James R. Docherty

Subtypes of functional α1- and α2-adrenoceptors

In this review, subtypes of functional α1- and α2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. α1-Adrenoceptors can be divided into α1A-, α1B- and α1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th α1-adrenoceptor, the α1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. α2-Adrenoceptors can be divided into α2A-, α2B- and α2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory α2-adrenoceptors are predominantly of the α2A-adrenoceptor subtype (the α2D-adrenoceptor is a species orthologue), although α2C-adrenoceptors may also occur prejunctionally. Although α2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of α-adrenoceptor subtypes.

Investigation of the subtypes of α1-adrenoceptor mediating contractions of rat aorta, vas deferens and spleen

1 The subtypes of α1‐adrenoceptor mediating contractions to exogenous noradrenaline (NA) or phenylephrine in rat vas deferens, spleen and aorta, and mediating contractions to endogenous NA in rat vas deferens have been examined. 2 In rat vas deferens, the competitive antagonists prazosin, WB 4101, benoxathian and 5‐methyl‐urapidil inhibited contractions to NA with pA2 values of 9.26, 9.54, 9.02 and 8.43, respectively. The irreversible antagonist chloroethylclonidine (CEC) (100 μm) failed to affect contractions to NA. 3 In rat vas deferens in the presence of nifedipine (10 μm), contractions to NA were significantly attenuated and under these conditions, CEC (100 μm) significantly reduced the maximum response to NA. 4 In rat spleen, the competitive antagonists prazosin, WB 4101 and benoxathian inhibited contractions to phenylephrine with pA2 values of 9.56, 8.85 and 7.60, respectively, and 5‐methyl‐urapidil had a KB of 6.62. CEC (100 μm) significantly reduced the maximum contraction to phenylephrine. 5 In rat aorta, the competitive antagonists, prazosin, WB 4101, benoxathian and 5‐methyl‐urapidil inhibited contractions to NA with pA2 values of 9.45, 9.21, 8.55 and 8.12, respectively. CEC (100 μm) produced an approximately parallel shift in the potency of NA, without significantly reducing the maximum response. 6 In epididymal portions of rat vas deferens in the presence of nifedipine (10 μm), the isometric contraction to a single electrical pulse was significantly reduced by CEC (100 μm), and by the competitive antagonists prazosin, WB 4101, benoxathian and 5‐methyl‐urapidil at concentrations of 1 nm. 7 In prostatic portions of rat vas deferens, the α1‐adrenoceptor agonist, amidephrine, produced concentration‐dependent increases in the isometric contraction to a single electrical stimulus and the maximum increase in the evoked response produced by amidephrine was unaffected by CEC (100 μm). 8 Contractions of rat vas deferens produced by NA (and amidephrine) are mediated predominantly by α1A‐adrenoceptors as shown by the high potency of α1A‐adrenoceptor selective antagonists and the lack of effect of CEC. A small CEC‐sensitive response, particularly in epididymal portions, was revealed in the presence of nifedipine. Contractions of rat spleen are mediated by α1B‐adrenoceptors since α1A‐selective antagonists showed low potency and CEC significantly reduced the maximum contraction to phenylephrine. Contractions of rat aorta to NA are mediated by non‐α1A, non‐α1B‐adrenoceptors, due to the high potency of the α1A‐selective antagonists and sensitivity to CEC. 9 The noradrenergic contraction of epididymal portions of rat vas deferens in the presence of nifedipine is CEC‐sensitive, but the α1A‐selective antagonists showed high potency, suggesting that this response is mediated by non‐α1A, non‐α1B‐adrenoceptors. 10 In conclusion, at least three subtypes of functional α1‐adrenoceptors have been demonstrated in these studies.

Subtypes of functional α1-adrenoceptor

In this review, subtypes of functional α1-adrenoceptor are discussed. These are cell membrane receptors, belonging to the seven-transmembrane-spanning G-protein-linked family of receptors, which respond to the physiological agonist noradrenaline. α1-Adrenoceptors can be divided into α1A-, α1B- and α1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A fourth α1-adrenoceptor, the α1L-, represents a functional phenotype of the α1A-adrenoceptor. α1-Adrenoceptor subtype knock-out mice have refined our knowledge of the functions of α-adrenoceptor subtypes, particuarly as subtype-selective agonists and antagonists are not available for all subtypes. α1-Adrenoceptors function as stimulatory receptors involved particularly in smooth muscle contraction, especially contraction of vascular smooth muscle, both in local vasoconstriction and in the control of blood pressure and temperature, and contraction of the prostate and bladder neck. Central actions are now being elucidated.

Effects of experimental diabetes on the responsiveness of rat aorta

1 Vascular responsiveness was examined in aortic ring preparations, with or without endothelium, from rats with experimental diabetes induced by streptozotocin and from vehicle‐treated (control) rats. 2 There were no significant differences between diabetic tissues and control tissues in the responsiveness to the vasoconstrictors noradrenaline, 5‐hydroxytryptamine and KCl, and to the vasodilators sodium nitroprusside, isoprenaline and acetylcholine. 3 When maximum contractions to vasoconstrictors was expressed relative to tissue weight, maximum contractions were significantly greater in diabetic tissues. 4 When expressed in terms of the KCl contraction, there were no significant differences between diabetic and control tissues in the maximum contraction to vasoconstrictors. 5 These results demonstrate that diabetic‐induced changes in vascular responsiveness, if any, do not occur at the receptor level.

Functional evidence for heterogeneity of peripheral prejunctional α2-adrenoceptors

1 We have examined the potencies of a series of α2‐adrenoceptor antagonists in functional studies of prejunctional α2‐adrenoceptors in rat atrium and vas deferens, and compared potencies with affinities for the α2A‐ligand binding site of human platelet and the α2B‐ site of rat kidney. 2 Antagonist potency in rat atrium was expressed as an EC30 (concentration producing 30% increase in the stimulation‐evoked overflow of tritium in tissues pre‐incubated with [3H]‐noradrenaline). Antagonist potency in rat vas deferens was expressed as a pA2 or KB at antagonizing the inhibition by the α2‐adrenoceptor agonist xylazine of the isometric twitch to a single stimulus, or as an EC30. 3 In ligand binding studies, Ki values were obtained for the displacement by α‐adrenoceptor antagonists of [3H]‐yohimbine binding to human platelet or rat kidney membranes. 4 In functional studies, three antagonists (ARC 239, prazosin and chlorpromazine) distinguished between prejunctional α2‐adrenoceptors of rat atrium (EC30) and rat vas deferens (pA2) and showed 49, 12 and 7 times higher potency in rat atrium, respectively. ARC 239 was also 17 times more potent in rat atrium than rat vas deferens when EC30 values were compared. 5 The correlation of affinity for the α2A‐ site of human platelet was better with prejunctional potency in rat vas deferens than rat atrium. 6 The correlation of affinity for the α2B‐site of rat kidney was better with prejunctional potency in rat atrium than rat vas deferens. 7 It is concluded that prejunctional α2‐adrenoceptors of rat vas deferens and rat atrium differ, and these receptors may resemble the α2A‐ and α2B‐ligand binding sites, respectively.

Experimental design and analysis and their reporting II: updated and simplified guidance for authors and peer reviewers

This article updates the guidance published in 2015 for authors submitting papers to British Journal of Pharmacology (Curtis et al., 2015) and is intended to provide the rubric for peer review. Thus, it is directed towards authors, reviewers and editors. Explanations for many of the requirements were outlined previously and are not restated here. The new guidelines are intended to replace those published previously. The guidelines have been simplified for ease of understanding by authors, to make it more straightforward for peer reviewers to check compliance and to facilitate the curation of the journals efforts to improve standards.

The role of monoamines in the changes in body temperature induced by 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) and its derivatives.

Hyperthermia is probably the most widely known acute adverse event that can follow ingestion of 3,4‐methylenedioxymethamphetamine (MDMA, ecstasy) by recreational users. The effect of MDMA on body temperature is complex because the drug has actions on all three major monoamine neurotransmitters [5‐hydroxytryptamine (5‐HT), dopamine and noradrenaline], both by amine release and by direct receptor activation. Hyperthermia and hypothermia can be induced in laboratory animals by MDMA, depending on the ambient temperature, and involve both central thermoregulation and peripheral changes in blood flow and thermogenesis. Acute 5‐HT release is not directly responsible for hyperthermia, but 5‐HT receptors are involved in modulating the hyperthermic response. Impairing 5‐HT function with a neurotoxic dose of MDMA or p‐chlorophenylalanine alters the subsequent MDMA‐induced hyperthermic response. MDMA also releases dopamine, and evidence suggests that this transmitter is involved in both the hyperthermic and hypothermic effects of MDMA in rats. The noradrenergic system is also involved in the hyperthermic response to MDMA. MDMA activates central α2A‐adrenoceptors and peripheral α1‐adrenoceptors to produce cutaneous vasoconstriction to restrict heat loss, and β3‐adrenoceptors in brown adipose tissue to increase heat generation. The hyperthermia occurring in recreational users of MDMA can be fatal, but data reviewed here indicate that it is unlikely that any single pharmaceutical agent will be effective in reversing the hyperthermia, so careful body cooling remains the principal clinical approach. Crucially, educating recreational users about the potential dangers of hyperthermia and the control of ambient temperature should remain key approaches to prevent this potentially fatal problem.

An investigation of presynaptic α-adrenoceptor subtypes in the pithed rat heart

1 The presynaptic cardio‐inhibitory and postsynaptic pressor responses to the α‐adrenoceptor agonists xylazine and cirazoline, and the interaction with the antagonists yohimbine and prazosin, were examined in the pithed rat. 2 Evidence was found to suggest that, as well as the already established pre‐ and postsynaptic α2‐and postsynaptic α1‐receptors, presynaptic α1‐receptors are present.

Are the prejunctional α2-adrenoceptors of the rat vas deferens and submandibular gland of the α2A-orα2D-subtype?

We have investigated the effects of antagonists at functional prejunctional alpha 2-adrenoceptors of rat was deferens and rat submandibular gland and compared potencies with affinities for the alpha 2A-, alpha 2B- and putative alpha 2D-ligand binding sites of human platelet, rat kidney, and rat submandibular gland, respectively. Prejunctional potency of antagonists was expressed as an EC30 (concentration producing a 30% increase in stimulation-evoked release of tritium in tissues pre-incubated with [3H]noradrenaline) in rat vas deferens and submandibular gland, and also as a pA2 (antagonist concentration producing a 2-fold shift in the inhibition by the alpha 2-adrenoceptor agonist xylazine of the electrically-evoked isometric twitch) in rat vas deferens. The functional prejunctional alpha 2-adrenoceptors of rat vas deferens and submandibular gland are alpha 2A-like since there was a good correlation between affinity for the alpha 2A-ligand binding site in human platelet and prejunctional potency in rat vas deferens (r = 0.90, n = 7, P less than 0.001), and submandibular gland (r = 0.84, n = 7, P less than 0.05). However, there was a better correlation between affinity for the alpha 2-ligand binding site of rat submandibular gland and prejunctional potency in rat vas deferens (r = 0.95, n = 7, P less than 0.001), and submandibular gland (r = 0.97, n = 7, P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacology of stimulants prohibited by the World Anti-Doping Agency (WADA)

This review examines the pharmacology of stimulants prohibited by the World Anti‐Doping Agency (WADA). Stimulants that increase alertness/reduce fatigue or activate the cardiovascular system can include drugs like ephedrine available in many over‐the‐counter medicines. Others such as amphetamines, cocaine and hallucinogenic drugs, available on prescription or illegally, can modify mood. A total of 62 stimulants (61 chemical entities) are listed in the WADA List, prohibited in competition. Athletes may have stimulants in their body for one of three main reasons: inadvertent consumption in a propriety medicine; deliberate consumption for misuse as a recreational drug and deliberate consumption to enhance performance. The majority of stimulants on the list act on the monoaminergic systems: adrenergic (sympathetic, transmitter noradrenaline), dopaminergic (transmitter dopamine) and serotonergic (transmitter serotonin, 5‐HT). Sympathomimetic describes agents, which mimic sympathetic responses, and dopaminomimetic and serotoninomimetic can be used to describe actions on the dopamine and serotonin systems. However, many agents act to mimic more than one of these monoamines, so that a collective term of monoaminomimetic may be useful. Monoaminomimietic actions of stimulants can include blockade of re‐uptake of neurotransmitter, indirect release of neurotransmitter, direct activation of monoaminergic receptors. Many of the stimulants are amphetamines or amphetamine derivatives, including agents with abuse potential as recreational drugs. A number of agents are metabolized to amphetamine or metamphetamine. In addition to the monoaminomimetic agents, a small number of agents with different modes of action are on the list. A number of commonly used stimulants are not considered as Prohibited Substances.