Affinity of detomidine, medetomidine and xylazine for alpha-2 adrenergic receptor subtypes

α₂-Adrenergic receptor agonists are widely used in veterinary medicine as sedative/hypnotic agents. Four pharmacological subtypes of the α₂-adrenergic receptor (A, B, C and D) have been identified based primarily on differences in affinity for several drugs. The purpose of this study was to examine the affinities of the sedative agents, xylazine, detomidine and medetomidine at the four α₂-adrenergic receptor subtypes. Saturation and inhibition binding curves were performed in membranes of tissues containing only one subtype of a₂-adrenergic receptor. The K_D for the α₂-adrenergic receptor radioligand, [³H]-MK-912, in HT29 cells (α_2A-), neonatal rat lung (α_2B-), OK cells (α_2C-) and PC12 cells transfected with RG20 (α_2D-) were 0.38 ± 0.08 n m , 0.70 ± 0.5 n m , 0.07 ± 0.02 n m and 0.87 ± 0.03 n m , respectively. Detomidine and medetomidine had approximately a 100 fold higher affinity for all the α₂-adrenergic receptors compared to xylazine but neither agonist displayed selectivity for the α₂-adrenergic receptor subtypes. These data suggest that available sedative/hypnotic α₂-adrenergic receptor agonists can not discriminate between the four known α₂-adrenergic receptor subtypes.     

The effect of xylazine on plasma thromboxane B2 concentration in sheep

α₂-adrenoceptor agonist drugs can cause respiratory changes leading to a short period of hypoxaemia in sheep. It has been suggested that this is due to transient platelet aggregation and pulmonary microembolism. If platelet aggregation were to follow platelet activation in response to the administration of α₂ agonists, plasma thromboxane levels would be expected to rise. This study was carried out to measure plasma thromboxane B₂ concentrations before and after the intravenous administration of the α₂-agonist drug xylazine at a dose of 0.1 mg/kg. It was found that the plasma thromboxane concentration rose by 320% and, furthermore, the rise was prevented by the prior administration of atipamezole hydrochloride (0.125 mg/kg), an α₂-adrenoceptor antagonist.     

Xylazine-induced mydriasis: possible involvement of a central postsynaptic regulation of parasympathetic tone

Intravenous injection of xylazine (0.01 – 1 mg/kg) produced a dose-dependent mydriasis associated with a depression of tonic ciliary nerve activity in anesthetized cats. Xylazine-induced mydriasis was apparent in the sympathectomized iris but was absent in the parasympathectomized, physostigmine-treated iris. Epinephrine (30 μg/kg, i.v.) produced a slighdy greater mydriasis in the sympathectomized iris than in the parasympathectomized, physostigmine-treated iris. The α₂-adrenergic blocking agent, yohimbine (0.5 mg/kg, i.v.) antagonized the pupillary dilation and reversed the depression of ciliary nerve activity induced by xylazine administration.
In rats pretreated with reserpine (7.5 mg/kg, s.c., 20 h) and α-methyl-p-tyrosine (250 mg/kg, i.p., 5 h), intravenous injection of xylazine (0.01 – 1 mg/kg) resulted in mydriasis of similar magnitude as control animals. However, xylazine induced bradycardia in the control group but not in die pretreated animals.
The results suggest that pupillary dilation produced by i.v. xylazine is primarily die result of a central inhibition of parasympathetic tone to the iris. It also appears that xylazine produces this effect via postsynaptic α₂-adrenergic mechanisms, while it produces bradycardia through a presynaptic α₂-adrenergic mechanism.     

Radioreceptor assay for determination of xylazine and medetomidine in sheep plasma

A radioreceptor assay technique is described for the measurement of xylazine and medetomidine in sheep plasma. The assay was based on the displacement of tritiated clonidine from a 2-adrenoceptors in a rat brain homogenate by xylazine or medetomidine extracted from plasma. Plasma samples from sheep which had been given xylazine and medetomidine were treated with alumina to remove endogenous catecholamines which would otherwise have bound to α₂-_- adrenoceptors and interfered with the assay. The drugs were then extracted using chloroform, reconstituted in buffer and used to displace [³H]clonidine. The concentration of α₂-agonist was calculated by reference to standard curves. The method had a detection limit of 2.5 ng/mL for xylazine and 0.24 ng/mL for medetomidine. The assay could also be used to detect metabolites capable of binding to α₂-receptors.     

The effects of opioid and α2 adrenergic blockade on non-steroidal anti-inflammatory drug analgesia in sheep

The analgesic effects of the non-steroidal anti-Inflammatory drugs (NSAIDs) flunixin and dipyrone were assessed in healthy sheep with no pre-existing inflammation, and in sheep with a chronic inflammatory lesion, using a mechanical noxious stimulus. Saline and dexamethasone were given as controls. Blood taken from healthy sheep after NSAID administration was assayed for thromboxane B₂ (TxB₂) to compare the ability of these drugs to inhibit cyclo-oxygenase. Both flunixin and dipyrone produced a small but statistically significant rise in pain thresholds (18% and 21% of maximum possible effect respectively) in the healthy sheep which peaked at 30 min and had returned to pre-drug values by 2–3 h. In the lame sheep a similar effect occurred but the response was smaller, much more variable and tended to be prolonged. Saline and dexamethasone had no effect on thresholds over 6 h in either group of sheep. The rise in thresholds was prevented by pre-treatment with naloxone (an opioid antagonist) or attpamezole (an α₂-adrenergic antagonist) in the healthy sheep. Naloxone and atipamezole had no effect on thresholds when given alone to healthy sheep. Both NSAIDs Inhibited the production of TxB₂ to a similar extent. These results indicate that central mechanisms may be involved in NSAID analgesia.     

Effects of medetomidine on intestinal and colonic motility in the dog

The motor responses of the jejunum and colon to stimulation of α₂-adrenoceptors by medetomidine and clonidine were investigated in four dogs. In fasting dogs, medetomidine, at a dose rate of 30 μg/kg i.v., disrupted the migrating myoelectric complex (MMC) pattern of the small intestine for about 2 h. Similar, but shorter-lasting effects were also induced by clonidine (30 μg/kg i.v.) on the jejunum. The administration of α₂-agonists inhibited colonic motility in fasting dogs, although medetomidine-induced inhibition was preceded by a short period of increased muscle tone. All these effects were reversed by the α₂-antagonists atipamezole (0.15 mg/kg i.v.) and yohimbine (0.20 mg/kg i.v.). In fed dogs, medetomidine (30 μg/kg i.v.) induced a strong increase of the tone on the proximal colon, while the activity of the medium and distal colon was completely suppressed. Yohimbine (0.50 mg/kg i.v.) immediately restored the activity of the colon and induced a propagated giant contraction and defaecation by the animal. These data confirm the importance of a₂-adrenergic receptors in the control of intestinal and colonic motility in the dog.     

Characterization of the antinociceptive and sedative effect of amitraz in horses

Amitraz, an acaricide used to control ectoparasites in animals has a complex pharmacological activity, including α₂-adrenergic agonist action. The purpose of this research was to investigate the possible antinociceptive and/or sedative effect of amitraz in horses. The sedative effect of the intravenous (i.v.) injection of dimethylformamide (DMF, 5 mL, control) or amitraz (0.05, 0.10, 0.15 mg/kg), was investigated on the head ptosis test. The participation of α₂-adrenergic receptors in the sedative effect provoked by amitraz was studied by dosing yohimbine (0.12 mg/kg, i.v.). To measure the antinociception, xylazine hydrochloride (1 mg/kg, i.v., positive control) and the same doses of amitraz and DMF were used. A focused radiant light/heat directed onto the fetlock and withers of a horse were used as a noxious stimulus to measure the hoof withdrawal reflex latency (HWRL) and the skin twitch reflex latency (STRL). The three doses of amitraz used (0.05, 0.10 and 0.15 mg/kg) provoked a dose-dependent relaxation of the cervical muscles. The experiments with amitraz and xylazine on the HWRL showed that after i.v. administration of all doses of amitraz there was a significant increase of HWRL up to 150 min after the injections. Additionally, there was a significant difference between control (DMF) and positive control (xylazine) values up to 30 min after drug injection. On the other hand, the experiments on the STRL show that after administration of amitraz at the dose of 0.15 mg/kg, a significant increase in STRL was observed when compared with the control group. This effect lasted up to 120 min after injection. However, no significant antinociceptive effect was observed with the 0.05 and 0.10 mg/kg doses of amitraz or at the 1.0 mg/kg dose of xylazine.     

Seizures during medetomindine sedation and local anaesthesia in two dogs undergoing skin biopsy

Each of two dogs presented for multiple skin biopsies were sedated with intravenous medetomidine and lignocaine was injected subcutaneously to provide local anaesthesia for skin biopsy. One dog had a seizure during skin biopsy and again immediately following reversal of medetomidine with atipamezole. The other dog developed seizures 2 h following skin biopsy at which time the medetomidine was reversed with atipamezole. Both dogs were neurologically normal with no history of seizures prior to the procedure and remained neurologically normal for 14 weeks and 9 months, respectively, following the procedure. A drug interaction between the α₂-adrenergic agonist medetomidine and lignocaine is suspected and highlights the potential for seizures following the subcutaneous administration of relatively large doses of lignocaine under medetomidine sedation.     

Relaxation of equine tracheal muscle in vitro by different adrenoceptor drugs

Strips of tracheal smooth muscle from 12 horses were contracted by carbachol in tissue baths under isometric conditions. This contraction (≅50% of maximum: EC₅₀) was relaxed completely with adrenoceptor drugs. The only exception was clenbuterol, where the degree of relaxation was ≅90%. In all horses the EC₅₀-value for isoprenaline (mean 1.6 × 10⁻⁸M) was less than that for adrenaline (mean 9.6 × 10^{− 8}M) and noradrenaline (mean 1. 8 × 10_{- 6}M). The potency ratio was 1 < 6 < 110 which indicates that the β₂-subtype dominates among the β-adrenoceptors of equine airways. All preparations were also very sensitive to the specific and potent β₂-receptor agonists clenbuterol (mean 5.7 × 10^{− 9}M) and procaterol (mean 3.6 × 10⁻¹⁰M). No differences in EC₅₀-values due to age, sex and breed were observed in this material. The standard deviation of the mean EC₅₀-values seems to be larger for the specific β₂-adrenoceptor agonists than for the unspecific. A reason for this could be differences in the pattern of the β-adrenoceptor population.     

The pharmacokinetics of xylazine hydrochloride: an interspecific study

The pharmacokinetic disposition of xylazine hydrochloride is described after both intravenous and intramuscular injection of a single dose, in four domestic species: horse, cattle, sheep and dog, by an original high performance liquid chromatographic technique. Remarkably small interspecific differences are reported. After intravenous administration, systemic half-life ( t _{1/2 β}) ranged between 22 min (sheep) and 50 min (horse) while the distribution phase is transient with half-life ( t _{1/2 α}) ranging from 1.2 min (cattle) to 5.9 min (horse). The peak level of drug concentration in the plasma is reached after 12–14 min in all the species studied following intramuscular administration. Xylazine bioavailability, as measured by the ratios of the areas under the intravenous and intramuscular plasma concentration versus time curves, ranged from 52% to 90% in dog, 17% to 73% in sheep and 40% to 48% in horse. The low dosage in cattle did not permit calculation. Kinetic data are correlated with clinical data and the origins of interspecific differences are discussed.     

The actions of medetomidine may not be mediated exclusively by α2-adrenoceptors in the equine saphenous vein

Spirals of endothelially denuded equine saphenous vein were used to study the pre- and post-junctional effects of medetomidine in vitro . The pD₂ values were calculated for noradrenaline (6.7 /pm 0.1), phenylephrine (5.6 /pm 0.1), BHT 920 (6.2 /pm 0.2) and UK 14304 (5.7 /pm 0.2). Medetomidine produced a biphasic response, with a pD₂1 of 8.2 /pm 0.1 and a pD₂2 of 5.7 /pm 0.1 in the equine saphenous vein ( n = 6 ). Prazosin (10⁻⁷ m) significantly shifted the second phase of the medetomidine concentration-response curve to the right (pD₂1 was 8.1 /pm 0.2 and pD₂2 was 5.0 /pm 0.2, P < 0.05). Rings of equine saphenous vein were electrically stimulated to investigate the pre-junctional effects of medetomidine. Increasing concentrations of the α₂-adrenoceptor agonist BHT 920 reduced the response to electrical stimulation in a concentration dependent manner to a maximum of 40 /pm 5%. whereas medetomidine (0.1-100 nm) caused a concentration dependent enhancement to a maximum of 490 /pm 150%. These results suggest α₁- and α₂-adrenoceptors are functional in the equine saphenous vein, but that medetomidine is not acting exclusively as an α₂-adrenoceptor agonist.     

The α2-adrenoceptor agonists xylazine and guanfacine exert different central nervous system, but comparable peripheral effects in calves

Acute pharmacodynamic effects of the α₂-adrenoceptor agonists, xylazine and guanfacine, were investigated in nine healthy calves in an open crossover trial. Xylazine (100 μg/kg body weight intravenously (i.v.)) and guanfacine (20 μg/kg body weight i.v.) were equi-effective in lowering heart rate by 25–30% at 5 min. Under these conditions, xylazine induced strong sedation and increased plasma growth hormone levels, indicating central nervous system mediated actions, whereas guanfacine was not sedative and did not induce release of growth hormone. Oxygen consumption was decreased by both drugs, but respiratory exchange ratio decreased only in response to xylazine. However, in response to both drugs, plasma levels of noradrenaline, adrenaline, insulin and non esterified fatty acids decreased similarly and glucose increased comparably. These results demonstrate marked differences in the central nervous system-mediated effects of the two α₂-adrenoceptor agonists, whereas peripheral actions are similar.     

Atipamezole increases medetomidine clearance in the dog: an agonist—antagonist interaction

Medetomidine, an α₂-adrenoceptor agonist, is a potent sedative and analgesic agent in the dog. When necessary, its action can be effectively antagonized by atipamezole. The present work was designed to study the effects of these drugs on each others' pharmacokinetics when a single intramuscular dose of medetomidine (50 μg kg^-1) was followed by a dose of atipamezole (250 μg kg^-1). Three different treatments were used: medetomidine alone, atipamezole alone, and atipamezole after medetomidine. Drug concentrations in plasma were measured by GC-MS. Statistical analysis of the results (anova) revealed significant differences between treatments in the kinetic parameters of medetomidine. Atipamezole decreased the AUC of medetomidine from 41.3 to 28.6 ng h ml"¹(P = 0.005), t_1/4 from 1.44 to 0.87 h ( P = 0.015), and increased Cl from 21 to 31 ml min^-1kg^-1(P = 0.017). Differences in V₂ did not reach statistical significance. The only statistically significant effects of medetomidine on the pharmacokinetics of atipamezole in this study were the slight decrease of Cl and C _max as well as the increase of AUC . It is suggested that the large dose of medetomidine used caused haemodynamic changes, resulting in decreased hepatic circulation and slower drug metabolism. Antagonism by atipamezole restored the hepatic blood flow and, consequently, increased the elimination of medetomidine by biotransformation.     

Cardiopulmonary effects of clonidine, diazepam and the peripheral α2 adrenoceptor agonist ST-91 in conscious sheep

The cardiopulmonary effects of the intravenous administration of clonidine (15 μg/kg), ST-91 (30 μg/kg) and diazepam (0.4 mg/kg) were compared in five healthy sheep using a randomized cross-over design, to determine whether the hypoxaemic effects of α₂ adrenoceptor agonists are due to sedation, or to peripheral α₂ adrenoceptor stimulation. All three drugs significantly lowered arterial oxygen tension (PaO₂) levels within 2 min of their administration; however, clonidine and ST-91 produced long lasting and severe hypoxaemia with mean PaO₂ levels of ≈40 mm Hg and 50 mm Hg (5.3 kPa and 6.6 kPa), respectively. The fall in PaO₂ was considerably less with diazepam (63 mm Hg or 8.4 kPa at 2 min) and by 15 min the values did not differ from placebo treated animals. None of the drugs increased arterial carbon dioxide tension (PaCO₂) levels when compared to saline treatment and the acid base variables did not show any significant change. A significant increase was recorded in the packed cell volume of the ST-91 treated group throughout the study. Within 2 min of their administration, all drugs caused a significant increase in mean arterial pressure (MAP) as compared to the placebo treated group. The MAP remained significantly increased for 5 and 60 min after clonidine and ST-91 treatment, respectively. The study shows that ST-91 and clonidine produce a greater degree of hypoxaemia than occurs with diazepam sedation, and that the hypoxaemic effect of α₂ adrenoceptor agonists in sheep are mainly mediated by peripheral α₂ adrenoceptors.     

Reversal of medetomidine-induced sedation in reindeer (Rangifer tarandus tarandus) with atipamezole increases the medetomidine concentration in plasma

The pharmacokinetics of two potent α₂-adrenoceptor agents that can be used for immobilization (medetomidine) and reversal (atipamezole) of the sedation in mammals, were studied in three reindeer ( Rangifer tarandus tarandus) in winter and again in summer. Medetomidine (60 μg/kg) was injected intravenously (i.v.), followed by atipamezole (300 μg/kg) intravenously 60 min later. Drug concentrations in plasma were measured by HPLC. The administration of atipamezole resulted in an immediate 2.5–3.5 fold increase in the medetomidine concentration in plasma. Clearance for medetomidine (median 19.3 mL/min·kg) was lower than clearance for atipamezole (median 31.0 mL/min·kg). The median elimination half-lives of medetomidine and atipamezole in plasma were 76.1 and 59.9 min, respectively. The animals became resedated 0.5–1 h after the reversal with atipamezole. Resedation may be explained by the longer elimination half-life of medetomidine compared to atipamezole.     

The effects of medetomidine and xylazine on gastrointestinal motility and gastrin release in the dog

The effect of medetomidine, a potent and highly selective α₂-adrenoceptor agonist, on the motility of the gastric antrum, duodenum, mid-jejunum and ileum was investigated in ten dogs. Its effect on the release of gastrin was also determined. Administration of medetomidine intramuscularly (i.m.) at a dose of 40 μg/kg inhibited the motility of the gastric antrum, duodenum, mid-jejunum and ileum significantly, in comparison to administration of xylazine intramuscularly at a dose of 2.0 mg/kg. The release of gastrin was also significantly decreased in dogs receiving medetomidine. It was found to inhibit the motility in the gastric antrum and duodenum longer than in the mid-jejunum and ileum, presumably by acting specifically on α₂-adrenoceptors, likely at the peripheral level. Medetomidine also inhibited the gastric contraction associated with gastrin secretion.     

Gastro-intestinal motor disturbances induced by lysine-acetylsalicylate treatment in sheep

The effects of intravenous (i.v.), intramuscular (i.m.) and oral administration of lysine-acetylsalicylate (Lys-ASA) on gastro-intestinal motility were investigated in sheep using electromyography. A dose of 20 mg/kg Lys-ASA intravenously reduced the frequency of reticular contractions for 86 ± 18 min, produced abomasal hypomotility and caused a disruption of the cyclical pattern of intestinal motility for at least 120 min. The frequency of reticular contractions measured from 20 to 30 min after Lys-ASA administration was negatively correlated (ß= 0.97; PΔ0.01) to the log of the dose used for doses varying from 10 to 40 mg/kg. Similar effects were observed with intramuscular and oral dose rates of 40 and 80 mg/kg, respectively. Previous i.v. administration of phentolamine (0.1 mg/kg) or tolazoline (2 mg/kg) abolished the effects of Lys-ASA (20 mg/kg) administered intravenously on both reticular contractions and abomaso-intestinal motility.
It was concluded that Lys-ASA administered at therapeutic doses in sheep produced gastro-intestinal motor disturbances and that α-and α₂-adrenergic antagonists are able to block them.     

Steroid—receptor interaction in a canine anal adenoma

An anal adenoma from a dog has been examined for cytosol binding proteins for androgens and oestrogens. Sucrose-density gradient analysis revealed low capacity binding moieties for testosterone with sedimentation coefficients of 2S and 8S, for 5α-dihydrotestosterone (8S) and for oestradiol-17β (4S and 8S). In each case it was shown that the steroids could be translocated to the nuclei and were found there to be protein-bound. The dissociation constants for 5α-dihydrotestosterone and oestradiol-17β were 5×1 × 10^-10M and 2×6 × 10^-10M respectively and the corresponding receptor site concentrations were 9 and 38 fmol/mg cytosol protein.
The possible significance of these findings with regard to the induction and maintenance of such tumours is discussed.     

Functional characterization of post-junctional adrenergic receptor subtypes in bovine intra-mammary arteries

We examined the functional role of adrenergic receptor subtypes (ARs) in bovine intra-mammary arteries (IMAs), 1.5–2.5 mm internal diameter. Norepinephrine (NE) and phenylephrine (PE) produced concentration-dependent increases in tone in segments maintained at a previously determined optimal basal tension in vitro . The sensitivity of the tissue to NE and PE, based on -log molar ED_50s was 6.87 ± 0.17 and 7.05 ± 0.35, respectively. In addition a Schild analysis yielded antagonist affinities for the receptor mediating contractile responses to NE (pA2 value) of 10.46 ± 0.85 for prazosin and 6.29 ± 0.18 for yohimbine. These data indicate a dominance of functional alpha 1 (α₁) over alpha 2 (α₂)-ARs in this tissue. Based on the inhibitory effects of chloroethylclonidine (CEC) on PE responses and the further reduction in sensitivity when nifedipine was added to the CEC, also in the presence of PE, we conclude that there is more than one α₁-AR subtype, with a predominant role for α_1B-ARs in phenylephrine responses. Stimulation of beta (β)-ARs, resulted in relatively small reductions in tone (the highest magnitude of response was 25.94 ± 6.46% of the papaverine maximum at 3×10⁻⁶ M isoproterenol); in addition, propranolol did not significantly alter tissue sensitivity to NE. Additional characterization of functional autonomic receptor populations in this circulatory bed will form a basis for future studies on circulatory dynamics in the mammary gland.     

Effects of xylazine on canine coronary artery vascular rings

OBJECTIVE: To determine the effects of xylazine on canine coronary artery smooth muscle tone. SAMPLE POPULATION: Hearts of 26 healthy dogs. PROCEDURE: Dogs were anesthetized with pentobarbital, and vascular rings of various diameters were prepared from the epicardial coronary arteries. Vascular rings were placed in tissue baths to which xylazine was added (cumulative concentrations ranging from 10(-10) to 10(-4) M), and changes in vascular ring tension were continuously recorded. Effects of the nitric oxide inhibitor NG-nitro-L-arginine methyl ester (L NAME; 5 mM), the alpha1-adrenoceptor antagonist prazosin (10 mM), and the alpha2-adrenoceptor antagonist atipamezole (10 mM) on xylazine-induced changes in vascular ring tension were determined. Results were expressed as percentage of maximal contraction for each vascular ring preparation. RESULTS: Xylazine induced vasoconstriction of small (< 500-microm-diameter) and medium (500- to 1,000-microm-diameter) vascular rings but not of large (> 1,000-microm-diameter) rings. For large vascular rings, L-NAME, atipamezole, and prazosin did not significantly affect the contractile response to xylazine. For small vascular rings, the contractile response following addition of xylazine to rings treated with L-NAME was not significantly different from the contractile response following addition of xylazine to control rings, except at a xylazine concentration of 10(-6) M. Xylazine-induced vasoconstriction of small vascular rings was blocked by atipamezole, but the addition of prazosin had no effect on xylazine-induced vasoconstriction. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that xylazine increases smooth muscle tone of small canine coronary arteriesand that this effect is predominantly mediated by stimulation of alpha2adrenoceptors.