Mini-review. The postsynaptic alpha 2-adrenoreceptor

The Discovery of Novel α2a Adrenergic Receptor Agonists Only Coupling to Gαi/O Proteins by Virtual Screening

Most α2-AR agonists derived from dexmedetomidine have few structural differences between them and have no selectivity for α2A/2B-AR or Gi/Gs, which can lead to side effects in drugs. To obtain novel and potent α2A-AR agonists, we performed virtual screening for human α2A-AR and α2B-AR to find α2A-AR agonists with higher selectivity. Compound P300-2342 and its three analogs significantly decreased the locomotor activity of mice (p < 0.05). Furthermore, P300-2342 and its three analogs inhibited the binding of [3H] Rauwolscine with IC50 values of 7.72 ± 0.76 and 12.23 ± 0.11 μM, respectively, to α2A-AR and α2B-AR. In α2A-AR-HEK293 cells, P300-2342 decreased forskolin-stimulated cAMP production without increasing cAMP production, which indicated that P300-2342 activated α2A-AR with coupling to the Gαi/o pathway but without Gαs coupling. P300-2342 exhibited no agonist but slight antagonist activities in α2B-AR. Similar results were obtained for the analogs of P300-2342. The docking results showed that P300-2342 formed π-hydrogen bonds with Y394, V114 in α2A-AR, and V93 in α2B-AR. Three analogs of P300-2342 formed several π-hydrogen bonds with V114, Y196, F390 in α2A-AR, and V93 in α2B-AR. We believe that these molecules can serve as leads for the further optimization of α2A-AR agonists with potentially few side effects.

Noradrenergic alterations in Parkinson's disease: a combined 11C-yohimbine PET/neuromelanin MRI study

Degeneration of the noradrenergic system is now considered a pathological hallmark of Parkinson's disease, but little is known about its consequences in terms of parkinsonian manifestations. Here, we evaluated two aspects of the noradrenergic system using multimodal in vivo imaging in patients with Parkinson's disease and healthy controls: the pigmented cell bodies of the locus coeruleus with neuromelanin sensitive MRI; and the density of α2-adrenergic receptors (ARs) with PET using 11C-yohimbine. Thirty patients with Parkinson's disease and 30 age- and sex-matched healthy control subjects were included. The characteristics of the patients' symptoms were assessed using the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS). Patients showed reduced neuromelanin signal intensity in the locus coeruleus compared with controls and diminished 11C-yohimbine binding in widespread cortical regions, including the motor cortex, as well as in the insula, thalamus and putamen. Clinically, locus coeruleus neuronal loss was correlated with motor (bradykinesia, motor fluctuations, tremor) and non-motor (fatigue, apathy, constipation) symptoms. A reduction of α2-AR availability in the thalamus was associated with tremor, while a reduction in the putamen, the insula and the superior temporal gyrus was associated with anxiety. These results highlight a multifaceted alteration of the noradrenergic system in Parkinson's disease since locus coeruleus and α2-AR degeneration were found to be partly uncoupled. These findings raise important issues about noradrenergic dysfunction that may encourage the search for new drugs targeting this system, including α2-ARs, for the treatment of Parkinson's disease.

6-Nitrodopamine is an endogenous selective dopamine receptor antagonist in Chelonoidis carbonaria aorta

Chelonoidis carbonaria aortic rings present endothelium-derived release of dopamine, noradrenaline, adrenaline and 6-nitrodopamine (6-ND). Here it was investigated whether 6-ND release is coupled to nitric oxide (NO) synthesis and its action on the vascular smooth muscle reactivity. Basal release of 6-ND from aortic rings in the absence and presence of the NO synthesis inhibitor L-NAME was quantified by LC-MS-MS. Aortic rings were suspended vertically between two metal hooks in 10-mL organ baths containing Krebs-Henseleit's solution and attached to isometric transducers. The tissues were allowed to equilibrate for 1 h before starting the experiments. The release of 6-ND was significantly reduced by previous incubation with L-NAME. 6-ND (up to 300 μM) had no contractile activity in the aortic rings. 6-ND (1, 3 and 10 μM) produced significant rightward shifts of the concentration-response curves to dopamine in endothelium-intact (pA₂ 6.09) and L-NAME pre-treated endothelium-intact (pA₂ 7.06) aortic rings. Contractions induced by noradrenaline and adrenaline were not affected by pre-incubation with 6-ND. The EFS (16 Hz)-induced aortic contractions were significantly inhibited by incubation with 6-ND (10 μM). In the thromboxane A₂ mimetic U-46619 (30 nM) pre-contracted endothelium intact aortic rings, 6-ND (1 nM-1 μM) and the dopamine D₂-receptor antagonist haloperidol (1 nM-1 μM) induced concentration-dependent relaxations. The relaxations were not present in endothelium-removed aortic rings but they were not affected by incubation with L-NAME in endothelium-intact aortic rings. The results indicate that the synthesis of this novel catecholamine in Chelonoidis carbonaria aortic rings is coupled to NO release and that 6-ND acts as a highly selective dopamine D₂-like receptor antagonist.

Imidazoline receptors agonists: possible mechanisms of endothelioprotection

Imidazoline receptor agonists are one of the groups of contemporary antihypertensive drugs with the pleiotropic cardiovascular effects. In this review, the historical, physiological, pathophysiological aspects concerning imidazoline receptor agonists and possible mechanisms for their participation in endothelioprotection were considered. Illuminated the molecular biology of each subtype of imidazoline receptors and their significance in the pharmacological correction of cardiovascular disease.

IR type 1 are localized in the brain nucleus, carrying out the descending tonic control of sympathetic activation, as well as in the endothelial cells of the vessels and kidneys. Their activation leads to a decrease in blood pressure, slowing the remodeling of the vascular wall and increasing sodium nares. IR type 2 is expressed predominantly in the adrenal gland, fat and muscle tissues. The physiological effects of their stimulation are associated with an increase in glucose utilization by peripheral tissues. IR type 3 are mainly present in pancreatic cells and are associated with the regulation of insulin secretion. Their stimulation leads to an increase in insulin liberation. Thus, IR agonists are able to improve endothelial function through various mechanisms, including blood pressure reduction, improvement in metabolic profile, and direct positive effects on the vascular wall.

Current information on the pharmacological effects of this group compounds allows us to conclude that they are a promising group for correcting endothelial dysfunction and complications associated with it.

Cooling-Evoked Hemodynamic Perturbations Facilitate Sympathetic Activity with Subsequent Myogenic Vascular Oscillations via Alpha2-Adrenergic Receptors

This study extends our previous work by examining the effects of alpha2-adrenoceptors under cold stimulation involving the increase of myogenic vascular oscillations as increases of very-low-frequency and low-frequency of the blood pressure variability. Forty-eight adult male Sprague-Dawley rats were randomly divided into four groups: vehicle; yohimbine; hexamethonium+yohimbine; guanethidine+yohimbine. Systolic blood pressure, heart rate, power spectral analysis of spontaneous blood pressure and heart rate variability and spectral coherence at very-low-frequency (0.02 to 0.2 Hz), low-frequency (0.2 to 0.6 Hz), and high-frequency (0.6 to 3.0 Hz) regions were monitored using telemetry. Key findings are as follows: 1) Cooling-induced pressor response was attenuated by yohimbine and further attenuated by hexamethonium+yohimbine and guanethidine+yohimbine, 2) Cooling-induced tachycardia response of yohimbine was attenuated by hexame-thonium+yohimbine and guanethidine+yohimbine, 3) Different patterns of power spectrum reaction and coherence value compared hexamethonium+yohimbine and guanethi-dine+yohimbine to yohimbine alone under cold stimulation. The results suggest that sympathetic activation of the postsynaptic alpha2-adrenoceptors causes vasoconstriction and heightening myogenic vascular oscillations, in turn, may increase blood flow to prevent tissue damage under stressful cooling challenge.

Haplotype-based analysis of alpha 2A, 2B, and 2C adrenergic receptor genes captures information on common functional loci at each gene

The alpha 2-adrenergic receptors (alpha2-AR) mediate physiological effects of epinephrine and norepinephrine. Three genes encode alpha2-AR subtypes carrying common functional polymorphisms (ADRA2A Asn251Lys, ADRA2B Ins/Del301-303 and ADRA2C Ins/Del322-325). We genotyped these functional markers plus a panel of single nucleotide polymorphisms evenly spaced over the gene regions to identify gene haplotype block structure. A total of 24 markers were genotyped in 96 Caucasians and 96 African Americans. ADRA2A and ADRA2B each had a single haplotype block at least 11 and 16 kb in size, respectively, in both populations. ADRA2C had one haplotype block of 10 kb in Caucasians only. For the three genes, haplotype diversity and the number of common haplotypes were highest in African Americans, but a similar number of markers (3-6) per block was sufficient to capture maximum diversity in either population. For each of the three genes, the haplotype was capable of capturing the information content of the known functional locus even when that locus was not genotyped. The alpha2-AR haplotype maps and marker panels are useful tools for genetic linkage studies to detect effects of known and unknown alpha2-AR functional loci.

Clonidine induces rat aorta relaxation by nitric oxide-dependent and -independent mechanisms

The alpha1- and alpha2-adrenoceptors coexist in vascular smooth muscle cells producing vascular contraction and relaxation. This study was designed to investigate which is the mechanism activated by clonidine in the rat aorta, and the endothelial factors possibly involved in the relaxation induced by clonidine. The alpha2-adrenoceptors agonist clonidine relaxed rat aortas pre-contracted with phenylephrine, with or without endothelium. In non-contracted denuded arteries, clonidine produced contractions instead of relaxation. In intact endothelium aortic rings, clonidine induced greater relaxation than in denuded aortic rings. In aortas with intact endothelium, the NO-synthase inhibitor L-NAME (10 micromol/L) and the NO-scavenger hemoglobin (10 micromol/L) reduced the relaxation to clonidine. On the other hand, indomethacin (10 micromol/L) failed to alter the relaxation induced by clonidine. These results suggest the participation of NO, but not prostacyclin in clonidine-induced relaxation. In aortic rings pre-contracted with KCl (60 mmol/L) the relaxation induced by clonidine was abolished; however, the K+ channel blockers glibenclamide (K(ATP)), tetraethylamonium (K(Ca)), and the combination of apamin and charybdotoxin (K(Ca)) did not change the relaxation induced by clonidine. The relaxation induced by clonidine on PGF2alpha-contracted arteries was not affected by prazosin. However, in the absence of prazosin, clonidine had an additional contractile effect in PGF2alpha-contracted arteries. In conclusion, our results show that in rat aorta clonidine can activate alpha2-adrenoceptors in the smooth muscle cells and alpha2-adrenoceptors in the endothelial cells that activates NO production, but not prostacyclin and/or EDHF. In the absence of phenylephrine and prazosin, clonidine can also activate alpha1-adrenoceptors and rat aorta contraction.

Alpha(1L)-, but not alpha(1H)-, adrenoceptor antagonist prevents allergic bronchoconstriction in guinea pigs in vivo

alpha-Adrenoceptors have been classified into alpha(1)- and alpha(2)-adrenoceptors. Recently, the alpha(1)-adrenoceptors were divided into two subtypes: alpha(1L) with low affinity and alpha(1H) with high affinity for prazosin. Little is known concerning the role of each subtype of alpha(1)-adrenoceptor in asthma. We investigated the effects of specific antagonists of alpha(1)- and alpha(2)-, alpha(1H)-, alpha(1L)-, and alpha(2)-adrenoceptors, namely moxisylyte, prazosin, 3-[N-[2-(4-hydroxy-2-isopropyl-5-methylphenoxy) ethyl]-N-methylaminomethyl]-4-methoxy-2, 5, 6-trimethylphenol hemifumarate (JTH-601), and yohimbine, respectively, on antigen-induced airway reactions in guinea pigs. Fifteen minutes after intravenous administration of moxisylyte (0.01, 0.1 or 1 mg/kg), prazosin (0.01, 0.1, 1 or 10 mg/kg), JTH-601 (1, 3, 6 or 10 mg/kg) or yohimbine (0.1 or 1 mg/kg), passively sensitized and artificially ventilated animals received an aerosolized antigen challenge. Bronchial responsiveness to inhaled methacholine was assessed as the dose of methacholine required to produce a 200% increase in the pressure at the airway opening (PC(200)) in non-sensitized animals. JTH-601 and moxisylyte, but not prazosin or yohimbine, dose dependently inhibited antigen-induced bronchoconstriction. None of the tested drugs altered PC(200). JTH-601 significantly reduced leukotriene C(4) levels in bronchoalveolar lavage fluid obtained 5 min after antigen challenge, but prazosin did not. These results indicate that prevention of antigen-induced bronchoconstriction by blockade of alpha-adrenoceptors is due to the inhibition of mediator release via alpha(1L)-adrenoceptor antagonism.

Pre- and postjunctional alpha(2)-adrenergic receptors in fetal and adult ovine cerebral arteries

In ovine cerebral arteries, adrenergic-mediated vasoconstrictor responses differ significantly with developmental age. We tested the hypothesis that, in part, these differences are a consequence of altered alpha(2)-adrenergic receptor (alpha(2)-AR) density and/or affinity. In fetal (approximately 140 days) and adult sheep, we measured alpha(2)-AR density and affinity with the antagonist [(3)H]idazoxan in main branch cerebral arteries and other vessels. We also quantified contractile responses in middle cerebral artery (MCA) to norepinephrine (NE) or phenylephrine in the presence of the alpha(2)-AR antagonists yohimbine and idazoxan and contractile responses to the alpha(2)-AR agonists clonidine and UK-14304. In fetal and adult cerebral artery homogenates, alpha(2)-AR density was 201 +/- 18 and 52 +/- 6 fmol/mg protein, respectively (P < 0.01); however, antagonist affinity values did not differ. In fetal, but not adult, MCA, 10(-7) M yohimbine significantly decreased the pD(2) for NE-induced tension in the presence of 3 x 10(-5) M cocaine, 10(-5) M deoxycorticosterone, and 10(-6) M tetrodotoxin. In fetal, but not adult, MCA, UK-14304 induced a significant decrease in pD(2) for the phenylephrine dose-response relation. In addition, stimulation-evoked fractional NE release was significantly greater in fetal than in adult cerebral arteries. In the presence of 10(-6) M idazoxan to block alpha(2)-AR-mediated inhibition of prejunctional NE release, the fractional NE release was significantly increased in both age groups. We conclude that in fetal and adult ovine cerebral arteries, alpha(2)-AR appear to be chiefly prejunctional. Nonetheless, the fetal cerebral arteries appear to have a significant component of postjunctional alpha(2)-AR.

Dual alpha(2)-adrenergic agonist and alpha(1)-adrenergic antagonist actions of dexmedetomidine on human isolated endothelium-denuded gastroepiploic arteries

The actions of dexmedetomidine (DEX) on human vascular smooth muscle are unclear. We investigated its effects on isolated, endothelium-denuded human gastroepiploic arteries in vitro and compared them with clonidine (CLO). DEX had little direct effect on resting tension, whereas CLO produced small contractile responses, an effect which is blocked by the alpha(1)-adrenergic antagonist prazosin. DEX markedly enhanced the high K(+) (40 mmol/L)-induced contraction, and this effect was reversed by the alpha(2)-adrenergic antagonists yohimbine and rauwolscine but unaffected by prazosin. However, CLO had little effect on the K(+) contractions. Interestingly, larger concentrations (>10(-7) mol/L) of both alpha(2)-adrenergic stimulants significantly inhibited the contractions elicited by the alpha(1)-adrenergic agonist phenylephrine (10(-6) mol/L) and, to a lesser extent, those elicited by the alpha(1)/alpha(2)-agonist norepinephrine (10(-6) mol/L). These results suggest the possibility that DEX and CLO each have a high affinity for alpha(1)-adrenoceptors in human isolated gastroepiploic arteries, resulting in a reduced efficacy of alpha(1)-adrenergic activation by alpha-agonists. The differing affinities of the drugs for alpha(1)- and alpha(2)-adrenoceptors may help explain their additional actions: 1) DEX enhances the high K(+)-induced contraction presumably through alpha(2)-adrenoceptor activation, and 2) CLO acts on alpha(1)-adrenoceptors as a partial agonist when present alone.

IMPLICATIONS

Dexmedetomidine may not directly affect smooth muscle in human peripheral resistance vessels within the usual range of plasma concentrations (<10(-7) mol/L) achieved in clinical practice. However, in large doses, it could enhance the response to nonadrenergic vasoconstrictor agonists while antagonizing the vasoconstrictor response to alpha(1)-adrenoceptor agonists.

Differential neural activation of vascular alpha-adrenoceptors in oral tissues of cats

The aim of this study was to determine the relative contribution of alpha(1)- and alpha(2)-adrenoceptors involved in sympathetic-evoked vasoconstrictor responses in tissues perfused by the lingual arterial circulation in pentobarbital anesthetized cats. Blood flow in the lingual artery was measured by ultrasonic flowmetry. Laser-Doppler flowmetry was utilized to measure oral tissue vasoconstrictor responses in the maxillary gingiva and from the surface of the tongue. Electrical stimulation of the preganglionic superior cervical sympathetic nerve resulted in frequency-dependent blood flow decreases at all three sites. These responses were stable over time and were uniformly antagonized by administration of phentolamine (0.3 - 3.0 mg kg(-1)). The selective alpha(1)-adrenoceptor antagonist, prazosin (10 - 300 microg kg(-1)), attenuated vasoconstriction in the lingual artery and gingiva, but was ineffective in blocking vasoconstriction in the tongue. Subsequent administration of rauwolscine (300 microg kg(-1)) antagonized remaining vasoconstrictor responses. In contrast, rauwolscine (10 - 300 microg kg(-1)), given alone, blocked evoked vasoconstriction in the tongue, and was without effect on gingival or lingual artery vasoconstrictor responses. Subsequent administration of prazosin (300 microg kg(-1)) largely antagonized remaining neurally elicited responses. These results suggest that neural vasoconstrictor responses in some regional vascular beds in the cat oral cavity are mediated by both alpha(1)- and alpha(2)-adrenoceptors. In contrast, tongue surface vasoconstrictor responses to sympathetic nerve activation appear to be mediated primarily by alpha(2)-adrenoceptors.

Effects of sympathetic nerve stimulation on long posterior ciliary artery blood flow in cats

A new technique using ultrasonic flowmetry was developed in order to directly measure blood flow in the long posterior ciliary artery (LPCA) of anesthetized cats. Basal LPCA blood flow averaged about 0.6 ml/min and was stable over the experimental period. Electrical stimulation of the cervical preganglionic cervical sympathetic nerve produced frequency-dependent anterior segment ocular vasoconstrictor responses. Ipsilateral nictitating membrane contractions were simultaneously measured as a well-established index of neural sympathetic activation. LPCA frequency-response relationships were shifted to the right in comparison with those for the nictitating membrane. When elicited at two min intervals, submaximal evoked responses of both systems were stable for more than 90 min. Ocular vasoconstrictor and nictitating membrane responses were blocked in a dose-dependent fashion by intravenous treatment with the non-selective a-adrenoceptor antagonist, phentolamine (0.3-3.0 mg/kg), as well as with the selective alpha1-adrenoceptor antagonist, prazosin (3-30 microg/kg). In contrast, neither evoked response was further antagonized by subsequent administration of the alpha2-adrenoceptor antagonist, yohimbine (500 microg/kg). These results demonstrate the usefulness of ultrasonic flowmetry to study mechanisms controlling ocular anterior segment circulation and suggest that, as previously established for the nictitating membrane and anterior choroid, adrenergic neurogenic vasoconstriction in tissues perfused by the LPCA is mediated predominantly by alpha1-adrenoceptors.

Sympathetic control of nasal blood flow in the rat mediated by alpha(1)-adrenoceptors

Experiments were undertaken, using laser-Doppler flowmetry, to determine the nature of adrenoceptors mediating sympathetic nerve evoked nasal vasoconstrictor responses in anesthetized rats. Presence of sympathetic tone was confirmed by a large (330%) increase of nasal blood flow following section of the ipsilateral preganglionic cervical sympathetic nerve. Electrical nerve stimulation produced reproducible, frequency-related nasal vasoconstrictor responses with near maximal response, observed at less than 10 Hz. Evoked nasal vasoconstrictor responses were largely blocked with intravenous treatment with the non-selective alpha-adrenoceptor antagonists, phentolamine (5 mg kg(-1)) and phenoxybenzamine (2 mg kg(-1)), as well as with the selective alpha(1)-adrenoceptor antagonist, prazosin (300 microg kg(-1)). alpha(2)-Adrenoceptor antagonism with rauwolscine (500 microg kg(-1)) potentiated neurally evoked nasal vasoconstriction. Neither atropine (1 mg kg(-1)) nor propranolol (1 mg kg(-1)) altered the evoked responses. Rats with intact cervical sympathetic nerves responded to rauwolscine with a modest constriction. Subsequent prazosin administration produced an increase of nasal blood flow of approximately 275%. These results suggest that the nasal vasculature of the rat is under intense sympathetic tone and that the resulting neurogenic vasoconstriction is mediated exclusively by activation of alpha(1)-adrenoceptors.

Platelet alpha2-adrenoceptor alterations in patients with essential hypertension are normalized after treatment with doxazosin but not propranolol

OBJECTIVE

Marked alterations have been demonstrated to occur in the platelet alpha2-adrenoceptors of patients with essential hypertension. The purpose of this study was to determine whether antihypertensive treatment with alpha-adrenergic blocker doxazosin or beta-adrenergic blocker propranolol can affect the affinity and the density of platelet alpha2-adrenoceptors in such patients.

SUBJECTS AND METHODS

In two groups of 22 previously untreated, essential hypertensive patients, the mean affinity (Kd) and density (B(max)) of platelet alpha2-adrenoceptors were studied by [3H]-UK 14304 binding assays; the first assays were performed before any medication was begun, the second were performed after treatment for up to 13 weeks with doxazosin or propranolol. A third group of 22 healthy normotensive volunteers matched by age, sex and body mass index was used as control.

RESULTS

Blood pressure did not differ significantly in the two hypertensive groups, and treatment with the two drugs resulted in closely similar, normal blood pressure levels. Kd and B(max) values were significantly higher in the two hypertensive groups than in controls. After treatment with propranolol the binding parameters did not change significantly, whereas after treatment with doxazosin Kd and B(max) returned to normotensive values.

CONCLUSIONS

In previously untreated, essential hypertensive patients platelet alpha2-adrenoceptors have a lower affinity but a higher density than in normotensive subjects. Despite similar effects on blood pressure, the treatment with the alpha-adrenergic blocker doxazosin is followed by restoration of normal findings in the binding assays of platelet alpha2-adrenoceptors whereas the treatment with the beta-adrenergic blocker propranolol does not alter the Kd and B(max) values.

Nitric oxide activity in the peripheral vasculature during normotensive and preeclamptic pregnancy

We investigated the role of nitric oxide (NO) in the vascular resistance changes of normotensive and preeclamptic pregnancy. Forearm blood flow (FBF) responses to brachial artery infusion of N(G)-monomethyl-L-arginine (L-NMMA), an NO synthase inhibitor, and angiotensin II (ANG II), an NO-independent vasoconstrictor, were determined by plethysmography in 20 nonpregnant women, 20 normotensive primigravidae, and 15 primigravidae with untreated preeclampsia. In pregnant subjects, FBF was reduced to nonpregnancy levels by infusion of norepinephrine (NE), which was then coinfused with ANG II (2, 4, and 8 ng/min) and L-NMMA (200, 400, and 800 microgram/min) each for 5 min. In separate studies, responses to NE (20, 50, and 100 ng/min) were determined in 8 nonpregnant women, with FBF elevated to pregnancy levels by concomitant infusion of glyceryl trinitrate, and 10 pregnant women. Vasoconstrictor responses to L-NMMA were increased in pregnant compared with nonpregnant subjects [mean +/- SE summary measure (in arbitrary units): 60 +/- 7 vs. 89 +/- 8, respectively; P < 0.01], whereas responses to ANG II were blunted (125 +/- 11 vs. 79 +/- 7, respectively; P < 0.001). Compared with normotensive pregnant subjects, preeclamptic subjects had an enhanced response to ANG II (79 +/- 7 vs. 103 +/- 8, respectively; P < 0.05) but no difference in response to L-NMMA (89 +/- 8 vs. 73 +/- 10, respectively; P = 0.30). Responses to NE were similar in pregnant and nonpregnant subjects (110 +/- 20 vs. 95 +/- 33, respectively; P = 0.66). During the third trimester of pregnancy, forearm constrictor responses to L-NMMA are increased. The responses to NE are unchanged, whereas responses to ANG II are blunted. Increased NO activity contributes to the fall in peripheral resistance. In preeclampsia, forearm constrictor responses to ANG II but not L-NMMA are increased compared with those in normal pregnancy. Changes in vascular NO activity are unlikely to account for the increased vascular tone in this condition.

One hundred years of adrenaline: the discovery of autoreceptors

The active principle of suprenal extract that produces its pressor effects was isolated by the joint research of John Abel in 1899 and Jokichi Takamine in 1901. Within three years Elliott, working in Langley's laboratory, suggested that this active principle, referred to by British physiologists as "adrenaline" and named "Adrenalin" by Takamine, was released from sympathetic nerve terminals to act on smooth muscle cells. However, it was not until 1946 that von Euler showed that demythelated adrenaline (noradrenaline) rather than adrenaline is a sympathetic transmitter. The possibility that this sympathetic transmitter could also act on nerve terminals was not developed until 1971. Research on autoreceptors culminated in the identification of adrenergic receptors on nerve terminals different to those on muscle cells. This paper assesses the contributions that established the idea of the adrenergic autoreceptor, 100 years after the discovery of adrenaline.

Sympathetic vasoconstriction in the rat anterior choroid is mediated by alpha1-adrenoceptors

These experiments were undertaken in an attempt to use laser-Doppler flowmetry to measure anterior choroidal blood flow in the anesthetized rat and to study the mechanism by which sympathetic nerve stimulation might produce vasoconstriction in this vascular bed. Electrical stimulation of the preganglionic cervical sympathetic nerve produced reproducible, frequency-related ocular vasoconstrictor responses with maximal vasoconstriction seen at about 32 Hz. Ocular vasoconstrictor responses were blocked by intravenous treatment with the nonselective alpha-adrenoceptor antagonists, phentolamine (5 mg kg(-1)) and phenoxybenzamine (2 mg kg(-1)), as well as with the selective alpha1-adrenoceptor antagonist, prazosin (0.3 mg kg(-1)). In contrast, the selective alpha2-adrenoceptor blocker, rauwolscine (0.5 mg kg(-1)), only potentiated the vasoconstriction. Neither intravenous atropine (1 mg kg(-1)) nor propranolol (1 mg kg(-1)) altered the magnitude of neurally evoked vasoconstriction. These results demonstrate the usefulness of laser-Doppler flowmetry in studies of the rat anterior choroidal circulation and suggest that adrenergic neurogenic vasoconstriction in the anterior segment of the rat eye is mediated almost exclusively by alpha1-adrenoceptor mechanisms.

Effects of intrathecal alpha1- and alpha2-noradrenergic agonists and norepinephrine on locomotion in chronic spinal cats

Noradrenergic drugs, acting on alpha adrenoceptors, have been found to play an important role in the initiation and modulation of locomotor pattern in adult cats after spinal cord transection. There are at least two subtypes of alpha adrenoceptors, alpha1 and alpha2 adrenoceptors. The aim of this study was to investigate the effects of selective alpha1 and alpha2 agonists in the initiation and modulation of locomotion in adult chronic cats in the early and late stages after complete transection at T13. Five cats, chronically implanted with an intrathecal cannula and electromyographic (EMG) electrodes were used in this study. Noradrenergic drugs including alpha2 agonists (clonidine, tizanidine, and oxymetazoline) and an antagonist, yohimbine, one alpha1 agonist (methoxamine), and a blocker, prazosin, as well as norepinephrine were injected intrathecally. EMG activity synchronized to video images of the hindlimbs were recorded before and after each drug injection. The results show differential effects of alpha1 and alpha2 agonists in the initiation of locomotion in early spinal cats (i.e., in the first week or so when there is no spontaneous locomotion) and in the modulation of locomotion and cutaneous reflexes in the late-spinal cats (i.e., when cats have recovered spontaneous locomotion). In early spinal cats, all three alpha2 agonists were found to initiate locomotion, although their action had a different time course. The alpha1 agonist methoxamine induced bouts of nice locomotor activity in three spinal cats some hours after injection but only induced sustained locomotion in one cat in which the effects were blocked by the alpha1 antagonist prazosin. In late spinal cats, although alpha2 agonists markedly increased the cycle duration and flexor muscle burst duration and decreased the weight support or extensor activity (effects blocked by an alpha2 antagonist, yohimbine), alpha1 agonist increased the weight support and primarily the extensor activity of the hindlimbs without markedly changing the timing of the step cycle. Although alpha2 agonists, especially clonidine, markedly reduced the cutaneous excitability and augmented the foot drag, the alpha1 agonist was found to increase the cutaneous reflex excitability. This is in line with previously reported differential effects of activation of the two receptors on motoneuron excitability and reflex transmission. Noradrenaline, the neurotransmitter itself, increased the cycle duration and at the same time retained the cutaneous excitability, thus exerting both alpha1 and alpha2 effects. This work therefore suggests that different subclasses of noradrenergic drugs could be used to more specifically target aspects of locomotor deficits in patients after spinal injury or diseases.

Role of alpha 2-adrenoceptors in the regulation of intestinal water transport

1. The influence of the sympathetic nervous system on intestinal fluid transport by the jejunum and ileum of the anaesthetized rat was investigated under basal conditions and during active secretion induced by intra-arterial infusion of vasoactive intestinal peptide (VIP). 2. Intra-arterial infusion of noradrenaline (3, 10, 30 nmol min-1, i.a.) and i.v. injection of the selective alpha 2-adrenoceptor agonist UK 14,304 (1 mumol kg-1, i.v.) increased the rate of basal fluid absorption. The effect of UK 14,304 was blocked by yohimbine (10 mumol kg-1, i.v.). However, the selective alpha 1-adrenoceptor agonist phenylephrine (5 mumol kg-1, i.v.) did not alter either the jejunal or ileal absorption rate. 3. The alpha 2-adrenoceptor antagonists yohimbine (0.3, 1.0, 3 and 10 mumol kg-1, i.v.) and rauwolscine (10 mumol kg-1, i.v.) decreased the basal absorption rate, while the alpha 1-adrenoceptor antagonist prazosin (3 mumol kg-1, i.v.) was without effect. Intracerebroventricular injection of yohimbine (3 mumol kg-1) caused a significant antiabsorptive effect in the jejunum but not ileum. 4. Peripheral chemical sympathectomy induced by pretreating animals with 6-hydroxydopamine (150 mg kg-1, i.p., total dose) induced a trend towards impaired absorption in the jejunum and ileum. 5. The findings provide evidence that the sympathetic nervous system exerts tonic control on intestinal fluid transport and that the effect is mainly through peripheral alpha 2-adrenoceptors. 6. The subtype determination of alpha 2-adrenoceptors in modulating intestinal fluid transport was assessed by determining the effects of alpha 2-adrenoceptor agents on intestinal fluid secretion induced by i.a. infusion of VIP (0.8 microgram min-1). 7. Intravenous administration of UK 14,304 caused a dose-dependent reversal of the secretory phase of the VIP-induced response, but failed to restore fluid transport to the control level of net absorption. EC50 values were 0.17 mumol kg-1 in the jejunum and 0.22 mumol kg-1 in the ileum. 8. The effect of UK 14,304 was blocked by the selective alpha 2A/D antagonist BRL 44408 and the nonselective alpha 2 antagonist yohimbine (each 10 mumol kg-1). The selective alpha 2B/C antagonist ARC 239 (10 mumol kg-1) did not affect the antisecretory action of UK 14,304. It is suggested that the alpha 2-adrenoceptors in the rat intestinal epithelium are the alpha 2D or alpha 2A-like subtype.

Effects of modulatory agents on neurally-mediated responses of trout intestinal smooth musclein vitro

Mediators and mechanisms responsible for the inhibitory modulation of trout intestinal smooth muscle were examined using a series of putative mediators and substances known to modulate neurotransmission in mammalian systems. Frequency response relationships to transmural stimulation and concentration response relationships to 5-hydroxytryptamine, carbachol, and substance P were established on paired segments of rainbow trout intestinein vitro in the presence and absence of putative modulatory agents. Modulation of neurally-mediated contractions of trout intestine was achieved with dibutyryl cyclic AMP and forskolin, agents that increase intracellular levels of cyclic AMP. The effect appears to be at the level of the smooth muscle, since the adenylate cyclase activator, forskolin, inhibited muscarinic and serotoninergic contractions as well as transmurally stimulated contractions. Substance P-induced contractions were unaffected by forskolin. The endogenous agonists/neurotransmitters which would increase cyclic AMP levels in rainbow trout intestinal smooth muscle are as yet unknown. The effects do not appear to be modulated by vasoactive intestinal peptide (VIP), calcitonin, calcitonin gene-related peptide (CGRP), or agents that activate β-adrenoceptors. Prostaglandin E2 (PGE2) and α2-adrenergenic agonists are possible agents which will decrease contractility of the smooth muscle. They were only active in the proximal intestine and on transmurally stimulated contractions. The effects of both PGE2 and α2-agonists appear to be prejunctional, decreasing release of contractile neurotransmitters in the enteric nervous system.

Preclinical assessment of a new transdermal delivery system for clonidine (M-5041T)

The effects of a new transdermal delivery system for clonidine (M-5041T) on hypotensive effect, urine volume, plasma renin activity (PRA) and antidiuretic hormone (ADH) in spontaneously hypertensive rats (SHRs) were compared to the effects of the continuous infusion of clonidine. Both M-5041T (1.5 and 4.5 mg/kg) and the continuous infusion of clonidine (250 micrograms/kg/24 h) elicited hypotensive effects persisting for 12 hours or more. These effects were based on consistent plasma concentrations of clonidine. These two treatments produced diuresis followed by antidiuresis, which was remarkably observed by continuous infusion of clonidine. Single subcutaneous injection of clonidine (50 micrograms/kg) produced diuresis accompanied by increases in electrolytes corresponding to plasma levels of clonidine. M-5041T at 1.5 mg/kg did not affect PRA until 12 h, and produced an increase in PRA at 24 h. M-5041T at 4.5 mg/kg and the continuous infusion of clonidine resulted in a decrease in PRA at 2 and 1 h followed by an increase at 12 and 24 h, respectively. M-5041T at 1.5 mg/kg did not affect plasma levels of ADH. Plasma ADH did increase at 2 and 4 h accompanied by diuresis following M-5041T at 4.5 mg/kg or the continuous infusion of clonidine, respectively. Clonidine-induced diuresis was not at least due to the inhibition of ADH release. The decrease in urine volume observed by continuous infusion of clonidine may be due to decrease in renal blood flow based on stimulation of peripheral adorenoceptors of clonidine. These findings suggest that the increases in ADH and PRA are due to the compensatory effects related to both diuresis and the long-lasting hypotensive effect induced by high plasma concentrations of clonidine. Thus, it can be expected that M-5041T at 1.5 mg/kg showing the minimum effective plasma concentration of clonidine will not result in tolerance to the hypotensive effect of clonidine associated with the retention of sodium in SHRs.

Effect of the vascular endothelium on noradrenaline-induced contractions in non-pregnant and pregnant guinea-pig uterine arteries

1. The effect of pregnancy on noradrenaline-mediated contraction of guinea-pig uterine artery rings with both intact and denuded endothelium was investigated. 2. Noradrenaline (25 nM-100 microM) induced concentration-dependent contraction of non-pregnant and pregnant guinea-pig uterine arterial rings with intact endothelium with similar pD2 and maximal response values (non-pregnant: pD2 = 5.85 +/- 0.02, maximal response = 121 +/- 8.2%; pregnant: pD2 = 5.81 +/- 0.04, maximal response = 122 +/- 9.1%). Removal of endothelium did not affect noradrenaline-induced contractions in non-pregnant guinea-pig uterine artery (pD2 = 5.97 +/- 0.02, maximal response = 119 +/- 8.6%). In contrast, in arteries from pregnant guinea-pigs, removal of endothelium shifted concentration-response curve for noradrenaline to the left, without affecting maximal response value (pD2 = 6.36 +/- 0.03, maximal response = 120 +/- 9.0%). 3. The pKA values for noradrenaline were: 5.76 +/- 0.09 and 5.82 +/- 0.10 for non-pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively and 5.74 +/- 0.09 and 5.72 +/- 0.07 for pregnant guinea-pig uterine artery with intact and denuded endothelium, respectively. 4. The receptor occupancy-response relationship for noradrenaline was linear for all types of vessels, except for pregnant guinea-pig uterine artery with denuded endothelium, since half-maximal response to noradrenaline was obtained with 44.8 +/- 6.9% (non-pregnant guinea-pig uterine artery with intact endothelium), 43.3 +/- 6.1% (non-pregnant guinea-pig uterine artery with denuded endothelium) and 44.3 +/- 6.3% (pregnant guinea-pig uterine artery with intact endothelium) receptor occupancy. In pregnant guinea-pig uterine artery with denuded endothelium, occupancy-response relationship for noradrenaline was non-linear since half-maximal response to noradrenaline was obtained with 19.7 +/- 3.3% receptor occupancy. 5. NG-monomethyl-L-arginine (100 microM) and indomethacin (10 microM) did not affect concentration-response curve for noradrenaline in guinea-pig uterine arteries, regardless of pregnancy status or endothelial condition. 6. In quiescent preparations, the alpha-adrenoceptor antagonists, prazosin (5-50 nM) and yohimbine (1-10 microM) produced parallel rightward shifts of the curves for noradrenaline and the slopes of the Schild plots were not significantly different from unity. The plots constrained to a slope of unity gave the following - log Kb values: prazosin vs. yohimbine 8.78 +/- 0.03 vs. 6.41 +/- 0.02 for non-pregnant guinea pig uterine artery with intact endothelium, 8.95 +/- 0.03 vs. 6.34 +/- 0.02 for non-pregnant guinea-pig uterine artery with denuded endothelium, 8.91 +/- 0.01 vs. 6.44 +/- 0.03 for pregnant guinea-pig uterine artery with intact endothelium and 9.07 +/- 0.01 vs. 6.52 +/- 0.03 for pregnant guinea-pig uterine artery with denuded endothelium.7. It is concluded that initially there is no difference in noradrenaline action between uterine arteries from non-pregnant and pregnant guinea-pigs, but after removal of the endothelium the pregnant guinea-pig uterine artery is more sensitive to noradrenaline, which is related to increased receptor reserve for noradrenaline in this tissue. It is probable that relaxing factor derived from the endothelium mediates this effect, but it is unlikely to be nitric oxide or prostacyclin. Antagonist affinities and affinity of noradrenaline itself suggests that an identical subtype of alpha-adrenoceptor, probably the alpha 1 subtype, is involved in the noradrenaline-induced contraction of non-pregnant and pregnant guinea-pig uterine artery with or without endothelium.

The role of calcium and alpha-adrenoceptors in contractile response of chick expansor secundariorum muscle to field stimulation

1. The effects of some alpha-adrenergic agonists and antagonists on electrically-evoked contractions and tension of chick expansor secundariorum muscle (ESM), and dependence of these events on extracellular calcium was investigated. 2. Both train and continuous electrical stimulation can produce regular contractions in preparations obtained from 40-60 day old chicks. 3. Clonidine had a biphasic action on the contractions produced by train electrical stimulation. In concentrations ranging from 10(-8) to 3 x 10(-7) M, clonidine decreased the contraction amplitude, but in higher concentrations, it caused an increase in both the muscle tension and the contraction amplitude. These effects were reversed by application of yohimbine although yohimbine by itself had no effect on the contractions. 4. Introduction of calcium free isotonic high potassium medium decreased muscle tone which was followed by further dose-dependent increase in tension, along with the addition of cumulative doses of CaCl2 (ED50 = 2.8 x 10(-3) M). 5. Nifedipine reduced the amplitude of ESM contractions produced by continuous electrical stimulation in a dose dependent manner (IC50 = 6.7 x 10(-7) M). 6. Methoxamine induced a completely dose dependent increase in muscle tension which was dependent on extracellular calcium and was inhibited by nifedipine. In the presence of 10(-8) M nifedipine, ED50 of methoxamine stimulatory effect increased from the control value of 2.2 x 10(-7) to 8.4 x 10(-7) M).(ABSTRACT TRUNCATED AT 250 WORDS)

Autonomic regulation of nasal vessels during changes in body position

The effects of postural changes on nasal airflow and nasal capillary blood flow were investigated in 15 healthy volunteers. Measurements were performed following nasal application of saline solution (control), the alpha-1 receptor antagonist prazosin, the alpha-2 receptor antagonist yohimbine, and after application of both prazosin and yohimbine. Nasal airflow in the control experiments did not significantly differ in the upright (362 +/- 166 ml/s), dorsally recumbent (350 +/- 167 ml/s) and 70 degrees head down position (311 +/- 167 ml/s). Following application of prazosin, nasal airflow was reduced to 223 +/- 121 ml/s in the upright position. Prazosin treatment significantly reduced nasal airflow to 177 +/- 111 ml/s when subjects were placed in dorsally recumbent positions and to 117 +/- 104 ml/s in 70 degrees head down positions (P < 0.001). Following application of yohimbine, nasal airflow remained stable when subjects were turned from upright (348 +/- 165 ml/s) to supine position (352 +/- 186 ml/s), whereas it was reduced to 199 +/- 137 ml/s in the head-down position. Application of both prazosin and yohimbine significantly increased nasal capillary blood flow in laser Doppler flowmetry measurements (P < 0.05). Changes in body position with or without application of the active drugs did not alter nasal capillary blood flow. These findings suggest that nasal congestion due to increased filling pressure in nasal capacitance vessels following postural changes is mainly prevented by alpha-1 adrenergic mechanisms.

Ocular effects of alpha 2-adrenoceptor activation in anesthetized cats

The present study was undertaken to determine the effects of direct administration of the selective alpha 2-adrenoceptor stimulant, B-HT 933, on choroidal blood flow, intraocular pressure and pupil size in anesthetized cats. Anterior segment choroidal blood flow was measured using laser-Doppler flowmetry. B-HT 933 administered by intra-arterial, topical and intracameral routes produced a significant depression of ocular blood flow which was largely abolished by pretreatment with rauwolscine. B-HT 933 did not lower IOP in any of these preparations. The largest doses of B-HT 933 caused a modest mydriasis when given intracamerally. However, this pupillary dilation was not blocked by rauwolscine. These results demonstrate that alpha 2-adrenoceptor activation can produce pronounced depression of anterior segment choroidal blood flow but does not cause a concomitant lowering of IOP or mydriasis in anesthetized cats.

Ocular alpha 2-receptor subclasses and antiglaucoma efficacy

An overview of the ocular hypotensive actions of some alpha 2-agonists with imidazoline structures is presented. These agents inhibit isoproterenol-stimulated adenylate cyclase (AC) activity in ciliary process membrane through a Na+ and GTP-dependent mechanism. Receptor binding studies with the alpha 2-agonist radioligand [125I] p-iodoclonidine ([125I]PIC) in rabbit ciliary body membranes indicate that the alpha 2-receptor subtype is alpha 2A. Gpp(NH)p and NaCl dose-dependently decreased the number of [125I]PIC binding sites by shifting the receptor-G protein complexes from the high affinity state to the low affinity state for agonist binding. This is consistent with the observations that inhibition of AC was Na+ and GTP dependent. The NaCl and Gpp(NH)p effects on binding appeared to be through different mechanisms. The alpha 2-receptor in ciliary process thus appears to be an alpha 2A-receptor that is negatively coupled to the AC-cAMP generating system.

Blockade of retinal NMDA receptors by sodium nitroprusside is probably due to nitric oxide formation

Effects of a nitric oxide (NO)-producing agent, sodium nitroprusside, on N-methyl-D-aspartate (NMDA) receptor activation in the cultured retinal neurons of rats were examined. NMDA in a Mg(2+)-free medium evoked inward currents at the resting membrane potential. Inward currents were also evoked by kainate. Sodium nitroprusside markedly reduced the NMDA-induced currents without affecting those induced by kainate. These results suggest the possible existence of a negative feed back system of NO which serves to regulate the activation of NMDA receptors in retinal neurons.

Dominant antagonistic action of alpha 2-adrenoceptor agonists on alpha 1-agonist-induced vasoconstriction

The action of clonidine and xylazine to suppress constrictor responses to norepinephrine (NE) and phenylephrine was analysed in isolated, perfused dog and monkey lingual arteries. In both kinds of arteries, alpha 1-adrenoceptor agonists, NE and phenylephrine induced strong vasoconstrictor responses, whereas alpha 2-adrenoceptor agonists, clonidine and xylazine, produced only a slight but long-lasting increase in perfusion pressure. Pretreatment with bolus injections of 300 micrograms clonidine or 1000 micrograms xylazine caused a significant inhibition of vasoconstrictor responses to NE and phenylephrine and shifted the dose-responses curves to the right. Clonidine induced a more potent inhibition than did xylazine. However, neither clonidine nor xylazine pretreatment inhibited 5-HT- and KCl-induced vasoconstriction. In preparations preconstricted with phenylephrine, clonidine and xylazine induced vasodilatation dose relatedly, but in preparations preconstricted with prostaglandin F2 alpha (PGF2 alpha), clonidine and xylazine never induced vasodilatation but only vasoconstriction. The vasoconstrictor effect of clonidine was readily blocked by bunazosin (an alpha 1-adrenoceptor antagonist), but was not affected by midaglizole (an alpha 2-antagonist). It was now demonstrated that alpha 2-agonists act as partial agonists on alpha 1-adrenoceptors when added alone but antagonized alpha 1 activation by alpha 1-agonists, suggesting that alpha 2-agonists have a high affinity for alpha 1-adrenoceptors in isolated lingual arteries.

Characterization of postsynaptic alpha-adrenoceptors in the arteries supplying the oviduct

1. In vitro experiments were designed to characterize postjunctional alpha-adrenoceptor subtypes in ring segments (1 mm length; outer diameter 300-500 microns) from arteries supplying the oviduct of the heifer. 2. Noradrenaline, adrenaline and phenylephrine evoked concentration-dependent contractile responses. The pD2 values were 5.67, 5.89 and 5.93, respectively. Medetomidine clonidine and B-HT 920 (2-amino-6-allyl-5,6,7,8-tetra-hydro-4H-(thiazo)-4,5-d-azepoine ) were ineffective. 3. The alpha-adrenoceptor selective antagonists, prazosin (1 nM-0.1 microM) and rauwolscine (0.1-10 microM) competitively antagonized the response to noradrenaline. The pA2 values were 9.38 and 6.83, respectively. 4. The dissociation constant (KD) for noradrenaline calculated by use of the irreversible antagonist, dibenamine, was 3.95 (2.09-5.81) microM. The occupancy-response relationship was non-linear. Half-maximal response to noradrenaline was obtained with 22% receptor occupancy while maximal response required 100% occupancy. 5. B-HT 920 evoked a biphasic contractile concentration-dependent response in preparations incubated in a physiological solution containing 20 mM K+, 0.1 microM prazosin and 1 microM propranolol. Rauwolscine 0.1 microM significantly (P less than 0.01) blocked the first component of the B-HT 920 concentration-response curve with an apparent pA2 value of 8.52 (7.86-9.18). 6. These results strongly suggest that alpha-adrenoceptors in oviductal arteries are mainly of the alpha 1 subtype, although a possible role for alpha 2-adrenoceptors cannot be excluded.

Functional distribution of alpha 1- and alpha 2-adrenergic receptors in the coronary microcirculation

BACKGROUND

The goal of this study was to determine the functional distribution of alpha 1- and alpha 2-adrenergic receptors in the epicardial coronary microcirculation. This goal was accomplished by intracoronary administration of the selective alpha 1-adrenergic agonist phenylephrine and the selective alpha 2-adrenergic agonist BHT-933 during measurements of coronary microvascular diameters in the beating heart.

METHODS AND RESULTS

Experimental measurements were made under conditions with intact vasomotor tone and during coronary hypoperfusion (i.e., under conditions with autoregulatory mechanisms intact and blunted, respectively). Administration of selective alpha 1- and alpha 2-adrenergic antagonists, prazosin and SKF 104078, respectively, confirmed that the agonists were preferentially activating the desired adrenergic receptor subtype because the vasoconstrictor effects of the agonists were completely blocked by the appropriate antagonist. With baseline coronary vasomotor tone intact, phenylephrine caused constriction (8 +/- 3% decrease in diameter, p less than 0.05) of small coronary arteries (vessels greater than 100 microns in diameter) but did not produce constriction of coronary arterioles (vessels less than 100 microns in diameter). During coronary hypoperfusion, phenylephrine caused constriction (p less than 0.05) of both small coronary arteries and arterioles, 6 +/- 2% and 11 +/- 3% decreases in diameter, respectively. BHT-933 did not cause significant changes in microvascular diameters under control conditions but substantially and selectively decreased arteriolar diameters during hypoperfusion (24 +/- 6% decrease in diameter, p less than 0.05).

CONCLUSIONS

In the intact, autoregulating coronary circulation, coronary arterioles escape from the effects of adrenergic activation but coronary arteries do not; rather, they can exhibit alpha 1-adrenergic coronary vasoconstriction. During coronary hypoperfusion, when autoregulatory adjustments are blunted, coronary arterioles are sensitive to both alpha 1- and alpha 2-adrenergic agonists, demonstrating significant constrictor responses. Also, the magnitude of coronary alpha 2-adrenergic arteriolar constriction (24% decrease in diameter) is significantly greater than that of alpha 2-adrenergic constriction (11% decrease in diameter) (p less than 0.05). Thus, alpha 1- and alpha 2-adrenergic activation produce different constrictor effects in the coronary microcirculation under baseline conditions when autoregulatory adjustments are intact and during coronary hypoperfusion when autoregulation is blunted. The data suggest that alpha 2-adrenergic receptors are preferentially distributed in arterioles, whereas alpha 1-adrenergic receptors are located throughout the coronary microcirculation. Importantly, the data also suggest that intrinsic autoregulatory adjustments in tone (i.e., autoregulatory escape) can override either alpha 1- or alpha 2-adrenergic constriction in coronary arterioles.

The effects of nifedipine on alpha 2-adrenoceptor-mediated contractions in several isolated blood vessels from the rabbit

1. The effects of the dihydropyridine calcium channel blocker, nifedipine, on noradrenaline-induced contractile responses have been examined in several isolated blood vessels from the rabbit, with particular emphasis on responses mediated via postjunctional alpha 2-adrenoceptors. 2. In the isolated renal vein, ear vein, distal saphenous artery, saphenous vein and plantaris vein, 0.1 microM and 1 microM nifedipine reduced responses elicited by 54 mM KCl by more than 70%. The remaining responses were abolished by alpha-adrenoceptor blockade, suggesting the involvement of noradrenaline released from neurones activating a dihydropyridine-resistant mechanism. 3. In the renal vein (alpha 1-), ear vein (predominantly alpha 2-), distal saphenous artery (alpha 1- greater than alpha 2-), saphenous vein and plantaris vein (alpha 2- greater than alpha 1-), 0.01 microM and 0.1 microM nifedipine produced concentration-related reductions in the maximum response to noradrenaline. However, 1 microM nifedipine was no more effective than 0.1 microM nifedipine and the reduction in the maximum varied from 10-25% of the control response. Thus, a sizeable component of the alpha-adrenoceptor-mediated response in all blood vessels is resistant to dihydropyridine calcium channel blockers and this appears to be unrelated to the alpha-adrenoceptor subtype involved. 4. Following irreversible inactivation of alpha 1-adrenoceptors and isolation of functional alpha 2-adrenoceptors in the saphenous vein, plantaris vein and distal saphenous artery (the latter requiring the presence of angiotensin II), the effect of nifedipine on responses to noradrenaline was increased. However, a component of the alpha 2-adrenoceptor response in each preparation was present even after the concentration of nifedipine was increased to 1 microM. 5. In the saphenous vein, a preparation in which it has been demonstrated previously that alpha 2-adrenoceptor-mediated responses are highly dependent upon the presence of extracellular calcium ions, partial depolarization with 20mM KCl failed to increase the inhibitory effect of 0.1 microM nifedipine. This suggests the involvement of dihydropyridine-resistant Ca2+ channels. The possible relationship between these dihydropyridine-resistant Ca2+ channels, alpha-adrenoceptor subtypes and 'receptor-operated' Ca2 + channels is discussed.

Characterization of alpha-adrenoceptors mediating contraction in isolated ovine umbilical vein

Responses to alpha-adrenoceptor agonists were studied in isolated umbilical vessels obtained from fetal lambs within 2 weeks of term (term = 145 days). Norepinephrine produced concentration-dependent contractions of the umbilical vein, but not in the umbilical artery. The contraction elicited by norepinephrine in the umbilical vein was not potentiated by cocaine (10(-5) M). Epinephrine also produced contractions of the umbilical vein with a potency similar to that of norepinephrine. Phenylephrine was less potent than norepinephrine while clonidine and xylazine were inactive in producing contractions of the umbilical vein. The dissociation constant (KA) of norepinephrine in the umbilical vein was 172 +/- 41 nM. There was little, if any, alpha-adrenoceptor reserve in the umbilical vein. Contraction produced to norepinephrine in the umbilical vein was effectively blocked by prazosin and phentolamine. Yohimbine was 500 times less potent than prazosin in blocking norepinephrine-induced contraction in the umbilical vein. Idazoxan only minimally antagonized the contractions elicited to norepinephrine. We conclude that alpha 1-adrenoceptors are present and mediate vasoconstriction in the ovine umbilical vein and that alpha 2-adrenoceptors are not present in this tissue.

Adrenergic vasomotion in the coronary microcirculation

The goal of this study was to determine the alpha-adrenergic receptor subtype(s) responsible for constriction at different microvascular levels in the coronary circulation. To accomplish these goals, the epicardial coronary microcirculation of intact beating hearts was viewed through an intravital microscope using stroboscopic epi-illumination. An initial study was designed to establish sites of alpha-adrenergic constriction to norepinephrine in preparations with intact vasomotor tone. For the primary experimental goal, coronary microvascular responses to selective alpha 1-adrenergic (phenylephrine) or alpha 2-adrenergic (BHT-933) agonists were evaluated, when coronary autoregulatory escape mechanisms were blunted during hypoperfusion. Infusion of norepinephrine decreased diameter of arterial vessels greater than 100 microns in diameter, but downstream coronary arterioles dilated significantly, representing autoregulatory escape from adrenergic vasoconstriction. In studies designed to examine the adrenergic receptor subtype (during hypoperfusion), phenylephrine produced modest constriction of vessels throughout the microcirculation (6-9% decrease in diameter), whereas BHT-933 produced marked constriction of small coronary microvessels, those less than 100 microns in diameter (24% decrease in diameter). From these results we conclude: 1) norepinephrine infusion causes disparate responses in the coronary microvasculature: constriction occurs in vessels greater than 100 microns in diameter, but dilation, via autoregulatory escape, predominates in vessels less than 100 microns in diameter; 2) alpha 1-adrenergic receptors are located in coronary arterioles and arteries; and 3) alpha 2-adrenergic receptors are preferentially located in small coronary arterioles. Thus, alpha 1- and alpha 2-adrenergic activation can produce dissimilar constrictor effects in the coronary microcirculation during hypoperfusion.

Isolated bovine cerebral arteries from rostral and caudal regions: distinct responses to adrenoceptor agonists

Responsiveness to norepinephrine and related agents was compared in isolated bovine anterior cerebral (ACA). middle cerebral (MCA), intracranial internal carotid (ICA), posterior communicating (PCOM), posterior cerebral (PCA) and basilar arteries (BA). Norepinephrine contracted the strips from ACA, MCA and ICA, but relaxed those from PCOM, PCA and BA. In the presence of propranolol, the amine-induced contractions tended to be potentiated in ACA, MCA and ICA, and the relaxations in PCOM, PCA and BA were reversed to contractions. The maximum contractions induced by norepinephrine in ICA and ACA treated with propranolol were significantly greater than those in PCA and BA, but the EC50 values did not differ among arteries. In ACA and MCA, the contractions induced by phenylephrine were greater than those induced by clonidine. The contractions induced by norepinephrine and phenylephrine were attenuated by prazosin but not influenced by yohimbine in ACA and MCA treated with propranolol. These findings indicate that the responses to norepinephrine evidently differ in bovine cerebral arteries of rostral (ACA, MCA and ICA) and caudal regions (PCOM, PCA and BA), possibly due to different functioning of alpha/beta receptors. The amine-induced contraction predominantly seen in the rostral arteries appears to be associated with activation of the alpha 1 adrenoceptor subtype.

Physiology and pharmacology of cardiovascular catecholamine receptors: implications for treatment of chronic heart failure

In the sympathetic nervous system the physiologic effects of the endogenous catecholamines noradrenaline (NA) and adrenaline (A) are mediated by alpha- and beta-adrenoreceptors (ARs). Both AR-types can be subdivided into two major subtypes: alpha-ARs into alpha-1 (predominant effect: vasoconstriction) and alpha-2 (presynaptic: inhibition of NA-release; postsynaptic: vasoconstriction), beta-ARs into beta-1 (cardiac effects, renal renin release, and lipolysis) and beta-2 (presynaptic: facilitation of NA-release; postsynaptic: vascular, bronchial, and uterine smooth muscle relaxation, glycogenolysis and possibly part of the A-mediated cardiac effects). During the last 30 years growing evidence has accumulated that dopamine (DA), the third endogenous catecholamine and the immediate precursor of NA, may also cause peripheral effects through stimulation of specific DA-receptors, in addition to its known action at alpha- and beta-ARs. It is now well accepted that at least two different DA-receptors are present in many peripheral tissues (DA1 and DA2), including those of the cardiovascular and autonomic nervous system. They seem to be involved in dilation of certain vascular beds, inhibition of NA-release during nerve stimulation, natriuresis, and aldosterone release. In chronic heart failure cardiac beta-AR function decreases (presumably due to endogenous "down-regulation" by the elevated catecholamines), and this decrease is related to the severity of heart failure (judged clinically by New York Heart Association functional class). The human heart contains both functional beta-1 and beta-2 ARs; cardiac beta-1 and beta-2 ARs seem to be differentially affected by different kinds of heart failure; in end-stage dilated cardiomyopathy beta-1 ARs are selectively reduced, whereas beta-2 ARs are nearly normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Reserpine induces vascular alpha 2-adrenergic supersensitivity and platelet alpha 2-adrenoceptor up-regulation in dog

1. The aim of the present study was to investigate the influence of catecholamine levels on the regulation of alpha 2-adrenoceptor sensitivity in dogs. 2. Blood pressure and heart rate values at rest, plasma catecholamine levels, platelet and adipocyte alpha 2-adrenoceptors as well as the alpha 2-mediated cardiovascular responses to clonidine (10 micrograms kg-1 i.v., after alpha 1-, beta-adrenoceptor plus muscarinic blockade) or noradrenaline (0.5, 1, 2 and 4 micrograms kg-1 i.v. after alpha 1- and beta-adrenoceptor blockade) were measured before and after reserpine treatment (0.1 mg kg-1 day-1 s.c. over 15 days). 3. Reserpine induced a significant decrease in resting systolic and diastolic blood pressures (213 +/- 2/87 +/- 6 mmHg before vs 158 +/- 5/59 +/- 3 mmHg after treatment) as well as in heart rate (91 +/- 2 beats min-1 before vs 76 +/- 3 beats min-1 after treatment). 4. A 5 min tilt test performed under chloralose anesthesia, failed to modify blood pressure before treatment whereas it induced a significant fall in the same animals after the 15 day treatment. Plasma levels of noradrenaline significantly decreased (262 +/- 58 vs 66 +/- 31 pg ml-1) whereas plasma adrenaline levels were unchanged. 5. The alpha 2-mediated pressor responses to noradrenaline were significantly increased after reserpine. Clonidine induced a marked pressor effect (+72 and +45% in systolic and diastolic blood pressures respectively) after reserpine treatment. This effect was suppressed by administration of RX-821002, a new specific alpha 2-adrenoceptor antagonist. 6. Reserpine treatment significantly increased platelet alpha 2-adrenoceptor number (identified with [3H]- yohimbine or [3H]-RX821002) with no change in Kd values. alpha 2-Adrenoceptor number remained unchanged in adipocytes (identified with [3H]-RX821002). 7. These results show that a 15 day treatment with reserpine induces a vascular alpha 2-adrenergic supersensitivity and an up-regulation in platelet alpha 2-adrenoceptors. In contrast, this phenomenon does not involve all the tissues since adipocyte alpha 2-adrenoceptors escape the effect of reserpine. We suggest that the levels of plasma noradrenaline play an important role in the regulation of the platelet and vascular alpha 2-adrenoceptors. In contrast, adipocyte alpha 2-adrenoceptors are not affected by changes in plasma noradrenaline levels.

Effects of the alpha 2-adrenoceptor agonist UK14304 on pressor responses in pithed rats

1. Intravenous infusions of UK14304 (0.3-10 micrograms/kg per min) in pithed rat produced dose-dependent pressor responses which were not affected by prazosin (10 micrograms/kg) but were reduced by yohimbine (0.3 mg/kg). 2. Pressor responses to noradrenaline (0.1 micrograms/kg), phenylephrine (1 micrograms/kg) and vasopressin (10 mU/kg) were enhanced during infusions of UK14304 (0.03-1 micrograms/kg per min). Likewise, pressor responses to spinal sympathetic stimulation were enhanced during infusions of low concentrations of UK14304 (0.03-0.3 microgram/kg per min) but were reduced during infusion of a higher concentration of UK14304 (10 micrograms/kg per min). 3. After administration of yohimbine (0.3 mg/kg) or the calcium channel blocking drug diltiazem (infused at 50 micrograms/kg per min), pressor responses to noradrenaline and UK14304 were reduced, and responses to noradrenaline during infusion of UK14304 were not enhanced. 4. Prazosin (10 micrograms/kg) revealed a secondary depressor component in the response to sympathetic stimulation which is due to beta-adrenoceptor activation, since it was abolished by ICI 118551 (0.3 mg/kg). In the presence of ICI 118551 plus prazosin, pressor responses to sympathetic stimulation were enhanced during infusions of UK14304. 5. The depressor response to nitroprusside and the depressor component of responses to sympathetic stimulation after prazosin were enhanced during infusions of UK14304 at concentrations that increased the blood pressure. 6. The findings show that alpha 2-adrenoceptor activation enhanced the pressor responses to sympathetic nerve stimulation, noradrenaline, phenylephrine and vasopressin in the pithed rat and beta-adrenoceptor activation produced depressor responses which increased with increasing blood pressure.

The effectiveness of alpha 2-adrenoceptor activation increases from the distal to the proximal part of the veins of canine limbs

1. The effectiveness of alpha 1- and alpha 2-adrenoceptor activation was compared at different levels of the saphenous and cephalic vein of the dog in vitro. 2. Helically cut strips were used to determine concentration-response curves to phenylephrine, noradrenaline, UK-14,304 (5-bromo-6-(imidazoline-2-ylamino)-quinoxaline) and B-HT 920 (2-amino-6-allyl-5,6,7,8-tetra-hydro-4H-(thiazo)-4,5-d-azepine). The effect of prazosin and yohimbine on these curves was also studied. 3. At the distal level, the maximum response to UK-14,304 amounted to 33 and 50% of those to noradrenaline in the saphenous and cephalic vein, respectively, while at the proximal level the maximum response to UK-14,304 amounted to 72 and 78% of those to noradrenaline, in the saphenous and cephalic vein, respectively. 4. In both vessels, the results obtained with B-HT 920 were very similar to those for UK-14,304. 5. The pD2 values for UK-14,304 - which were identical at the three levels of both vessels - and the pA2 values for the antagonism exerted by either prazosin or yohimbine against the responses to UK-14,304 indicate that the alpha 2-adrenoceptors are identical at the different levels of both vessels. 6. These results show that the effectiveness of alpha 2-adrenoceptor stimulation increases from the distal to the proximal regions of canine limb veins. Apparently, this is due to a greater density of alpha 2-adrenoceptors in the proximal regions. 7. Yohimbine is much more potent against phenylephrine distally than proximally in both vessels. However, after 30 nm phenoxybenzamine - a concentration which eliminates the vast majority of alpha,-adrenoceptors without affecting alpha 2-adrenoceptors - yohimbine became equally potent at both levels, suggesting that the difference existing before phenoxybenzamine depended on alpha,-adrenoceptors. Hence it is concluded that alpha,-adrenoceptors in distal and proximal portions may differ.

Pharmacological analysis of vasodilator responses to alpha 2-adrenoceptor agonists in isolated rat common carotid arteries

Using the cannula inserting method, vasodilator responses to alpha 2-adrenoceptor agonists (clonidine, guanabenz, DJ7141 and xylazine) were investigated in isolated and perfused rat common carotid arteries. Alpha 2-adrenoceptor agonists dose-dependently induced a vasodilation in preparations preconstricted by noradrenaline. The potencies were in the order of clonidine greater than guanabenz greater than DJ-7141 greater than or equal to xylazine. Removal of the endothelium inhibited ACh-induced vasodilation, but not the alpha 2-agonist-induced dilation. Atropine treatment inhibited ACh-induced vasodilation, but not the alpha 2-agonist-induced dilation. Alpha 2-agonist-induced dilations were not modified by beta-blockade, which significantly suppressed isoprenaline-induced vasodilations. The potent alpha 2-adrenoceptor antagonist DG5128 did not influence the alpha 2-agonist-induced vasodilation. In preparations preconstricted by PGF2 alpha, clonidine and xylazine never induced a vasodilation, and clonidine frequently induced vasoconstrictions that were completely blocked by bunazosin. It is concluded that alpha 2-adrenoceptor agonist-induced vasodilation is independent from the existence of the endothelium, and that it is not related to vascular beta- and alpha 2-adrenoceptors and muscarinic receptors, suggesting that the alpha 2-adrenoceptor agonist-induced vasodilation is due to an antagonistic activity towards the vascular alpha 1-adrenoceptors.