The pharmacology of alpha 1- and alpha 2-adrenoceptors: evidence for and against a further subdivision

Treatment of maple syrup urine disease: Benefits, risks, and challenges of liver transplantation

Maple syrup urine disease (MSUD) is caused by a deficiency in the activity of the branched-chain α-ketoacid dehydrogenase (BCKD) complex, promoting the accumulation of the branched-chain amino acids (BCAA) leucine, isoleucine, and valine, as well as their respective α-keto acids. MSUD is an autosomal recessive hereditary metabolic disorder characterized by ketoacidosis, ataxia, coma, and mental and psychomotor retardation. The mechanisms involved in the brain damage caused by MSUD are not fully understood. Early diagnosis and treatment, as well as proper control of metabolic decompensation crises, are crucial for patients' survival and for a better prognosis. The recommended treatment consists of a high-calorie diet with restricted protein intake and specific formulas containing essential amino acids, except those accumulated in MSUD. This treatment will be maintained throughout life, being adjusted according to the patients' nutritional needs and BCAA concentration. Because dietary treatment may not be sufficient to prevent neurological damage in MSUD patients, other therapeutic strategies have been studied, including liver transplantation. With transplantation, it is possible to obtain an increase of about 10% of the normal BCKD in the body, an amount sufficient to maintain amino acid homeostasis and reduce metabolic decompensation crises. However, the experience related to this practice is very limited when considering the shortage of liver for transplantation and the risks related to the surgical procedure and immunosuppression. Thus, the purpose of this review is to survey the benefits, risks, and challenges of liver transplantation in the treatment of MSUD.

Gestational purple grape juice consumption does not change fetal ductus arteriosus constriction in Wistar rats

Grape juice consumption may influence the early occurrence of ductal constriction during pregnancy, since the consumption of foods rich in polyphenols can be linked to the premature constriction of the ductus arteriosus. This study aimed to evaluate the effect of purple grape juice consumption during gestation on fetal ductus arteriosus closure, prostaglandin levels, and oxidative stress markers in Wistar rats. We divided 18 pregnant rats into four groups: a control group (C), a single-dose grape juice group (SDGJ), a two-dose grape juice group (TDGJ) of 7 μl/g body weight per day, and an indomethacin group (I). Blood was collected on gestational day (GD) 0, 14, and 20. Prostaglandin levels were measured, and the livers and hearts were removed from the mothers and fetuses for oxidative stress analysis; histology of the fetal ductus arteriosus was performed. Prostaglandin levels (pg/ml) at GD 20 were (C:1462.10 ± 314.61); (SDGJ:987.66 ± 86.25); (TDGJ:1290.00 ± 221.57), and (I:584.75 ± 46.77). Fetal ductus arteriosus closure occurred only in the indomethacin group. Lipid peroxidation evaluated through thiobarbituric acid reactive substances (nmol/mg protein) in maternal livers was lower in the grape juice groups (C: 4.11 ± 0.76 nmol/mg protein), (SDGJ: 2.34 ± 0.36), (TDGJ: 1.52 ± 0.18), and (I: 4.20 ± 0.76). Sulfhydryls (nmol/mg protein) were lower in the TDGJ group (C:763.59 ± 61.38 nmol/mg protein), (SDGJ:978.88 ± 158.81), (TDGJ:385.32 ± 86.78), and (I:727.72 ± 49.12). Also, superoxide dismutase activity (USOD/mg protein) was higher in fetal hearts in this group: (C:5.29 ± 0.33), (SDGJ:4.48 ± 0.47), (TDGJ:7.35 ± 0.43), and (I:6.00 ± 0.18). We conclude that grape juice consumption in pregnancy does not induce ductus arteriosus closure in the fetus and presented potential antioxidant effects.

Chemical profile of Lippia thymoides, evaluation of the acetylcholinesterase inhibitory activity of its essential oil, and molecular docking and molecular dynamics simulations

The essential oils of the fresh and dry flowers, leaves, branches, and roots of Lippia thymoides were obtained by hydrodistillation and analyzed using gas chromatography (GC) and GC-mass spectrometry (MS). The acetylcholinesterase inhibitory activity of the essential oil of fresh leaves was investigated on silica gel plates. The interactions of the key compounds with acetylcholinesterase were simulated by molecular docking and molecular dynamics studies. In total, 75 compounds were identified, and oxygenated monoterpenes were the dominant components of all the plant parts, ranging from 19.48% to 84.99%. In the roots, the main compounds were saturated and unsaturated fatty acids, having contents varying from 39.5% to 32.17%, respectively. In the evaluation of the anticholinesterase activity, the essential oils (detection limit (DL) = 0.1 ng/spot) were found to be about ten times less active than that of physostigmine (DL = 0.01ng/spot), whereas thymol and thymol acetate presented DL values each of 0.01 ng/spot, equivalent to that of the positive control. Based on the docking and molecular dynamics studies, thymol and thymol acetate interact with the catalytic residues Ser203 and His447 of the active site of acetylcholinesterase. The binding free energies (ΔGbind) for these ligands were -18.49 and -26.88 kcal/mol, demonstrating that the ligands are able to interact with the protein and inhibit their catalytic activity.

Receptors Involved in the Modulation of 5-Hydroxytryptamine Release in Bovine Cerebral Arteries

The uptake of [3H]5-hydroxytryptamine (5-HT) in bovine cerebral arteries was reduced by cocaine (1 μm), ouabain (100 μm), pretreatment with 6-hydroxydopamine (6-OHDA) (1·46 Mm, 10 min) and metitepine (1 μm). Electrically-stimulated tritium release was decreased by tetrodotoxin (0·8 μm), Ca-free medium, denervation with 6-OHDA (1·46 Mm, 10 min), 5-HT (10 μm), noradrenaline (1 μm) and the agonist of α2-adrenoceptors B-HT 920 (0·1 and 1 μm), enhanced by metitepine (1 μm, antagonists of presynaptic 5-HT1 receptors) and rauwolscine (1 μm, antagonist at α2-adrenoceptors, and also of 5-HT,1d receptors) and not affected by ketanserin (1 μm, antagonist of 5-HT2 receptors), methysergide (0·1 μm, antagonist of 5-HT1 and 5-HT2 receptors) and phentolamine (1 and 3 μm antagonist of α-adrenoceptors and less potent of 5-HT1 receptors). The inhibitory action of 10 μm 5-HT was partially reversed by phentolamine (3 μm) and cocaine (1 μm) and completely reversed by both metitepine (1 μm) and rauwolscine (1 μm). Ketanserin (1 μm), methysergide (0·1 μm) or phentolamine (1 μm) had no effect. Rauwolscine (1 μm) antagonized the inhibition induced by both noradrenaline (1 μm) and B-HT 920 (0·1 and 1 μm). 5-HT induced tritium release which was inhibited by cocaine (an antagonist of 5-HT3 receptors) and denervation with 6-OHDA. These results suggest that 5-HT is mainly accumulated in adrenergic nerve endings, that evoked [3H]5-HT release is modulated by 5-HT1-like receptors, but the participation of α2-adrenoceptors cannot be discounted, or more probably both types of receptors have features in common, and evoked [3H]5-HT release elicited by 5-HT may be partially mediated by activation of 5-HT3 receptors.

Subtypes of functional alpha1-adrenoceptor

In this review, subtypes of functional alpha1-adrenoceptor are discussed. These are cell membrane receptors, belonging to the seven-transmembrane-spanning G-protein-linked family of receptors, which respond to the physiological agonist noradrenaline. alpha1-Adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A fourth alpha1-adrenoceptor, the alpha1L-, represents a functional phenotype of the alpha1A-adrenoceptor. alpha1-Adrenoceptor subtype knock-out mice have refined our knowledge of the functions of alpha-adrenoceptor subtypes, particuarly as subtype-selective agonists and antagonists are not available for all subtypes. alpha1-Adrenoceptors function as stimulatory receptors involved particularly in smooth muscle contraction, especially contraction of vascular smooth muscle, both in local vasoconstriction and in the control of blood pressure and temperature, and contraction of the prostate and bladder neck. Central actions are now being elucidated.

Orchidectomy modulates alpha2-adrenoceptor reactivity in rat mesenteric artery through increased thromboxane A2 formation

The present study evaluates the effect of endogenous male sex hormones on the reactivity to alpha2-adrenoceptor activation, and to analyze the role of the endothelium in this response in intact and endothelially denuded superior mesenteric arteries from control and orchidectomized male Sprague-Dawley rats. The concentration-dependent constriction induced by clonidine was analyzed in the absence and presence of the nitric oxide synthase (NOS) inhibitor, Nomega-nitro-L-arginine (L-NAME), cyclooxygenase (COX-1 and COX-2) inhibitors, indomethacin, the specific COX-2 inhibitor NS-398, the thromboxane-prostanoid receptor antagonist SQ29,548 and the thromboxane A2 (TXA2) synthase inhibitor, furegrelate. Endothelial NOS (eNOS), COX-2 and TXA2 synthase protein expression was studied by Western blot analysis. In addition, the basal and clonidine-stimulated production of TXB2, the stable TXA2 metabolite, was also measured. In intact vessels from control male rats, the concentration-dependent constriction induced by clonidine was increased by both L-NAME or endothelial removal, unaltered by indomethacin and decreased by NS-398; in denuded vessels, the clonidine response was decreased by NS-398 and unaltered by L-NAME, indomethacin, SQ29,548 or furegrelate. In intact vessels from orchidectomized rats, the constriction induced by clonidine was increased by L-NAME but practically abolished by indomethacin or NS-398; in endothelially denuded segments the clonidine response was unaltered by L-NAME, but was decreased by indomethacin, NS-398, SQ29,548 or furegrelate. Orchidectomy failed to modify eNOS,COX-2 and TXA2 synthase expression, and increased basal and clonidine-stimulated TXB2 release. These results show that TXA2 produced in smooth muscle cells is increased in mesenteric arteries from orchidectomized rats compared to their controls, and that this prostanoid is functionally involved in the vasoconstrictor response to clonidine only in arteries from the orchidectomized rats.

Investigation of the subtypes of alpha1-adrenoceptor mediating contractions of rat vas deferens

1 The subtypes of alpha1-adrenoceptor mediating contractions of rat vas deferens to endogenous and exogenous noradrenaline and to the exogenous agonists methoxamine, phenylephrine and A61603 have been examined. 2 The effects of antagonists on the shape of concentration-response curves, both tonic and phasic, to the four agonists were analysed. Prazosin produced parallel shifts in all cases. Particularly for RS 17053 against noradrenaline, there was some evidence for a resistant component of the agonist response. High concentrations of RS 17053 (1-10 microM) virtually abolished tonic contractions but phasic contractions were resistant. 3 A series of nine antagonists (the above and WB4101, benoxathian, phentolamine, BMY 7378, HV 723, spiperone) were investigated against contractions to noradrenaline. The correlation with the potency of the series of alpha1-adrenoceptor antagonists against contractions to noradrenaline was significant only for the alpha1A-adrenoceptor ligand binding site (r=0.88, n=9, P<0.01). 4 In epididymal portions (nifedipine 10 microM), the isometric contraction to a single electrical pulse is alpha1-adrenoceptor mediated. The correlation with ligand binding sites for 11 antagonists (the above plus ARC 239 and (+)-niguldipine) was significant only for the alpha1D-adrenoceptor subtype (r=0.65, n=11, P<0.05). 5 In conclusion, tonic contractions of rat vas deferens produced by exogenous agonists are mediated predominantly by alpha1A-adrenoceptors, although a second subtype of receptor may additionally be involved in phasic contractions. Nerve-stimulation evoked alpha1-adrenoceptor mediated contractions seem to predominantly involve non-alpha1A-adrenoceptors, and the receptor involved resembles the alpha1D-receptor.

Estrogen replacement modulates resistance artery smooth muscle and endothelial alpha2-adrenoceptor reactivity

The purpose of this study was to evaluate the effects of estrogen replacement on ovariectomized rats on the reactivity to alpha2-adrenoceptor activation, and to analyze the role of the endothelium in modulating this response. Third order branches of the superior mesenteric artery from ovariectomized untreated (OvX) and estrogen-replaced (E2) Sprague-Dawley rats were cannulated and pressurized to 50 mmHg. Under relaxed conditions (0.1 mM papaverine), there were no differences in lumen diameter. Intact vessels from E2 rats were unresponsive to clonidine (0.01-10 microM); incubation in indomethacin (1 microM), a cyclooxygenase inhibitor, produced intermediate constriction that was significantly augmented by L-NNA (0.3 mM), a NO synthase inhibitor, or by endothelial denudation. Conversely, intact vessels from OvX animals constricted to clonidine in a concentration-dependent manner. This effect was significantly diminished by endothelial removal or indomethacin, but was not affected by L-NNA. Yohimbine (1 microM), an beta2 receptor antagonist, significantly diminished arterial sensitivity to, and efficacy of clonidine. These results suggest that estrogen replacement enhanced vasoconstriction induced by smooth muscle alpha2 adrenoceptor activation, although this effect was obscured in intact vessels due to an overriding influence of endothelial dilator substances, primarily NO. In arteries from OvX animals, smooth muscle was less sensitive to alpha2 agonist stimulation, however, the release of a vasoconstrictor prostanoid from the endothelium was predominant, and induced significant vasoconstriction.

Subtypes of functional alpha1- and alpha2-adrenoceptors

In this review, subtypes of functional alpha1- and alpha2-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline. Alpha1-adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A 4th alpha1-adrenoceptor, the alpha1L-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. Alpha2-adrenoceptors can be divided into alpha2A-, alpha2B- and alpha2C-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha2-adrenoceptors are predominantly of the alpha2A-adrenoceptor subtype (the alpha2D-adrenoceptor is a species orthologue), although alpha2C-adrenoceptors may also occur prejunctionally. Although alpha2-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of alpha-adrenoceptor subtypes.

Effects of the selective cyclooxygenase-2 inhibitor nimesulide on vascular contractions in endothelium-denuded rat aorta

We have examined the effects of the selective cyclooxygenase-2 inhibitor nimesulide and the non-selective cyclooxygenase inhibitor indomethacin on vascular responsiveness of endothelium-denuded rat aorta. Isometric contractions were obtained to the alpha-adrenoceptor agonists phenylephrine (full agonist) and clonidine (partial agonist relative to phenylephrine) and to endothelin-1 and KCl. Maximum contractile responses to the partial agonist clonidine were significantly reduced by nimesulide (10 microM) and by indomethacin (10 microM) to 60.8 +/- 8.5% (n = 8) and 69.0 +/- 9.6% (n = 12) of control, respectively, as compared with the effects of vehicle (99.0 +/- 5.8%; n = 17). The inhibitors had lesser effects against contractions to phenylephrine: nimesulide had no significant effect, whereas indomethacin caused a small but significant reduction in the maximum contraction to phenylephrine to 90.3 +/- 5.0% (n = 12) of control (vehicle: 108.0 +/- 5.2%, n = 15 nimesulide: 111.8 +/- 5.9%, n = 5). Neither nimesulide nor indomethacin had any effect on contractions to endothelin-1 or KCl. These actions differed from the effects of the Ca2+ entry blocker nifedipine, which significantly reduced contractions to clonidine and KCl to a similar extent. The maximum contraction to clonidine was also significantly reduced by the thromboxane receptor antagonist SQ 29548 (1 microM) to 83.4 +/- 6.4% of control (n = 7) (vehicle 115.5 +/- 7.5%, n = 7). It is concluded that the cyclooxygenase inhibitors nimesulide or indomethacin reduce vascular responsiveness to alpha-adrenoceptor agonists in endothelium-denuded rat aorta, presumably by preventing the formation of vasoconstrictor prostaglandins in aortic smooth muscle by cyclooxygenase-2. This reduced vascular responsiveness was most clearly seen with the partial agonist clonidine.

Chloroethylclonidine and alpha-adrenoceptor agonist interaction in blood vessels following heart failure

This study examined the interaction of chloroethylclonidine with alpha-adrenoceptor agonists in canine endothelium-denuded dorsal pedal artery and saphenous vein before (non-paced) and at end-stage heart failure which was induced by rapid ventricular pacing (250 bpm for no more than four weeks). The interaction was heterogeneous in both non-paced and heart failure blood vessels. In the dorsal pedal artery, only chloroethylclonidine (10(-4) M) reduced the maximum response to noradrenaline. At 10(-6) and 10(-5) M, chloroethylclonidine potentiated the response to noradrenaline. In the saphenous vein, chloroethylclonidine was not surmountable against noradrenaline before heart failure, but produced competitive antagonism of noradrenaline in the heart failure group (pA2 = 5.7). In the dorsal pedal artery, chloroethylclonidine potentiated the response to low concentrations of methoxamine, but inhibited the response of higher concentrations. In the saphenous vein, chloroethylclonidine (10(-6) and 10(-5) M) potentiated the response to methoxamine from non-paced dogs but did not significantly effect the response at heart failure. In the dorsal pedal artery, chloroethylclonidine (10(-4) M) potentiated low concentrations and inhibited higher concentrations of phenylephrine from non-paced animals but had no significant effect at heart failure. In contrast, in the saphenous vein, chloroethylclonidine (at all concentrations tested) inhibited the response to phenylephrine in non-paced dogs, whereas the inhibitory effect was not as marked in heart failure. In conclusion, these results indicate that differences in alpha1-adrenoceptor populations and distribution are blood vessel dependent and dependent on the pathological state.

The alpha-adrenoceptor-mediated actions of chloroethylclonidine

1. Chloroethylclonidine (CEC) has an affinity for all 6 subtypes of alpha-adrenoceptor, but binds irreversibly particularly to alpha 1B-, alpha 1D-, alpha 2C-, and alpha 2A/D-adrenoceptors. 2. Functionally, CEC behaves as an irreversible alpha 1-adrenoceptor antagonist, reducing the maximum response to noradrenaline (NA), and shows subtype selectivity in that alpha 1A-adrenoceptors are relatively insensitive to CEC. CEC also behaves as an irreversible alpha 2-adrenoceptor agonist, both prejunctionally in the rat vas deferens and postjunctionally in the dog saphenous vein. 3. In the rat aorta, CEC does not produce direct contractions, but following exposure to CEC concentrations of NA of 10 microM and above produce contractions resistant to alpha 1- and alpha 2-adrenoceptor blockade. We have investigated this phenomenon in detail. 4. Receptor protection experiments were carried out in the rat aorta, in which the protecting agent was present prior to and during exposure to CEC. The component of the contraction to NA resistant to alpha-blockade was still present following receptor protection with the alpha 1-adrenoceptor antagonist prazosin, but absent following receptor protection with NA and reduced following receptor protection with alpha 2-adrenoceptor antagonists. The resistant response may represent an irreversible agonist interaction between CEC, NA, and normally silent alpha 2-adrenoceptors, that cannot be affected by subsequent competitive antagonism, but that can be prevented by receptor protection with the agonist NA prior to CEC. 5. CEC has two major classes of action at alpha-adrenoceptors: irreversible antagonism at alpha 1-adrenoceptors, and irreversible agonism at alpha 2-adrenoceptors. Both actions can be demonstrated in the rat aorta.

Effects of catecholamines on isolated human colonic smooth muscle

1. The effects of catecholamines and some adrenoceptor agonists and antagonists on isolated preparations of human colonic smooth muscle obtained from surgical resections were examined. 2. Strips of circular smooth muscle displayed rhythmic myogenic spontaneous contractions which were inhibited by catecholamines with an order of potency of isoprenaline (1.0) > noradrenaline (0.32) > adrenaline (0.2). Phentolamine (0.7 microM) significantly shifted the noradrenaline concentration-response curve (CRC) to the right but had no significant effect on isoprenaline or adrenaline. Propranolol (1 microM) significantly shifted the isoprenaline to the right but had no significant effect on noradrenaline or adrenaline. 3. Salbutamol (30 microM) had no inhibitory effect on the spontaneous activity and ICI 118,551 (1 microM) had no effect on inhibitory responses to isoprenaline. Betaxolol (1 microM) significantly shifted the CRC to isoprenaline to the right. BRL 37344 had no effect on spontaneous activity. 4. Responsiveness of circular strips to catecholamines was not affected by age of the patient and no consistent differences between males and females were shown. 5. Strips of taenia coli exhibited little or no spontaneous phasic activity. Noradrenaline and isoprenaline relaxed KCl-induced tone. The effects of noradrenaline and isoprenaline were antagonized by propranolol but not by phentolamine. BRL 37344 had no effect on KCl-induced tone. 6. In conclusion, catecholamines relaxed spontaneous activity of human colon circular smooth muscle through an action on both alpha- and beta-adrenoceptors. The alpha-adrenoceptors were of the alpha 1-subtype. The beta-adrenoceptor-mediated relaxation appeared to be primarily beta 1. In taenia coli, catecholamines relaxed KCl-induced tone via beta-adrenoceptors only.

Molecular pharmacology of alpha 2-adrenoceptor subtypes

alpha 2-adrenergic receptors mediate many of the physiological actions of the endogenous catecholamines adrenaline and noradrenaline, and are targets of several therapeutic agents. alpha 2-adrenoceptor agonists are currently used as antihypertensives and as veterinary sedative anaesthetics. They are also used in humans as adjuncts to anaesthesia, as spinal analgesics, and to treat opioid, nicotine and alcohol dependence and withdrawal. Three human alpha 2-adrenoceptor subtype genes have been cloned and designated alpha 2-C10, alpha 2-C4, and alpha 2-C2, according to their location on human chromosomes 10, 4 and 2. They correspond to the previously identified pharmacological receptor subtypes alpha 2A, alpha 2C and alpha 2B. The receptor proteins share only about 50% identity in their amino acid sequence, but some structurally and functionally important domains are very well conserved. The most obvious functionally important differences between the receptor subtypes are based on their different tissue distributions; e.g. the alpha 2A subtype appears to be an important modulator of noradrenergic neurotransmission in the brain. The three receptors bind most alpha 2-adrenergic drugs with similar affinities, but some compounds (e.g. oxymetazoline) are capable of discriminating between the subtypes. Clinically useful subtype selectivity cannot be achieved with currently available pharmaceutical agents. The second messenger pathways of the three receptors show many similarities, but small functional differences between the subtypes may turn out to have important pharmacological and clinical consequences. All alpha 2-adrenoceptors couple to the pertussis-toxin sensitive inhibitory G proteins Gi and G(o), but recent evidence indicates that also other G proteins may interact with alpha 2-adrenoceptors, including Gs and Gq/11. Inhibition of adenylyl cyclase activity, which results in decreased formation of cAMP, is an important consequence of alpha 2-adrenoceptor activation. Many of the physiological effects of alpha 2-adrenoceptor activation cannot, however, be explained by decreases in cAMP formation. Therefore, alternative mechanisms have been sought to account for the various effects of alpha 2-adrenoceptor activation on electrophysiologic, secretory and contractile cellular responses. Recent results obtained from studies on ion channel regulation point to the importance of calcium and potassium channels in the molecular pharmacology of alpha 2-adrenoceptors.

Investigation of the actions of chloroethylclonidine in rat aorta

1. The interaction between chloroethylclonidine (CEC) and noradrenaline (NA) has been examined at alpha-adrenoceptors mediating contractions of rat aorta. 2. In rat aorta, the competitive antagonist prazosin, over the concentration-range 0.01-10 microM, produced concentration-dependent shifts in the contractile potency of NA, so that there was no component of the NA contraction resistant to prazosin. 3. The irreversible alpha 1-adrenoceptor antagonists, phenoxybenzamine (PBZ) (1-10 microM) and benextramine (10 microM) produced shifts in potency of NA and reduced the maximum response in a concentration-dependent manner. 4. The irreversible alpha 1-adrenoceptor antagonist, CEC (100 microM), produced a non-parallel shift in the NA concentration-response curve so that low concentrations of NA produced relatively small contractions but relatively high concentrations produced further contractions, so that the maximum response was not significantly reduced. 5. The combination of CEC pretreatment and subsequent prazosin (0.1 microM) produced a parallel shift in the potency of NA. However, prazosin (10 microM) failed to produce any further effect on the response to high concentrations of NA following CEC pretreatment. Hence, a component of the contraction to NA in the presence of CEC was resistant to subsequent prazosin. Likewise, this component was resistant to a combination of prazosin (10 microM) and yohimbine (10 microM). 6. Receptor protection experiments were carried out in which tissues were exposed to NA (100 microM), yohimbine (10 microM) or prazosin (0.1 microM) prior to and during exposure to CEC. Receptor protection with NA, yohimbine or prazosin (0.1 microM), followed by washout prevented the shift in potency of NA produced by CEC. 7. Further experiments examined the effects of prazosin (10 microM) on responses to NA following receptor protection with NA (100 microM), yohimbine (10 microM), prazosin (10 microM), or xylazine (100 microM). In receptor protection studies with NA, subsequent prazosin (10 microM) produced a shift in response to NA following CEC which was not signficantly different from the shift produced by prazosin alone in the absence of receptor protection. In receptor protection studies with prazosin, yohimbine or xylazine, subsequent prazosin (10 microM) produced shifts in the response to NA following CEC which were significantly less than the shift produced by prazosin alone in the absence of receptor protection.8. It is concluded that CEC has two actions in the rat aorta. Firstly, it behaves as an irreversible a,-adrenoceptor antagonist, reducing the response to low concentrations of NA (up to 10 microM). However,after exposure to CEC, concentrations of NA of 10 microM and above produced contractions resistant toprazosin. This resistant component was still present following receptor protection with alpha1,- or alpha2-adrenoceptor antagonists, but absent following receptor protection with NA. Hence, the latter response may represent an irreversible agonist interaction between CEC, NA and alpha-adrenoceptors which cannot be affected by subsequent competitive antagonism, but which can be prevented by receptor protection with the agonist NA prior to CEC.

Involvement of alpha 2-adrenoceptors and protein kinase C on nicotine-induced facilitation of noradrenaline release in bovine cerebral arteries

1. Incubation of bovine cerebral vessels (previously exposed to [3H]-noradrenaline) with nicotine for 30 sec produced a facilitation of the electrically-induced noradrenaline release, which was antagonized by hexamethonium, a blocker of nicotinic receptors. This facilitation was not observed when the incubation time was increased to 20 or 75 min. 2. Rauwolscine, an alpha 2-adrenoceptor blocker, enhanced and phorbol 12, 13-dibutyrate reduced the facilitator effect produced by 30 sec exposure to nicotine. 3. These data suggest: (1) presynaptic nicotinic receptors produce a facilitation of stimulated noradrenaline release; these receptors are easily desensitized by increasing the incubation time with nicotine; (2) protein kinase C and alpha 2-adrenoceptors appear to be involved in this process.

Effects of alpha-adrenoceptor agonists and antagonists on metabolic rate in cattle

Brahman steers (Bos indicus) were treated with the alpha 2-adrenergic agonists, guanfacin.HCl (4-440 micrograms/kg), UK14304.HCl (20-125 micrograms/kg) and clonidine.HCl (0.2-5 micrograms/kg). All three agonists produced dose-dependent reductions in metabolic rate, heart rate and rectal temperature (P < 0.001). Brahman heifers were infused with idazoxan.HCl (10 micrograms/kg/hr), an alpha 2-adrenergic antagonist, alone and in combination with an intramuscular injection of guanfacin.HCl (80 micrograms/kg). Idazoxan alone did not alter rectal temperature but it blocked the guanfacin-induced lowering of rectal temperature (P = 0.05 for the interaction between the two drugs). Idazoxan alone raised metabolic rate (P = 0.01). Guanfacin lowered metabolic rate (P = 0.007) and heart rate (P = 0.03), but the blocking of the guanfacin effect by idazoxan could not be demonstrated (P > 0.05) for either. The same heifers treated with 0.5, 1.0 and 5 micrograms/kg prazosin.HCl, an alpha 1-adrenergic antagonist, had significant changes in metabolic rate (P = 0.003) and heart rate (P = 0.008) at 0.5 and 5 micrograms/kg. Metabolic rate and heart rate decreased at the lower dose and increased at the higher dose. These results with cattle parallel previous results in rats (Gazzola, 1993) where a minimal, conceptual model for the partial control of resting metabolic rate by the sympathetic nervous system was postulated. The model indicates points of control in the sympathetic nervous system which could be manipulated so as to alter the metabolic rate of farm animals.

Age-related changes in vascular responses: a review

Normal aging is associated with different changes in the cardiovascular system that lead to an increase in pathological processes, such as hypertension, coronary artery disease, heart failure, and postural hypotension with enhancement of both morbidity and mortality. The vascular alterations consist of changes in the function and structure of the arteries, and increasing vascular stiffness, mainly when atherosclerosis is present, whose incidence is increased with age. The arteries accumulate lipids, collagen, and minerals. Cerebral perfusion may be reduced in the elderly, mainly regional cerebral blood flow, which leads to a deterioration of mental and physical functions. The degree of deterioration is increased when aging is associated with hypertension. Aging alters endothelial cells, which play an important role in vascular tone regulation. Such a process tends to reduce endothelium-dependent relaxations, and clearly reduces the vasodilation elicited by beta-adrenoceptor agonists. The contractions induced by different agents, such as 5-hydroxytryptamine, histamine, high potassium and angiotensin are barely affected with aging, whereas those elicited by noradrenaline or endothelin are usually reduced. However, plasma noradrenaline levels are increased with age, mainly due to a reduction in the sensitivity of presynaptic alpha 2-adrenoceptors and also of noradrenaline uptake. Sodium pump activity, that controls cellular ionic homeostasis, may be altered depending on animal species. Finally, vascular Ca2+ regulation appears to be altered and the extracellular Ca2+ dependence of contractile responses elicited by agonists is increased, which justifies the enhanced sensitivity to Ca2+ antagonists in senescence.

Pharmacological profiles of a novel alpha 1-adrenoceptor agonist, PNO-49B, at alpha 1-adrenoceptor subtypes

The effects of a newly synthesized compound, PNO-49B, (R)-(-)-3'-(2-amino-1-hydroxyethyl)-4'-fluoromethanesulfonanilide hydrochloride, on alpha 1-adrenoceptor subtypes were examined in various tissues in which the following distribution of alpha 1-adrenoceptor subtypes has been suggested: dog carotid artery (alpha 1B), dog mesenteric artery (alpha 1N), rabbit thoracic aorta (alpha 1B + alpha 1L), rat liver (alpha 1B), rat vas deferens (alpha 1A + alpha 1L), rat cerebral cortex (alpha 1A + alpha 1B) and rat thoracic aorta (controversial subtype). PNO-49B (0.1-100 microM) produced concentration-dependent contractions in dog mesenteric artery, rabbit thoracic aorta, rat thoracic aorta and rat vas deferens; and the maximal amplitudes of contraction were almost the same as or slightly less than those of noradrenaline. By contrast, the maximal response to PNO-49B in dog carotid artery was markedly smaller than the response to noradrenaline. In rabbit thoracic aorta, the contractile response to PNO-49B was not affected by inactivation of the alpha 1B subtype with chloroethylclonidine (CEC), although the response to noradrenaline was attenuated by that treatment. The dissociation constants (KA) of PNO-49B were not different among the rat thoracic aorta, dog carotid and mesenteric arteries and rabbit thoracic aorta (CEC-pretreated). The contractile responses to PNO-49B were inhibited competitively by prazosin, HV723 (alpha-ethyl-3,4,5-trimethoxy-alpha-(3-((2-(2-methoxyphenoxy)-ethyl)- amino(propyl)benzeneacetonitrile fumarate) and by WB4101 (2-(2,6-dimethoxyphenoxyethyl)-aminomethyl-1,4- benzodioxane).(ABSTRACT TRUNCATED AT 250 WORDS)

Prejunctional actions of N-ethyl-maleimide and phenoxybenzamine in rat vas deferens

In studies of electrically evoked isometric contractions of rat vas deferens, N-ethyl-maleimide (30 microM) pretreatment significantly reduced the prejunctional inhibitory potencies of xylazine and 5-hydroxytryptamine but failed to affect the potency of the alpha 1-adrenoceptor agonist amidephrine. Phenoxybenzamine (1 microM) or N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (10 microM) produced significant shifts in the potency of xylazine and significantly reduced the maximum inhibition, but the combination of phenoxybenzamine or EEDQ and N-ethyl-maleimide (30 microM) produced no further alteration in the effects of xylazine. In displacement studies, N-ethyl-maleimide displaced the binding of [3H]MK 912 ((2S,12bS)1',3'-dimethylspiro- (1,3,4,5',6,6',7,12b-octahydro-2H-benzo[b]furo[2,3-a]quinazoline)- 2,4'- pyrimidin-2'one) to rat renal cortex membranes with a Ki of 466 +/- 133 microM (n = 5), and so does not bind to alpha 2-adrenoceptors in the concentration range in which it affects prejunctional receptor mediated responses. This may suggest that N-ethyl-maleimide has actions other than inactivation of G-proteins or that the irreversible alpha 2-adrenoceptor antagonists phenoxybenzamine and EEDQ inactivate G-proteins sensitive to N-ethyl-maleimide in concentrations at which they bind to alpha 2-adrenoceptors.

Heterogeneity of alpha 1-adrenoceptor subtypes involved in adrenergic contractions of dog blood vessels

1. We determined the alpha 1-adrenoceptor subtypes involved in adrenergic contractions of eight different blood vessels isolated from the dog. 2. Noradrenaline produced concentration-dependent contractions in all the blood vessels tested, which were competitively inhibited by prazosin, WB4101, HV723 and 5-methylurapidil. However, there was considerable difference between the vessels with regard to the pKB values for all the antagonists. The alpha 1-adrenoceptors of dog vertebral and carotid arteries had high affinity for prazosin (pKB > 9.0) but low affinity for WB4101 (< 8.5), 5-methylurapidil (< 7.5) and HV723 (< or = 8.5). By contrast, HV723 had higher affinity (> 9.0) than prazosin (< 8.3), WB4101 (< 8.7) and 5-methylurapidil (< 8.2) in the portal vein, mesenteric artery and vein, and renal artery. In the femoral artery and vein, however, the four antagonists showed pKB values in the range 8.0-8.7. 3. Chloroethylclonidine (10 microM) produced a remarkable reduction of the contractile responses to noradrenaline in the vertebral and carotid arteries as compared with those in the other vessels. Nifedipine inhibited the responses to noradrenaline in all the tissues tested, and had marked effects in the portal vein. 4. Sympathetic adrenergic contractions induced by transmural electrical stimulation were also inhibited by prazosin and HV723 at different potencies among tissues. The relative potencies of both the antagonists paralleled the relationship in inhibiting the responses to exogenous noradrenaline in each vessel. 5. According to recent alpha l-adrenoceptor subclassification, the present results suggest that the contractions of blood vessels induced by endogenous and exogenous noradrenaline are mediated through different alpha l-adrenoceptor subtypes heterogeneously distributed in each vessel; presumably, the alpha 1 B subtype in the carotid and vertebral arteries, the alpha IN subtype in the visceral region and the alpha IL subtype in the femoral region. Regionally different expression of alpha 1-adrenoceptor subtypes may be in part associated with the regional heterogeneity of sympathetic responses in the blood vessels.

Aging and vasoconstrictor responses mediated by alpha 2-adrenoceptors and 5-HT1 and 5-HT2 receptors

The potencies of noradrenaline (NA) at producing alpha 2-adrenoceptor mediated contractions, and of 5-hydroxytryptamine (5-HT) at producing 5-HT1 and 5-HT2 receptor mediated contractions were investigated in rings of human saphenous vein obtained from varicose vein surgery. There was a significant negative correlation between agonist potency and age for NA at alpha 2-adrenoceptors (r = 0.52, n = 21, p < 0.05) and for 5-HT at both 5-HT1 (r = 0.47, n = 19, p < 0.05) and 5-HT2 (r = 0.54, n = 19, p < 0.05) receptors, so that both agonists were less potent with increasing age. This demonstrates an age-related decrease in alpha 2-adrenoceptor and 5-HT1 and 5HT2 mediated contractile responsiveness in the human saphenous vein.

Analysis of agonism at functional prejunctional alpha 2-adrenoceptors of rat vas deferens using operational and null approaches

The alpha 2-adrenoceptors located prejunctionally on the postganglionic neurons that innervate the smooth muscle of the prostatic portion of the rat vas deferens were examined. For this purpose, three imidazolidine derivatives (structurally related to clonidine) were studied for their effects on twitch contractions elicited by electrical field stimulation of this tissue. In this study, operational model-fitting and the nested hyperbolic method were used to analyse the effects of irreversible receptor alkylation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on the alpha 2-adrenoceptor-mediated effects of clonidine (2-[2,6-dichlorophenylimino]imidazolidine) in stimulated vas deferens. The operational model provided an estimate of KA for clonidine which was not significantly different from the estimate obtained by using the nested hyperbolic method (null approach). The data indicate a large receptor reserve at prejunctional alpha 2-adrenoceptors for clonidine. The estimates of apparent affinity for St-587 (2-[2-chloro-5-trifluoromethylphenylimino]imidazolidine) and St-591 (2-[2-chloro-5-methylphenylimino]imidazolidine) did not depend on the method of calculation as the 'null' method and the 'operational' method gave similar answers. Further, estimates of the ratio of tau values for these partial agonists with respect to clonidine were numerically the same as those of their relative efficacies. Therefore, no limitations in the ability of the operational model to fit experimental data and provide reproducible estimates of affinity and efficacy have been revealed for agonists acting at prejunctional alpha 2-adrenoceptors.

Frequency- and train length-dependent variation in the roles of postjunctional alpha 1- and alpha 2-adrenoceptors for the field stimulation-induced neurogenic contraction of rat tail artery

The present paper examines the roles of postjunctional alpha 1- and alpha 2-adrenoceptors for the noradrenaline (NA)-induced neurogenic contractile response to field stimulation mainly with 1-100 pulses at 2 or 20 Hz, in the tail artery of adult normotensive rats. Pharmacological tools were employed to isolate and characterize the alpha 1- and alpha 2-adrenoceptor-mediated components of this response. The degree to which the drugs influenced NA release or reuptake was assessed by their effects on the electrochemically determined, stimulation-induced rise in the NA concentration at the innervated outer surface of the media. This response was unaffected by alpha,beta-methylene ATP (10 microM) or suramin (500 microM), added to desensitize or block P2-purinoceptors, respectively prazosin (0.1 microM) or SK&F 104078 (6-chloro-9-[(3-methyl-2-butenyl)oxyl]-3-methyl- 1H-2,3,4,5-tetrohydro-3-benzazepine, 0.1 microM), used to block postjunctional alpha 1- and alpha 2-adrenoceptors respectively, nifedipine (10 microM), blocker of Ca2+ influx through L-type channels, and ryanodine (10 microM), which blocks mobilization of Ca2+ from intracellular stores; it was moderately enhanced by yohimbine (0.1 microM), blocker of pre- and postjunctional alpha 2-adrenoceptors, and strongly enhanced by cocaine (3 microM) or desipramine (1 microM), blockers of NA reuptake. Judging from their inhibitory effects on the contractile responses to the alpha 1- and alpha 2-adrenoceptor agonists, phenylephrine and xylazine, prazosin (0.1 microM) and SK&F 104078 (0.1 microM) could be used to selectively block alpha 1- and alpha 2-adrenoceptors respectively, while yohimbine (0.1 microM) was less selective, strongly depressing alpha 2- and slightly depressing alpha 1-adrenoceptor-mediated responses. The alpha 1-adrenoceptor-mediated component of the contractile response to short trains at 20 Hz was fast in onset, brief in duration and abolished by ryanodine; that mediated by alpha 2-adrenoceptors was more delayed, prolonged and insensitive to ryanodine. Both components were dose-dependently depressed by nifedipine (0.1-10 microM). The small contractile responses to single pulses, or up to 50 pulses at 2 Hz, or short train (< 4 pulses) at 20 Hz, were more markedly depressed by 0.1 microM yohimbine or SK&F 104078 than by 0.1 microM prazosin and, hence, mediated mainly by alpha 2-adrenoceptors. The reverse was true of the much larger response to longer trains at 20 Hz, which thus probably was mediated mainly by alpha 1-adrenoceptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Investigation of the subtypes of alpha 1-adrenoceptor mediating contractions of rat aorta, vas deferens and spleen

1. The subtypes of alpha 1-adrenoceptor mediating contractions to exogenous noradrenaline (NA) or phenylephrine in rat vas deferens, spleen and aorta, and mediating contractions to endogenous NA in rat vas deferens have been examined. 2. In rat vas deferens, the competitive antagonists prazosin, WB 4101, benoxathian and 5-methyl-urapidil inhibited contractions to NA with pA2 values of 9.26, 9.54, 9.02 and 8.43, respectively. The irreversible antagonist chloroethylclonidine (CEC) (100 microM) failed to affect contractions to NA. 3. In rat vas deferens in the presence of nifedipine (10 microM), contractions to NA were significantly attenuated and under these conditions, CEC (100 microM) significantly reduced the maximum response to NA. 4. In rat spleen, the competitive antagonists prazosin, WB 4101 and benoxathian inhibited contractions to phenylephrine with pA2 values of 9.56, 8.85 and 7.60, respectively, and 5-methyl-urapidil had a KB of 6.62. CEC (100 microM) significantly reduced the maximum contraction to phenylephrine. 5. In rat aorta, the competitive antagonists, prazosin, WB 4101, benoxathian and 5-methyl-urapidil inhibited contractions to NA with pA2 values of 9.45, 9.21, 8.55 and 8.12, respectively. CEC (100 microM) produced an approximately parallel shift in the potency of NA, without significantly reducing the maximum response. 6. In epididymal portions of rat vas deferens in the presence of nifedipine (10 microM), the isometric contraction to a single electrical pulse was significantly reduced by CEC (100 microM), and by the competitive antagonists prazosin, WB 4101, benoxathian and 5-methyl-urapidil at concentrations of 1 nM. 7. In prostatic portions of rat vas deferens, the alpha l-adrenoceptor agonist, amidephrine, produced concentration-dependent increases in the isometric contraction to a single electrical stimulus and the maximum increase in the evoked response produced by amidephrine was unaffected by CEC (100 microM).8. Contractions of rat vas deferens produced by NA (and amidephrine) are mediated predominantly by alpha lA-adrenoceptors as shown by the high potency of alpha lA-adrenoceptor selective antagonists and the lack of effect of CEC. A small CEC-sensitive response, particularly in epididymal portions, was revealed in the presence of nifedipine. Contractions of rat spleen are mediated by alpha lB-adrenoceptors since alpha 1A selective antagonists showed low potency and CEC significantly reduced the maximum contraction to phenylephrine. Contractions of rat aorta to NA are mediated by non-alpha lA, non-alpha lB-adrenoceptors, due to the high potency of the aMA-selective antagonists and sensitivity to CEC.9. The noradrenergic contraction of epididymal portions of rat vas deferens in the presence of nifedipine is CEC-sensitive, but the alpha 1 A-selective antagonists showed high potency, suggesting that this response is mediated by non-alpha lA, non-alpha 1B-adrenoceptors.10. In conclusion, at least three subtypes of functional alpha 1-adrenoceptors have been demonstrated in these studies.

Chloroethylclonidine: an irreversible agonist at prejunctional alpha 2-adrenoceptors in rat vas deferens

1. The possibility that chloroethylclonidine (CEC) activates prejunctional alpha 2-adrenoceptors was studied in the isolated vas deferens of the rat. Tissues were stimulated electrically and both the stimulation-evoked overflow of tritium (after preincubation with [3H]-noradrenaline) and the purinergic contraction component (isolated by prazosin 0.3 microM) were measured. 2. CEC (0.1-3 microM) concentration-dependently reduced the overflow of tritium evoked by trains of 6 pulses/100 Hz. The inhibition by CEC was not altered by prazosin (0.3 microM) but was prevented by pre-exposure to rauwolscine (0.3 microM). The inhibition, once established, did not fade upon washout of CEC, even when the washout fluid contained rauwolscine (0.3 microM). 3. CEC (0.1-3 microM) concentration-dependently reduced the purinergic component of contractions elicited by single pulses. The inhibition, again, was prevented by pre-exposure to rauwolscine (0.3 microM) and once established, did not fade upon washout of CEC, even when the washout fluid contained rauwolscine (0.3 microM). 4. CEC (3 microM) reduced the overflow of tritium evoked by 20 pulses/10 Hz, did not alter the overflow evoked by 100 pulses/10 Hz and increased the overflow evoked by 500 pulses/10 Hz. 5. CEC (3 microM) reduced the early peak, but increased the late plateau phase, of purinergic contractions elicited by 100 pulses/10 Hz. 6. It is concluded that CEC reduces the release of noradrenaline and a purinergic co-transmitter by irreversible activation of prejunctional alpha 2-adrenoceptors. CEC seems to be a partial alpha 2-agonist with an efficacy lower than that of noradrenaline. The prejunctional inhibitory effect limits the suitability of CEC for the characterization of postjunctional alpha 1-adrenoceptors mediating responses to sympathetic nerve stimulation.

Subclassification of release-regulating alpha 2-autoreceptors in human brain cortex

1. Release-regulating alpha 2-autoreceptors in human brain were characterized pharmacologically in cortical slices from patients undergoing neurosurgery to remove subcortical tumours; the slices were prelabelled with [3H]-noradrenaline ([3H]-NA) and stimulated electrically (3 Hz, 2 ms, 24 mA) under superfusion conditions. 2. The stimulus-evoked tritium overflow was almost totally Ca(2+)-dependent and tetrodotoxin-sensitive. 3. Clonidine and oxymetazoline 0.01 to 1 microM inhibited in a concentration-dependent manner the evoked overflow of tritium. The two drugs were equipotent (EC50 = 0.03 microM) and their maximal effect was approx. 45%. Phenylephrine and methoxamine, up to 1 microM, did not affect tritium overflow. 4. Yohimbine (0.01-0.1 microM) shifted the concentration-response curve of clonidine to the right. The calculated pA2 value was 8.29. 5. Prazosin and 2-[2-[4-(o-methoxyphenyl)piperazine-1-yl]ethyl]-4,4- dimethyl-1,3(2H,4H)-isoquinolinedione (AR-C 239), tested at 0.3 microM, did not modify the concentration-response curve of clonidine. 6. The effect of clonidine was antagonized by (+)-mianserin (pA2 = 7.74), but not by up to 0.3 microM of the (-)-enantiomer. The concentration-response curve of clonidine was shifted to the right by the novel alpha 2-adrenoceptor antagonist, 5-chloro-4-(1-butyl-1,2,5,6-tetrahydropyridin-3-yl)-thiazole-2-ami ne (Z)-2-butenedioate (1:1) salt (ORG 20350) (pA2 = 7.55). 7. Yohimbine, (+)-mianserin and ORG 20350, but not prazosin and (-)-mianserin, increased the electrically-evoked tritium overflow, suggesting that autoreceptors may be tonically activated by endogenous NA. 8. Desipramine (1 microM) increased evoked tritium overflow from human cortex slices. The effect of clonidine (0.01- 1 g1M) on the evoked overflow of tritium was reduced in presence of 1 muM desipramine.9. It is proposed that autoregulation of NA release can occur in human cerebral cortex. The process involves activation of alpha 2-adrenoceptors which may be either the alpha2X or the alpha2D subtype.

Subtype classification of presynaptic alpha 2-adrenoceptors

1. Presynaptic alpha 2-adrenoceptors in a few tissues have recently been pharmacologically classified in functional studies. 2. Autoreceptors are of alpha 2A-subtype in rabbit occipito-parietal cortex, rat cerebral cortex, vas deferens, submandibular gland, kidney, guinea-pig ileum submucosal arterioles and urethra. 3. Heteroreceptors are of alpha 2A-subtype in rat cerebral cortex, vas deferens, guinea-pig ileum submucosal plexus and Auerbach's plexus. 4. In rat atria autoreceptors have been shown to be of alpha 2B-subtype. 5. Classification is done mainly with alpha 2A-adrenoceptor-selective oxymetazoline, WB 4101 and BRL 44408, and the alpha 2B-adrenoceptor-selective prazosin, AR-C 239, chlorpromazine and BRL 41992. 6. With four alpha 2-adrenoceptor subtypes to consider, a larger number of subtype-selective compounds may have to be characterized in the classification in the many tissues, where presynaptic alpha 2-adrenoceptors are found.

Sympathetic innervation and alpha-adrenoceptor profile of blood vessels in the posterior region of the rabbit knee joint

1. Experiments were performed to determine the nature of adrenoceptors mediating neurally-induced vasoconstriction of blood vessels in the posterior region of the rabbit knee joint capsule. 2. Electrical stimulation of the posterior articular nerve resulted in frequency-dependent vasoconstriction which was maximal at 10 Hz. This response was mediated predominantly by alpha 2-adrenoceptors as it was only slightly reduced by prazosin administration and was not only abolished but converted into a dilator response by the alpha 2-adrenoceptor antagonist rauwolscine. Further experiments with another specific alpha 1-adrenoceptor antagonist YM-12617 showed that the frequency-response curve in the presence of this antagonist did not differ significantly from control. 3. Neurally-induced vasoconstriction did not appear to have a purinergic component as it was unaffected by the P2x-purinoceptor desensitiser alpha, beta methylene ATP. 4. The rank-order of potency of alpha-adrenoceptor agonists given as a bolus by close intra-arterial injection was: adrenaline = UK-14304 > clonidine > phenylephrine, suggesting that the vasoconstrictor effects were mediated predominantly by postjunctional alpha 2-adrenoceptors. 5. The alpha 2-adrenoceptor antagonist rauwolscine converted the constrictor response to close intra-arterial injection of adrenaline into a dilator response. The vasoconstrictor responses to UK-14304, clonidine and phenylephrine were substantially inhibited by rauwolscine. The alpha 1-adrenoceptor antagonist prazosin failed to inhibit the vasoconstrictor responses to adrenaline, clonidine and UK-14304 and resulted in enhancement of their constrictor effects. 6. The enhancement of the responses to the a, and a2 agonists by prazosin appeared to be specifically related to this agent as administration of YM-12617 did not show such enhancement. The dose-response curves to both clonidine and UK-14304 in the presence of YM-12617 did not differ significantly from control responses. Responses to phenylephrine were significantly reduced by YM-12617, indicating the presence of post-junctional a,-adrenoceptors.7. These results show almost complete reversal of the adrenoceptor profile compared to results obtained in an earlier in vitro study, where responses were mediated predominantly by ax,-adrenoceptors with a small population of postjunctional a2-adrenoceptors (Ferrell & Khoshbaten, 1989). This suggests that the differing environment in vitro may not completely reflect the conditions prevailing in vivo.

Pharmacological characterization of presynaptic alpha 2-autoreceptors in rat submaxillary gland and heart atrium

1. The pharmacological properties of presynaptic alpha 2-autoreceptors were studied in rat isolated submaxillary glands and atria. Tissue pieces were preincubated with [3H]-noradrenaline, then superfused with medium containing desipramine, and stimulated electrically. In one series of experiments, pEC30 values of 12 alpha-adrenoceptor antagonists were determined, i.e., negative logarithms of concentrations that increased the electrically evoked overflow of tritium by 30%. In another series, pKD values of 9 alpha-adrenoceptor antagonists against the release-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), and of 3 antagonists against the release-inhibiting effect of methoxamine, were determined. 2. In submaxillary glands, the pEC30 values of the antagonists correlated well with their pKD values against UK 14304 (r = 0.93). The same was true for atria (r = 0.92). 3. In submaxillary glands, the pKD values of 3 antagonists against UK14304 were very similar to their pKD values against methoxamine, with a maximal difference of 0.4. The same was true for atria where the maximal difference was 0.3. 4. The pEC30 values obtained in submaxillary glands correlated significantly with those obtained in atria (r = 0.81). The same was true for the pKD values (r = 0.79). However, the pEC30 and pKD values also indicated consistent differences between the two tissues. 5. It is concluded that the sites of action of the imidazoline UK 14304 (alpha 2-selective), the phenylethylamine noradrenaline, and the phenylethylamine methoxamine (alpha 1-selective) are exclusively alpha 2-adrenoceptors. There is no indication for presynaptic alpha 1-adrenoceptors or for an effect of UK 14304 mediated by presynaptic imidazoline receptors.The 02-autoreceptor population in the submaxillary gland differs from that in the atrium.6. Comparison with studies from the literature indicates that the submaxillary autoreceptors are closely similar to the a2D radioligand binding site found in the bovine pineal gland and probably the rat submaxillary gland. The atrial autoreceptors also conform best to this site, but the agreement is more limited; the atrial autoreceptors may represent a type related to, but distinct from, the a2D site, or a mixture of different types.

Differential expression of two alpha 2-adrenergic receptor subtype mRNAs in human tissues

Genetic subtypes of alpha 2-adrenergic receptors (AR) may mediate distinct physiological functions, and undergo differential cell type-specific regulation. Thus, these distinct receptor subtypes are possible targets for the development of subtype-selective drugs. We have analyzed the tissue distribution of two human alpha 2-adrenoceptor subtype gene mRNAs, alpha 2-C4 and alpha 2-C10, in normal human fetal and adult tissues. Both receptor subtype mRNAs were abundantly expressed in fetal brain and choroid plexus. In non-neural fetal tissues, alpha 2-C10 mRNA was detected in spleen, kidney, adrenal gland, and skin, while alpha 2-C4 transcripts were observed only in kidney and skin. Most regions of the adult brain also expressed both subtypes, but with marked quantitative differences. For example, cerebral cortex contained predominantly alpha 2-C10 mRNA, whereas the caudate nucleus expressed mostly alpha 2-C4 mRNA. In adult peripheral tissues, alpha 2-C10 mRNA expression was most abundant in spleen and renal cortex, and expression of alpha 2-C4 mRNA was strongest in renal cortex and medulla. These different expression patterns provide evidence for the differential regulation of the two alpha 2-adrenergic receptor genes and warrant further investigation with techniques capable of improved anatomical resolution. Regional differences in receptor subtype expression may be valuable for the development of new, subtype-selective pharmacological agents with more targeted actions compared to currently used alpha 2-adrenoceptor agonists and antagonists.

Different types of norepinephrinergic receptors are involved in preoptic area mediated independent modulation of sleep-wakefulness and body temperature

The preoptic area is known to regulate sleep-wakefulness and body temperature. It was suggested earlier that though sleep-wakefulness and body temperature may affect each other, the preoptic area mediated influence on those two physiological phenomena is likely to be independent of alteration in each other. Since intrapreoptic area norepinephrine could modulate both those functions, study of that system was undertaken. It was hypothesized that since the preoptic area has different types of norepinephrinergic receptors (viz. alpha 1, alpha 2 and beta), independent modulation of those two functions was probably due to activation or inactivation of separate receptors. Hence, the effects of different agonist and antagonist of those receptors individually as well as in combination into the preoptic area were studied on those two functions in freely moving rats. The results suggest that norepinephrine induced preoptic area mediated influence on the body temperature is primarily regulated by the alpha 1 receptors while the sleep and wakefulness are regulated by alpha 2 and beta receptors, respectively. The finding should help in explaining several poorly understood observations reported earlier and it suggests that similar phenomena may possibly exist in other system involving other neurotransmitters as well.

Reconstitution of alpha 1-adrenoceptors having high affinity for prazosin

Using a variety of detergents we find that soluble alpha 1-adrenoceptor recovery from aortic and hepatic membranes is markedly enhanced if the receptor is first prelabelled with prazosin. Moreover, prelabelling prevents the reduction in prazosin affinity induced by solubilizing concentrations of digitonin, possibly by stabilizing the receptor's conformation. 20-25% of alpha 1-adrenoceptors solubilized in sodium cholate were reconstituted into brain lipids. Specific [3H]prazosin binding to the reconstituted receptor was saturable and of high affinity (KD = 0.019 +/- 0.008 nM). We conclude that prelabelling is essential to preserve the receptor in detergents. However, once the detergent is removed brain lipids alone are able to maintain the receptor in a form with high affinity for prazosin.

Studies on the mechanism of 5-HT1 receptor-induced smooth muscle contraction in dog saphenous vein

1. We have investigated the mechanism of smooth muscle contraction evoked by activation of 5-HT1-like receptors in dog isolated saphenous vein. 2. In the presence of the 5-HT2 receptor antagonist, ritanserin (0.1 microM), concentration-effect curves (10 nM-300 microM) for 5-hydroxytryptamine (5-HT)-induced smooth muscle contraction were biphasic. This could be attributed to a direct action on 5-HT1-like receptors at low concentrations of 5-HT (10 nM-10 microM) and an indirect (through the release of noradrenaline from sympathetic neurones) activation of postjunctional alpha-adrenoceptors at higher 5-HT concentrations. In contrast, concentration-effect curves (100 nM-100 microM) for sumatriptan-induced contractions were not biphasic, and were due solely to activation of 5-HT1-like receptors. 3. Smooth muscle contractions evoked either by low concentrations of 5-HT or by sumatriptan were abolished by removal of extracellular calcium and were markedly inhibited, but not abolished, by the calcium channel blocker, verapamil (1-30 microM). In contrast, contractions evoked by high concentrations of 5-HT were markedly less sensitive to removal of extracellular calcium or to verapamil. 4. 5-HT and sumatriptan also inhibited (to a maximum of about 50%) prostaglandin E2 (PGE2, 5 microM)-stimulated adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation. This effect was mimicked by the alpha 2-adrenoceptor agonist, azepexole (B-HT933) but not by the alpha 1-adrenoceptor agonist, methoxamine.5. In contrast to mediation of smooth muscle contraction, the 5-HT1-like receptor-mediated inhibition of PGE2-stimulated cyclic AMP formation evoked by 5-HT or sumatriptan was not attenuated by removal of extracellular calcium or by verapamil (1 microM).6. A directly-acting inhibitor of adenylyl cyclase, 2',3'-dideoxyadenosine (1 mM) inhibited PGE2-stimulated cyclic AMP formation but did not produce smooth muscle contraction.7. These results suggest that contractile responses of dog isolated saphenous vein arising through activation of 5-HT1-like receptors are associated with both an influx of extracellular calcium ions (to a large extent via voltage-dependent channels) and an inhibition of adenylyl cyclase. However, although these two responses are coupled to the same receptor, they appear to be independent.

Functional characterization of alpha-adrenoceptors on pancreatic islets of fa/fa Zucker rats

Recently, a defect in pertussis toxin-independent actions of epinephrine on pancreatic B-cells of fa/fa Zucker rats was reported (Cawthorn and Chan (1991) Mol. Cell. Endocrinol. 75, 197-204). We now report studies of islet alpha 2-adrenoceptor function of fa/fa rats. Insulin and cAMP production by islets of obese rats were both inhibited by the alpha 2-adrenoceptor agonist clonidine. Calculated pD2 values for clonidine were 9.57 +/- 0.59 and 9.43 +/- 0.33 for lean and fa/fa rat islets, respectively. Yohimbine reversed clonidine effects equipotently in lean and obese rat islets (pA2 values of 7.48 +/- 0.57 vs 7.43 +/- 0.58). Unexpectedly, the alpha 1-antagonist prazosin stimulated insulin secretion from islets of obese but not lean rats. Functional characteristics of the alpha-adrenoceptors on fa/fa islets are thus similar to those recently designated alpha 2B. Altered expression of alpha-adrenoceptors on pancreatic islets of fa/fa rats may contribute to changes in the pertussis toxin-independent pathway of epinephrine action previously observed.

Characterization of the subtype of presynaptic alpha 2-adrenoceptors modulating noradrenaline release in cat and bovine cerebral arteries

The possible existence of a heterogeneous population of alpha 2-adrenoceptors (alpha 2A and alpha 2B, demonstrated by binding studies) in adrenergic nerve endings of cat and bovine cerebral arteries modulating noradrenaline release was investigated. Electrical field stimulation elicited an increase of tritium secretion from these vessels preincubated with (+/-)-[3H]noradrenaline, which was reduced by the alpha 2-agonists, clonidine (1 microM) and B-HT 920 (0.01 and 0.1 microM), in cat cerebral arteries but only by B-HT 920 in bovine cerebral arteries. This reduction was inhibited by the antagonist of the alpha 2B-subtype, prazosin, and the antagonists of alpha 2A- and alpha 2B-subtypes yohimbine and particularly rauwolscine. The effect of B-HT 920 was partially inhibited by clonidine in bovine, but not in cat cerebral arteries. In both types of arteries, prazosin, yohimbine and the alpha 1-agonist methoxamine (all at 1 microM) failed to modify the stimulated radioactivity liberation, whereas it was increased by 1 microM rauwolscine, and by yohimbine plus prazosin in cat cerebral arteries. The basal tritium release was enhanced by rauwolscine and prazosin in cat cerebral arteries but only by the latter in bovine cerebral arteries. These results suggest: (1) the existence of presynaptic alpha 2-adrenoceptors, mainly of the alpha 2B-subtype, in these vessels negatively modulating noradrenaline release, their activity being greater in cat than in bovine cerebral arteries, and (2) clonidine has no agonistic but a weak antagonistic action in the latter vessels.

Contraction-mediating alpha 2-adrenoceptors in the mouse vas deferens

The question of the existence of postjunctional, contraction-mediating alpha 2-adrenoceptors, in addition to the known alpha 1-adrenoceptors, was studied in the mouse isolated vas deferens. Both the alpha 1-selective agonist phenylephrine and the alpha 2-selective agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) caused contraction of the vas deferens. In the presence of the alpha 1-selective antagonist prazosin (added in order to prevent an alpha 1 component in the effect of high concentrations of UK 14,304), the alpha 2-selective antagonist yohimbine and idazoxan shifted the concentration-response curve of UK 14,304 to the right in a manner compatible with competitive antagonism and with dissociation constants KB indicating the involvement of alpha 2-adrenoceptors. The maximal contraction elicited by UK 14,304 (in the presence of prazosin) was much lower than the maximal contraction elicited by phenylephrine. The effect of UK 14,304 was not changed by the P2-purinoceptor agonist alpha,beta-methylene-ATP and was reduced by neuropeptide Y, but was markedly enhanced by relatively low concentrations of phenylephrine. When the sympathetic fibres of the vas deferens were stimulated by trains of ten widely spaced (0.5 Hz) electric pulses, the tissue responded with ten separate twitches in which purinergic and adrenergic components were isolated by prazosin and suramin, respectively. Prazosin reduced the first adrenergic twitch in these trains at concentrations close to its KB value at alpha 1-adrenoceptors, whereas yohimbine and idazoxan reduced the first adrenergic twitch at concentrations far lower than their KB values at alpha 1-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Subclassification of the presynaptic alpha 2-autoreceptors in rabbit brain cortex

1. alpha 2-Adrenoceptor binding sites have been subclassified into alpha 2A sites of which a main characteristic is very low affinity for prazosin, and alpha 2B sites with relatively high affinity for prazosin. The presynaptic alpha 2-autoreceptors in rabbit brain cortex were studied in order to classify them in terms of alpha 2A and alpha 2B. Release of [3H]-noradrenaline in cortical slices was elicited by trains of 4 pulses delivered at 100 Hz. 2. Clonidine caused concentration-dependent inhibition of the stimulation-evoked overflow of tritium, with an EC50 of 7.5 nM and a maximal inhibition by 96%. 3. The following alpha-adrenoceptor antagonists shifted the concentration-response curve of clonidine to the right (antagonist-receptor dissociation constants KD in brackets): yohimbine (14 nM), 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazo le (BRL 44408; 15 nM) and 1,2-dimethyl-2,3,9,13betetrahydro-1H-dibenzo[c,f]imidazo[1,5-a]aze pine (BRL 41992; 630 nM). Prazosin 1 microM and 2-[2-[4-(o-methoxyphenyl)piperazine-1-yl]-ethyl]-4,4-dimethyl-1,3 (2H,4H)-isoquinolinedione (AR-C 239) 1 microM failed to antagonize the effect of clonidine. Higher concentrations of prazosin and AR-C 239 greatly accelerated the basal efflux of tritium. 4. The method used permits the functional determination of antagonist affinities undistorted by endogenous alpha 2-autoinhibition. A comparison with affinities derived from radioligand binding experiments indicates that the presynaptic alpha 2-autoreceptors in rabbit brain cortex are markedly different from the alpha 2B-subtype and probably belong to the prazosin-insensitive alpha 2A-subtype.

Alpha 2-adrenoceptor blocking profile of SK&F 104078: further evidence for receptor subtypes

1. The ability of the putative, selective post-junctional alpha 2-adrenoceptor antagonist, SK&F 104078 to antagonize the effects of structurally-diverse agonists at pre-junctional alpha 2-adrenoceptors in the guinea-pig ileum and rat vas deferens in vitro and in the rat heart in vivo, and at post-junctional alpha 2-adrenoceptors in the rabbit ear vein in vitro, was examined. Results obtained with SK&F 104078 were compared with those obtained with yohimbine. 2. Xylazine and B-HT933 each caused a concentration-dependent inhibition of the field-stimulation-evoked twitch responses of the guinea-pig ileum and rat vas deferens. SK&F 104078 did not antagonize either agonist in the guinea-pig ileum and exerted only minimal blocking activity against xylazine in the rat vas deferens. In contrast, SK&F 104078 competitively antagonized B-HT933 in the rat vas deferens (pA2 = 6.45). Yohimbine competitively antagonized both agonists in each tissue (pA2 values ranged between 7.46 and 7.88). 3. In the pithed rat xylazine and B-HT933, injected intravenously, caused a dose-dependent reduction in the tachycardia elicited by stimulation of the cardiac preganglionic sympathetic nerves. SK&F 104078 (10 mg kg-1, i.v.) caused a 20-30 fold rightward displacement of the dose-response curve to xylazine, but did not affect responses to B-HT933. Yohimbine (1 mg kg-1, i.v.) antagonized both agonists to a similar degree. 4. In the rabbit ear vein xylazine, B-HT933, noradrenaline and UK 14304 elicted vasoconstrictor responses. Prazosin was without effect, but in contrast, SK&F 104078 was a competitive antagonist of each of the agonists (pA2 values ranged between 6.63 and 6.72). Yohimbine also competitively antagonized each of the agonists in this preparation (pA2 values ranged between 7.81 and 8.07). 5. SK&F 104078 was also a competitive antagonist (pA2 = 6.20) against noradrenaline at post-junctional alpha 1-adrenoceptors in the rabbit aorta. 6. These data show that SK&F 104078 is a competitive antagonist at post-junctional alpha l- and alpha 2-adrenoceptors. Its antagonist potency at pre-junctional alpha 2-adrenoceptors is agonist- and tissuedependent. Yohimbine does not discriminate between pre- and post-junctional alpha 2-adrenoceptors. The findings are discussed in terms of the possible existence of subclasses of OC2-adrenoceptors.

Identification of an additional gene belonging to the alpha 2 adrenergic receptor family in the human genome by PCR

We here describe the cloning of an additional gene, called alpha 2-1.8, which is similar to the previously cloned human alpha 2-adrenergic receptor located on chromosome 4. The alpha 2-1.8 gene was identified by using the polymerase chain reaction with primers specific for sequences in transmembrane regions 2 and 5 of the previously isolated human alpha 2-C4 and alpha 2-C10 adrenoceptor genes, which are localized on chromosomes 4 and 10, respectively. The new gene was identified by amplifying the 1.8 kb size fractionated region of PstI restriction cut human genomic DNA. The previously cloned alpha 2-C10 and alpha 2-C4 genes were recovered at their expected locations, 0.96 and 5.9 kb, respectively. We have identified 387 bases of the new alpha 2-1.8 gene, and its sequence is identical to the previously described alpha 2-C4 gene, but it is distinct from the alpha 2-C10 and alpha 2-C2 genes. Our results demonstrate that the alpha 2-C4 adrenergic receptor exists in more than one copy in the human genome.

Functional evidence for heterogeneity of peripheral prejunctional alpha 2-adrenoceptors

1. We have examined the potencies of a series of alpha 2-adrenoceptor antagonists in functional studies of prejunctional alpha 2-adrenoceptors in rat atrium and vas deferens, and compared potencies with affinities for the alpha 2A-ligand binding site of human platelet and the alpha 2B-site of rat kidney. 2. Antagonist potency in rat atrium was expressed as an EC30 (concentration producing 30% increase in the stimulation-evoked overflow of tritium in tissues pre-incubated with [3H]-noradrenaline). Antagonist potency in rat vas deferens was expressed as a pA2 or KB at antagonizing the inhibition by the alpha 2-adrenoceptor agonist xylazine of the isometric twitch to a single stimulus, or as an EC30. 3. In ligand binding studies, Ki values were obtained for the displacement by alpha-adrenoceptor antagonists of [3H]-yohimbine binding to human platelet or rat kidney membranes. 4. In functional studies, three antagonists (ARC 239, prazosin and chlorpromazine) distinguished between prejunctional alpha 2-adrenoceptors of rat atrium (EC30) and rat vas deferens (pA2) and showed 49, 12 and 7 times higher potency in rat atrium, respectively. ARC 239 was also 17 times more potent in rat atrium than rat vas deferens when EC30 values were compared. 5. The correlation of affinity for the alpha 2A-site of human platelet was better with prejunctional potency in rat vas deferens than rat atrium. 6. The correlation of affinity for the alpha 2B-site of rat kidney was better with prejunctional potency in rat atrium than rat vas deferens. 7. It is concluded that prejunctional alpha 2-adrenoceptors of rat vas deferens and rat atrium differ, and these receptors may resemble the alpha 2A- and alpha 2B-ligand binding sites, respectively.