A New β2-Adrenoceptor Agonist with α1-Adrenoceptor Blocking Properties

Abstract: The phenylethanolamine D2343 exhibits a dualistic adrenoceptormediated effect, i.e. a β-agonistic effect combined with an α-antagonistic one. Tracheal smooth muscles and heart preparations were used to gauge the agonistic effect on adrenergic β-receptors. Rabbit aorta and rat vas deferens were used to determine the α-adrenoceptor blocking activity. The β-adrenoceptor activity of D2343 was classified as β₂-type with about the same efficacy as the β₂-selective terbutaline on tracheal muscle. The effect on isolated heart preparations was greater than that produced by terbutaline. The α-receptor blocking capacity was directed against the α₁-receptor type and was of nearly the same potency as for phentolamine.     

Characterization of α-Adrenoceptor Activity in Term-newborn Piglet Mesentery

For further characterization of neonatal mesenteric α₁-adrenoceptor populations, an extracorporeal perfusion circuit was established to control intestinal blood flow in 0-2 day old piglets. Activation of α₁-adrenoceptors was first documented by observing dose-dependent increases in mesenteric perfusion pressure after intra-mesenteric arterial injection of methoxamine and noradrenaline. Peripheral intravenous injections of WB 4101 (a competitive α_1A-adrenoceptor antagonist), but not clorethylclonidine (CEC, an α_1B-adrenoceptor antagonist), significantly (P < 0.05, analysis of variance) blunted mesenteric vasoconstrictor responses to those agonists. That the mesenteric vasoconstrictor response to mesenteric plexus stimulation was unaltered by CEC, but was muted by both WB 4101 and SK&F 104856 (a post-junctional α₁- and α₂-adrenoceptor antagonist) suggests that pre- and post-junctional α_1A-adrenoceptors are present and functional at birth.     

Neuroendocrine Effects of Dexmedetomidine: Evidence of Cross-Tolerance Between a μ-Opioid Agonist and an α2-Adrenoceptor Agonist in Growth Hormone Secretion of the Male Rat

Abstract: The role of α₂-adrenergic receptors (adrenoceptors) in the secretion of growth hormone, prolactin and thyrotropin was studied using highly selective agonists and antagonists of the α₂-adrenoceptor. The interplay between opiates and α₂-adrenergic drugs in the acute secretion of growth hormone and prolactin, as well as the possible cross-tolerance between morphine (μ-opioid receptor agonist) and dexmedetomidine (α₂-adrenoceptor agonist) in growth hormone secretion were also evaluated. Dexmedetomidine dose-dependently increased plasma growth hormone and prolactin levels and decreased thyrotropin levels. The enhanced secretion of both growth hormone and prolactin was antagonized by atipamezole (an α₂-adrenoceptor antagonist) but not by prazosin (an α₁-adrenoceptor antagonist). Morphine (5 mg/kg)-induced stimulation of growth hormone secretion was antagonized by both naloxone (u-opioid antagonist) and atipamezole. Naloxone, but not atipamezole, antagonized the morphine-induced increase in prolactin secretion. Dexmedetomidine increased growth hormone secretion in the saline pretreated rats, but did not do so in the morphine-tolerant rats. The stimulation of α₂-adrenoceptor enhances secretion of both growth hormone and prolactin. The adrenergic regulation of thyrotropin secretion still remains unclear. Evidently, adrenergic mechanisms are involved in the morphine-induced stimulation of growth hormone secretion, but not in the morphine-induced stimulation of prolactin secretion. In addition, there is a clear cross-tolerance between dexmedetomidine and morphine in growth hormone secretion of the rat.     

α1-Adrenoceptor Blocking Activity of Some Ring-open Analogues of Prazosin

Synthesis and structural characterization of some ring-open analogues of Prazosin containing either the guanidine substructure or urea-equivalent groups are described. The opening of the pyrimidine ring in Prazosin is very important as far as the affinity for α₁-adrenoceptor is concerned. The pA₂ values of the ring-open derivatives are 10⁴–10⁵ fold lower than that of the parent. It is probable that the affinity decrease principally reflects a negative influence of the conformational factors in the interaction with the α₁-receptor. The derivative 5 containing the guanidine moiety, charged at physiological pH, is as active as the other derivatives containing the uncharged urea-equivalent groups. This behaviour indicates, in this class of compounds, the importance of H-bonding interactions with the receptor. When in the ring-open models the ethanediamino substructure is substituted for the piperazine ring additional decrease in activity occurs.     

An Analysis of the β2-Adrenoceptor Selectivity in Three Series of β-Adrenoceptor Agonists

The aim of the study was to analyse the β₂-adrenoceptor selectivity earlier found in two series of catecholamines and one series of resorcinolamines (Johansson et al. 1986). The affinity of the compounds was assessed in binding studies in preparations from the guinea-pig left heart ventricle (β₁-adrenoceptors) and the soleus muscle (β₂-adrenoceptors) using ³H-CGP-12177 as radioligand. Further, the activation of the adenylate cyclase by the compounds was studied in the same preparations. Selectivity quotients were obtained from both functional effects and from affinity and adenylate cyclase activating studies. There was a good correlation between the selectivity quotients obtained in these two ways. Tertiary butyl substitution on the amino nitrogen gave the highest β₂-adrenoceptor selectivity in both the catechol and resorcinol series. In comparison with their isopropyl substituted analogues the β₂-adrenoceptor selectivity of these compounds (KWD 2026 and terbutaline) was mainly due to a change in affinity for the β₁- and β₂-adrenoceptors and, to a lesser degree, a change in intrinsic efficacy.     

α1 (but not α2)-Adrenoceptor Agonists in Combination with the Dopamine D2 Agonist Quinpirole Produce Locomotor Stimulation in Dopamine-Depleted Mice

Mice were premedicated with reserpine and α-methyl-p-tyrosine to deplete stores of dopamine (DA) (and other neurotransmitters) and to stop DA (and noradrenaline (NA)) synthesis. In DA-depleted mice, the mixed α₁/α₂ agonist clonidine potentiated locomotor stimulation induced by a low dose of apomorphine as measured in automated activity cages. Clonidine and the slightly αl-selective agonist ST587, but not ST91, an α-agonist which does not readily cross the blood brain barrier, produced marked stimulation when combined with the selective D₂ agonist quinpirole. The D₁ -selective agonist SKF38393 also produced marked excitation when combined with quinpirole. All the selective agonists, bar quinpirole which in some cases produced a significant locomotor stimulation, were relatively inactive when given alone. A “blind” observational analysis of the animals challenged with clonidine plus quinpirole indicated an increase in sniffing, rearing and shaking behaviour. In contrast, observation of the animals challenged with SKF38393 plus quinpirole indicated increased sniffing, rearing and biting and, in one case, increased grooming behaviour. Clonidine did not produce excitation (in automated cages) when combined with the selective D₁ agonist SKF38393. The excitation produced by clonidine plus quinpirole was blocked by the selective D₂ antagonist raclopride but not by the selective D₁ antagonist SCH23390. The stimulation was also blocked by the α₁ antagonist prazosin but not by the α₂ antagonists idazoxan or yohimbine. Biochemical analysis in the striata of mice challenged with clonidine plus quinpirole did not provide any obvious biochemical basis for the behavioural interaction. It is concluded that α₁ receptor agonists in combination with D₂ DA agonists can produce marked stimulation in DA depleted mice.     

Selectivity of ABT-089 for α4β2* and α6β2* nicotinic acetylcholine receptors in brain

Numerous pharmaceutical efforts have targeted neuronal nicotinic receptors (nAChRs) for amelioration of cognitive deficits. While α4β2 and α7 are the more prominent nAChR in brain, other heteromeric nAChR can have important impact on agonist pharmacology. ABT-089 is a pioneer nAChR agonist found to enhance cognitive function with an exceptionally low incidence of adverse effects. To further investigate the mechanism of action of ABT-089, we evaluated its function in mouse brain preparations in which we have characterized the subunit composition of native nAChR. Among α4β2*-nAChR, ABT-089 had partial agonist activity (7–23% of nicotine) and high selectivity for α4α5β2 nAChR as evidenced by loss of activity in thalamus of α5^−/− mice. ABT-089 stimulated [³H]-dopamine release (57%) exceeded the activity at α4β2* nAChR, that could be explained by the activity at α6β2* nAChR. The concentration–response relationship for ABT-089 stimulation of α6β2* nAChR was biphasic. EC₅₀ and efficacy values for ABT-089, respectively, were 28 μM and 98% at the less sensitive α6β2* nAChR and 0.11 μM and 36% at the more sensitive subtype (the most sensitive target for ABT-089 identified to date). ABT-089 had essentially no agonist or antagonist activity at concentrations ≤300 μM at α3β4-nAChR measured by [³H]-acetylcholine release from interpeduncular nucleus. Thus, ABT-089 is a β2* nAChR ligand with demonstrable agonist activity at α4β2* and α6β2* receptors. As one form of α6β2* nAChR is sensitive to sub-μM concentrations, we propose that this receptor in particular may contribute to the enhanced cognitive performance following low doses of ABT-089.     

Methadone is a Non‐Competitive Antagonist at the α4β2 and α3* Nicotinic Acetylcholine Receptors and an Agonist at the α7 Nicotinic Acetylcholine Receptor

Nicotine–methadone interactions have been studied in human beings and in various experimental settings regarding addiction, reward and pain. Most methadone maintenance treatment patients are smokers, and methadone administration has been shown to increase cigarette smoking. Previous in vitro studies have shown that methadone is a non‐competitive antagonist at rat α3β4 nicotinic acetylcholine receptors (nAChR) and an agonist at human α7 nAChRs. In this study, we used cell lines expressing human α4β2, α7 and α3* nAChRs to compare the interactions of methadone at the various human nAChRs under the same experimental conditions. A [³H]epibatidine displacement assay was used to determine whether methadone binds to the nicotinic receptors, and ⁸⁶Rb⁺ efflux and changes in intracellular calcium [Ca²⁺]_i were used to assess changes in the functional activity of the receptors. Methadone displaced [³H]epibatidine from nicotinic agonist‐binding sites in SH‐EP1‐hα7 and SH‐SY5Y cells, but not in SH‐EP1‐hα4β2 cells. The K_i values for methadone were 6.3 μM in SH‐EP1‐hα7 cells and 19.4 μM and 1008 μM in SH‐SY5Y cells. Methadone increased [Ca²⁺]_i in all cell lines in a concentration‐dependent manner, and in SH‐EP1‐hα7 cells, the effect was more pronounced than the effect of nicotine treatment. In SH‐EP1‐hα4β2 cells, the effect of methadone was negligible compared to that of nicotine. Methadone pre‐treatment abolished the nicotine‐induced response in [Ca²⁺]_i in all cell lines expressing nAChRs. In SH‐EP1‐hα4β2 and SH‐SY5Y cells, methadone had no effect on the ⁸⁶Rb⁺ efflux, but it antagonized the nicotine‐induced ⁸⁶Rb⁺ ion efflux in a non‐competitive manner. These results suggest that methadone is an agonist at human α7 nAChRs and a non‐competitive antagonist at human α4β2 and α3* nAChRs. This study adds further support to the previous findings that opioids interact with nAChRs, which may underlie their frequent co‐administration in human beings and might be of interest to the field of drug discovery.     

β-Adrenoceptor mRNA Levels Can Be Increased Via β-Adrenoceptor-independent Events

The drug (-)-oxaprotiline has been used as a tool to study the regulation of the β-adrenoceptor in rat C₆ glioma cells. Treatment with (-)-oxaprotiline for 30 min results in an increase in steady-state β-adrenoceptor mRNA levels. The effect is β-adrenoceptor-independent, is not additive or synergistic with isoprenaline treatment, and does not involve activation of adenylate cyclase. The data show that (-)-oxaprotiline can affect β-adrenoceptor mRNA levels via a mechanism that bypasses the receptor, perhaps involving direct activation of protein kinase A.     

β-Adrenoceptor antagonist activity of 3-methoxyisoprenaline

1. The beta-adrenoceptor antagonist activity of 3-methoxyisoprenaline, the O-methylated metabolite of isoprenaline, was studied on isolated driven atrial strip and tracheal chain preparations of the guinea-pig and on the hind limb blood flow of the dog.2. On both the atrial strip and tracheal chain preparations the blockade of responses to isoprenaline fulfilled the criteria for simple competitive inhibition.3. 3-Methoxyisoprenaline decreased the vasodilator response to isoprenaline in the dog hind limb, but did not affect the response to noradrenaline.4. 3-Methoxyisoprenaline had about 1/3,700 of the potency of propranolol as a beta-adrenoceptor antagonist on the tracheal chain preparation, 1/1,000 on the atrial strip preparation and less than 1/400 on the hind limb blood flow.5. The antagonist activity of 3-methoxyisoprenaline showed a slight specificity for cardiac beta-adrenoceptors, being 4.3 times more active on guinea-pig atria than on trachea.6. Although 3-methoxyisoprenaline antagonized the actions of isoprenaline in the three preparations, its activity was extremely weak. It is unlikely that the formation of 3-methoxyisoprenaline from isoprenaline, administered therapeutically, could lead to beta-adrenoceptor blockade.     

Effect of Prenalterol,a β-1-Adrenoceptor Agonist,on Renin Secretion Rate in the Anaesthetized Dog

Abstract: The effects of the β-1-adrenoceptor agonist, prenalterol, 20 μg/kg intravenously on renin secretion rate (RSR), renal haemodynamics and sodium excretion were examined in anaesthetized dogs with innervated or denervated kidneys. In dogs with innervated kidneys, prenalterol increased RSR from 1.1±0.2 to 7.9±0.1 unitsXmin.^?1Xg^?1 (P<0.01). Prenalterol did not affect mean arterial pressure, renal blood blow, glomerular filtration rate or sodium excretion. Heart rate was increased by 53±17 beats/min. (P<0.01). The increase in RSR produced by prenalterol was independent of intact renal innervation as RSR increased to the same extent in dogs with denervated kidneys. Pretreatment with the β-1-adrenoceptor antagonist, metoprolol 0.5 mg/kg intravenously, abolished the increase in RSR produced by prenalterol. These findings suggest that prenalterol directly activates renal β-1-adrenoceptors to increase RSR.     

Assessment of the Sedative Effects of Dexmedetomidine,an α2-Adrenoceptor Agonist,with Analysis of Saccadic Eye Movements

Abstract: Single intravenous doses (0.5 μg/kg and 1.0 μg/kg) of dexmedetomidine (4(5)-(1-(2,3-dimethylphenyl)ethyl)imidazole), a selective α₂-adrenoceptor agonist, and saline placebo were administered to six healthy volunteers (4 males and 2 females) in a double-blind, placebo-controlled cross-over study. The effects on vigilance were assessed using both subjective estimation (visual analogue scale, VAS) and objective tests (critical flicker fusion frequency, CFF; the Maddox wing; saccadic eye movement analysis). Dose-dependent subjective sedation was seen in VAS measurements, and impairment of vigilance was observed in CFF, Maddox wing and peak saccadic velocity, while saccade latency was not influenced by dexmedetomidine. The changes in vigilance were concurrent with moderate reductions in blood pressure and heart rate. CFF, the Maddox wing and peak saccadic velocity all proved sensitive in the assessment of sedation induced by dexmedetomidine.     

Comparative Effects of β-Adrenoceptor Partial Agonists on

Abstract: The chronotropic effect of three β-1-adrenoceptor partial agonists prenalterol, xamoterol and epanolol has been compared on the right atria of the rat in order to evaluate their intrinsic activity and to place them in rank order of effectiveness. The results show that prenalterol, xamoterol and epanolol are all partial agonists. The intrinsic activities relative to that of isoprenaline are 0.84 for prenalterol, 0.59 for xamoterol and 0.29 for epanolol. This rank order of intrinsic activities should remain the same in different species and in man. Both atenolol and propranolol reversed the chronotropic effects of the three agonists. The K_B of the two blockers was similar against prenalterol and xamoterol, which indicates that the two partial agonists are probably competing for the same population of receptors. The EC50 is twice as large than K_B for xamoterol, which is consistent with isoprenaline working through both β-1- and β-2- receptors and xamoterol finds it more difficult to block the β-2-receptors.     

Qualitative Differences between the Inotropic Responses in Rat Papillary Muscles to α-Adrenoceptor and β-Adrenoceptor Stimulation by both Noradrenaline and Adrenaline

Abstract: Mammalian heart has obviously both α- and β-adrenergic inotropic mechanisms, and stimulation of these two mechanisms by appropriate agonists lead to qualitatively different inotropic responses. This difference can serve to recognize the two adrenergic inotropic effects. In order to explore if the endogenous catecholamines, noradrenaline and adrenaline, could activate both these mechanisms, and if so, to characterize them qualitatively and quantitatively, the mechanical responses in electrically driven rat left ventricular papillary muscles were examined in the absence or presence of appropriate receptor blockade. Isometric tension (T), rate of rise and decline of tension (first derivative = T) and rate of transition from tension rise to tension decline (negative part of second derivative=T) were recorded. α-Adrenergic and β-adrenergic inotropic effects were demonstrated both for noradrenaline and adrenaline. Maximal β-adrenoceptor stimulation (agonist in the presence of an α-adrenoceptor blocker) caused a small increase in T_max, intermediate increases in T'_max and T'_min, and a considerable increase in T_min (β-type effect). Maximal α-adrenoceptor stimulation (agonist in the presence of a β-adrenoceptor blocker) increased all parts of the contraction-relaxation cycle by about the same degree (T_minT_minT'_maxT_max α-type effect). While β-adrenoceptor stimulation gave a dose dependent and pronounced increase in the ratio T_min/T_max (relaxation-onset index), α-adrenoceptor stimulation decreased it to subcontrol values. The time course of the response to the α-adrenoceptor stimulation was characterized by a transient decrease in all qualities followed by an increase which reached maximum at 4–5 min. β-Adrenoceptor stimulation gave a monophasic inotropic response which developed in the course of 1–2 min. Both agents alone gave a monophasic response with the characteristics of a β-type effect (i.e. relative maximal increase of T_minT_minT_maxT_max), and a marked increase in the relaxation-onset index (T_min/T_max). Thus the β-adrenergic inotropic component was the dominating one when the amines were used alone. The two different response patterns probably reflect a dual mechanism of action of the endogenous catecholamines: the β-adrenergic component which is dependent upon an increase in cyclic AMP levels and the α-adrenergic component which is independent on cyclic AMP.