradrenaline were determined in all tissues. To determine the evoked release, the tissues were pretreated with pargyline (1 mmol · 1^–1) and then exposed to ³H-noradrenaline, washed out and transmurally stimulated (1 Hz). In addition, the influence of inhibition of neuronal uptake by desipramine (40nmo1·1^–1) on pre- and postjunctional effects of adrenaline and -methylnoradrenaline (and/or noradrenaline) was compared.The noradrenaline content of the tissues averaged: 17.4, 23.2, 3.1, and 4.8 g·g^–1 for the epididymal and the prostatic portions of the rat vas deferens and for the ear and the jejunal arteries of the rabbit, respectively. The fractional electrically-evoked release of ³H-noradrenaline was 2.02 and 2.04 × 10^–5 for the epididymal and the prostatic portions of the rat vas deferens, respectively, and 3.33 and 3.26 × 10^–5 for the ear and the jejunal arteries of the rabbit, respectively. Desipramine enhanced much more the postjunctional effect of noradrenaline, adrenaline, and -methylnoradrenaline in the epididymal than in the prostatic portion of the rat vas deferens. Moreover, desipramine similarly enhanced the effects of noradrenaline and adrenaline in the ear artery, while it did not modify the responses to these amines in the jejunal artery. The influence of inhibition of neuronal uptake on the prejunctional effect of sympathomimetic agonists was studied in the rat vas deferens only. Desipramine caused very marked and similar enhancements of the inhibitory effect of the amines in the epididymal and the prostatic portions (72.5 and 93.5 times, respectively, for adrenaline, and 34.7 and 41.4 times, respectively, for -methylnoradrenaline).These results are compatible with the hypothesis that postjunctional adrenoceptors are situated closer to the varicosities in the epididymal than in the prostatic portion of the rat vas deferens and also closer in the rabbit ear than in the rabbit jejunal artery. Therefore, the variation in the relative importance of the noradrenergic and the purinergic components may depend on the distance between the sites from where the transmitters are released and the sites where they act. Send offprint requests to J. Gonçalves at the above address     

Acute dilation to α2-adrenoceptor antagonists uncovers dual constriction and dilation mediated by arterial α2-adrenoceptors

Background and purpose:

In mouse tail arteries, selective α₂-adrenoceptor antagonism with rauwolscine caused powerful dilation during constriction to the α₁-adrenoceptor agonist phenylephrine. This study therefore assessed phenylephrine''s selectivity at vascular α-adrenoceptors and the mechanism(s) underlying dilation to rauwolscine.

Experimental approach:

Mouse isolated tail arteries were assessed using a pressure myograph.

Key results:

The α₂-adrenoceptor agonist UK14,304 caused low-maximum constriction that was inhibited by rauwolscine (3 × 10⁻⁸ M) but not by the selective α₁-adrenoceptor antagonist prazosin (10⁻⁷ M). Concentration–effect curves to phenylephrine, cirazoline or noradrenaline were unaffected by rauwolscine but were inhibited by prazosin, which was more effective at high compared with low levels of constriction. In the presence of prazosin, rauwolscine inhibited the curves and was more effective at low compared with high levels of constriction. Although rauwolscine alone did not affect concentration–effect curves to phenylephrine, noradrenaline or cirazoline, it caused marked transient dilation when administered during constriction to these agonists. Dilation was mimicked by another α₂-adrenoceptor antagonist (RX821002, 3 × 10⁻⁸ M), was dependent on agonist selectivity, and did not occur during adrenoceptor-independent constriction (U46619). During constriction to UK14,304 plus U46619, rauwolscine or rapid removal of UK14,304 caused transient dilation that virtually abolished the combined constriction. Endothelial denudation reduced these dilator responses.

Conclusions and implications:

Inhibition of α₂-adrenoceptors caused transient dilation that was substantially greater than the contribution of α₂-adrenoceptors to the constriction. This reflects a slowly reversing α₂-adrenoceptor-mediated endothelium-dependent dilation and provides a rapid, sensitive test of α₂-adrenoceptor activity. This approach also clearly emphasizes the poor selectivity of phenylephrine at vascular α-adrenoceptors.     

β(2) -adrenoceptor agonists: current and future direction

Despite the passionate debate over the use of β(2) -adrenoceptor agonists in the treatment of airway disorders, these agents are still central in the symptomatic management of asthma and COPD. A variety of β(2) -adrenoceptor agonists with long half-lives, also called ultra long-acting β(2) -adrenoceptor agonists (ultra-LABAs; indacaterol, olodaterol, vilanterol, carmoterol, LAS100977 and PF-610355) are currently under development with the hopes of achieving once-daily dosing. It is likely that the once-daily dosing of a bronchodilator would be a significant convenience and probably a compliance-enhancing advantage, leading to improved overall clinical outcomes. As combination therapy with an inhaled corticosteroid (ICS) and a LABA is important for treating patients suffering from asthma, and a combination with an inhaled long-acting antimuscarinic agent (LAMA) is important for treating COPD patients whose conditions are not sufficiently controlled by monotherapy with a β(2) -adrenoceptor agonist, some novel once-daily combinations of LABAs and ICSs or LAMAs are under development.     

Long-acting beta2-adrenoceptor agonists: a smart choice for asthma?

An endogenous defect in beta(2)-adrenoceptors that results in impaired relaxation of airway smooth muscle was originally proposed as a putative underlying cause of the asthmatic condition. Short-acting beta(2)-adrenoceptor agonists such as salbutamol are still recommended for relieving acute episodes of bronchial smooth muscle spasm. Twice-daily long-acting beta(2)-adrenoceptor agonists (LABAs) such as salmeterol are advocated for use as add-on bronchodilator therapies to inhaled corticosteroids such as beclomethasone, which are used as first-line anti-inflammatory therapy. There is recent evidence of increased asthma exacerbations and associated deaths in subjects taking salmeterol compared with those taking placebo. My opinion is that, in certain situations, the use of LABAs could have adverse effects on asthma control because of the particular pharmacological properties of this class of drug. In this article, I examine the possible pharmacological mechanisms and use of LABAs in certain situations, which are relevant to the benefits and risks of these drugs in asthma.     

A comparison of the effects of TMB-8 and nifedipine on pressor responses to α1- and α2-adrenoceptor agonists in pithed rats

TMB-8 has been characterized as an inhibitor of the release of Ca⁺ from intracellular pools. We have studied the modification of the pressor responses to selective _l-adrenoceptor agonists (methoxamine and phenylephrine), and to selective ₂-adrenoceptor agonists (B-HT 920 and B-HT 933) in pithed rats, produced by TMB-8. We have compared this modification with that produced by the calcium antagonist nifedipine. Nifedipine (100 g/kg, 300 g/kg, and 1000 g/kg) inhibited in a dose-dependent manner the pressor responses to the ₁- and ₂-adrenoceptor agonists, the dose-response curves to the ₂-adrenoceptor agonists being shifted further to the right. TMB-8 at a dose of 3000 g/kg did not modify the pressor effects of the _l-adrenoceptor agonists, and neither did it reinforce the inhibition of such responses produced by nifedipine. By contrast, TMB-8 pretreatment (0.03 g/kg, 0.3 g/kg, 3 g/kg, 30 g/kg, 300 g/kg and 3000 g/kg) inhibited the responses to both ₂-adrenoceptor agonists, the inhibition being more pronounced with B-HT 920. A similar effect was obtained with 0.03 g/kg TMB-8 and 0.3 g/kg TMB-8, particularly in the case of B-HT 920. It was stronger with higher doses, but similar for all doses over 3 g/kg. The inhibition of the pressor responses mediated by the stimulation of ₂-adrenoceptors by TMB-8 was less in rats treated with the Ca²⁺ entry promoter BAY K 8644 (300 g/kg), and could also be reduced by the continuous infusion of CaCl₂ (0.25 g/min). These results suggest that in pithed rats TMB-8 may also behave as an inhibitor of the Ca⁺ influx into vascular smooth muscle.     

Inhibition by α2-adrenoceptor agonists of the secretion of catecholamines from isolated adrenal medullary cells

Summary In adrenal medullary cells, carbachol evokes the secretion of catecholamines with simultaneous uptake of⁴⁵Ca. Highly selective agonists for ₂-adrenoceptors, clonidine, naphazoline, guanfacine, tramazoline and oxymetazoline inhibited carbachol evoked secretion of catecholamines in a dose-dependent manner. These ₂-agonists also inhibited the uptake of⁴⁵Ca evoked by carbachol with similar dose-response curve to inhibition of catecholamine secretion. On the contrary, highly selective agonists for ₁-adrenoceptors, phenylephrine and norfenefrine did not inhibit the secretion of catecholamines and cellular uptake of⁴⁵Ca. The inhibition by ₂-agonists was not restored either by the increase in carbachol, or Ca concentrations, suggesting that the mode of inhibition was distinct from competition at cholinergic receptors or Ca-channels. It is likely that ₂-agonists inhibited the secretion of catecholamines via the inhibition of Ca uptake which was probably caused through the activation of ₂-adrenoceptors.