Alpha 2-adrenoceptor Antagonist Yohimbine Research Articles

Changes in plasma catecholamine and neuropeptide Y levels after sympathetic activation in dogs

1. Plasma levels of noradrenaline (NA) and neuropeptide Y (NPY) were evaluated in two experimental models associated with an increase in sympathetic tone: conscious dogs which were subject to either sinoaortic denervation or acute administration of the alpha 2-adrenoceptor antagonist yohimbine. 2. Dogs that had undergone sinoaortic denervation exhibited a two fold increase in plasma NA without any change in NPY levels. 3. Yohimbine (0.05 mg kg-1 i.v. as a bolus) produced similar effects. A higher dose of yohimbine (0.5 mg kg-1 i.v.) increased both plasma NA (7 fold) and NPY (6.5 fold) levels. 4. The present results indicate that changes in plasma catecholamines and NPY are not always concomitant. They suggest that the simultaneous release of NA and NPY is only observed under in vivo conditions for a marked increase in sympathetic tone.

Neurotransmission in human resistance arteries: contribution of alpha 1- and alpha 2-adrenoceptors but not P 2-purinoceptors.

The contribution of postjunctional alpha 1- and alpha 2-adrenoceptors and P2-purinoceptors to the neuroeffector response was examined in isolated segments of human subcutaneous resistance arteries. Electrical field stimulation (EFS, 20 V, 0.2 ms, 1-25 Hz) elicited a maximum contractile response which was 38.2 +/- 1.6% of that elicited by exogenously applied (5 microM) noradrenaline (n = 56). Tetrodotoxin (1 microM), used to inhibit neurotransmission, reduced the electrically evoked response to 24.7 +/- 4.4% (n = 10) of the control response. The alpha 1-adrenoceptor antagonist prazosin (1 microM) reduced the maximum EFS contractile response to 64.8 +/- 5.5% of the control response (n = 17). Application of the alpha 2-adrenoceptor antagonist yohimbine (0.1 microM) reduced the maximum EFS response to 68.2 +/- 8.2% of the control response (n = 9). In the presence of prazosin plus yohimbine at the above-mentioned concentrations the maximum response to EFS was reduced to 47.6 +/- 6.7% (n = 11). Responses following alpha-blockade were not statistically different from those in the presence of tetrodotoxin, but the mean responses indicate that a non-adrenergic component to the EFS response cannot be discounted. Desensitisation of P 2-purinoceptors with alpha, beta-methylene ATP had no effect on responses to EFS; therefore under the conditions studied these receptors do not appear to be involved in neurotransmission. These results confirm the presence of postjunctional alpha 1- and alpha 2-adrenoceptors in human resistance arteries and for the first time demonstrate that the postjunctional alpha 2-adrenoceptor is important in modulating vascular responses elicited by intramural sympathetic nerve fibres.

The activation of Ca(2+)-dependent K+ conductance by adrenaline in mouse peritoneal macrophages.

Responses to adrenaline in mouse peritoneal macrophages were investigated with perforated and cell-attached patch-clamp recording, and with a combination of the perforated-patch recording and fura-2 fluorescence measurements. Extracellularly applied adrenaline induced a transient outward current (4-10 s in duration, 100-500 pA in amplitude) at -40 mV associated with a marked increase in conductance. The adrenaline-induced current [Io (Adr)] reversed polarity near -80 mV. The reversal potential depended distinctly on the external K+ concentration but not on external Cl- concentration. Removal of external Ca2+ did not affect Io(Adr) within 2-4 min but subsequent responses to adrenaline were progressively depressed. In contrast, treatment with an intracellular Ca2+ chelator, the acetoxymethyl ester of 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid completely abolished Io(Adr). Furthermore, Io(Adr) was blocked by bath-applied quinidine and charybdotoxin, but not by tetraethylammonium or apamin. Extracellular application of an alpha 1-adrenoceptor agonist phenylephrine and of noradrenaline mimicked Io(Adr). On the other hand, Io(Adr) was antagonized by a non-selective alpha-adrenoceptor antagonist phentolamine (0.2 microM) and an alpha 1-adrenoceptor antagonist prazosin (0.2 microM), but was not affected by an alpha 2-adrenoceptor antagonist yohimbine (1 microM) or a beta-adrenoceptor antagonist propranolol (1 microM). Cell-attached single-channel recordings with the pipette solution containing 145 mM KCl revealed the activation of single-channel currents with a conductance of 40 pS during application of adrenaline outside the patch. Parallel measurements of membrane current and fura-2 fluorescence in the same cell demonstrated a correlation between the rise in [Ca2+]i and an increase in K+ conductance. Therefore, it is concluded that adrenaline activates a Ca(2+)-dependent K+ conductance by release of Ca2+ from internal stores through an activation of an alpha 1-adrenoceptor.

Studies on the interaction between presynaptic alpha 2-adrenoceptors and adenosine A1 receptors located on noradrenergic nerve terminals.

The aim of the present study was to obtain a more detailed understanding of the interaction between presynaptic alpha 2-adrenoceptors and A1 adenosine receptors mediating inhibition of noradrenaline release in the central nervous system. Slices of rabbit hippocampus, prelabelled with [3H]noradrenaline, were superfused in the presence of the re-uptake inhibitor (+)-oxaprotiline and electrically stimulated during superfusion. During stimulation with 36 pulses at 3 Hz the alpha 2-adrenoceptor antagonist yohimbine induced a five-fold increase of noradrenaline release indicating a pronounced autoinhibition of approximately 80%. In these experiments the inhibition of release caused by R-PIA, a preferential A1 agonist, as well as its facilitation caused by DPCPX, a selective A1 antagonist, were smaller in comparison to the effects of these compounds on release virtually free of autoinhibition (i.e. by stimulating the tissue with 4 pulses at 100 Hz (POP-stimulation) or with 36 pulses at 3 Hz in presence of yohimbine). Clonidine, an alpha 2-adrenoceptor agonist, was used to impose a distinct alpha 2-adrenoceptor-mediated inhibition of release elicited by POP-stimulation. Only, however, in the presence of 30 nmol/l clonidine, causing maximum inhibition of approximately 80% of 3H-overflow, but not in the presence of 6 nmol/l clonidine, causing approximately 50% inhibition, a significant diminution of the inhibitory effect of R-PIA was seen. Similarly, the alpha 2-adrenoceptor mechanism was affected only by 10 mumols/l R-PIA causing maximum inhibition of approximately 80%, but remained unchanged in the presence of 30 nmol/l R-PIA diminishing release by 50%.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of NG-monomethyl-L-arginine on arcade arterioles of rat spinotrapezius muscles

The effect of the specific inhibitor of nitric oxide (NO) formation NG-monomethyl-L-arginine (L-NMMA) on resting arteriolar diameter and on actions of both endothelium-dependent and -independent vasoactive substances was investigated using intravital microscopy in rats. The spinotrapezius muscle of anaesthetized normotensive rats was suspended in a Krebs-Henseleit bath containing tetrodotoxin (3 x 10(-7) M), indomethacin (2.8 x 10(-5) M), and propranolol (10(-6) M) to block sympathetic nerve conduction, prostacyclin formation, and beta-adrenergic receptors, respectively. Acetylcholine (ACh), nitroprusside (NP), norepinephrine (NE), phenylephrine (PE), and guanabenz (GB) were topically applied before and after superfusion of L-NMMA (10(-5) to 10(-4) M). Superfusion of L-NMMA reduced arteriolar diameter and caused dose-dependent increases in arteriolar tone. The onset of action of L-NMMA was nearly immediate. L-NMMA inhibited vasodilator responses to the endothelium-dependent vasodilator ACh but not to the endothelium-independent NP. NE induced dose-related vasoconstriction that was significantly potentiated by L-NMMA. These effects were partially reversed by addition of L-arginine (10(-3) M). Potentiation of vasoconstriction elicited by NE was still observed after inhibition of alpha 1-adrenoceptors with prazosin, but potentiation was abolished by the alpha 2-adrenoceptor antagonist yohimbine. L-NMMA potentiated arteriolar vasoconstriction elicited by the alpha 2-adrenergic receptor agonist GB but not by the alpha 1-adrenoceptor agonist PE. These findings with L-NMMA suggest that resting diameter of arterioles is modulated by endogenous NO biosynthesis and that endothelium-dependent vasodilators act through the formation of endogenous NO to exert their action in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrically evoked noradrenaline release from cultured chick sympathetic neurons: modulation via presynaptic ?2-adrenoceptors and lack of autoinhibition

Sympathetic neurons from twelve day old chick embryos were plated on polystyrol discs and kept in culture for five days. After incubation with 3H-noradrenaline the discs were transferred to small chambers and superfused. Electrical field stimulation (36 pulses at 3 Hz) increased the outflow of tritium. The evoked overflow of tritium was abolished in the absence of extracellular calcium and was diminished by about 90% in the presence of tetrodotoxin (1 mumol/l). The alpha 2-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline (UK-14,304) caused a concentration-dependent decrease in overflow, whereas the alpha 1-adrenoceptor agonist methoxamine was ineffective at up to 1 mumol/l. The concentration-response curve of UK-14,304 was shifted to the right by the alpha 2-adrenoceptor antagonist yohimbine (0.03 mumol/l). Yohimbine on its own caused no significant change. The noradrenaline reuptake inhibitor cocaine (10 mumol/l) caused a small (20%) increase in evoked overflow. The results indicate that cultured chick sympathetic neurons possess release-modulating alpha 2-adrenoceptors and that the electrically induced overflow of transmitter occurs under conditions virtually free of autoinhibition.

Biphasic effect of tricyclic antidepressants on the release of norepinephrine from the adrenergic nerves of the rabbit heart

The release of norepinephrine (NE) from the right atrium of the rabbit heart was used as a model to investigate biphasic effects due to tricyclic antidepressants, similar to those clinically observed in the treatment of depression and known as "therapeutic window". Strips of the atrium were loaded with 3H-NE, and then superfused by Krebs solution. The basal release and the electrical stimulation evoked release of 3H-NE were measured in the presence and absence of four clinically used tricyclic antidepressants: imipramine, amitriptyline, desipramine and nortriptyline. In addition, guanethidine, an adrenergic neuron blocker, was also studied. At lower concentrations (0.5-10 microM) tricyclic antidepressants increased, whereas higher concentrations (50-100 microM), inhibited the evoked release of NE. This inhibition was not prevented by the alpha2 adrenoceptor antagonist yohimbine, excluding the possibility of alpha 2 adrenoceptor-mediated inhibition of NE release. In higher concentrations the tricyclic antidepressants increased the basal release of NE in a Ca-independent way. Secondary amine derivatives were more potent inhibitors of the evoked release, and enhance the resting basal release of NE to a greater extent than the tertiary ones. Similarly, guanethidine (1-50 microM) also decreased the evoked release and increased the basal release of NE in a concentration dependent manner. Yohimbine failed to counteract the inhibition caused by guanethidine and the increment of the basal release was Ca-independent. It is concluded that the effect of tricyclic antidepressants in potentiating the release of NE is masked by their adrenergic neuron blocking properties, i.e. they inhibit the release of NE.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of food intake in rats by the alpha 2-adrenoceptor agonists xylazine and detomidine

Effects of the alpha 2-adrenoceptor agonists xylaziine and detomidine on food intake in freely feeding male rats were evaluated. Intraperitoneal (i.p.) injection of xylazine (0.25-3 mg/kg) and detomidine (25 and 50 μg/kg) significantly increased the 2 and 24 h food intake from control values. Treatment of rats with the alpha 2-adrenoceptor antagonist yohimbine (1 mg/kg, i.p.) 15 min before xylazine (0.5 mg/kg, i.p.) or detomidine (50 μg/kg, i.p.) significantly inhibited the increase in food intake. Yohimbine treatments alone at 0.5, 1 and 2 mg/kg, i.p. did not significantly change the 2 and 24 h food intake from control values. The results suggest that the alpha 2-agonists xylazine and detomidine enhance food intake in rats.

Differential effects of electrical stimulation, blockade of neuronal amine uptake and activation of α2-adrenoceptors on the release of endogenous noradrenaline and 5-hydroxytryptamine from the isolated rat pineal gland

Isolated rat pineal glands were incubated in vitro and the release of endogenous noradrenaline or 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was determined by HPLC with electrochemical detection. In the absence of test drugs, the spontaneous outflow of noradrenaline was about 10 fmol/10 min and electrical stimulation (5 Hz, 1500 pulses) evoked the release of about 70 fmol noradrenaline. Nomifensine enhanced the spontaneous outflow of noradrenaline about threefold and the electrically evoked release of noradrenaline about sixfold. In the presence of nomifensine, the alpha 2-adrenoceptor antagonist yohimbine markedly increased the electrically evoked release of noradrenaline, whereas the alpha 1-adrenoceptor antagonist prazosin had no effect. Clonidine inhibited the electrically evoked release of noradrenaline by about 65%, and this was antagonized by yohimbine in a competitive manner. In the absence of drugs, the initial spontaneous outflow of 5-HT was (compared with noradrenaline) very high 64 pmol/10 min). It declined by 80% within 1 h of incubation in vitro. The outflow of 5-HIAA amounted initially to 38 pmol 10 min and declined by 40% within 1 h of incubation. Addition of L-tryptophan (10 mumol/l) after 1 h of incubation in vitro largely enhanced the outflow of 5-HT and 5-HIAA within 30 min of incubation (about ten- and fourfold, respectively). When L-tryptophan was present from the onset of incubation the initial outflow of 5-HT and 5-HIAA was only slightly elevated, but the decline was largely attenuated. Neither omission of calcium nor addition of nomifensine, clonidine or yohimbine significantly affected the spontaneous outflow of 5-HT or 5-HIAA.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of desipramine (DMI) and alpha-2 adrenoceptor agonists on flexor reflex activity (FRA) in the spinalized and decerebrate rat

In the spinalized and decerebrate rat, electrically evoked flexor reflex activity (FRA) of the right anterior tibialis muscle was facilitated by DMI (0.1-3 mg/kg i.v.). DMI (3 mg/kg i.v.) had not effect on twitches of the left ant. tib. muscle evoked by electrical stimulation of the distal stump of the severed sciatic nerve (directly evoked twitches). The alpha-1 adrenoceptor antagonist prazosin (300 micrograms/kg i.v.) and the alpha-2 adrenoceptor agonists clonidine (5 and 10 micrograms/kg i.v.) and BHT-920 (100 micrograms/kg i.v.) all suppressed FRA facilitated by DMI (3 mg/kg i.v.). The alpha-2 adrenoceptor antagonists yohimbine (100 micrograms/kg i.v.) and mianserin (30 micrograms/kg i.v.) antagonized the suppressant effect of clonidine (5 micrograms/kg i.v.) on DMI-facilitated FRA. Clonidine (10 micrograms/kg i.v.) had no effect on directly evoked twitches, and at 3-300 micrograms/kg i.v. had no effect on FRA facilitated by L-DOPA (100 mg/kg i.p.). It is suggested that DMI-facilitated FRA can be used as a test for alpha-2 adrenoceptor agonists which may act at presynaptic alpha-2 adrenoceptors in the spinal cord.

Pharmacodynamic effects of chronic yohimbine treatment in healthy volunteers

The effects of 14 days of treatment with the alpha 2-adrenoceptor antagonist yohimbine on cardiovascular parameters (blood pressure and heart rate), plasma catecholamines, insulin and non-esterified fatty acids (NEFA), and the number of alpha 2-adrenoceptors on platelets have been evaluated. The treatment increased plasma noradrenaline but not adrenaline and did not change the other cardiovascular, metabolic or routine blood parameters. It caused a decrease in the velocity of adrenaline-induced platelet aggregation, although [3H]-yohimbine binding to platelets remained unchanged. Acute administration of yohimbine before the experiment led to a significant increase in plasma noradrenaline and NEFA levels. After the 15 day treatment, acute administration of the same dose of yohimbine had less effect on plasma NEFA and there was no significant change in plasma noradrenaline. The acute lipid-mobilizing effect of yohimbine in healthy volunteers persisted after the 14 days of treatment with no change in other parameters. The prolonged alpha 2-adrenoceptor blockade with yohimbine did not induce any change in platelet alpha 2-adrenoceptors.

Amitraz-induced glucose intolerance in rats: antagonism by yohimbine but not by prazosin

Amitraz (N'-[2,4-dimethylphenyl]-N-[(2,4-dimethylphenyl)imino]-N- methylmethanimidamide) is a formamidine insecticide/acaricide that increases plasma glucose and decreases plasma insulin concentrations in dogs when applied topically. Because amitraz activates alpha 2-adrenoceptors in numerous tissues, in this study we used rats as a model to determine whether these effects of amitraz are mediated by alpha 2-adrenoceptors. The i.v. injection of amitraz (0.1, 0.3, and 1 mg/kg) followed by i.v. glucose injection (1 g/kg) induced a dose-dependent glucose intolerance characterized by hypoinsulinemia. At 1 mg/kg, amitraz completely blocked the insulin release induced by i.v. glucose administration. The alpha 2-adrenoceptor antagonist yohimbine (1 mg/kg, i.v.) prevented the effects of amitraz, but the alpha 1-adrenoceptor antagonist prazosin (0.3 mg/kg, i.v.) did not. The results suggested that one mechanism by which amitraz prolongs glucose-induced hyperglycemia is via inhibition of insulin release and this effect is mediated by alpha 2-adrenoceptors.

Xylazine Enhances Porcine Myometrial Contractility in Vitro: Possible Involvement of α2-Adrenoceptors and Ca2+ Channels

The effects of xylazine on porcine myometrial contractility were studied in vitro using uterine strips to determine the alpha 2-adrenergic influences during the diestrous stage of the estrous cycle. Xylazine (10(-8)-10(-5) M) caused a dose-dependent increase in the amplitude of myometrial contractility. The alpha 2-adrenoceptor antagonists idazoxan and yohimbine (10(-8)-10(-6) M) blocked the effects of xylazine in a dose-dependent manner. Yohimbine was approximately 10 times more potent than idazoxan in this regard. In contrast, an alpha 1-adrenoceptor antagonist prazosin (10(-7) and 10(-6) M) did not block the xylazine-induced increase in myometrial contractility, but a higher dose of prazosin (10(-5) M) did reduce the effects of xylazine. When the porcine uterine strips were pretreated with Ca2(+)-free Tyrod's solution or verapamil, a Ca2+ channel blocker, the effects of xylazine on myometrial contractility were completely abolished, whereas those of carbachol were only moderately reduced. The results suggest that the xylazine-induced myometrial contractility is mediated by alpha 2-adrenoceptors and that this effect is mediated, at least in part, by Ca2+ channels, whereas the effect of carbachol is attributed to an increase in both Ca2+ entry and release of Ca2+ from intracellular pools.

?1- and ?-adrenoceptor stimulation potentiate the anticonflict effect of a benzodiazepine

Interactions between different noradrenaline (NA)-active drugs and the benzodiazepine alprazolam (APZ) were examined in a modified Vogel's drinking conflict test in the rat. In a dose (0.5 mg/kg) which did not alter the behavior by itself, the alpha 2-adrenoceptor antagonist yohimbine consistently was found to enhance the anticonflict effect of APZ (0.5 mg/kg). The yohimbine induced potentiation of the APZ effect was counteracted both by the selective alpha 1-adrenoceptor antagonist prazosin (0.25 mg/kg) and the beta-adrenoceptor antagonist propranolol (2.0 mg/kg), but not by the selective beta 1-adrenoceptor antagonist metoprolol (2.0 mg/kg). Similar potentiating phenomena were obtained after co-administration of APZ (0.5 mg/kg) with the selective alpha 1-adrenoceptor agonist ST 587 (0.5-1.0 mg/kg) as well as with the suggested beta 2-adrenoceptor agonist clenbuterol (1.0 mg/kg). The results indicate that the potentiative effects of alpha 2-adrenoceptor antagonists on BDZ induced anticonflict action may be due to increased stimulation of alpha 1- and beta-adrenoceptors, via enhanced NA release. The findings are discussed in relation to the signal-to-noise hypothesis of NA function, and in relation to the suggested NA involvement in anxiety-related behavior.

Effect of bromocriptine on neurogenic vasoconstriction in the isolated autoperfused hindquarters of the rat

The effects of local administration of bromocriptine were studied in the isolated autoperfused hindquarters of the rat, and compared to the actions of apomorphine and pergolide. Local injection of bromocriptine (1 microgram kg-1) (into the hindquarters) did not alter perfusion pressure, but reduced the pressor response to electrical stimulation of the lumbar sympathetic chains at all frequencies used (0.5-10 Hz; 5 ms; 35 V). Bromocriptine (1 microgram kg-1) did not alter the increases in perfusion pressure induced by local administration of noradrenaline. The effects of local administration of apomorphine (1 microgram kg-1) and pergolide (1 microgram kg-1) were similar to that of bromocriptine. The inhibitory effect by bromocriptine, apomorphine and pergolide of the stimulation-evoked pressor responses was completely antagonized by intravenous administration of the dopamine receptor antagonist sulpiride (0.3 mg kg-1) but was not by the alpha 2-adrenoceptor antagonist yohimbine (1 mg kg-1). In this dose, yohimbine antagonized the inhibitory effect of the alpha-adrenoceptor agonist clonidine (1 microgram kg-1). The inhibitory effect of clonidine was not altered by sulpiride but was antagonized by yohimbine. The results indicate that bromocriptine like apomorphine and pergolide inhibit neurally-induced pressor responses in the autoperfused hindquarters of the rat by stimulation of presynaptic dopamine receptors. Stimulation of these receptors leading to a fall in noradrenaline release and consequently of vasomotor tone, might at least in part explain the vasodilatator effects of bromocriptine in the rat.

Characteristics of chloride currents activated by noradrenaline in rabbit ear artery cells.

1. Responses to noradrenaline were studied in isolated rabbit ear artery cells with the nystatin method of whole-cell patch-clamp recording. With this technique it was possible to obtain reproducible responses to noradrenaline which was not possible with traditional whole-cell recording. 2. With NaCl as the major constituent of the bathing solution (potassium-free pipette and external solutions) the reversal potential (Er) of the noradrenaline-evoked current was about 0 mV. When external chloride was replaced by thiocyanate, iodide, nitrate and bromide, Er was shifted to more negative potentials which indicates that a chloride conductance increase contributes to the current activated by noradrenaline. 3. When sodium was substituted by Tris, N-methyl-D-glucamine, choline or barium, Er of the noradrenaline-evoked current did not alter. This result suggests that a cation conductance is not implicated in the response to noradrenaline recorded with the nystatin method of whole-cell recording. 4. The chloride current activated by noradrenaline was blocked by the selective alpha 1-adrenoceptor antagonist prazosin but was not affected by the alpha 2-adrenoceptor antagonist yohimbine. 5. When cells were exposed to zero calcium bathing solutions the amplitude of the current elicited by noradrenaline was unaffected when measured within 1-2 min in zero calcium conditions. Continued exposure to 0 Ca + 1 mM-EGTA solution reversibly abolished the chloride current to noradrenaline. 6. In the presence of caffeine, which releases Ca2+ from internal stores and itself induced an inward current (at a holding potential of -50 mV), noradrenaline did not elicit a current. These data suggest that the chloride current evoked by noradrenaline results from an increase in intracellular concentration of calcium derived from internal stores. 7. The chloride channel blocking agents 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; 0.5 mM) and furosemide (0.5 mM) produced partial reduction of the noradrenaline-evoked chloride current whereas 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS), anthracene-9-carboxylic acid (A-9-C) and picrotoxin were ineffective in concentrations of up to 0.5 mM. However DIDS and furosemide were non-selective blockers as both agents were more effective against the adenosine triphosphate-induced cation current.

Pharmacological effects and pharmacokinetics of atipamezole, a novel alpha 2‐adrenoceptor antagonist‐a randomized, double‐blind cross‐over study in healthy male volunteers.

1. Single doses (10, 30 and 100 mg) of atipamezole (MPV-1248), a new potent and selective imidazole-type alpha 2-adrenoceptor antagonist, and saline placebo were administered as 20 min intravenous infusions to six healthy male volunteers in a randomized double-blind, cross-over phase I study. Later, 100 mg atipamezole was given orally to the same subjects in an open fashion. 2. The i.v. doses resulted in linearly dose-related concentrations of atipamezole in plasma. Pharmacokinetic calculations revealed an elimination half-life of 1.7-2.0 h, an apparent volume of distribution of 3.0-3.5 l kg-1 and a total plasma clearance of 1.1-1.5 l h-1 kg-1. No atipamezole could be detected in plasma after oral dosing. 3. Subjective drug effects were seen mainly after the largest i.v. dose and included increased alertness and nervousness, coldness and sweating of hands and feet, tremor and shivering, motor restlessness, and increased salivation. Salivation was also quantitated using dental cotton rolls, with dose-related increases produced by the i.v. doses. 4. The 100 mg i.v. dose increased plasma noradrenaline concentrations on average by 484 +/- 269 (s.d.)%, and also elevated both systolic and diastolic blood pressure (mean increases 17 +/- 7/14 +/- 2 mm Hg). The 30 mg dose had minor and the 10 mg dose no effects on these variables. Adrenaline and cyclic AMP levels in plasma were increased only after the largest dose. No drug effects were observed after oral dosing. 4. Plasma C-peptide and blood glucose levels were not markedly influenced by the drug, and cortisol secretion was not stimulated. 5. The observed effects are compatible with the presumed alpha 2-adrenoceptor antagonistic action of atipamezole and are in general concordance with the reported results of other alpha 2-adrenoceptor antagonists (yohimbine and idazoxan). 6. Although not orally active, atipamezole may prove to be a useful agent in studies of alpha 2-adrenoceptor function in man.

Role of adrenoceptors in vitro and in vivo in the effects of lithium on blood glucose levels and insulin secretion in the rat

1. Pretreatment of rats with the non-selective alpha-adrenoceptor antagonist dihydroergotamine counteracts the inhibition of glucose-induced insulin secretion caused by lithium both in vitro and in vivo. The present study was therefore carried out to specify further which type of adrenoceptor is involved in lithium-induced hyperglycaemia and inhibition of insulin secretion. 2. The lithium-induced effects were reversibly blocked by pretreatment of rats with the alpha 2-adrenoceptor antagonist yohimbine or a combination of yohimbine and the non-selective beta-receptor antagonist propranolol, whereas the alpha 1-receptor antagonist prazosin and propranolol alone were ineffective in blocking these effects. 3. These findings suggest that the effects of lithium on plasma glucose and insulin levels are mediated mainly by the stimulation of alpha 2-adrenoceptors.

Duodenal alkaline secretion in rabbits: influence of artificial ventilation

The effect of artificial ventilation on duodenal alkaline secretion and blood flow was studied in sodium pentobarbital-anaesthetized rabbits. A duodenal segment (approximately 3 cm) was cannulated in situ and continuously perfused with isotonic saline, and the bicarbonate secretion was titrated by pH-stat. Compared with the spontaneous breathing state, artificial ventilation improved the respiratory status of the animal, increasing Po2 and decreasing both Pco2 and plasma bicarbonate. Duodenal blood flow as measured with laser-Doppler flowmetry was not altered but the alkaline secretion was reduced. Pretreatment with the alpha 2-adrenoceptor antagonist yohimbine (0.5 mg kg-1 i.v., followed by 0.5 mg kg-1 h-1 i.v.) or the ganglionic blocker hexamethonium (10 mg kg-1 i.v.) did not affect the decline in duodenal alkaline secretion in response to artificial ventilation. Nor did these pretreatments affect the changes in plasma bicarbonate and Pco2 or significantly alter the blood flow. Increasing Pco2 in the respirator air increased the plasma Pco2 and bicarbonate concentration as well as the duodenal bicarbonate secretion. Pretreatment with the carbonic anhydrase inhibitor acetazolamide (80 mg kg-1 i.v.) reduced the bicarbonate secretion, and artificial ventilation induced a further reduction. Increasing Pco2 in the respirator in the animals pretreated with acetazolamide did not affect the bicarbonate secretion. Duodenal alkaline secretion was thus always reduced on artificial ventilation. The mechanism for this reduction does not seem to involve the sympathetic nervous system or the blood flow, but appears to be the consequence of alterations in the plasma concentration of bicarbonate and the plasma Pco2.

α2‐Adrenoceptor subtypes and imidazoline‐like binding sites in the rat brain

1. The binding of [3H]-yohimbine and [3H]-idazoxan to rat cortex and hippocampus is rapid, reversible and of high affinity. Saturation data indicate that a single population of binding sites exist for [3H]-yohimbine in the cortex (Bmax 121 +/- 10 fmol mg-1, protein; Kd 5.2 +/- 0.9 nM) and hippocampus (Bmax 72 +/- 6 fmol mg-1 protein; Kd 5.8 +/- 0.7 nM). [3H]-idazoxan labels one site in the cortex (Bmax 87 +/- 8 fmol mg-1 protein; Kd 4.1 +/- 0.9 nM) and hippocampus (Bmax 30 +/- 6 fmol mg-1 protein; Kd 3.5 +/- 0.5 nM), when 3 microM phentolamine is used to define non-specific binding. A second distinct [3H]-idazoxan binding site (Bmax 110 +/- 21 fmol mg-1 protein; Kd 3.6 +/- 0.07 nM) is identified in rat cortex if 0.3 microM cirazoline is used to define non-specific binding and 3 microM yohimbine is included to prevent binding to alpha 2-adrenoceptors. 2. Displacement studies indicate that the alpha 1-adrenoceptor antagonist prazosin and the 5-HT1 ligands 8-OH-DPAT, RU 24969 and methysergide differentiate [3H]-yohimbine binding into two components; a high and low affinity site. In contrast the displacement of [3H]-idazoxan by each ligand was monophasic. 3. The affinities of 8-OH-DPAT, RU 24969 and methysergide determined against [3H]-idazoxan binding to the cortex and hippocampus correlate significantly with the binding site displaying low affinity for prazosin and previously designated alpha 2A. In contrast, a poor correlation exists for the high affinity site for prazosin designated alpha 2B. 4. [3H]-idazoxan, in the presence of 3 microM yohimbine, labels a site that displays high affinity towards cirazoline, naphazoline and guanabenz, but low affinity towards clonidine, p-aminoclonidine, adrenaline, noradrenaline and the alpha 2-adrenoceptor antagonists yohimbine, rauwolscine, WY 26703 and BDF 6143. 5. The results of this study indicate that [3H]-yohimbine labels two sites; the alpha 2A- and alpha 2B-adrenoceptors whereas [3H]-idazoxan labels an alpha 2-adrenoceptor with a profile consistent with the alpha 2A-adrenoceptor subtype. In addition, [3H]-idazoxan labels an imidazoline binding site in the rat cortex that is pharmacologically distinct from alpha 2-adrenoceptors. The low affinity of clonidine and p-aminoclonidine indicates that the imidazoline-like binding site in rat cortex is different from the site labelled by [3H]-clonidine and [3H]-p-aminoclonidine in human, rat and bovine brain stem, providing evidence of potential heterogeneity within this class of binding sites.