Selective Alpha 2-adrenoceptor Antagonist Research Articles

Doxazosin for the Treatment of Benign Prostatic Hyperplasia in Patients with Mild to Moderate Essential Hypertension: A Double-Blind, Placebo-Controlled, Dose-Response Multicenter Study

A total of 248 hypertensive patients 45 years old or older with benign prostatic hyperplasia (BPH) was included in this 16-week, multicenter, double-blind, placebo-controlled, parallel-group dose-response study. Doxazosin, a selective alpha 1-adrenoceptor antagonist, produced a significant increase in maximum urinary flow rate (2.3 to 3.6 ml. per second) at doses of 4 mg., 8 mg. and 12 mg., and in average flow rate (8 mg. and 12 mg.) compared with placebo. The increase in maximum flow rate was significant with doxazosin versus placebo within week of initiating double-blind therapy. Doxazosin compared to placebo significantly decreased patient-assessed total, obstructive and irritative BPH symptoms. Blood pressure was significantly lower with all doxazosin doses compared with placebo. Adverse events, primarily mild to moderate in severity, were reported in 48 percent of patients on doxazosin and 35 percent on placebo. Our results strongly support the use of doxazosin as a nonoperative therapeutic alternative in the management of uncomplicated BPH. Doxazosin would also be particularly useful in the management of patients who have BPH and hypertension.

Importance of imidazoline receptors in the cardiovascular actions of centrally acting antihypertensive agents.

Increasing evidence indicates that the hypotensive effect of centrally acting antihypertensive drugs is not due to stimulation of alpha 2-adrenoceptors but to action on imidazoline receptors (IR). This has led to the development and recent clinical use of second generation agents such as rilmenidine and moxonidine that possess a much greater selectivity toward these nonadrenergic receptors. However, relatively few studies have examined the role of these receptors in conscious animals or have adequately accounted for the alpha 2-adrenoceptor antagonist properties of IR antagonists such as idazoxan. We have taken the approach of initially calibrating the alpha 2-adrenoceptor antagonist potency of intracisternally (ic) administered idazoxan and the IR-1 receptor antagonist efaroxan against 2-methoxyidazoxan, a highly selective alpha 2-adrenoceptor antagonist with little or no imidazoline antagonist effect. This was done using alpha-methyldopa, a hypotensive agent affecting only alpha 2-adrenoceptors. Thus, we chose doses of the antagonists with equal alpha 2-adrenoceptor blocking action such that differences in the ability of idazoxan or efaroxan compared to 2-methoxy-idazoxan to reverse the hypotension produced by rilmenidine, moxonidine, or clonidine indicate an interaction with IR. By this method we found that the hypotensive effects of rilmenidine and moxonidine at moderate intracisternal doses were more readily reversed by the imidazoline antagonists than by 2-methoxy-idazoxan, indicating that IR were largely responsible for their hypotensive actions. By contrast, clonidine's effects were equally reversed by all antagonists, suggesting interaction mainly with alpha 2-adrenoceptors. In conscious rabbits with chronic renal sympathetic nerve electrodes we examined the effect of rilmenidine and alpha-methyldopa on the renal sympathetic baroreflex. Both drugs reduced renal sympathetic nerve activity and sympathetic baroreflex responses, but only the effect of rilmenidine was preferentially reversed by idazoxan. Thus, both IR and central alpha 2-adrenoceptor receptors can influence the renal baroreflex, but the former are relatively more important for the actions of rilmenidine. We recently examined the possible sites of action of rilmenidine in anesthetized rabbits and showed that sixfold lower doses were required to reduce blood pressure when the drug was injected into the rostral ventrolateral medulla compared to intracisternal administration. At this site rilmenidine also reduced renal sympathetic tone and inhibited renal sympathetic baroreflex responses. By contrast, rilmenidine was relatively ineffective when injected into the nucleus of the solitary tract. These experiments support the view that rilmenidine acts primarily at IR in the rostral ventrolateral medulla to reduce sympathetic tone and modulate sympathetic baroreflexes.

Evidence for a functional alpha 1A- (alpha 1C-) adrenoceptor mediating contraction of the rat epididymal vas deferens and an alpha 1B-adrenoceptor mediating contraction of the rat spleen.

1. The alpha 1-adrenoceptor subtype mediating contraction of the rat epididymal vas deferens and rat spleen has been investigated by use of alpha 1-adrenoceptor antagonists that have shown selectivity between the different cloned receptor subtypes. 2. In the rat epididymal vas deferens the potency of noradrenaline and phenylephrine was increased in the presence of neuronal and extra-neuronal uptake blockers, cocaine and beta-oestradiol, but these did not alter that of methoxamine. The order of potency of the agonists in the presence or absence of uptake blockade was noradrenaline > phenylephrine > methoxamine. In the rat spleen the potency of these agonists was not altered in the presence of cocaine and beta-oestradiol, and their order of potency was the same as in the vas deferens. 3. The non subtype selective alpha 1-adrenoceptor antagonist prazosin (up to 1 x 10(-7) M) was found to antagonize contractions to noradrenaline in the vas deferens competitively (pA2 9.2), but only in a non competitive manner in the spleen. Contractions to phenylephrine in the spleen however were competitively antagonized by prazosin (up to 1 x 10(-7) M) with a pA2 of 9.2. This suggests that there is an alpha 1- and a non alpha 1-adrenoceptor response to noradrenaline in the rat spleen. 4. Pretreatment with chlorethylclonidine (10(-4) M for 30 min) did not alter the noradrenaline contractions in the vas deferens, but contractions to noradrenaline and phenylephrine in the spleen were shifted 30 and 300 fold to the right of the control curve, respectively. This suggests that only the contractions in the spleen were mediated by alpha 1B-adrenoceptors. 5. The noradrenaline contractions in the vas deferens were competitively antagonized by WB 4101 (pA29.6), 5-methyl-urapidil (pA2 8.7), phentolamine (pA2 8.3), benoxathian (pA2 9.4), spiperone (pA2 7.5),indoramin (pA2 8.4) and BMY 7378 (pA2 6.7), consistent with the affinities of these antagonists in binding studies on tissue alpha 1A-adrenoceptors. These values correlated best with their published affinities on the expressed alpha 1c-adrenoceptor clone and poorly with those at either the expressed alpha lb- or alpha 1d adrenoceptor clones. Therefore the classical alpha 1A-adrenoceptor appears to be the same as the expressed alpha lc-adrenoceptor clone.6. The phenylephrine contractions in the spleen were competitively antagonized by WB 4101 (pA2 8.1),5-methyl-urapidil (pA2 7.1), phentolamine (pA2 7.3), benoxathian (pA2 7.4), spiperone (pA2 7.9),indoramin (pA2 7.5) and BMY 7378 (pA2 7.4), consistent with the affinities of these antagonists in binding studies on tissue alB-adrenoceptors. The pA2 values correlated best with the published affinities of these compounds on the expressed alb-adrenoceptor clone and poorly with those at either the expressed alpha ld- or alpha lc-adrenoceptor clones. Therefore the alpha lB-adrenoceptor appears to be the same as the expressed alpha lb-adrenoceptor clone.7. The results provide pharmacological evidence that the alpha1-adrenoceptor mediating noradrenaline contractions in the epididymal portion of the rat vas deferens is the alpha 1A-(alpha lC) subtype and that contractions to phenylephrine in the rat spleen are mediated by the alpha 1B-subtype.

Prejunctional facilitatory α1‐adrenoceptors in the rat urinary bladder

1. The effect of activation of alpha 1-adrenoceptors on acetylcholine (ACh) release and neurally evoked contractile responses induced by electrical field stimulation was investigated in smooth muscle strips from the rat urinary bladder. 2. Neurogenic contractions were facilitated by the alpha 1-adrenoceptor agonists, phenylephrine (PE) (2-128 microM) and methoxamine (2-128 microM) in a dose-dependent manner. These agents also increased small amplitude spontaneous contractions of bladder strips and in 10% of strips increased basal tone. However, contractions elicited by exogenous ACh (1-10 microM) were not affected by alpha 1-agonists. 3. The magnitude of the PE facilitation was higher at lower frequencies (1-5 Hz) or at submaximal intensities of stimulation and at lower Ca2+ concentrations (0.5-1 mM). The selective alpha 1-adrenoceptor antagonist, terazosin (TRZ) (0.05-1 microM), competitively inhibited (pA2 value: 8.6) the PE facilitation of the neurally evoked contractions but not the PE induced increase of spontaneous contractions. 4. [3H]-noradrenaline (NA) and [14C]-ACh release evoked by electrical field stimulation were increased (140% and 173%, respectively) by 2 microM PE. TRZ (0.05-0.1 microM) blocked the PE facilitation of ACh release but not the facilitation of NA release. TRZ alone did not alter the release of ACh or NA nor the amplitude of the neurogenic contractions. 5. PE (2 microM) did not alter the basal release of ACh but did increase (by 180%) the basal release of NA. Desipramine (2 microM) blocked this effect of PE and also the PE-facilitation of evoked ACh and NA release. 6. It is concluded that cholinergic terminals in the rat urinary bladder exhibit alpha 1-adrenoceptors which can facilitate the release of transmitter. However, under the conditions of our experiments it appears that cholinergic transmission is not modulated by alpha 1 adrenergic mechanisms. Further studies are necessary to determine whether these receptors can be activated by endogenous noradrenaline released within the bladder.

Contrasting effects of the imidazol(in)e alpha 2-adrenoceptor agonists, medetomidine, clonidine and UK 14,304 on extraneuronal levels of noradrenaline in the rat frontal cortex: evaluation using in vivo microdialysis and synaptosomal uptake studies.

1. In vivo microdialysis in halothane-anaesthetized rats and synaptosomal [3H]-noradrenaline uptake studies in vitro were used to evaluate the effects of imidazole (medetomidine) and imidazoline (clonidine and UK 14,304) alpha 2-adrenoceptor agonists on extraneuronal levels of noradrenaline in the frontal cortex. 2. Levels of noradrenaline in the dialysate were increased by a depolarizing concentration of K+ (60 mM for 20 min) and substantially attenuated by reducing Ca2+ supply in the perfusate. These results suggest that spontaneous efflux of noradrenaline in the cortex is regulated predominantly by cation-dependent exocytotic mechanisms. 3. At a low perfusion concentration (0.5 microM), medetomidine, clonidine and UK 14,304 all reduced the level of noradrenaline in cortical dialysates. Continuous perfusion of the selective alpha 2-adrenoceptor antagonist, atipamezole (0.5 microM) caused a sustained increase in noradrenaline efflux and reversed the inhibitory effects of medetomidine. All these changes are consistent with drug actions at presynaptic alpha 2-adrenoceptors. 4. Higher concentrations of medetomidine (5-50 microM), but not clonidine or UK 14,304, evoked a non-desensitizing increase in noradrenaline efflux. This effect was not antagonized by 0.5 microM atipamezole. 5. The tricyclic noradrenaline reuptake inhibitor, desmethylimipramine (0.5-50 microM), increased noradrenaline efflux in a concentration-dependent manner. 6. The specific uptake of [3H]-noradrenaline into cortical synaptosomes was inhibited by medetomidine and desmethylimipramine with IC50 values of approximately 7 microM and 8 microM respectively. Neither clonidine nor UK 14,304 inhibited [3H]-noradrenaline uptake. 7. These results indicate that micromolar concentrations of the selective alpha2-adrenoceptor agonist,medetomidine, can augment extraneuronal levels of noradrenaline in the rat frontal cortex; this effect seems to involve an inhibition of noradrenaline reuptake rather than an action at alpha2-adrenoceptors.

Pharmacological characterization of noradrenaline‐induced contractions of the porcine isolated palmar lateral vein and palmar common digital artery

1. The aim of this study was to examine the pharmacological characteristics of alpha-adrenoceptor-mediated contractions in two porcine isolated blood vessels, the palmar lateral vein (PLV) and the palmar common digital artery (PCDA). This was carried out with noradrenaline used as the agonist throughout, and either phentolamine (non-selective alpha-adrenoceptor antagonist), prazosin and YM-12617 (selective alpha 1-adrenoceptor antagonists) or rauwolscine and CH-38083 (selective alpha 2-adrenoceptor antagonists). 2. Noradrenaline (0.003-10 microM) produced concentration-dependent contractions in both vessels, with the PCDA (pD2 = 6.33 +/- 0.07, n = 10) being approximately 10 fold less sensitive to noradrenaline compared to the PLV (pD2 = 7.39 +/- 0.09, n = 8). Also, the maximal response to noradrenaline was greater in the PCDA compared to the PLV. Phentolamine (0.03-30 microM) produced parallel rightward shifts in the CRC to noradrenaline in both tissue preparations. The pA2 values were similar and slopes of the Schild plots were not significantly different from unity, indicating an interaction between phentolamine and a single receptor in each preparation. 3. In the PCDA the alpha 1-adrenoceptor antagonists, prazosin (0.01-1 microM) and YM-12617 (0.01-1 microM) produced non-parallel rightwards shifts in the CRC to noradrenaline, with the lower 10-15% of the CRC exhibiting greater resistance to the effects of these antagonists compared to the upper part. In contrast, rauwolscine (1-10 microM) and CH-38083 (10 microM) produced parallel displacement of the CRC to noradrenaline. In the PLV, low concentrations of either alpha l- (0.01 microM) or alpha2-adrenoceptor antagonists(0.1-1 microM) produced a large shift in the CRC, but subsequent higher concentrations had only small additional effects. Based upon pKB values estimated from the effects of the lower concentrations of antagonists, the results are consistent with a large population of alpha1-adrenoceptors in the PCDA and a mixture of alpha l- and alpha2-adrenoceptors in the PLV.4. In both tissues, when an ac,- and an a2-adrenoceptor antagonist were used in combination the effect produced was greater than that with either agent alone. In contrast, the combination of the alpha1-adrenoceptor antagonists (prazosin and YM-12617 together) or the alpha2-adrenoceptor antagonists (CH-38083 and rauwolscine together) were no more effective than that produced by the individual antagonists. These findings suggest the presence of functional alpha l- and alpha2-adrenoceptors in the PLV andPCDA.5. Phenoxybenzamine (0.3-3 microM, 60min exposure) produced a concentration-dependent reduction in the maximal response to noradrenaline which was more pronounced in the PCDA than the PLV. After a 60 min exposure to a combination of phenoxybenzamine (1 microM) and rauwolscine (1 microM), the remaining NA-induced contraction after washout was resistant to prazosin (0.1 microM) and sensitive to rauwolscine(1 microM) in both tissue preparations, indicating the existence of functional alpha2-adrenoceptors in both vessels.6. Evidence suggests that post-junctional alpha l- and alpha2-adrenoceptors contribute to noradrenaline-induced contractions in the PCDA and PLV, with the latter possessing a larger population of functional alpha2-adrenoceptors.

Effects of yohimbine and desipramine on adrenal catecholamine release in response to splanchnic nerve stimulation in anesthetized dogs.

The effects of yohimbine and desipramine on adrenal catecholamine (CA) release in response to splanchnic nerve stimulation (SNS) were examined in anesthetized dogs. SNS and 3 Hz produced frequency-dependent increases in epinephrine (EPI) and norepinephrine (NE) output determined from adrenal venous blood. Yohimbine (30 and 100 micrograms/kg, i.v.), a selective alpha 2-adrenoceptor antagonist, enhanced the SNS-induced increases in both EPI and NE output. Desipramine (100 and 300 micrograms/kg, i.v.), an amine pump inhibitor, enhanced the SNS-induced increases in NE output, whereas no enhancement of EPI output was produced. After desipramine treatment, yohimbine further enhanced the SNS-induced increases in EPI and NE output. After yohimbine treatment, desipramine further enhanced the SNS-induced increase in NE output. These results suggest that the release of adrenal CA in response to SNS is inhibited by alpha 2-adrenoceptors, and that released NE, rather than EPI, is predominantly taken up into the dog adrenal medullary cells.

Examination of the role of inhibition of cyclic AMP in α2‐adrenoceptor mediated contractions of the porcine isolated palmar lateral vein

1. We have examined the effect of elevation of cellular adenosine 3':5'-cyclic monophosphate (cyclic AMP) on alpha 1- and alpha 2-adrenoceptor-mediated contraction of the isolated palmar lateral vein of the pig. Cellular cyclic AMP was increased by either inhibition of phosphodiesterase by rolipram, or direct activation of adenylyl cyclase by forskolin. 2. Noradrenaline (1 nM-10 microM) caused concentration-dependent contractions of the porcine isolated palmar lateral vein (pD2 7.32 +/- 0.07, n = 10). The selective alpha 1-adrenoceptor antagonist, prazosin (0.1 microM) and the selective alpha 2-adrenoceptor antagonist, rauwolscine (1 microM) caused a 10 fold rightward displacement of the concentration-response curve and a combination of the two antagonists caused a 200 fold rightward displacement of the concentration-response curve. The selective alpha 2-adrenoceptor agonist, UK-14304, also produced concentration-dependent contractions of the palmar lateral vein (pD2 7.70 +/- 0.15, n = 5), but the maximum response was 55.5 +/- 7.6% (n = 5) of that produced by noradrenaline. Prazosin (0.1 microM) failed to affect responses to UK-14304 but rauwolscine, 1 microM, caused a 200 fold rightward displacement. The estimated pKB value for rauwolscine (8.28 +/- 0.19, n = 10) is consistent with inhibition of alpha 2-adrenoceptors. Thus, the porcine isolated palmar lateral vein has a population of alpha 1- and alpha 2-adrenoceptors capable of producing a contraction. 3. Rolipram, 10 micro M, and forskolin, 1 micro M, caused a 2-3 fold rightward displacement of the noradrenaline concentration-response curve (CRC), but 1,9-dideoxyforskolin, 1 micro M, a forskolin analogue which does not activate adenylyl cyclase, failed to produce a significant inhibition of noradrenaline induced contractions. The combination of forskolin (1 micro M) and rolipram (10 micro M) were additive, producing a 20 fold rightward displacement of the noradrenaline CRC.4. Responses to noradrenaline were similarly affected by a combination of rolipram (10 micro M) and prazosin (0.1 micro M) (isolation of alpha 2-adrenoceptors) and the combination of rolipram (10 micro M) and rauwolscine(1 micro M) (isolation of alpha l-adrenoceptors), resulting in a 100 fold rightward displacement of the noradrenaline CRC. Although forskolin inhibited both alpha l- and alpha 2-adrenoceptor-mediated contractions,the effects produced were not similar. In particular, noradrenaline, 0.3-3 micro M, produced a significant contraction in the presence of forskolin (1 micro M) and prazosin (0.1 micro M) (an alpha 2-adrenoceptor-mediated response) but not in the presence of forskolin (1 micro M) and rauwolscine (1 micro M) (an alpha l-adrenoceptor mediated response).5. Five minute exposure to either rolipram (10 micro M) or forskolin (1 micro M) elevated [3H]-cyclic AMP of the porcine isolated palmar lateral vein by approximately 70% and 150-200%, respectively. Neither noradrenaline (1 nM- 100 micro M) nor UK-14304 (1 nM- 100 micro M) affected basal levels of [3H]-cyclic AMP,but both produced a concentration-dependent inhibition of forskolin-stimulated [3H]-cyclic AMP accumulation with a pKi of 7.43 +/- 0.1 (n = 3) and 7.97 +/- 0.18 (n = 3), respectively. The effect of noradrenaline against forskolin-stimulated [3H]-cyclic AMP accumulation was reversed by rauwolscine(1 micro M) but not by prazosin (0.1 micro M). In contrast, alpha 2-adrenoceptor activation did not affect rolipram induced elevation of [3H]-cyclic AMP.6. These findings indicate that M2-adrenoceptor contractions of the porcine isolated palmar lateral vein are not produced by reduction in cellular cyclic AMP per se. It is proposed that this response involves a novel signal transduction mechanism. However, when cellular cyclic AMP has been elevated by agents that stimulate adenylyl cyclase, rather than through inhibition of phosphodiesterase, the ability of alpha 2-adrenoceptors to inhibit cyclic AMP formation may be of functional importance in vascular smooth muscle.

Regulation of prostatic smooth muscle contractility by intracellular second messengers: implications for the conservative treatment of benign prostatic hyperplasia.

The increased sympathetic neurotransmission in benign prostatic hyperplasia (BPH) results in a alpha 1C-adrenoceptor-mediated increase in prostatic smooth muscle tone which seems to be responsible for the dynamic infravesical obstruction occurring in BPH. The prostatic smooth muscle contractions evoked by norepinephrine can be efficiently blocked by alpha 1-adrenoceptor blockers. Moreover, an impressive number of clinical trials illustrated the beneficial results of alpha 1-adrenoceptor blockers in the treatment of BPH. However, despite knowledge of alpha 1-adrenergic neurotransmission and the clinical application of its blockade by selective alpha 1-adrenoceptor antagonists, very little is known about the intracellular pathways involved in the regulation of prostatic smooth muscle contractility. To study the intracellular mechanism of the alpha 1C-adrenoceptor-induced prostatic smooth muscle contraction, the patch-clamp technique in the whole-cell configuration mode combined with the Fura-II fluorescence technique was used in human, enzymatically isolated smooth muscle cells obtained from patients undergoing transurethral resection of the prostate because of symptomatic BPH. Furthermore changes in prostatic smooth muscle contractility were registered in organ bath experiments. Application of the selective alpha 1-adrenoceptor agonist phenylephrine (PE) increased the L-type Ca(2+)-channel current (ICa) dose dependently from 8 up to 18.5 microA/cm2, simultaneously elevating the free cytoplasmic Ca2+ concentration ([Ca2+]i) up to 1.9 microM. Pretreating the myocytes with pertussis toxin, an exotoxin of Bordetella pertussis which inactivates GTP-binding proteins (G proteins) of the Gi and G(o) family by ADP ribosylation, reduced the PE-induced ICa stimulation by 71.5 +/- 5.6% (n = 21). Dialysis of the cytosol with the second messenger inositol-1,4,5-trisphosphate (IP3), which releases Ca2+ from intracellular non-mitochondrial, IP3-sensitive Ca2+ pools, imitated the PE-evoked responses. Pretreating the myocytes with the Ca(2+)-release blockers ryanodine (10-100 microM, n = 8), thapsigargin (0.1 microM, n = 11) or low-molecular weight heparin (n = 14) largely attenuated the PE-evoked responses. The experimental results suggest a coupling of alpha 1-adrenoceptors to phospholipase C-converting phosphoinositol-4,5-bisphosphate into diacylglycerol, an endogenous activator of the protein kinase C and IP3 which releases Ca2+ from intracellular stores stimulating ICa via Ca(2+)-calmodulin-dependent protein kinase induced phosphorylation of voltage-dependent Ca2+ channels. This knowledge could be of interest for conservative treatment in symptomatic BPH.

Potentiation of the anti‐obesity effect of the selective β3‐adrenoceptor agonist BRL 35135 in obese Zucker rats by exercise

1. The effects of chronic treatments with a selective beta 3-adrenoceptor agonist and a selective alpha 2-adrenoceptor antagonist and their interactions with physical exercise training were studied in experimental obesity. 2. BRL 35135 (beta 3-agonist, 0.5 mg kg-1 day-1 p.o.), atipamezole (alpha 2-antagonist, 4.0 mg kg-1 day-1 p.o.) and placebo were given to genetically obese male Zucker rats. Half of the rats were kept sedentary whereas the other half were subjected to moderate treadmill exercise training. Body weight gain, cumulative food intake, the neuropeptide Y content of the hypothalamic paraventricular nucleus, brown adipose tissue thermogenic activity (measured as GDP binding), plasma insulin and glucose levels were measured after 3 weeks' treatment and exercise. 3. Treatment with BRL 35135 reduced weight gain by 19%, increased brown adipose tissue thermogenic activity 45-fold and reduced plasma insulin by 50%. Atipamezole slightly increased food intake and neuropeptide Y content in the paraventricular hypothalamic nucleus but had no effect on the other measured parameters. Exercise alone had no effect on weight gain, food intake or thermogenic activity, whereas it reduced plasma insulin and glucose levels. 4. The effect of BRL 35135 on weight gain and thermogenic activity was significantly potentiated by exercise; the reduction in weight gain was 56% in comparison with 19% in sedentary animals. Food intake was significantly reduced in the BRL 35135-treated-exercise-trained animals, although neither beta 3-agonist nor exercise alone affected it. 5. Based on the present results in genetically obese Zucker rats, combination of 03-agonist treatment with a moderate physical training may offer a new feasible approach to the therapy of obesity.- BRL 35135; atipamezole; P3-adrenoceptor agonism; M2-adrenoceptor antagonism; brown adipose tissue; thermogenesis;genetic obesity; Zucker rat; exercise; neuropeptide Y

Involvement of alpha-adrenoceptors in the excitatory effect of the A2 adenosine receptors agonist 5'-N-ethylcarboxamidoadenosine (NECA) on cardiac automaticity in the isolated right ventricle of the rat.

The effects of the non-selective A2 adenosine receptor agonist 5'-N-ethyl-carboxamidoadenosine (NECA) were studied on ventricular automaticity induced by a local injury in the isolated right ventricle of the rat. In concentrations ranging from 0.1 to 100 nM, NECA significantly increased ventricular automaticity. This effect was not apparent when the nonselective alpha-adrenoceptor blocker phenoxybenzamine was present at a concentration of 10 microM, which antagonizes both alpha 1- and alpha 2-adrenoceptors, as well as when rats were pretreated with reserpine. In non-reserpinized rats, the excitatory effect of NECA was also abolished in the presence of the selective alpha 1-adrenoceptor antagonist prazosin, but not in the presence of the alpha 2-adrenoceptor antagonist idazoxan. In reserpinized rats, the excitatory effect of NECA was restored in the presence of the non specific alpha-adrenoceptor agonist phenylephrine as well as in the presence of the selective alpha 1-adrenoceptor agonist amidephrine but not in the presence of the selective alpha 2-adrenoceptor agonist clonidine. These results suggest that the excitatory effect of NECA on ectopic ventricular automaticity is dependent on endogenous catecholamines and that alpha-adrenoceptors of type 1 are, in some way, involved in this effect.

Atipamezole, an alpha 2 antagonist, augments opiate-induced muscle rigidity in the rat

Atipamezole is a new, highly selective alpha 2-adrenoceptor antagonist currently undergoing clinical trials as an antagonist for dexmedetomidine, a potent alpha 2 agonist with sedative and analgesic properties. It has previously been demonstrated that dexmedetomidine, acting at central alpha 2 adrenoceptors, antagonizes opiate-induced muscle rigidity. However, the role of endogenous alpha 2-adrenergic systems in opiate-induced rigidity remains to be elucidated. The present study was designed to assess the effects of atipamezole on basal muscle tone and on alfentanil-induced muscle rigidity in the rat. Muscle tone was measured using gastrocnemius electromyography (EMG). After a 15-min baseline, saline or atipamezole (0.3 or 1.0 mg/kg) was administered, and 10 min later, saline or alfentanil (50, 150, or 300 μg/kg) was injected subcutaneously. Data were collected for an additional 60 min. Atipamezole (1.0 mg/kg) pretreatment (in the absence of alfentanil) produced a small increase in tonic EMG activity when compared with saline pretreatment. After saline pretreatment, significant muscle rigidity occurred in the two highest alfentanil dose groups. Atipamezole (0.3 and 1.0 mg/kg) augmented alfentanil-induced muscle rigidity. The ability of the alpha 2 antagonist to potentiate both basal muscle tone and alfentanil-induced rigidity suggests that endogenous adrenergic activity and/or direct alpha 2-adrenoceptor interaction with opioid receptors mediate opiate-induced muscle rigidity. These findings may be of clinical as well as basic neuropharmacological interest.

Regionally specific changes in extracellular noradrenaline following chronic idazoxan as revealed by in vivo microdialysis.

The selective alpha 2-adrenoceptor antagonist idazoxan was administered chronically (0.8 mg/kg per h) to rats for a period of 10 days via osmotic minipumps. On day 11, 24 h after removal of the pumps, the rats were anaesthetised and microdialysis probes were implanted into either the frontal cortex or hippocampus. Basal noradrenaline release in the frontal cortex was significantly elevated compared with the saline control group. Each animal was then challenged with idazoxan (10 mg/kg s.c.). Inhibition of presynaptic alpha 2-adrenoceptors resulted in a significant increase in noradrenaline release in the saline control group. However, animals treated chronically with idazoxan, showed a markedly attenuated response to the single dose idazoxan challenge in the frontal cortex. No significant change in either basal release or in response to idazoxan challenge was observed in the hippocampus in the chronic idazoxan-treated animals as compared with the chronic saline control group. Chronic idazoxan administration results in selective enhancement of noradrenaline release in the frontal cortex but not in the hippocampus. This would be consistent with a down-regulation of presynaptic alpha 2-adrenoceptors with the subsequent loss of presynaptic noradrenergic negative feedback inhibition.

Covariation of α2-adrenoceptor density and function following irreversible antagonism with EEDQ

1. Administration of the irreversible antagonist, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline, (EEDQ, 2 mg kg-1, i.p.) to mice reduced binding of [3H]-RX 821002 (2-methoxy-idazoxan) to alpha 2-adrenoceptors in whole mouse brain by 75% 24 h later. The receptor binding returned over time only being reduced by 25% by 16 days post administration; the time taken for binding to return to 50% of control levels was estimated to be 5.25 days. 2. EEDQ administration also resulted in the loss of the sedative effect of the alpha 2-adrenoceptor agonist, medetomidine, measured by the holeboard test of directed exploration and locomotor activity. Agonist-induced sedation returned to control values by 8 days post EEDQ administration. 3. EEDQ administration also resulted in the loss of the hypothermic response to medetomidine (0.1 mg kg-1, i.p.). Medetomidine-induced hypothermia returned to control values by 12 days post EEDQ administration. 4. Pretreatment with the selective alpha 2-adrenoceptor antagonist, RX 821002 (0.1-3.0 mg kg-1, i.p.) 45 min before EEDQ prevented the loss of alpha 2-adrenoceptors as well as the blockade of medetomide-induced sedation and hypothermia by EEDQ. 5. The results of these experiments indicate that there is significant receptor reserve for alpha 2-adrenoceptor-mediated behavioural and physiological responses.

Effects of imidazolines on noradrenaline release in rat isolated kidney.

The aim of the present study was to investigate whether or not activation of imidazoline receptors modulates noradrenaline release in the rat isolated kidney. Kidneys were pre-exposed to 3H-noradrenaline and the renal nerves were stimulated with 6 pulses at 100 Hz. The stimulation induced (S-I) outflow of radioactivity was taken as an index of endogenous noradrenaline release. The imidazoline derivatives clonidine (1-1000 nmol/l) and moxonidine (10-1000 nmol/l) inhibited S-I outflow of radioactivity with an EC50 of 6.8 nmol/l and 62.5 nmol/l and a maximum of 88% and 97%, respectively. The concentration response curves for clonidine and moxonidine were shifted to the right by the selective alpha 2-adrenoceptor antagonist rauwolscine (0.1 mumol/l) in a parallel manner with identical pKB's of 8.52 and 8.46, respectively. Furthermore, the alpha-adrenoceptor agonist (-)-alpha-methylnoradrenaline (0.1-30 nmol/l), which has no affinity for imidazoline binding sites, inhibited S-I outflow of radioactivity with and EC50 of 2.4 nmol/l and a maximum of 94%. Rauwolscine (0.1 mumol/l) again shifted the concentration response curve for this alpha-adrenoceptor agonist to the right with a pKB of 8.40. Moreover, the selective alpha 2-adrenoceptor antagonist 2-[2-(2-methoxy-1,4- benzo-dioxanyl)]imidazoline HCl (RX821002, 0.01 mumol/l) shifted the concentration response curves for clonidine and moxonidine to the right with pKB's of 9.46 and 9.18, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of the Responses of Isolated and Perfused Canine Splenic Arteries to Vasoactive Substances and to Periarterially Electrical Stimulation

Pharmacological characteristics of the canine isolated splenic artery were investigated by the cannula insertion method for observing vascular responses to vasoactive agents and periarterial nerve stimulation. Four alpha-adrenoceptor agonists and tyramine induced vasoconstrictions in a dose-dependent manner, and the order of potency was noradrenaline (NA) > phenylephrine > clonidine > methoxamine > tyramine. Xylazine (a selective alpha 2-adrenoceptor agonist) did not elicit any vasoconstriction. Several autacoids and KCl also constricted the splenic artery dose-dependently, and the order of potency was 5-hydroxytryptamine (5-HT) >> ATP = histamine >> KCl. The dose-response curves for clonidine and NA were shifted to the right by bunazosin (a selective alpha 1-adrenoceptor antagonist), but were not affected by midaglizole (a selective alpha 2-adrenoceptor antagonist). The parameters of electrical stimulation to elicit a clear and constant vasoconstriction were 0.2 msec of pulse duration, 6 V and 0.1 Hz. The vasoconstrictive responses to electrical stimulation at 6-12 V, 0.1-10 Hz and 0.2-1 msec of pulse duration were completely inhibited by tetrodotoxin (TTX) and strongly inhibited by guanethidine. The results in this study suggest that: 1) in contrast with other regional arteries, the canine splenic artery has an alpha 1-adrenoceptor-related and clonidine-sensitive vasoconstrictive response, 2) this artery has no functional postsynaptic alpha 2-adrenoceptors, 3) it may be easier to observe the vascular responses to vasoactive agents in the isolated and perfused arterial segments, and 4) the isolated and perfused canine splenic artery is useful as a preparation to study the sympathetic nerve transmission.

Functional evidence that alpha 2A-adrenoceptors are responsible for antilipolysis in human abdominal fat cells.

The effects of alpha 2-adrenoceptor agonists (dexmedetomidine, oxymetazoline), alone or in combination with various alpha-adrenoceptor subtype-selective antagonists (CH-38083, idazoxan, WB4101, BRL44408, ARC-239, prazosin), on noradrenaline- and isoprenaline-induced lipolysis were investigated in human isolated abdominal subcutaneous fat cells. The rank order of potency of antagonists in preventing dexmedetomidine- and oxymetazoline-evoked suppression of isoprenaline-induced lipolysis was (pA2-values): CH-38083 (7.69 and 7.48) congruent to idazoxan (7.5 and 7.41) > BRL44408 (7.23 and 7.19) congruent to WB4101 (7.13 and 7.12) > prazosin (5.18 and 5.17) > ARC-239 (4.72, 4.9). While CH-38083 and idazoxan, non-subtype selective alpha 2-adrenoceptor antagonists and BRL44408, a selective alpha 2A-adrenoceptor antagonist as well as WB4101 potentiated the lipolytic effect of noradrenaline, ARC-239, the selective alpha 2B-adrenoceptor antagonist failed to affect it. In addition since the alpha 2A-adrenoceptor selective agonist, oxymetazoline concentration dependently inhibited the lipolytic effect of isoprenaline, and WB4101 and BRL44408 (alpha 2A-adrenoceptor antagonists) antagonised the effect of oxymetazoline in a competitive manner, it is concluded that the alpha 2A-adrenoceptor subtype is involved in antilipolysis. In addition, functional evidence was obtained that there is an interaction between alpha 2A- and beta-adrenoceptors located on the cell surface of adipocytes, through which locally released noradrenaline and/or circulating circulating adrenaline influence lipolysis.

The effects of a peripherally selective alpha 2‐adrenoceptor antagonist, MK‐467, on the metabolic and cardiovascular response to exercise in healthy man.

1. A double-blind, placebo controlled study has been conducted to investigate the consequences of i.v. treatment with MK-467, a peripherally selective alpha 2-adrenoceptor antagonist in exercising healthy male subjects. In particular, the effects on blood pressure, heart rate, circulating catecholamines (noradrenaline (NA) and adrenaline (A)), insulin, glucose and free fatty acids (FFA) were determined. 2. Exercise produced increases in catecholamines, blood pressure and heart rate. FFA increased at the start of the exercise but then declined as exercise progressed. 3. MK-467 significantly increased NA, in a dose-dependent manner, before and during exercise without altering A. Blood pressure and heart rate were not affected by drug treatment. 4. The insulin and FFA response to exercise was significantly enhanced by MK-467 although glucose was unaltered by drug. 5. It is concluded that both pre- and post-junctional peripheral alpha 2-receptors play an important role in the metabolic response to exercise in man.

Effects of pertussis and cholera toxins on alpha-adrenoceptor function in rat tail artery: differences in hypertension.

The alpha 1- and alpha 2-adrenoceptor-stimulated contractile responses of rat tail artery rings were compared in Sprague-Dawley (SD), spontaneously hypertensive (SHR), and Wistar-Kyoto (WKY) rats that were untreated, treated with pertussis toxin, or treated with cholera toxin. The maximal responses, expressed as milligrams of tension, induced by clonidine (an alpha 2-adrenoceptor agonist) and cirazoline (a selective alpha 1-adrenoceptor agonist) were significantly greater in SHR than in SD or WKY, and the tissues were more sensitive to the agonists in SHR or SD than in WKY. Yohimbine (0.1 microM), a selective alpha 2-adrenoceptor antagonist, shifted the dose-response curves for clonidine to the right. The effects of yohimbine were greater in SD than in WKY or SHR, but not different between WKY and SHR. Prazosin (0.05 microM), a selective alpha 1-adrenoceptor antagonist, shifted the dose-response curves of cirazoline to the right, but the effects of prazosin were not different among these three strains of rats. Nifedipine (0.05 microM) completely blocked the response to clonidine in SD and WKY; however, in SHR, approximately one-third of the response to clonidine was resistant to nifedipine. Nifedipine, at 0.05 microM, only partially inhibited responses to cirazoline in SD, SHR, and WKY, and no differences were noted between the strains. Pertussis toxin pretreatment (50 micrograms/kg, 3 days before experiment) almost completely blocked the responses to clonidine, but only partially inhibited those to cirazoline. After pertussis toxin pretreatment, the responses (maximal effects and EC50s) to clonidine and cirazoline were not significantly different in arteries from the three strains of rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of adrenoceptor agonists on striated muscle strips of the canine oesophagus

1. Acute psychological stress, which could be related to the release of a large amount of catecholamines, may cause oesophageal motility disorders. Therefore, the aim of our study was to elucidate the influence of adrenoceptor agonists on the striated muscle portion of the oesophagus by use of isolated strips from dogs. 2. Contractions were evoked in isolated striated muscle strips by electrical field stimulation (1 pulse min-1, 1 ms/pulse, submaximal voltage). The effects induced by administration of adrenoceptor agonists alone or in the presence of antagonists were tested to determine the nature of the adrenoceptors on this muscle preparation. 3. The administration of both the natural adrenoceptor agonists, adrenaline and noradrenaline, and the synthetic beta-adrenoceptor agonists, isoprenaline (beta 1 + beta 2), dobutamine (beta 1) or ritodrine (beta 2), enhanced the amplitude of the contractions induced by electrical stimulation in a concentration-dependent manner. The maximum responses were 82.6 (adrenaline), 66.2 (noradrenaline), 86.2 (isoprenaline), 34.6 (dobutamine) and 80.8% (ritodrine). The EC20 values obtained were respectively 2 nM, 0.2 microM, 0.91 nM, 3 microM and 80 nM. The administration of the alpha 1-adrenoceptor agonist, phenylephrine, also enhanced the contractile response in a concentration-dependent manner (EC20 value = 0.3 microM) and the maximum response was 64.6%, but the administration of the alpha 2-adrenoceptor agonist, clonidine, did not influence the contractile response. These data suggest the involvement of beta 2- and possibly alpha 1-adrenoceptors in the responses of these adrenoceptor agonists. 4 The selective P2-adrenoceptor antagonist ICI 118551 (3-100nM) shifted the concentration-effect curves for noradrenaline, phenylephrine and ritodrine to the right in a concentration-dependent manner.ICI 118551 (3 nM) also shifted the concentration-effect curves for adrenaline and isoprenaline to the right, but increasing the concentration of ICI 118551 did not cause any further antagonist activity until a concentration of 100 nM, when a further rightward shift was obtained.5. The selective alpha 1-adrenoceptor antagonist, prazosin (30-300 nM), did not affect the increased contractile responses induced by adrenaline, noradrenaline, phenylephrine, isoprenaline or ritodrine.6. In conclusion, it appears that beta2-adrenoceptors are present in the striated muscle portion of the canine oesophagus, where they mediate an enhancement of contractile responses evoked by electrical stimulation. The alpha l-agonist, phenylephrine, appears to interact with beta2-adrenoceptors on this preparation.beta 3-Adrenoceptors have already been demonstrated in smooth muscle from various parts of the gastrointestinal tract, and our study does not exclude the possibility that there is an additional population of beta 3-receptors in the canine striated muscle part of the oesophagus.