Alpha 2-adrenoceptor Agonist Research Articles

P02-315 - Alpha2C-adrenoceptor activation reduces dopa uptake in human neuroblastoma SH-SY5Y cells

Alpha2C-adrenoceptor antagonists have been proposed has having therapeutic value in the treatment of neuropsychiatric disorders such as schizophrenia and drug withdrawal. Mice with targeted deletion or selective pharmacological inhibition of alpha2C-adrenoceptors presented higher brain tissue levels of dopamine and its precursor 3, 4-dihydroxyphenylalanine (DOPA). In this study we investigated the effect of alpha2C-adrenoceptor activation on DOPA uptake in a human neuroblastoma cell line (SH-SY5Y). DOPA levels in cells were evaluated by high performance liquid chromatography with electrochemical detection. Results are presented as arithmetic mean ± standard error. SH-SY5Y cells take up DOPA in a time dependent (linear until 6 minutes) and concentration dependent (2.5–2500 μM) manner. Non-linear analysis of the saturation curves revealed for DOPA a Km (in μM) of 435 ± 91 and a Vmax (in nmol/ mg protein/6 min) of 465 ± 30. The uptake of DOPA was inhibited by BCH and neutral amino acids but no by MeAIB and the acidic and basic aminoacids. DOPA uptake was unaltered by lowering the pH. In the absence of sodium there was a 20% reduction in the Vmax values for DOPA uptake. For a single concentration of DOPA, incubation with different concentrations of the alpha2-adrenoceptor agonist medetomidine (0.1–1000nM) produced a concentration dependent decrease in DOPA uptake (IC50 in nM: 8 ± 0.3; Emax in % of inhibition: 55 ± 5). Pre-incubation with the selective alpha2C-antagonist JP-1302 (300nM) abolished the reduction produced by incubation medetomidine. In conclusion, in SH-SY5Y cells DOPA uptake is promoted by the LAT (sodium independent) and B0 (sodium dependent) systems and is inhibited by activation of alpha2C-adrenoceptors.

Effect of prazosin and guanfacine on stress-induced reinstatement of alcohol and food seeking in rats

Relapse to alcohol use during abstinence or maladaptive eating habits during dieting is often provoked by stress. The anxiogenic drug yohimbine, which causes stress-like responses in humans and non-humans, reliably reinstates alcohol and food seeking in a rat relapse model. Yohimibine is a prototypical alpha-2 adrenoceptor antagonist, but results from studies on noradrenaline's role in yohimbine-induced reinstatement of drug and food seeking are inconclusive. Here, we further addressed this issue by studying the effect of the alpha-1 adrenoceptor antagonist prazosin and the alpha-2 adrenoceptor agonist guanfacine on yohimbine-induced reinstatement. In exp. 1, we trained rats to self-administer alcohol (12% w/v, 1 h/day), and after extinction of alcohol-reinforced lever pressing, we tested prazosin's (0.5, 1.0, and 2.0 mg/kg, i.p.) or guanfacine's (0.125, 0.25, and 0.5 mg/kg, i.p.) effect on yohimbine (1.25 mg/kg, i.p.)-induced reinstatement; we also examined prazosin's effect on intermittent-footshock-stress-induced reinstatement. In exp. 2, we trained food-restricted rats to self-administer 45 mg food pellets and first examined prazosin's or guanfacine's effects on food-reinforced responding, and then, after extinction of lever presses, on yohimbine-induced reinstatement. Prazosin (0.5-2.0 mg/kg) blocked yohimbine-induced reinstatement of food and alcohol seeking, as well as footshock-induced reinstatement of alcohol seeking. Guanfacine attenuated yohimbine-induced reinstatement of alcohol seeking at the highest dose (0.5 mg/kg), but its effect on yohimbine-induced reinstatement of food seeking was not significant. Neither prazosin nor guanfacine affected high-rate food-reinforced responding. Results demonstrate an important role of postsynaptic alpha-1 adrenoceptors in stress-induced reinstatement of alcohol and food seeking.

Icilin-evoked behavioral stimulation is attenuated by alpha2-adrenoceptor activation

Icilin is a transient receptor potential cation channel subfamily M (TRPM8) agonist that produces behavioral activation in rats and mice. Its hallmark overt pharmacological effect is wet-dog shakes (WDS) in rats. The vigorous shaking associated with icilin is dependent on NMDA receptor activation and nitric oxide production, but little else is known about the biological systems that modulate the behavioral phenomenon. The present study investigated the hypothesis that alpha2-adrenoceptor activation inhibits icilin-induced WDS. Rats injected with icilin (0.5, 1, 2.5, 5mg/kg, i.p.) displayed dose-related WDS that were inhibited by pretreatment with a fixed dose of clonidine (0.15mg/kg, s.c.). Shaking behavior caused by a fixed dose (2.5mg/kg) of icilin was also inhibited in a dose-related manner by clonidine pretreatment (0.03–0.15mg/kg, s.c.) and reduced by clonidine posttreatment (0.15mg/kg, s.c.). Pretreatment with a peripherally restricted alpha2-adrenoceptor agonist, ST91 (0.075, 0.15mg/kg), also decreased the incidence of shaking elicited by 2.5mg/kg of icilin. Pretreatment with yohimbine (2mg/kg, i.p.) enhanced the shaking induced by a low dose of icilin (0.5mg/kg). The imidazoline site agonists, agmatine (150mg/kg, i.p.) and 2-BFI (7mg/kg, i.p.), did not affect icilin-evoked shaking. These results suggest that alpha2-adrenoceptor activation inhibits shaking induced by icilin and that increases in peripheral, as well as central, alpha2-adrenoceptor signaling oppose the behavioral stimulant effect of icilin.

New route for Tizanidine administration: a pharmacokinetics and light microscope autoradiography study

Spasticity could represent a complication of several central nervous system (CNS) disorders. Multiple sclerosis and painful paroxysmal syndromes (e.g. trigeminal neuralgia) are pathologies in which anti-spastic drugs are used to a greater extent . Baclofen, tizanidine (TIZ), benzodiazepines, dantrolene, and, more recently, gabapentin are the pharmacological agents more widely used. Baclofen, TIZ, benzodiazepines, gabapentin, clonidine but not dantrolene are active on CNS. The alpha2 adrenoceptor agonist TIZ is one of the most effective and largely used anti-spastic drugs. Oral treatment is the only route of anti-spastics administration, although it may cause problems of bioavailability and/or compliance in spastic patients with impaired deglutition. This study was designed to assess the possibility of develop a new route of administration of TIZ and to identify its targets in the spinal cord. New Zealand rabbits were treated with oral (OR , n=6), intramuscular (IM, n=6) or intranasal (IN, n=6) TIZ (3.2 mg/kg/day). Plasma concentration was measured by HPLC on samples collected at 0, 30, 60, 90, 120 and 480 min after treatment. Curves of average concentrations of TIZ vs. time were constructed. In plasma, TIZ reached a peak between 45-65 min after administration. C max was in the range of 268.33–1213.64 ng/ml for IM treatment, of 73.95–135.92 ng/ml for IN treatment and of 16.86–857.25 ng/ml for OR treatment. After pharmacokinetic studies, different spinal cord tracts were removed and used for radioligand binding assay and autoradiography. Using [3H]-RX821002 ([3H]-RX) (0.1-14nM), the Kd and Bmax for every spinal cord segment were calculated. The non-specific binding was obtained with 100µM of (-)-epinephrine. [3H]-RX sites were accumulated in the superficial laminae of dorsal horn. Dense [3H]-RX binding in control was seen over the superficial dorsal horn (laminae I-II) and centrally located lamina X. The ventral horn showed moderate levels of binding. [3H]-RX was displaced after TIZ treatment using different administration routes. The heaviest accumulation of silver grains (lowest displacement of TIZ) occurred after OR administration of the compound, the lowest after IM administration. These results suggest that both IM and IN administration of TIZ may represent routes of administration of the drug alternative to the OR one. Considering predictable adverse effects of IM treatment, the IN administration could represent the elective route to administrate this kind of drugs.

Comparison of SHR, WKY and Wistar rats in different behavioural animal models: effect of dopamine D1 and alpha2 agonists

Spontaneously hypertensive rats (SHR) and its counterpart, the Wistar-Kyoto rats (WKY), are probably the most often used animal model of ADHD. However, SHR as model of ADHD have also been criticised partly because of not differing to outbred rat strains. In the present study, adolescent SHR, WKY and Wistar rats from Charles River were tested in open-field, elevated plus maze and novel object recognition and on gastrointestinal transport to more intensively evaluate the strain characteristics. Non-habituated SHR and Wistar rats were more active than WKY rats but contrary to Wistar rats SHR stay hyperactive in a familiar environment. SHR were more sensitive to the alpha2-adrenoceptor agonist guanfacine and the dopamine D1 agonist A-68930 than WKY and Wistar rats, whereas amphetamine, the D1/D5 agonist ABT431 and the D2 agonist quinpirole, similarly affected open-field activity in all strains. In the elevated plus maze, SHR and Wistar rats showed less anxiety-related behaviour than WKY rats. Guanfacine and amphetamine induced an anxiolytic-like activity in SHR but not in WKY and Wistar rats. SHR showed the highest long-term memory in the novel object recognition. Gastrointestinal transport was similar and comparably affected by guanfacine in all rat strains. The present study shows clear differences in the behaviour of SHR and Wistar rats but also of WKY and Wistar rats. The use of SHR as animal model of ADHD is supported.

Alpha‐adrenoceptor agonistic activity of oxymetazoline and xylometazoline

Oxymetazoline and xylometazoline are both used as nasal mucosa decongesting α-adrenoceptor agonists during a common cold. However, it is largely unknown which of the six α-adrenoceptor subtypes are actually present in human nasal mucosa, which are activated by the two alpha-adrenoceptor agonists and to what extent. Therefore, mRNA expression in human nasal mucosa of the six α-adrenoceptor subtypes was studied. Furthermore, the affinity and potency of the imidazolines oxymetazoline and xylometazoline at these α-adrenoceptor subtypes were examined in transfected HEK293 cells. The rank order of mRNA levels of α-adrenoceptor subtypes in human nasal mucosa was: α(2A) > α(1A) ≥ α(2B) > α(1D) ≥ α(2C) >> α(1B) . Oxymetazoline and xylometazoline exhibited in radioligand competition studies higher affinities than the catecholamines adrenaline and noradrenaline at most α-adrenoceptor subtypes. Compared to xylometazoline, oxymetazoline exhibited a significantly higher affinity at α(1A) - but a lower affinity at α(2B) -adrenoceptors. In functional studies in which adrenoceptor-mediated Ca(2+) signals were measured, both, oxymetazoline and xylometazoline behaved at α(2B) -adrenoceptors as full agonists but oxymetazoline was significantly more potent than xylometazoline. Furthermore, oxymetazoline was also a partial agonist at α(1A) -adrenoceptors; however, its potency was relatively low and it was much lower than its affinity. The higher potency at α(2B) -adrenoceptors, i.e. at receptors highly expressed at the mRNA level in human nasal mucosa, could eventually explain why in nasal decongestants oxymetazoline can be used in lower concentrations than xylometazoline.

Prehypoxic clonidine administration improves vasomotricity of isolated rat aorta during reoxygenation

Clonidine, an alpha-2 adrenoceptor (A2A) agonist, improves posthypoxic function of several organs in humans and animal models. Mechanisms underlying these effects are not fully understood. A competent vasomotricity during reperfusion participates in the organ's recovery. This study investigates the specific effect of clonidine administration before hypoxia on posthypoxic vasomotricity in a rodent model. Isolated aortic rings from young rats were submitted to hypoxia/reoxygenation (20/40 min). Clonidine (10(-5) mol l(-1)), alone or with various receptor antagonists such as rauwolscine (specific A2A antagonist), ARC239 (A2B/C-prefering A antagonist), BRL44408 (A2AA antagonist), nitrogen oxide synthase inhibitor (Nω-nitro-L-arginine methyl ester) or cyclooxygenase inhibitor (indometacin), was administered in two randomized baths and washed out before hypoxia. Two other baths constituted the control group. In some experiments, clonidine was substituted by UK14.304 (A2A agonist). In all baths, posthypoxic endothelium-dependent dilatation (PED) and posthypoxic contraction were assessed. Clonidine improves PED (P = 0.002) and posthypoxic contraction (P = 0.001). Rauwolscine (P = 0.803) and ARC239 (P = 0.236) abolished clonidine-induced PED improvement. In the presence of BRL44408, clonidine increases posthypoxic endothelial dysfunction (P = 0.001). UK14.304 (P = 0.954) does not mimic the clonidine effect on PED. Concerning posthypoxic contraction, rauwolscine (P = 0.917), endothelial removal (P = 0.687), ARC239 (P = 0.118) or BRL44408 (P = 0.204) prevents the clonidine effect, but UK14.304 induces it (P = 0.005). Nω-Nitro-L-arginine methyl ester (P = 0.026) does not abolish it. Indometacin prevents it on PED and posthypoxic contraction (P = 0.678 and P = 0.851, respectively). Clonidine improves posthypoxic vasomotricity on isolated aortic rings. Concerning PED, two endothelial A2A subtypes are required. The clonidine effect on posthypoxic contraction requires endothelial A2A. Nitrogen oxide does not seem to be involved. Concerning the clonidine effect on PED and posthypoxic contraction, cyclooxygenase metabolites seem to be involved.

Locus coeruleus stimulation and noradrenergic modulation of hippocampo-prefrontal cortex long-term potentiation

Stimulation of the subiculum/CA1 of the hippocampal formation evokes monosynaptic field potentials in the prefrontal cortex (PFC). High-frequency stimulation of the hippocampus (HPC) can induce long-term potentiation (LTP) in this hippocampo-prefrontal cortical (hippo-PFC) pathway. Previous studies have shown that dopamine and serotonin modulate hippo-PFC LTP. Here, we investigated whether the locus coeruleus (LC) and noradrenaline (NA) can modulate LTP in the rat hippo-PFC pathway. Stimulation of the LC in combination with stimulation of the HPC increased hippo-PFC LTP. Infusion of lidocaine into the LC reduced hippo-PFC LTP. Administration of the noradrenaline reuptake inhibitor, nisoxetine or the alpha2 adrenoceptor antagonist, idazoxan prior to high-frequency stimulation of the HPC enhanced hippo-LTP. In contrast, administration of clonidine, an alpha2 adrenoceptor agonist, impaired hippo-PFC LTP. Partial noradrenergic (NAergic) lesioning with DSP-4 also impaired hippo-PFC LTP. In conclusion, the LC and NAergic mechanisms modulate hippo-PFC LTP.

Synergism between prostanoids and other vasoactive agents.

Prostaglandin E2 is usually considered to be a vasodilator, but some vascular beds respond with weak vasoconstriction mediated by prostanoid EP3-receptors. We have used the guinea pig isolated thoracic aorta to examine the nature of the synergism between the EP3 agonist sulprostone and other vasoactive agents. Muscle tension was recorded from endothelium-denuded rings of aorta suspended in conventional organ baths. Indomethacin and the TP-receptor antagonist GR 32191 were usually present. Sulprostone (0.1-300 nM) showed two profiles: low-responder preparations (maximum response = 15-35% of 100 nM U-46619 response) were insensitive to the L-type Ca2+ blocker nifedipine, whereas high-responders (maximum = 35-70%) showed a nifedipine-sensitive component at higher sulprostone concentrations only. Charybdotoxin (CTX), a blocker of large-conductance Ca2+-activated K+ channels (BKCa), slightly enhanced threshold sulprostone responses and markedly enhanced larger responses; the enhancements were abolished by nifedipine. In contrast, threshold sulprostone responses were dramatically enhanced in the presence of established small contractions to phenylephrine (alpha1-adrenoceptor agonist), U-46619 (TP agonist), cyclopiazonic acid (sarcoplasmic Ca2+ pump inhibitor), and 4-aminopyridine (4-AP, Kv channel blocker). Nifedipine had no effect on enhancements of threshold sulprostone responses, and partially inhibited the enhancements of larger responses. BKCa channel activation appears to increase progressively as sulprostone-induced contraction increases. CTX removes this "BKCa brake," thereby providing an "L-type channel" Ca2+ flux to prime the EP3-receptor-driven Ca2+-sensitization mechanism (via Rhokinase activation, unpublished observations). In contrast, the other agents, including 4-AP, direct a non-L-type channel source of Ca2+ to the calmodulin-myosin light chain arm.

The Modulation of Vascular ATP-Sensitive K+Channel Function via the Phosphatidylinositol 3-Kinase–Akt Pathway Activated by Phenylephrine

The present study examined the modulator role of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway activated by the alpha-1 adrenoceptor agonist phenylephrine in ATP-sensitive K(+) channel function in intact vascular smooth muscle. We evaluated the ATP-sensitive K(+) channel function and the activity of the PI3K-Akt pathway in the rat thoracic aorta without endothelium. The PI3K inhibitor 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002) (10(-5) M) augmented relaxation in response to the ATP-sensitive K(+) channel opener levcromakalim (10(-8) to 3 x 10(-6) M) in aortic rings contracted with phenylephrine (3 x 10(-7) M) but not with 9,11-dideoxy-11alpha,9alpha-epoxy-methanoprostaglandin F(2alpha) (U46619; 3 x 10(-8) M), although those agents induced similar contraction. ATP-sensitive K(+) channel currents induced by levcromakalim (10(-6) M) in the presence of phenylephrine (3 x 10(-7) M) were enhanced by the nonselective alpha-adrenoceptor antagonist phentolamine (10(-7) M) and LY294002 (10(-5) M). Levels of the regulatory subunits of PI3K p85-alpha and p55-gamma increased in the membrane fraction from aortas without endothelium treated with phenylephrine (3 x 10(-7) M) but not with U46619 (3 x 10(-8) M). Phenylephrine simultaneously augmented Akt phosphorylation at Ser473 and Thr308. Therefore, activation of the PI3K-Akt pathway seems to play a role in the impairment of ATP-sensitive K(+) channel function in vascular smooth muscle exposed to alpha-1 adrenergic stimuli.

High concentrations of dexmedetomidine inhibit compound action potentials in frog sciatic nerves without α2 adrenoceptor activation

Dexmedetomidine, an alpha(2)-adrenoceptor agonist, exhibits anti-nociceptive actions at the spinal cord and enhances the effect of local anaesthetics in the peripheral nervous system. Although the latter action may be attributed in part to inhibition of nerve conduction produced by dexmedetomidine, this has not been fully examined yet. We examined the effects of various adrenoceptor agonists including dexmedetomidine, and tetracaine, a local anaesthetic, on compound action potentials (CAPs) recorded from the frog sciatic nerve, using the air-gap method. Dexmedetomidine reversibly and concentration-dependently reduced the peak amplitude of CAPs (IC(50)= 0.40 mmol x L(-1)). This action was not antagonized by two alpha(2)-adrenoceptor antagonists, yohimbine and atipamezole; the latter antagonist itself reduced CAP peak amplitude. Clonidine and oxymetazoline, two other alpha(2)-adrenoceptor agonists, also inhibited CAPs; the maximum effect of clonidine was only 20%, while oxymetazoline was less potent (IC(50)= 1.5 mmol x L(-1)) than dexmedetomidine. On the other hand, (+/-)-adrenaline, (+/-)-noradrenaline, alpha(1)-adrenoceptor agonist (-)-phenylephrine and beta-adrenoceptor agonist (-)-isoprenaline (each 1 mmol x L(-1)) had no effect on CAPs. Tetracaine reversibly reduced CAP peak amplitude (IC(50) of 0.014 mmol x L(-1)). Dexmedetomidine reduced CAP peak amplitude without alpha(2)-adrenoceptor activation (at concentrations >1000-fold higher than those used as alpha(2) adrenoceptor agonist), with a lower potency than tetracaine. CAPs were inhibited by other alpha(2) adrenoceptor agonists, oxymetazoline and clonidine, and also an alpha(2) adrenoceptor antagonist atipamezole. Thus, some drugs acting on alpha(2) adrenoceptors are able to block nerve conduction.

Phenylephrine Suppresses the Pain Modulation of Diffuse Noxious Inhibitory Control in Rats

Diffuse noxious inhibitory control (DNIC) is a phenomenon whereby wide dynamic range neurons are selectively and powerfully inhibited through the central nervous system by noxious stimuli heterotopically applied to a body area distant from their excitatory receptive fields. Previous work has shown that systemic administration of an alpha1-adrenoceptor agonist, phenylephrine (PE), blocked the DNIC. We hypothesized that descending inhibitory pathways mediate the DNIC mechanism and that the neural network of the DNIC loop exists in the middle brainstem, likely in a more rostral part than formerly assumed, possibly the nucleus raphe magnus (RMg). The aim of this study was to determine whether DNIC is directly modulated by PE when administered close to the RMg. The experiments were performed on anesthetized male Sprague-Dawley rats. For administration of different drugs close to the RMg, the tip of a 33-gauge cannula was placed into an area close to the RMg as determined using the atlas of Paxinos and Watson. Single square-wave electrical stimuli were applied to the digits of the left hindpaw. The C-fiber reflex response elicited by electrical stimulation within the receptive field of the ipsilateral sural nerve was recorded from the biceps femoris muscle in the absence and presence of noxious tail immersion in warm water at 50 degrees C. The DNIC effect was calculated from a recorded electromyogram as the "inhibition rate." Saline (0.05 microL) or PE (0.05 microg/0.05 microL) was microinjected close to the RMg through the cannula. The C-fiber reflex evoked by electromyographic activity was recorded the same way. The inhibition rate of the C-fiber reflex was compared before and after administration of drugs. A paired t test was used for statistical comparison between same drug administration groups, and 1-way analysis of variance and Bonferroni multiple comparison were used for statistical analysis between different drugs. At the end of all experiments, the tissue-contacting end of the cannula tip was cauterized with an electric current to localize the drug administration site. The brain was removed, sliced in coronal sections, and stained with hematoxylin and eosin. The C-fiber reflex inhibited by noxious thermal stimuli (DNIC) was significantly blocked after the injection of PE close to the RMg. Direct administration of PE close to the RMg inhibited DNIC, thereby affecting and modulating the intrinsic pain inhibition system. These findings suggest that the RMg may be involved in the regulation of DNIC.

Purinergic contraction of the rat vas deferens in L-NAME-induced hypertension: effect of sildenafil

Hypertension (HTN) is a risk factor for erectile dysfunction, but its effect on vas deferens (VD) contractility and the ejaculatory response has not been delineated. NG-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, was used for induction of nitric oxide (NO)-deficient HTN. Our aim was to evaluate the effects of L-NAME-induced HTN on rat VD contractility and to determine whether sildenafil affects VD contractility. A total of 36 male rats were divided into (1) control, (2) L-NAME-HTN, (3) sildenafil treated L-NAME-HTN groups. Group 2 was treated with L-NAME (40 mg kg(-1) per day) in drinking water for 4 weeks. Group 3 received sildenafil (1.5 mg kg(-1) per day, by oral gavage) concomitantly with L-NAME. The prostatic portion of the VD was subjected to electrical field stimulation (EFS, 1-20 Hz), and the P2X(1) agonist alpha,beta-methylene ATP (alpha,beta-meATP, 100 micromol L(-1)-1 micromol L(-1)) and the alpha1-adrenoceptor agonist phenylephrine (Phe, 100 micromol L(-1)-1 mmol L(-1)) were used to construct concentration-response curves. These experiments were repeated in the presence of P2X receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 30 micromol L(-1)). VD contractions in response to EFS, alpha,beta-meATP and Phe were significantly enhanced by L-NAME. Sildenafil treatment in the L-NAME group improved the contractile response of VD to EFS (20 Hz). In the presence of PPADS, the enhanced contractile response of VD to EFS and alpha,beta-meATP in hypertensive rats was reversed. In the rat model of chronic NO depletion, the purinergic and adrenergic components and EFS affect VD contractility. The VD contractile response may be mediated more by the purinergic system than the adrenergic system, and sildenafil may alter the ejaculatory response in men with PE.

Premedication with clonidine is superior to benzodiazepines. A meta analysis of published studies

Premedication is considered important in pediatric anesthesia. Benzodiazepines are the most commonly used premedication agents. Clonidine, an alpha2 adrenoceptor agonist, is gaining popularity among anesthesiologists. The goal of the present study was to perform a meta-analysis of studies comparing premedication with clonidine to Benzodiazepines. A comprehensive literature search was conducted to identify clinical trials focusing on the comparison of clonidine and Benzodiazepines for premedication in children. Six reviewers independently assessed each study to meet the inclusion criteria and extracted data. Original data from each trial were combined to calculate the pooled odds ratio (OR) or the mean differences (MD), 95% confidence intervals [95% CI] and statistical heterogeneity were accessed. Ten publications fulfilling the inclusion criteria were found. Premedication with clonidine, in comparison with midazolam, exhibited a superior effect on sedation at induction (OR=0.49 [0.27, 0.89]), decreased the incidence of emergence agitation (OR=0.25 [0.11, 0.58]) and produced a more effective early post-operative analgesia (OR=0.33 [0.21, 0.58]). Compared with diazepam, clonidine was superior in preventing post-operative nausea and vomiting (PONV). Premedication with clonidine is superior to midazolam in producing sedation, decreasing post-operative pain and emergence agitation. However, the superiority of clonidine for PONV prevention remains unclear while other factors such as nausea prevention might interfere with this result.

Highly variable pharmacokinetics of dexmedetomidine during intensive care: a case report

IntroductionDexmedetomidine is a selective and potent alpha2-adrenoceptor agonist licensed for use in the sedation of patients initially ventilated in intensive care units at a maximum dose rate of 0.7 μg/kg/h administered for up to 24 hours. Higher dose rates and longer infusion periods are sometimes required to achieve sufficient sedation. There are some previous reports on the use of long-term moderate to high-dose infusions of dexmedetomidine in patients in intensive care units, but none of these accounts have cited dexmedetomidine plasma concentrations.Case presentationWe describe the case of a 42-year-old Caucasian woman with severe hemorrhagic pancreatitis following laparoscopic cholecystectomy who received dexmedetomidine for 24 consecutive days at a maximum dose rate of 1.9 μg/kg/h. Samples for the measurement of dexmedetomidine concentrations in her plasma were drawn at intervals of eight hours. On average, the observed plasma concentrations were well in accordance with previous knowledge on the pharmacokinetics of dexmedetomidine. There was, however, marked variability in the concentration of dexmedetomidine in her plasma despite a stable infusion rate.ConclusionThe pharmacokinetics of dexmedetomidine appears to be highly variable during intensive care.

Inhibitory and facilitory actions of isocyanine derivatives at human and rat organic cation transporters 1, 2 and 3: A comparison to human α1- and α2-adrenoceptor subtypes

Organic cation transporters (OCTs), comprising OCT1, OCT2 and OCT3 subtypes, control absorption and elimination of xenobiotics and endogenous compounds in kidney, liver and placenta. In addition, they ensure "uptake2", low-affinity catecholamine clearance in sympathetically-innervated tissue and the CNS. The prototypical OCT ligand, disprocynium24 (D24), recognises OCT3, but its actions at OCT1 and OCT2 remain unknown. Herein, together with two other isocyanine derivatives (AAC291 and AAC301) and chemically-related adrenergic agents, we evaluated actions of D24 at OCTs, monoamine transporters and alpha(1)- and alpha(2)-adrenoceptors. D24 concentration-dependently suppressed [3H]-1-methyl-4-phenylpyridinium (MPP+) transport at human (h) and rat (r) OCT1, OCT2 and OCT3 in stably transfected HEK293 cells. Interestingly, low concentrations of D24 enhanced transport by h/rOCT2, a substrate-dependent effect suppressed by inhibition of protein kinase C. AAC291 and AAC301 likewise inhibited transport by all classes of h/r OCT and at low concentrations induced even more marked increases in transport by h/rOCT2. Further, by analogy to D24, they displayed antagonist properties at halpha(1A/B/D)-adrenoceptors (Ca2+-flux) and halpha(2A/B/C)-adrenoceptors ([35S]GTPgammaS binding). They were, however, less potent than D24 at serotonin transporters ([3H]citalopram binding) and AAC291 did not bind to dopamine and norepinephrine transporters. The preferential alpha(1B)-adrenoceptor antagonist, AH11110A, the alpha2-adrenoceptor agonist, RWJ52353, and the adrenergic neurotoxin DSP-4 likewise affected [3H]MPP+ transport, in an OCT-subtype and species-dependent manner. In conclusion, D24, other isocyanine congeners and chemically-related adrenergic agents inhibit OCT-mediated [3H]MPP+ transport, and all drugs display significant activity at alpha1- and alpha2-adrenoceptor subtypes, expanding previous reports of promiscuity between pharmacophores recognising alpha-adrenoceptors and OCTs.

Effects of Clonidine and Isoflurane on the Myocardial Contractility Behavior of Isolated Rat Hearts

Isoflurane is chosen as an anesthesia drug for cardiac surgeries, and its effectiveness and safety have been proved in countless clinical studies. Clonidine, a central alpha-agonist, has recently been added to isoflurane to attenuate sympathetic hyperactivity by acting directly on its site of origin in the central nervous system. The ability of alpha2-adrenoceptor agonists to inhibit central sympathetic outflow may benefit patients at risk of myocardial damage by improving myocardial oxygen demand and the supply ratio and contributing to hemodynamic stability. We investigated the effects of clonidine and isoflurane, alone and in combination, on the myocardial contractility of isolated rat hearts and found that use of clonidine plus isoflurane decreased the systolic pressure somewhat, but use of the drugs separately did not exhibit this effect. Clonidine plus isoflurane did not affect +(dP/dt)max, but it did decrease -(dP/dt)max significantly compared with the use of isoflurane alone. These results indicate that clonidine and isoflurane have the capacity to interact with each other. The capacity of clonidine to decrease isoflurane's inotropic effect could theoretically contribute to improving the myocardial oxygen demand and the supply ratio, decreasing surgical stress, and benefiting patients at risk of myocardial damage.

Thermographic study of in vivo modulation of vascular responses to phenylephrine and endothelin-1 by dexamethasone in the horse

In vitro, glucocorticoids potentiate vasoconstriction of equine digital vessels to catecholamines and this has been implicated as a mechanism of glucocorticoid-induced laminitis. This observation has never been confirmed in vivo. To study the effects of glucocorticoid therapy on vasoconstrictor responsiveness in the horse in vivo. In a blinded, randomised cross-over experiment, 9 horses were treated with either dexamethasone (0.1 mg/kg bwt i.v. q. 24 h) or saline i.v. for 6 days. The changes in local average skin temperature before (baseline) and after intradermal injections of the alpha1-adrenoceptor agonist phenylephrine (PHE; 10(-4), 10(-5), 10(-6), 10(-7) and 10(-8) mol/l), endothelin-1 (ET-1; 10(-5), 10(-6), 10(-7), 10(-8) and 10(-9) mol/l) or ET-1 plus a blocker (BQ-123 10(-6) mol/l; RES-701 10(-6) mol/l; and L-NAME 10(-4) mol/l) were investigated with a thermograph. Dexamethasone (DEX) decreased baseline skin temperatures, suggesting reduced blood flow as a consequence of an increase in vasomotor tone. This was accompanied by potentiation of the response to PHE as demonstrated by a left shift in the dose-response curve and a decrease in the EC50. Dexamethasone did not potentiate ET-1, but the interplay with the lower baseline temperature resulted in a significantly lower skin temperature for this vasoconstrictor after DEX. The different ET-1 blockers had no effect on ET-1 modulated skin temperatures. Dexamethasone decreases skin perfusion. This is accompanied by a potentiated alpha1-adrenoceptor agonist response and a greater response to ET-1. Glucocorticoid therapy probably decreases perfusion of the equine hoof. During disease states that already are characterised by hypoperfusion and/or increased levels of circulating catecholamines, glucocorticoid therapy could, according to the vascular model of laminitis, tilt the balance in favour of laminitis.

Alpha-2 adrenoceptor agonists and sepsis: improved survival?

Dr Pandharipande and colleagues should be commended for segregating the effect of dexmedetomidine, an alpha-2 adrenoceptor agonist, in sepsis [1]. However, three questions arise from their study: what are the P-values for the data reported in Table 1? Why was there such a discrepancy between the fentanyl dose given to patients on dexmedetomidine and those on lorazepam (1,114 versus 117 μg/day, P = 0.01) considering the 50 to 80% reduction in the use of opiates commonly observed in the literature when alpha-2 adrenoceptor agonists are administered? And how many days did the patients spend on spontaneous (for example, pressure support) versus controlled/assisted ventilation? In their study, survival was better (a 70% reduction in risk of dying at 28 days) in patients on dexmedetomidine (n = 31) than in those on lorazepam (n = 32). Improved survival was observed earlier in tetanus patients [2] (rate of death of 50% versus 11% in control (n = 10) versus clonidine-treated (n = 17) patients; P = 0.04; this 1998 reference is not cited in the bibliography). In the study of Dr Pandharipande and colleagues, baseline characteristics were slightly different (Table 1 in [1]): temperature, heart/respiratory rate, incidence of vasopressors (dexmedetomidine, 32%; lorazepam, 56%) and drotrecogin alpha (activated; P = 0.20) were higher and systolic pressure lower in the lorazepam group despite 'similar severity of illness'. Could bias explain partially improved survival? As concluded by the authors [1], a larger trial should demonstrate improved survival (for example, upon septic shock [3]). Secondly, the dexmedetomidine patients received ten times more fentanyl and had more ventilator-free days. Usually, alpha-2 adrenoceptor agonists reduce the need for opiates by 50 to 80% and preserve spontaneous ventilation. So why this discrepancy? Thirdly, vasopressor requirements were reduced in the dexmedetomidine group (Table 3 in [1]). A 2003 reference [4] showed previously a reduced vasopressor requirement and was not cited in the bibliography. Could more ventilator-free days lead to less infections [5,6], improved survival, lowered intrathoracic pressure and reduced vasopressor requirements?

Reactive oxygen species are involved in regulating α1-adrenoceptor-activated vascular smooth muscle contraction

BackgroundReactive oxygen species (ROS) were shown to mediate aberrant contractility in hypertension, yet the physiological roles of ROS in vascular smooth muscle contraction have remained elusive. This study aimed to examine whether ROS regulate α1-adrenoceptor-activated contraction by altering myosin phosphatase activities.MethodsUsing endothelium-denuded rat tail artery (RTA) strips, effects of anti-oxidants on isometric force, ROS production, phosphorylation of the 20-kDa myosin light chain (MLC20), and myosin phosphatase stimulated by α1-adrenoceptor agonist phenylephrine were examined.ResultsAn antioxidant, N-acetyl-L-cysteine (NAC), and two NADPH oxidase inhibitors, apocynin and VAS2870, dose-dependently inhibited contraction activated by phenylephrine. Phenylephrine stimulated superoxide anion production that was diminished by the pretreatment of apocynin, VAS2870, superoxide scavenger tiron or mitochondria inhibitor rotenone, but not by xanthine oxidase inhibitor allopurinol or cyclooxygenase inhibitor indomethacin. Concurrently, NADPH oxidase activity in RTA homogenates increased within 1 min upon phenylephrine stimulation, sustained for 10 min, and was abolished by the co-treatment with apocynin, but not allopurinol or rotenone. Phenylephrine-induced MLC20 phosphorylation was dose-dependently decreased by apocynin. Furthermore, apocynin inhibited phenylephrine-stimulated RhoA translocation to plasma membrane and phosphorylation of both myosin phosphatase regulatory subunit MYPT1Thr855 and myosin phosphatase inhibitor CPI-17Thr38.ConclusionsROS, probably derived from NADPH oxidase and mitochondria, partially regulate α1-adrenoceptor-activated smooth muscle contraction by altering myosin phosphatase-mediated MLC20 phosphorylation through both RhoA/Rho kinase- and CPI-17-dependent pathways.