Postsynaptic alpha 1- and alpha 2-adrenoceptors in the circulatory system of the pithed rat: selective stimulation of the alpha 2-type by B-HT 933

Sympathetic Overactivity in CKD Disrupts Buffering of Neurotransmission by Endothelium-Derived Hyperpolarizing Factor and Enhances Vasoconstriction

BACKGROUND

Hypertension commonly complicates CKD. Vascular smooth muscle cells (VSMCs) of resistance arteries receive signals from the sympathetic nervous system that induce an endothelial cell (EC)-dependent anticontractile response that moderates vasoconstriction. However, the specific role of this pathway in the enhanced vasoconstriction in CKD is unknown.

METHODS

A mouse model of CKD hypertension generated with 5/6-nephrectomy (5/6Nx) was used to investigate the hypothesis that an impaired anticontractile mechanism enhances sympathetic vasoconstriction. In vivo, ex vivo (isolated mesenteric resistance arteries), and in vitro (VSMC and EC coculture) models demonstrated neurovascular transmission and its contribution to vascular resistance.

RESULTS

By 4 weeks, 5/6Nx mice (versus sham) had augmented increases in mesenteric vascular resistance and mean arterial pressure with carotid artery occlusion, accompanied by decreased connexin 43 (Cx43) expression at myoendothelial junctions (MEJs), impaired gap junction function, decreased EC-dependent hyperpolarization (EDH), and enhanced contractions. Exposure of VSMCs to NE for 24 hours in a vascular cell coculture decreased MEJ Cx43 expression and MEJ gap junction function. These changes preceded vascular structural changes evident only at week 8. Inhibition of central sympathetic outflow or transfection of Cx43 normalized neurovascular transmission and vasoconstriction in 5/6Nx mice.

CONCLUSIONS

5/6Nx mice have enhanced neurovascular transmission and vasoconstriction from an impaired EDH anticontractile component before vascular structural changes. These neurovascular changes depend on an enhanced sympathetic discharge that impairs the expression of Cx43 in gap junctions at MEJs, thereby interrupting EDH responses that normally moderate vascular tone. Dysregulation of neurovascular transmission may contribute to the development of hypertension in CKD.

Renal sympathetic nerve activity and vascular reactivity to phenylephrine after lipopolysaccharide administration in conscious rats

It has been proposed that sympathoexcitation is responsible for vascular desensitization to α₁-adrenoceptor stimulation during lipopolysaccharide (LPS)-induced systemic inflammation. The present study tested this hypothesis by examining the effects of sympatho-deactivation with the α₂-adrenoceptor agonist, dexmedetomidine, on mean arterial pressure (MAP), renal sympathetic nerve activity (RSNA), and vascular reactivity to phenylephrine in conscious rats with cardiac autonomic blockade (methylatropine and atenolol) following LPS administration. In male, adult Sprague-Dawley rats (n = 5 per group), RSNA and MAP were continuously recorded over 1-h periods, before and after LPS administration (20 mg/kg iv), and finally after infusion of either saline or dexmedetomidine (5 μg/kg, then 5 μg/kg/h iv). A full dose-response curve to phenylephrine was constructed under each condition. After pooling data from both groups of rats (n = 10), LPS significantly (P = 0.005) decreased MAP (from 115 ± 1 to 107 ± 2 mmHg), increased RSNA (to 403 ± 46% of baseline values) and induced 4 to 5-fold increases in the half-maximal effective dose (ED₅₀) of phenylephrine (from 1.02 ± 0.09 to 4.76 ± 0.51 μg/kg). During saline infusion, RSNA progressively decreased while vascular reactivity did not improve. Treatment with dexmedetomidine decreased MAP, returned RSNA to near pre-endotoxemic levels, but only partially restored vascular reactivity to phenylephrine (ED₅₀ was still threefold increased as compared with baseline values). These findings indicate that only part of the decrease in vascular reactivity to α₁-adrenoceptor stimulation during endotoxemia can be accounted for by sympathetic activation, at least on a short-term basis.

New 2-Aminothiazoline derivatives lower blood pressure of spontaneously hypertensive rats (SHR) via I1-imidazoline and alpha-2 adrenergic receptors activation

2-Aminothiazolines share an isosteric relationship with imidazolines and oxazolines with antihypertensive activity mainly mediated by the imidazoline I₁-receptor. In the present work, we have prepared five aminothiazolines, following a previously described synthetic pathway. Aminothiazolines derived from dicyclopropylmethylamine (ATZ1) and cyclohexylamine (3) are unprecedented in the literature. Competitive radioligand assay was carried out with all synthetic compounds, and the I₁ receptor affinity in comparison to rilmenidine in PC12 cells was determined. Surprisingly, the rilmenidine isoster (ATZ1) showed no I₁-receptor interaction. Diethyl (ATZ4) and 2-ethyl-hexylamine (ATZ5) derivatives bind to the receptor with 11.98 and 10.94nmol/l, respectively. These compounds were selected for in vivo experiments. Both compounds reduced the blood pressure of spontaneously hypertensive rats (SHR). The hypotensive effect of these compounds was abrogated in the presence of α₂ adrenergic (yohimbine) and I₁ (efaroxan) receptor antagonists suggesting that both aminothiazolines bind to the adrenergic and imidazoline receptors. Lipinski's descriptors of the synthesized aminothiazolines were calculated and are similar to the known imidazoline I₁ receptor ligands. 3D-Similarity between ATZ5 and agmatine, the natural imidazoline receptor ligand, was also observed.

Pharmacological identification of α1- and α2-adrenoceptor subtypes involved in the vasopressor responses induced by ergotamine in pithed rats

Ergotamine has been used in clinical practice for the acute treatment of migraine for over 90 years. So far, it is known that ergotamine interacts with diverse receptors (including α1/2-adrenoceptors, 5-HT1, 5-HT2 and D2-like receptors) and that produces increases in mean blood pressure which are significantly blocked by yohimbine, a classical α2-adrenoceptor antagonist with a moderate affinity for α1-adrenoceptors. Since α1/2-adrenoceptors mediate vasopressor and vasoconstrictor responses in the cardiovascular system, this study was designed to identify the α-adrenoceptor subtypes (α1A, α1B, α1D, α2A, α2B and α2C) involved in ergotamine-induced vasopressor responses in pithed rats. In male Wistar pithed rats baseline heart rate and blood pressure were recorded. Then, the vasopressor responses to intravenous (i.v.) bolus injections of ergotamine were determined after administration of vehicle or several α1⧸2-adrenoceptor antagonists. I.v. administration of the antagonists prazosin (α1, 0.1-30 µg/kg), rauwolscine (α2, 0.3-300 µg/kg), prazosin (0.1 µg/kg) plus rauwolscine (0.3 µg/kg), 5-methylurapidil (α1A, 100 and 300 µg/kg), L-765,314 (α1B, 100 and 300 µg/kg), BMY 7378 (α1D, 100 and 300 µg/kg), BRL44408 (α2A, 300 and 1000 µg/kg) and JP-1302 (α2C, 300 µg/kg), significantly blocked the vasopressor responses to ergotamine, whereas imiloxan (α2B, 1000 and 3000 µg/kg), JP-1302 (100 µg/kg) or the corresponding vehicles (saline 0.9%, propylene glycol 20% or dimethyl sulfoxide 10%; 1ml/kg) failed to modify the responses to ergotamine. The above results suggest that the vasopressor responses to ergotamine in pithed rats are mainly mediated by α1A-, α1B-, α1D-, α2A- and α2C-adrenoceptors and may explain its adverse/therapeutic effects.

Endothelin modulates the cardiovascular effects of clonidine in the rat

Clonidine decreases mean arterial pressure (MAP) by acting as an α(2)-adrenergic receptor (AR) agonist in the central nervous system; it also acts on peripheral α-ARs to produce vasoconstriction. Endothelin (ET) has been shown to modulate the action of ARs. The present study was conducted to determine the involvement of ET in cardiovascular effects of clonidine. Intravenous administration of clonidine (10, 30 and 90μgkg(-1)) produced a dose-dependent decrease in MAP and heart rate (HR). Treatment with ET-1 (100, 300 and 900ngkg(-1)) significantly attenuated clonidine (10μgkg(-1)) induced fall in MAP and HR. Rats treated with ET-1 (900ngkg(-1)) showed an increase in MAP and HR after clonidine administration compared to untreated rats, while ET(A/B) antagonist, TAK-044 (1mgkg(-1)) and ET(A) antagonist, BMS-182874 (9mgkg(-1)) potentiated the hypotensive effect of clonidine. ET(B) receptor agonist, IRL-1620 (5μgkg(-1)) produced significant attenuation of clonidine induced fall in MAP and HR, while ET(B) receptor antagonist, BQ-788 (0.3mgkg(-1)), potentiated the hypotensive effect of clonidine. Prazosin (0.1mgkg(-1)) completely blocked ET-1 induced changes in cardiovascular effects of clonidine. Clonidine-induced contraction of rat abdominal aortic ring was potentiated by ET-1, which was completely blocked by prazosin. Clonidine produced an increase in ET(A) receptor expression in the brain and abdominal aorta while ET(B) receptors were not affected. It is concluded that ET enhances the responsiveness of vascular ARs to the constrictor effect of clonidine and ET antagonists potentiate the hypotensive effect of clonidine suggesting that a combination of ET antagonist with clonidine may be a useful option to treat hypertension.

Pharmacological analysis of the interaction of antimuscarinic drugs at M(2) and M(3) muscarinic receptors in vivo using the pithed rat assay

Muscarinic receptor antagonists form the mainstay of the therapeutic options for airway, bladder, and gastrointestinal smooth muscle disorders. Both M(2) and M(3) muscarinic receptors are involved in mediating smooth muscle contractility, although the relative functional contribution of each subtype, especially in the disease state, is unclear. Because the potency and selectivity of compounds for a given receptor in an in vivo setting can be dissimilar to that observed in an in vitro system, we developed an in vivo assay to simultaneously determine the absolute potency and selectivity of muscarinic receptor antagonists at M(2) and M(3) receptors using the pithed rat. Methacholine (MCh)-induced bradycardia and depressor responses were used as surrogate functional endpoints for M(2) and M(3) receptor activation, respectively. The influence of the muscarinic antagonists, tolterodine, oxybutynin, darifenacin, Ro 320-6206, solifenacin, or tiotropium on the MCh-induced responses were studied. The estimated DR(10) values (dose producing a tenfold shift in the MCh curve) of tolterodine, oxybutynin, darifenacin, Ro 320-6206, solifenacin, and tiotropium for the M(2) muscarinic receptor-mediated bradycardia were 0.22, 1.18, approximately 2.6, 0.025, 0.40, and 0.0026 mg/kg, respectively, and 0.14, 0.18, 0.11, 3.0, 0.18, and 0.0017 mg/kg, respectively, for the M(3) muscarinic receptor-mediated depressor response. In a separate set of experiments, a single intravenous dose of tiotropium was administered before a MCh curve at 1, 3, 6, or 9 h to determine if tiotropium exhibited time-dependent selectivity for the M(3) receptor as has been reported from in vitro studies. The results indicate a slight preference of tiotropium for the M(3) receptor at later time points. The pithed rat assay may serve useful for elucidating the functional contribution of M(2) and M(3) receptors to the in vivo pharmacological effects of antagonists in disease animal models.

Vasopressors for cardiopulmonary resuscitation. Does pharmacological evidence support clinical practice?

Adrenaline (epinephrine) has been used for cardiopulmonary resuscitation (CPR) since 1896. The rationale behind its use is thought to be its alpha-adrenoceptor-mediated peripheral vasoconstriction, causing residual blood flow to be diverted to coronary and cerebral circulations. This protects these tissues from ischaemic damage and increases the likelihood of restoration of spontaneous circulation. Clinical trials have not demonstrated any benefit of adrenaline over placebo as an agent for resuscitation. Adrenaline has deleterious effects in the setting of resuscitation, predictable from its promiscuous pharmacological profile. This article discusses the relevant pharmacology of adrenaline in the context of CPR. Experimental and clinical evidences for the use of adrenaline and alternative vasopressor agents in resuscitation are given, and the properties of an ideal vasopressor are discussed.

Pharmacological evidence that alpha2A- and alpha2C-adrenoceptors mediate the inhibition of cardioaccelerator sympathetic outflow in pithed rats

It has been suggested that the alpha(2)-adrenoceptors mediating cardiac sympatho-inhibition in pithed rats closely resemble the pharmacological profile of the alpha(2A)-adrenoceptor subtype. However, several lines of evidence suggest that more than one subtype may be involved. Thus, the present study has pharmacologically re-evaluated the receptor subtype(s) involved in the inhibitory effect of the alpha(2)-adrenoceptor agonist, B-HT 933, on the tachycardic responses elicited by selective cardiac sympathetic stimulation (0.03, 0.1, 0.3, 1 and 3 Hz) in desipramine-pretreated pithed rats. I.v. continuous infusions of B-HT 933 (30 microg/kg min), which failed to modify the tachycardic responses to exogenous noradrenaline, inhibited those induced by preganglionic (C(7)-T(1)) stimulation of the cardiac sympathetic outflow at all frequencies of stimulation (0.03-3 Hz). This cardiac sympatho-inhibitory response to B-HT 933 was: (1) unaltered by saline (1 ml/kg) or the antagonists BRL44408 (100 microg/kg; alpha(2A)) or imiloxan (3000 and 10,000 microg/kg; alpha(2B)); (2) partially antagonized by BRL44408 (300 microg/kg) or MK912 (10 microg/kg; alpha(2C)) given separately; and (3) completely antagonized by rauwolscine (300 microg/kg; alpha(2)), MK912 (30 microg/kg) or the combination of BRL44408 (300 microg/kg) plus MK912 (10 microg/kg). Moreover, the above doses of antagonists, which are high enough to block their respective receptors, failed to block per se the tachycardic responses to sympathetic stimulation. These results suggest that the cardiac sympatho-inhibition induced by B-HT 933 in pithed rats is mainly mediated by stimulation of alpha(2A)- and alpha(2C)-adrenoceptors.

Pertussis toxin-induced histamine sensitisation: an aspecific phenomenon independent from the nitric oxide system?

Mechanisms were studied initially to develop an in vitro safety test for detecting pertussis toxin toxicity in acellular pertussis vaccines based on the histamine sensitisation test. Maximal contractions and sensitivities to different agonists and adrenoceptor-induced contractions in Ca2+-free medium of isolated rat small mesenteric resistance arteries were significantly reduced by in vivo [30 microg/kg, intravenously (i.v.), day 5] or in vitro (10 microg/ml, 2 h) pertussis toxin pretreatment. Pertussis toxin-induced decrease in sensitivity of small mesenteric resistance arteries to noradrenaline was endothelium-dependent. Nomega-nitro-L-arginine methyl ester (L-NAME) (100 microM, 20 min) did not reestablish the sensitivity to noradrenaline. In vivo L-NAME treatment (0, 1, 10 or 30 mg/kg) of pertussis toxin-pretreated (15 microg/kg) rats did not reduce pertussis toxin-induced enhancement of the histamine-induced decrease in blood pressure and histamine (10, 30, 100 or 300 mg/kg) induced mortality. Finally, in vivo pertussis toxin pretreatment sensitises rats for sodium nitroprusside (50 microg/kg/min). We conclude that pertussis toxin-induced histamine sensitisation is caused by an interference of pertussis toxin with the contractile mechanisms of vascular smooth muscle of resistance arteries which indicates only an indirect role for histamine in the histamine sensitisation test.

Existence of alpha-adrenoceptor subtypes in isolated human gastroepiploic and omental arteries

Establishing the existence of alpha-adrenoceptor subtypes in isolated human gastroepiploic and omental arteries was the goal of the present study. Functional vascular reactivity of selective alpha(1)- and alpha(2)-adrenoceptor agonists and antagonists was studied, using a cannula inserting technique. Intraluminal administration of norepinephrine (NE), phenylephrine (PE) or BHT-933 caused a vasoconstrictive response in a dose-related manner. The relative potencies of the 3 agonists were almost the same in both arteries. NE-induced vasoconstrictions were significantly antagonized by either prazosin or rauwolscine. PE-induced responses were strongly inhibited by prazosin. BHT-933-induced constrictions were inhibited by rauwolscine. These results indicate that both alpha(1)- and alpha(2)-adrenoceptors exist in the human gastroepiploic and omental arteries.

Hemodynamic and cardiac anti-hypertrophic actions of clonidine in Goldblatt one-kidney, one-clip rats

In congestive heart failure, the chronic sympathetic hyperactivity contributes to a poor prognosis. In this respect, clonidine, a centrally acting sympathoinhibitory drug, has previously been tested in clinical trials. The aim of the current study was to evaluate the effects of clonidine on morbidity and mortality in an experimental model of cardiac hypertrophy associated with hypertension, renal failure, and intense sympathetic activation. One-kidney, one-clip Goldblatt hypertensive rats were treated orally with clonidine (200 microg/kg/d) during 30 days and were compared with untreated rats and with sham-operated animals. Cardiac hemodynamics, left ventricular volume and elasticity, cardiac morphometry and histology, and renal function were evaluated. A survival study was also performed. Clonidine normalized cardiac function, ventricular stiffness, and prevented ventricular structural remodeling. Moreover, despite a marked renal function impairment, survival of the animals was increased in the clonidine group. The centrally acting sympathoinhibitory drug clonidine exhibited marked cardioprotective properties. This study emphasized the interest of evaluating drugs whose aim is to treat congestive heart failure, in an experimental model of cardiac and renal failure.

Alpha-vascular responses after short-term and long-term inhibition of nitric oxide synthesis

Apart from the direct actions of nitric oxide (NO) on vascular smooth muscle, this factor may regulate cardiovascular functions through specific actions on alpha-adrenergic constrictor mechanisms. In this study we aim to establish whether the inhibition of the synthesis of this mediator could alter the vasoconstrictor responses mediated by alpha-adrenoceptor stimulation. We have been able to demonstrate that the blockage of the NO synthase really does exist, when both short- and long-term treatments with Nomega-nitro-Larginine methyl ester (L-NAME) are carried out. We have evaluated the concentration-dependent contractions induced by the selective alpha1-adrenoceptor agonists methoxamine and phenylephrine in isolated rat aorta rings in the following groups of animals: control, short-term L-NAME-treated (100 mg/kg i.p. 20 min before subjecting the animals to the experiments) and long-term L-NAME-treated (100 mg/kg per day in the drinking water for 7, 21, or 45 days). We have also evaluated the pressor responses to methoxamine and to the selective alpha-adrenoceptor agonist B-HT 920 using the pithed rat preparation in the same groups of animals. The contractile responses to methoxamine and phenylephrine were similar in the rat aorta preparations from control and short-term L-NAME-treated animals. On the contrary, in the rat aorta preparations from long-term L-NAME-treated animals these responses were clearly reduced when compared with the corresponding responses in those from control animals, the reduction being more marked when the treatment lasted longer. The pressor responses to methoxamine were also similar in control and short-term L-NAME-treated pithed rats. Nevertheless, the responses to B-HT 920 were greater in the latter. On the other hand, the dose-response curves to both alpha-adrenoceptor agonists were shifted to the right in a non-parallel manner in rats treated long term with L-NAME, the shift being, in the case of B-HT 920, more accentuated when the treatment lasted 21 or 45 days than when it lasted only 7 days. These results indicate that the short-term decrease in NO synthesis does not modify the vascular smooth muscle responses mediated by alpha1-adrenoceptor stimulation, but it does induce a potentiation of sympathetic vasoconstriction mediated by alpha2-adrenoceptors. Nevertheless, the long-term inhibition of NO synthesis causes a compensating decrease in the alpha1- and alpha2-vascular smooth muscle contractile responses.

Cardiovascular effects of clonidine-like drugs in pithed rabbits

Administration (3 to 100 microg/kg IV) of clonidine, rilmenidine, and an imidazoline derivative, 2-(2-chlorophenylamino)imidazoline, in pithed nonstimulated rabbits caused a dose-dependent increase in mean arterial pressure without affecting heart rate. Prazosin (0.1 mg/kg IV) almost abolished the pressor responses to 2-(2-chlorophenylamino)imidazoline, partially inhibited those induced by clonidine, but failed to affect those elicited by rilmenidine. In contrast, yohimbine (1 mg/kg IV) blunted the pressor responses of the 3 drugs. In sympathetically stimulated pithed rabbits, 2-(2-chlorophenylamino)imidazoline induced only pressor effects, whereas clonidine and rilmenidine caused a transient pressure increase followed by a dose-dependent depressor effect. Yohimbine abolished the depressor effect of both drugs, which may have involved presynaptic alpha(2)-adrenoceptors. In conclusion, peripheral effects of 2-(2-chlorophenylamino)imidazoline and clonidine involved at least alpha(1)- and alpha(2)-adrenoceptor activation, whereas pressor and depressor effects of rilmenidine were mediated by alpha(2)-adrenoceptors.

Characterization of postjunctional alpha-adrenoceptors in the pithed mouse

The adrenoceptor subtypes responsible for the pressor response to alpha1- and alpha2-adrenoceptor agonists have not yet been established, although gene knockout experiments in the mouse have provided evidence for a role of the alpha1B- and alpha2B-adrenoceptor. We have evaluated the blood pressure response to selective activation of postjunctional alpha1- and alpha2-adrenoceptors in the pithed mouse. The pressor response to phenylephrine was sensitive to blockade by terazosin, a selective alpha1-adrenoceptor antagonist, but insensitive to rauwolscine, an antagonist at alpha2-adrenoceptors. Phentolamine, a nonselective alpha-adrenoceptor antagonist, blocked the response to either phenylephrine or the selective alpha2-adrenoceptor agonist B-HT 933, whereas rauwolscine blocked only B-HT 933. A dose of terazosin effective against phenylephrine had no effect on B-HT 933; however, the B-HT 933 response was antagonized when the terazosin dose was increased tenfold. A high dose of doxazosin, an alpha1-adrenoceptor antagonist having no affinity for the alpha2B adrenoceptor, blocked the response to phenylephrine but not B-HT 933. Comparison of the potencies of these antagonists against the pressor response to phenylephrine with their affinities for recombinant alpha1-adrenoceptor subtypes suggests that this response is mediated by either alpha1B- or alpha1D-adrenoceptors. The alpha2B-adrenoceptor subtype is likely to take part in the response to B-HT 933. The ability of certain quinazoline alpha1-adrenoceptor antagonists to block the alpha2B adrenoceptor may contribute to their activity as antihypertensive agents.

Differential regulation of mesenteric and femoral blood flow in the rat as revealed by computerized data acquisition and evaluation

1. A set-up for computerized acquisition and evaluation of haemodynamic data was constructed. Blood flow (BF) in the superior mesenteric and femoral artery of urethane-anaesthetized rats was measured with the ultrasonic transit time shift technique. The signals for arterial blood pressure and BF were fed into a personal computer via an analogue-digital converter. Mean arterial blood pressure, heart rate and vascular conductance (CV) were calculated on-line. For subsequent analysis of the data, algorithms were programmed to filter the data, and to determine average and peak values for each parameter. 2. Systemic hypertension induced by phenylephrine (3-300 nmol kg-1), angiotensin II (0.1-3.0 nmol kg-1) and arginine vasopressin (0.03-1.0 nmol kg-1) was accompanied by constriction of the mesenteric artery. In contrast, the femoral artery responded to phenylephrine with constriction, to angiotensin II with dilatation and to arginine vasopressin with dilation followed by constriction. The haemodynamic effects of endothelin-1 (0.03-3.0 nmol kg-1) were generally biphasic, the initial hypotension being associated with dilatation, and the delayed hypertension being accompanied by constriction of both the mesenteric and femoral arterial bed. 3. Terbutaline (3-1.0 nmol kg-1) and calcitonin gene-related peptide (0.03-1 nmol kg-1) caused systemic hypotension along with mesenteric and femoral vasodilatation. 4. Telmisartan (1 mg kg-1), an angiotensin AT1 receptor antagonist, dilated the mesenteric artery, but had no effect on femoral VC. In contrast, the alpha 1-adrenoceptor antagonist prazosin (0.1 mg kg-1), dilated the femoral artery without altering mesenteric VC. Similarly, the beta-adrenoceptor antagonist propranolol (1 mg kg-1) had no effect on mesenteric VC, but constricted the femoral arterial bed. 5. These data demonstrate that the haemodynamic effects of exogenously administered drugs can widely differ between the mesenteric and femoral arterial beds of urethane-anaesthetized rats. Furthermore, vascular tone of these two arterial beds in maintained by different vasoconstrictor systems. While the femoral artery is mainly under adrenergic control, the renin-angiotensin axis is predominant in the mesenteric arterial bed. In addition, this study also demonstrates that computerized analysis enables quick and accurate estimation of haemodynamic drug effects, and is superior to 'by hand' evaluation of peak changes in the functional diameter of the vascular bed under study.

Postjunctional alpha 2-adrenoceptors are not present in proximal arterioles of rat intestine

The purpose of this study was to evaluate two potential stimuli for nitric oxide (NO) release in rat intestinal arterioles during sympathetic nerve activation. To determine whether these vessels contain endothelial alpha 2-adrenoceptors linked to the L-arginine-NO pathway, intravital microscopy was used to study the response of first-order arterioles (1As, 20-40 microns ID) to direct application of 1) the selective alpha 2-agonist BHT-933 and 2) norepinephrine (NE) or sympathetic nerve stimulation before and after alpha 1- or alpha 2-receptor blockade. The effect of sympathetic nerve stimulation on 1A wall shear rate (WSR) was also determined to evaluate the possibility of hemodynamic shear stress as a stimulus for NO release. BHT-933 had no effect on 1A diameter, whereas NE produced dose-dependent constrictions of 5 +/- 3 to 15 +/- 3 microns, which were usually abolished by the alpha 1-antagonist prazosin but unaffected by the alpha 2-antagonist idazoxan. Sympathetic nerve stimulation at 3, 8, and 16 Hz induced constrictions of 4 +/- 1, 8 +/- 2, and 17 +/- 4 microns, respectively, and these constrictions were also usually abolished by prazosin but unaffected by idazoxan. Resting WSR averaged 1,997 +/- 163 s-1 and decreased to 1,587 +/- 209, 1,087 +/- 195, and 537 +/- 99 s-1 during 3-, 8-, and 16-Hz nerve stimulation. These results suggest that alpha 2-adrenoceptor-dependent pathways do not influence either resting tone or sympathetic constriction of proximal arterioles in the intestinal submucosa and that luminal shear stress in these vessels significantly decreases with sympathetic constriction. It therefore appears unlikely that either alpha 2-receptor activation or changes in hemodynamic shear serve as stimuli for arteriolar NO release during periods of increased sympathetic nerve activity.

Alpha-adrenoceptors and vascular regulation: molecular, pharmacologic and clinical correlates

This manuscript is intended to provide a comprehensive review of the alpha-adrenoceptors (ARs) and their role in vascular regulation. The historical development of the concept of receptors and the division of the alpha-ARs into alpha 1 and alpha 2 subtypes is traced. Emphasis will be placed on current understanding of the specific contribution of discrete alpha 1- and alpha 2-AR subtypes in the regulation of the vasculature, selective agonists and antagonists for these receptors, the second messengers utilized by these receptors, the myoplasmic calcium pathways activated to initiate smooth muscle contraction, as well as the clinical uses of agonists and antagonists that work at these receptors. New information is presented that deals with the molecular aspects of ligand interactions with specific subdomains of these receptors, as well as mRNA distribution and the regulation of alpha 1- and alpha 2-AR gene transcription and translation.

Dilatation by angiotensin II of the rat femoral arterial bed in vivo via pressure/flow-induced release of nitric oxide and prostaglandins

1. The haemodynamic effects of angiotensin II (AII) and, for comparison, arginine vasopressin (AVP) in the femoral and superior mesenteric artery of urethane-anaesthetized rats were analysed with the ultrasonic transit time shift technique. 2. I.v. bolus injection of AII (0.1-3 nmol kg-1) and AVP (0.03-1 nmol kg-1) increased blood pressure which was accompanied by a decrease in blood flow through the superior mesenteric artery and an increase in femoral blood flow. The femoral hyperaemia was in part due to vasodilatation as indicated by a rise of femoral vascular conductance up to 200% relative to baseline. The femoral vasodilatation caused by AVP, but not AII, was followed by vasoconstriction. 3. Blockade of angiotensin AT1 receptors by telmisartan (0.2-20 mumol kg-1) prevented all haemodynamic responses to AII. 4. The femoral dilator responses to AII and AVP depended on the increase in vascular perfusion pressure since vasodilatation was reversed to vasoconstriction when blood pressure was maintained constant by means of a gravity reservoir. However, the AII-evoked femoral vasodilatation was not due to an autonomic or neuroendocrine reflex because it was not depressed by hexamethonium (75 mumol kg-1), prazosin (0.25 mumol kg-1) or propranolol (3 mumol kg-1). 5. The AII-induced femoral vasodilatation was suppressed by blockade of nitric oxide (NO) synthesis with NG-nitro-L-arginine methyl ester (L-NAME, 40 mumol kg-1) and reversed to vasoconstriction when L-NAME was combined with indomethacin (30 mumol kg-1), but was left unaltered by antagonism of endothelin ETA/B receptors with bosentan (37 mumol kg-1). 6. These results demonstrate that the effect of AII to increase systemic blood pressure and the resulting rise of perfusion pressure in the femoral artery stimulates the formation of NO and prostaglandins and thereby dilates the femoral arterial bed. This local vasodilator mechanism is sufficient to mask the direct vasoconstrictor response to AII.

Characterization of alpha-adrenoceptor subtype(s) mediating vasoconstriction in the perfused rabbit ovarian vascular bed

1. alpha 1-Adrenoceptor agonists, noradrenaline, phenylephrine, methoxamine, oxymetazoline and SDZ NVI 085 but not alpha 2-adrenoceptor agonists, UK 14304, tizanidine or clonidine evoked dose-dependent vasoconstriction of the isolated perfused rabbit ovarian vascular bed. The rank order of agonist potency was noradenaline > oxymetazoline > phenylephrine > SDZ NVI 085 > methoxamine. 2. Prazosin (10(-8) M - 10(-5) M) displaced agonist dose-response curves to the right. The pA2/pKB values ranged between 7.27 and 7.66 against noradrenaline, phenylephrine, methoxamine and SDZ NVI 085 and were not significantly different from each other. Prazosin was however significantly less potent against oxymetazoline (pA2 6.38). Yohimbine (10(-6) M - 10(-5) M) was not very effective against any of the agonists. 3. WB 4101 (10(-8) M - 10(-5) M) displaced agonist dose-response curves to the right. The pA2/ pKB values ranged between 7.08 and 7.93 against noradrenaline, phenylephrine, methoxamine and SDZ NVI 085. WB 4101 was significantly less potent against oxymetazoline (pKB 6.85). 4. SZL-49 (5 x 10(-6) M) but not chloroethylclonidine (3 x 10(-5) M) significantly reduced vasoconstrictor responses to all the agonists. 5. Electrical field stimulation of the ovarian bed produced frequency-dependent vasoconstrictor effects which were abolished by 6-OHDA. The responses were also antagonized in a concentration-dependent by prazosin (10(-7) M - 10(-5) M) and WB 4101 (3 x 10(-8) M - 3 x 10(-7) M). Yohimbine reduced the response to electrical stimulation by 20% at 10(-5) M. The vasoconstrictor effect was also inhibited by SZL-49 but not by chloroethylclonidine. 6. These results would suggest that the vasoconstrictor responses of the ovarian vascular bed to adrenergic agonists and to electrical stimulation are mediated via the alpha 1A-adrenoceptor subtype.

Intrathecal clonidine and tizanidine in conscious dogs: comparison of analgesic and hemodynamic effects

Intrathecal delivery of alpha(2)-adrenergic agonists produces an analgesic effect. However, hemodynamic side effects limit their clinical usage. To more fully characterize the effects on heart rate and arterial blood pressure of alpha(2)-adrenergic agonists, clonidine and tizanidine were injected intrathecally in conscious dogs. Both compounds produced a potent inhibition of thermal foot-withdrawal latencies at 1000 micrograms, which was blocked by the alpha(2)-adrenergic antagonist yohimbine. Tizanidine (250-500 micrograms) did not change heart rate. Clonidine (500 -2000 micrograms) and tizanidine (1000-2000 micrograms) decreased heart rate. The tizanidine effect was inhibited by yohimbine and the alpha(2)/imidazoline antagonist idazoxan, as well as the parasympathetic blocker glycopyrrolate. No drug completely inhibited the clonidine-induced bradycardia. Clonidine had a biphasic effect on arterial blood pressure, a decrease at 500 micrograms and an increase at 2000 micrograms. Tizanidine decreased arterial blood pressure at all doses. The results indicate that, while the analgesic effects of both drugs are similar, the hemodynamic responses differ. While the decrease in heart rate with tizanidine is consistent with alpha(2)-adrenergic binding and vagal action, the bradycardia induced by clonidine is more complex. In addition, the increased arterial blood pressure with high doses of clonidine, which is suggestive of a peripheral vasoconstrictive effect, does not occur with tizanidine.

Imidazoline receptor agonist drugs: a new approach to the treatment of systemic hypertension

The imidazoline receptors have recently been discovered to be involved in central nervous system control of blood pressure (I-1 receptor) and in neuroprotection for cerebral ischemia (I-2 receptor). A new class of central-acting antihypertensive agents has been developed, the imidazoline receptor agonists (rilmenidine and moxonidine), which control blood pressure effectively without the adverse effects of sedation and mental depression that are usually associated with central-acting antihypertensives. This new generation of central-acting antihypertensive agents are highly selective for the imidazoline receptor, while having a low affinity for alpha 2-adrenergic receptors.

Studies on the protective effect of azepexole on ouabain-induced cardiac arrhythmias and lethality in guinea-pig

Azepexole, an alpha 2-adrenoceptor agonist (125, 250 and 500 micrograms/kg i.v.), was examined for its effect on ouabain-induced ventricular premature beats, ventricular tachyarrhythmias and lethality in guinea-pigs. The doses of ouabain required to cause ventricular arrhythmias and lethality were significantly higher in azepexole-treated animals. However, it did not offer any protection in reserpinised guinea-pigs. Idazoxan, the alpha 2-adrenoceptor antagonist (100 micrograms/kg i.v.) inhibited the protective action of azepexole while corynanthine, the alpha 1-adrenoceptor antagonist (1 mg/kg i.v.), potentiated the effect. Azepexole inhibited the rate of the ouabain-induced rise in mean arterial blood pressure and the peak pressor response. In isolated paced left atria of guinea-pig, azepexole (2.76 x 10(-3) M) did not offer any protection against extrasystolic contractions induced by ouabain. Therefore the protective effect of azepexole may be mediated through the stimulation of alpha 2-adrenoceptors and the resultant suppression of the indirect neural components of ouabain toxicity.

Mediation of the hypotensive action of systemic clonidine in the rat by alpha 2-adrenoceptors

1. During the past few years it has been shown that the sympatholytic effect resulting from localized microinjection of clonidine and other imidazolines into the rostral ventrolateral medulla (RVL) results from activation of 'imidazoline' receptors (I1 receptors) rather than from an alpha 2-adrenoceptor-mediated effect. 2. The relative contributions of these two receptor systems to the hypotensive action of systemically administered clonidine have not been studied. Clonidine has affinity for both I1 and alpha 2-adrenoceptors; guanabenz represents a useful pharmacological tool, since it activates only the alpha 2-adrenoceptor. 3. Antagonists acting at both I1 and alpha 2-adrenoceptors (idazoxan) and at only alpha 2-adrenoceptors (SK&F 86466; 6-chloro-3-methyl-2,3,4,5-tetrahydro-3-benzazepine) are available. Idazoxan (1 mg kg-1, i.v.) and SK&F 86466 (3 mg kg-1, i.v.) produced an equivalent degree of blockade of the pressor response to guanabenz or clonidine in the pithed rat, a response mediated by the alpha 2-adrenoceptor. 4. Guanabenz (30 micrograms kg-1, i.v.) and clonidine (10 micrograms kg-1, i.v.) lowered blood pressure in the chloralose-anaesthetized spontaneously hypertensive rat by 48 +/- 4.6 mmHg and 44 +/- 5.4 mmHg, respectively; this response, for either agonist, was blocked by both idazoxan and SK&F 86466. 5. These data show that the hypotensive effect of intravenously administered clonidine results from activation of central alpha 2-adrenoceptors, with no significant contribution from an I1-mediated effect. Thus clonidine can lower blood pressure by different receptor mechanisms, dependent on the route of administration.

Endothelin antagonizes the hypotension and potentiates the hypertension induced by clonidine

Modification of clonidine-induced cardiovascular effects by endothelin-1 (ET-1) was studied in male Sprague-Dawley rats. A dose-dependent decrease in blood pressure and heart rate was produced by clonidine (100, 250 and 500 micrograms/kg i.v.). Lower doses produced only a fall in blood pressure (through central alpha-adrenoceptors) while higher doses of clonidine produced an initial hypertensive response (through peripheral alpha-adrenoceptors) and subsequent longer lasting hypotension and bradycardia. The hypotension and bradycardia induced by 100 and 250 micrograms/kg i.v. dose of clonidine were completely blocked by ET-1 (100 ng/kg i.v.) pretreatment. Conversely, the hypertensive response induced by high dose of clonidine (500 micrograms/kg i.v.) was significantly potentiated by ET-1 pretreatment. In cervical sectioned rats, i.v. administered clonidine failed to produce any hypotensive effect, indicating lack of central effect of clonidine. ET-1 significantly (P < 0.0005) potentiated the hypertensive response of a low dose (50 micrograms/kg i.v.) of clonidine in cervical-sectioned rats. I.c.v. administration of clonidine (1, 2, 4 and 6 micrograms) produced a dose-dependent decrease in blood pressure and heart rate. ET-1 pretreatment (25 ng i.c.v.) transiently blocked the clonidine-induced decrease in blood pressure and heart rate for about 10 min but the hypotension and bradycardia was observed subsequently. Since the major site of action of clonidine is the ventral surface of medulla, clonidine was applied directly to the ventral surface of medulla and produced a decrease in blood pressure and heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of sympathetic nerves exerts an inhibitory influence on afferent nerve-induced vasodilation unrelated to vasoconstriction in rat dental pulp

In order to elucidate a possible influence of the sympathetic nervous system on afferent nerve function, rat mandibular incisors were electrically stimulated and blood flow changes monitored in the incisor pulp of untreated and sympathectomized animals by a laser Doppler flowmeter. Monopolar electrical stimulation of the tooth (200 microA, 5 ms, 40 Hz, 1 s) in normal animals resulted in a transient reduction in pulpal blood flow (PBF) (16% reduction, n = 10) followed by a small but long-lasting increase (11% increase). After administration of phenoxybenzamine or phentolamine (3 mg kg-1, i.v.) the initial dip in PBF was reduced by 59% (P < 0.001) while the subsequent increase was enhanced by 185% (P < 0.001). Similarly, infusion of prazosin (50 micrograms kg-1, i.v.) and idazoxan (0.5 mg kg-1, i.v.) significantly enhanced the increase in PBF by 118 and by 79%, respectively. In chronically sympathectomized animals the increase in PBF was 250% larger than that seen in untreated animals (P < 0.001). This increase in PBF was not further enhanced after alpha-adrenergic blockade. Acute resection of the superior cervical sympathetic ganglion, also resulted in some enhancement (by 56%) of the stimulation-induced increase in PBF (P < 0.01, n = 6). The increase in PBF was unaffected by infusion of timolol (150 micrograms kg-1) and atropine (1 mg kg-1) but was totally abolished by intravenous pre-treatment with capsaicin (1-3 mg kg-1). The present results suggest that activation of sympathetic nerves exerts inhibitory effects on the afferent nerve-induced vasodilation in the rat incisor pulp unrelated to sympathetic vasoconstriction.

Evaluation of a rat model for the study of local regulation of intestinal blood flow: ex vivo asanguineous perfusion of the ileal vascular bed

A new model of ex vivo vascularly perfused, isolated rat ileum was developed and evaluated. Segments of distal ileum (approximately 5 cm) from male Wistar rats were isolated on their vascular pedicles. Perfusion through an aortic cannula with oxygenated (95% O2, 5% CO2) Krebs solution containing 5% bovine albumin, 5.6 mM glucose, and 25 mM mannitol at 37 degrees C was initiated immediately after interruption of blood flow. The bowel preparations, including the abdominal aorta, were then transferred to a perfusion chamber. Perfusion pressure was maintained by gravity at 40 mm Hg. Flow was measured with an electromagnetic flow probe. The portal vein, together with the lymphatics, drained freely into collection tubes. The bowel lumen was perfused at 0.85 ml/min with isotonic modified Krebs solution containing [14C]polyethylene glycol, and the luminal perfusion pressure was monitored. Luminal effluents were collected through a large-bore outlet tubing. As determined by histology, O2 consumption, vascular reactivity, and mucosal permeability, the preparations were viable for at least 60 min of perfusion. With this model, a vasoconstrictor effect of the alpha 2-adrenoceptor agonist clonidine was documented for the first time in isolated rat bowel.

Alpha 2-adrenoceptor-mediated contractions in the rabbit aorta treated with BAY K 8644

We tested the effect of the selective alpha 2-adrenoceptor agonists B-HT 920 and B-HT 933 on the rabbit aorta. These drugs had weak contractile effects in the tissue, which were inhibited by the selective alpha 1-adrenoceptor antagonist prazosin (10(-5) M). Their contractile effects were potentiated by the Ca2+ channel facilitator BAY K 8644 (10(-6) M). The selective alpha 2-adrenoceptor antagonist yohimbine (10(-5) M) reduced the contractions elicited by B-HT 920 and B-HT 933 in the presence of BAY K 8644 (10(-6) M), but did not alter the control effect of these drugs. In the rabbit aorta, the contractile effect of B-HT 920 and B-HT 933 in the presence of BAY K 8644 (10(-6) M) was partly caused by alpha 1-adrenoceptor stimulation, because prazosin (10(-5) M) relaxed the contractions elicited under these conditions. In the aorta preincubated with BAY K 8644 (10(-6) M) and prazosin (10(-5) M), B-HT 920 (3 x 10(-4) M) elicited non-sustained phasic contractions (1-5 g), which were probably due to alpha 2-adrenoceptor stimulation, as they were inhibited by yohimbine (10(-5) M). In similar experiments B-HT 933 (3 x 10(-4) M) caused inconsistent and slight contractions (< 0.5 g developed tension).

Influence of anaesthesia on the cardiovascular effects of rilmenidine and clonidine in spontaneously hypertensive rats

1. The acute cardiovascular effects of two alpha 2-adrenoceptor agonists, rilmenidine and clonidine, were studied in 15-week-old male spontaneously hypertensive rats (SHRs). The effects of these drugs were compared with intravenous (i.v.) and intracerebroventricular (i.c.v.) administration in conscious and pentobarbitone-anaesthetized SHRs, in which aortic blood pressure (BP) was continuously recorded. 2. In conscious SHRs, i.v. doses of either rilmenidine (30, 100, 300 micrograms kg-1) or clonidine (3, 10, 30 micrograms kg-1) induced dose-dependent short-lasting increases in BP followed by moderate decreases associated with bradycardia, while the same three doses of both drugs given i.c.v. were devoid of BP and heart rate (HR) effects. 3. Pentobarbitone-anaesthesia increased the sympathetic control of BP and suppressed the cardiac baroreflex sensitivity. 4. In anaesthetized SHRs, i.v. injections of the same 3 doses of rilmenidine and clonidine induced a slight increase in BP, rapidly followed by profound and long-lasting BP and HR decreases. Surprisingly, when given i.c.v., these 3 doses lowered BP and HR to the same extent but in a more progressive manner. 5. The lack of efficacy of both drugs in conscious SHRs after the i.c.v. administration of i.v. active doses and the lack of more marked and rapid effects in anaesthetized SHRs, after i.c.v. than after i.v. injections, question the involvement of a major central site of action for these antihypertensive alpha 2-adrenoceptor agonists. Moreover, these results show that the cardiovascular effects of these drugs are profoundly influenced by baseline sympathetic nervous system activity which is enhanced by pentobarbitone-anaesthesia.

Neurally evoked responses of human isolated resistance arteries are mediated by both alpha 1- and alpha 2-adrenoceptors

1. Human subcutaneous resistance arteries (internal diameter 113-626 microns) were mounted in an isometric myograph. Electrical field stimulation was applied either continuously in the form of a frequency-response curve or intermittently at 16 Hz. The magnitude of the maximum contraction induced by continuous stimulation expressed as a percentage of the response to a supramaximal concentration of noradrenaline (10 microM) was highly variable but unrelated to vessel calibre. Contractile responses to both continuous and intermittent stimulation were abolished by 1 microM tetrodotoxin. 2. Prazosin (100 nM and 1 microM, alpha 1-adrenoceptor antagonist) inhibited responses to continuous stimulation over a range of frequencies (2-8 Hz). The response to continuous stimulation at 8 Hz was inhibited by 78 +/- 6% by 1 microM prazosin. Rauwolscine (100 nM, alpha 2-adrenoceptor antagonist) had a smaller effect on contractions induced by continuous stimulation. Rauwolscine inhibited the response at 8 Hz by 36 +/- 6%. Rauwolscine at a higher concentration (1 microM) caused further inhibition of the response to continuous stimulation. Prazosin and rauwolscine in combination almost completely inhibited the response to continuous stimulation at concentrations of 1 microM. 3. Prazosin and rauwolscine inhibited responses to intermittent stimulation in a concentration-dependent manner. The IC50 for this action of prazosin was 3.7 +/- 1.6 nM and the maximum inhibition induced by 100 nM prazosin was 78 +/- 6%. The IC50 of rauwolscine was 12.0 +/- 1.3 nM and 100 nM rauwolscine caused a 86 +/- 7% reduction in the response to intermittent stimulation.Prazosin and rauwolscine in combination (each at 100 nM) caused marked inhibition of the response to intermittent stimulation leaving only 7.0 +/- 2.6% of the response.4. These data suggest that neurally released noradrenaline evokes contractions of human resistance arteries by activation of both alpha 1,- and alpha 2-adrenoceptors postjunctionally.

The effects of clonidine and three 2-imidazoline derivatives on the secretion of protein and some electrolytes by rat submandibular and parotid glands

1. Three imidazoline analogues of clonidine were potent secretagogues for the parotid and submandibular glands at relatively high doses. 2. Salivation in response to clonidine was completely abolished by prazosin, phentolamine, phenoxybenzamine and dihydroergotamine. 3. The gamma-type of proteins was secreted in response to three of the analogues, whereas with p-aminoclonidine the alpha-type of proteins was secreted by the submandibular gland. 4. Albumin was specifically secreted by the submandibular gland in response to clonidine but not to isoproterenol or phenylephrine.

Cardiovascular responses of sinoaortic-denervated rats to intracerebroventricular injection of α1- and α2-adrenoceptor agonists

The aim of the present work was to analyse the cardiovascular responses induced by i.c.v. administration of the alpha 1- and alpha 2-adrenoceptor agonists, phenylephrine and clonidine, respectively, in conscious normal and sinoaortic-denervated rats. Sinoaortic denervation involves changes in central and peripheral catecholaminergic pathways. Clonidine (1-10 micrograms) produced a dose-dependent rise in blood pressure and a bradycardiac response in sham-operated animals, whereas in sinoaortic-denervated rats it provoked a brief rise in blood pressure followed by a marked fall as well as bradycardia. The responses involved mostly activation of central alpha 2-adrenoceptors, but the blood pressure responses induced by clonidine in sinoaortic-denervated rats may also have involved alpha 1-adrenoceptors. The bradycardia induced by the alpha 2-agonist in both groups of rats involved preferentially central alpha 2-adrenoceptors but also partially stimulated alpha 1-adrenoceptors. Phenylephrine, at a dose of 10-60 micrograms, induced a rise in blood pressure and a bradycardiac response while 90 micrograms produced a biphasic pressure response (early transient rise followed by a fall) as well as bradycardia in both sham-operated and sinoaortic-denervated animals. Phenylephrine activated alpha 1-adrenoceptors in every case, but the fall in blood pressure and the bradycardia also involved alpha 2-adrenoceptors. The responses were significantly higher in the sinoaortic-denervated rats than in the sham-operated. Our findings suggest that arterial baroreceptor reflexes can modify the effects of alpha-agonists initiated in the central nervous system. Sinoaortic denervation preparations enable one to unmask the depressor response to clonidine and also demonstrate the true magnitude of the phenylephrine response.

Evidence for antagonistic activity of endothelin for clonidine induced hypotension and bradycardia

Effect of endothelin (ET) on clonidine induced cardiovascular effects was studied in male Sprague-Dawley rats. Clonidine (75 micrograms/kg, iv) produced significant decrease in blood pressure and heart rate. ET-1 (50 ng/kg, iv) pretreatment completely antagonized the hypotension and bradycardia induced by clonidine. ET-2 (50 ng/kg, iv) and ET-3 (50 ng/kg, iv) had similar antagonistic effect on clonidine induced hypotension and bradycardia. The antagonistic effect of ET lasted for several hours, however, 4 hours after ET pretreatment only partial blockade of clonidine induced hypotension and bradycardia was observed. This indicated that the antagonistic effect of ET was reversible. Initial hypertensive response induced by high dose of clonidine (750 micrograms/kg, iv) could not be antagonized by ET-1, ET-2 or ET-3, while phenoxybenzamine, an alpha adrenoceptor antagonist, blocked the hypertensive response of clonidine. Thus, ET has no antagonistic effect on the initial hypertensive response but antagonizes the hypotensive and bradycardic effect induced by clonidine. Clonidine induced hypotension and bradycardia are mediated through central alpha 2 adrenoceptors while hypertension is mediated through peripheral alpha 2 adrenoceptors. It is concluded that central alpha 2 adrenoceptors are different from peripheral alpha 2 adrenoceptors and ET antagonizes the effect of clonidine only on central alpha 2 adrenoceptors but has no antagonistic activity on peripheral alpha 2 adrenoceptors.

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

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

Action of noradrenaline on isolated proximal and distal coronary arteries of rat: selective release of endothelium-derived relaxing factor in proximal arteries

1. The effect of noradrenaline (NA) on the vascular smooth muscle and endothelial cells in isolated ring segments from the proximal and distal part of the left coronary artery (LCA) in rats was examined. 2. NA had a weak concentration-dependent contractile effect on proximal but relaxed distal segments of the LCA. The maximal NA-induced contraction of the proximal segments was doubled while the relaxation of the distal LCA segments was converted to a contraction after blockade of beta-adrenoceptors with propranolol 3 x 10(-6) M, thus indicating the presence of both alpha- and beta-adrenoceptors in the arteries, with dominance of alpha-adrenoceptors and of beta-adrenoceptors in the proximal and distal segments of the LCA, respectively. 3. The contractile effect of NA (beta-adrenoceptors blocked) was doubled in the proximal LCA segments after the endothelium was removed. Endothelial denudation had, in contrast, no potentiating effect on the contractile response of the distal arteries to NA. Both proximal and distal segments became more sensitive to the contractile action of NA after removal of the endothelium. 4. The spontaneous myogenic tone increased in both proximal and distal LCAs after endothelial removal, indicating spontaneous release of a relaxing endothelial factor in the vessels. 5. Following contraction with prostaglandin F2 alpha (PGF2 alpha), and in the presence of propranolol, 3 x 10(-6) M, and prazosin, 10(-6) M, NA induced an endothelium-dependent relaxation of only proximal but not distal segments of the precontracted LCA. The NA-induced relaxation of the proximal segments of the LCA was not altered by indomethacin 10- M but was completely abolished after incubation with methylene blue, 3 x 10-6 M, or following endothelium removal. These results are compatible with NA-induced release of EDRF in these arteries. 6. The selective alpha 2-adrenoceptor agonist, B-HT 933, only induced a weak relaxation of PGF2 alpha,-precontracted proximal (endothelium intact) LCA segments at a concentration of 10-4M. The NA-induced relaxation of these vessels was unaffected by incubating the vessels with 10- IM B-HT 933. The NA relaxation response curve was shifted ca 1.1 log unit to the right by rauwolscine, 1o- 6M, giving an estimated pA2-value of 7.12. The receptor through which NA activates the endothelium appears to be of an atypical alpha 2-subtype.

Responses to noradrenaline in human subcutaneous resistance arteries are mediated by both alpha 1- and alpha 2-adrenoceptors

1. In vitro experiments in a microvascular myograph were designed to characterize postjunctional alpha-adrenoceptors of human subcutaneous resistance arteries (normalised internal diameter 143-313 microns). 2. Both the alpha 1-selective agonist phenylephrine in the presence of 0.3 microM yohimbine and the alpha 2-selective agonist B-HT 933 in the presence of 0.3 microM prazosin elicited prominent and concentration-dependent contractions. The maximum response to phenylephrine and B-HT 933 was not different from the response to high K physiological salt solution (125 mM K+), and the pD2 values (-log EC50) were 5.90 and 6.11, respectively. 3. In the presence of the alpha 2-selective antagonist yohimbine (0.3 microM), the alpha 1-selective antagonist prazosin competitively antagonised the responses to phenylephrine; the pA2 of prazosin for the receptor which mediated the response to phenylephrine was 8.41. 4. Blockade of either alpha 2-adrenoceptors with 0.1 microM yohimbine or alpha 1-adrenoceptors with 0.1 microM prazosin caused shifts to the right of the noradrenaline concentration-response curves and the shifts in pD2 were 0.69 and 0.61, respectively. The combination of the two antagonists at the above-mentioned concentrations caused a marked, parallel shift to the right of the noradrenaline concentration-response curve, the shift of the pD2 was 2.68. 5. These results suggest that activation of both alpha 1- and alpha 2-adrenoceptors produces contractions in human subcutaneous resistance arteries, and that responses to noradrenaline in these vessels are mediated by both alpha-adrenoceptor subtypes.

Cardiovascular effects of rilmenidine, a new alpha 2-adrenoceptor agonist, and clonidine in conscious spontaneously hypertensive rats

1. The acute and chronic effects of rilmenidine, a partial agonist of alpha 1- and alpha 2-adrenoceptors with antihypertensive properties, were compared to those of clonidine on blood pressure (BP), heart rate (HR) and the urinary excretion of catecholamines, which was used as an index of sympathetic activity. 2. As these drugs are known to interfere centrally and peripherally with the sympathetic nervous system, long-term arterial blood pressure recordings in freely moving unstressed adult spontaneously hypertensive rats (SHR) were used. 3. Acute i.v. administrations of rilmenidine (0.3 mg/kg at 1200 h, 1.2 mg/kg at 1700 and 2200 h) and clonidine (12 micrograms/kg at 1200 h, 50 micrograms/kg at 1700 and 2200 h) induced short-lasting increases in BP associated with a decrease in HR, which were followed by prolonged, dose-dependent decreases in BP without bradycardia. The pressor effect was less marked and the associated bradycardia was more marked in active SHR with physiologically high sympathetic activity than in resting SHR. 4. A 12-day oral treatment with rilmenidine (6.0 mg/kg daily) or clonidine (150 micrograms/kg daily) induced moderate decreases in BP without change in HR. Rilmenidine but not clonidine decreased normetanephrine (NMN) excretion in active but not in resting SHR. 5. Finally, during the 24 h following the cessation of the treatments, BP returned to normal, without significantly exceeding that of untreated controls. However, upswings in BP or HR were observed, more markedly and frequently after clonidine than after rilmenidine. 6. In conclusion the effects of alpha 2-adrenoceptor agonists appear to be influenced by the pre-existing sympathetic tone. The general agreement between these data and those observed in patients demonstrates that the use of conscious unstressed animals is of value to determine the cardiovascular effects of drugs which act on the sympathetic nervous system.

Postjunctional alpha 2-adrenoceptors mediate vasoconstriction in human subcutaneous resistance vessels

1. In vitro studies have been performed on human medium-sized muscular arteries (internal diameter 1-4 mm) in a classical organ bath and with human subcutaneous resistance arteries (internal diameter 103-626 microns) in a microvascular myograph. 2. Although the medium-sized muscular arteries showed no response to either of the alpha 2-agonists B-HT 933 or UK 14304 in concentrations up to 10 microM, the subcutaneous resistance arteries from all regions examined showed well-pronounced and concentration-dependent responses to B-HT 933, the pD2 (-log EC50) being 5.11 +/- 0.09. 3. In the resistance arteries the alpha 2-antagonist yohimbine caused a parallel shift to the right of the B-HT 933 concentration-response curve; the yohimbine pA2 for the B-HT 933 receptor was 7.86 +/- 0.12. 4. There was an inverse relationship between the maximum response to B-HT 933 and the calibre of the resistance vessels. 5. These results indicate the presence of a postjunctional alpha 2-adrenoceptor in human subcutaneous resistance arteries and not in medium sized muscular arteries.

Differences in alpha-adrenergic responsiveness between human internal mammary arteries and saphenous veins

Little is known regarding specific biologic and pharmacologic differences between human internal mammary arteries and saphenous veins. To better define the role of alpha-adrenoceptor-mediated vasoconstriction in human internal mammary arteries and saphenous veins, we obtained fresh specimens of both vessels from 32 patients undergoing coronary artery bypass surgery. Dose-response curves were generated for the relatively selective alpha 1-receptor agonist phenylephrine, the alpha 2-receptor agonist BHT-920, and the alpha 1- and alpha 2-receptor agonist norepinephrine. Phenylephrine elicited similar contractile responses in internal mammary arteries and saphenous veins, with a mean EC50 (the effective concentration necessary to produce 50% of the maximal contraction) of 1.4 X 10(-6) M for internal mammary arteries and 1.8 X 10(-6) M for saphenous veins (p = NS). Selective stimulation of alpha 2-receptors with BHT-920 elicited a marked contractile response only in saphenous veins. Dose-response curves for phenylephrine and BHT-920 were shifted to the right for both vessels in the presence of the alpha 1-receptor antagonist prazosin and the alpha 2-receptor antagonist yohimbine, respectively. Norepinephrine elicited contraction at a lower concentration in saphenous veins than in internal mammary arteries with a mean EC50 of 7.8 X 10(-8) M for saphenous veins and a mean EC50 of 3.4 X 10(-7) M for internal mammary arteries (p less than 0.05). The results suggest that alpha-adrenoceptor-mediated vasoconstriction is caused primarily by alpha 1-receptors in human internal mammary arteries and by alpha 1- and alpha 2-receptors in human saphenous veins.

The inhibition of alpha 1-adrenoceptor-mediated contractions of rabbit pulmonary artery by Ca2+-withdrawal, pertussis toxin and N-ethylmaleimide is dependent on agonist intrinsic efficacy

Contractions were induced in rings of rabbit pulmonary artery with the preferential alpha 1-adrenoceptor agonists, phenylephrine, methoxamine and St 587 [2-(2-chloro-trifluoromethyl-phenylimino)imidazolidine and the preferential alpha 2-adrenoceptor agonists, clonidine and B-HT 920 [6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-(4,5-d) azepine] [corrected]. Phenylephrine and methoxamine acted as full agonists whereas St 587, clonidine and B-HT 920 were partial agonists (intrinsic activities 0.62, 0.38 and 0.42, respectively). Experiments with alpha 1- and alpha 2-adrenoceptor antagonists indicated that the receptors involved are of the alpha 1 type only. Removal of extracellular Ca2+ inhibited maximal contractions to phenylephrine and methoxamine by 30% and 49%, respectively. The remaining contraction components of the full agonists were abolished by the "intracellular Ca2+ antagonist" TMB-8 [8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate]. Contractions to St 587, clonidine and B-HT 920 were virtually abolished in Ca2+-free medium. Pretreatment of the donor rabbits with pertussis toxin (2.5 micrograms/kg i.v., 5-6 days before sacrifice) attenuated the efficacies of the full agonists, phenylephrine and methoxamine by only 24% and 17%, respectively, whereas maximal contractions to the partial agonists, St 587, clonidine and B-HT 920, were inhibited by 46%, 61% and 75%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Antinociceptive effects of azepexole (BHT 933) in mice

Subcutaneous (s.c.) administration of azepexole (BHT 933) at doses between 4 and 40 mg/kg produced dose-dependent antinociceptive effects in mice as assessed by tail-immersion, tail-pinch and acetic acid writhing tests. The ED16s were 5.6 +/- 0.4, 6.7 +/- 1.2 and 2.96 +/- 0.2 mg/kg respectively. Similarly, morphine produced analgesia in the same tests with ED16s of 0.87 +/- 0.03, 0.47 +/- 0.1 and 0.45 +/- 0.01 mg/kg respectively. In all instances naloxone (0.1 mg/kg s.c.) shifted the dose-response curves to morphine to the right in a parallel manner. Naloxone (0.1 and 1 mg/kg s.c.) partially antagonized the effect of azepexole in the tail-immersion and tail-pinch tests but significantly decreased the slope of the dose-response curve suggesting that a competitive interaction at the level of the opioid receptors did not occur. Naloxone had no effect on the antinociceptive action of azepexole in the acetic acid writhing test.

Yohimbine's anxiogenic action: evidence for noradrenergic and dopaminergic sites

Yohimbine (2.5 or 4 mg/kg) reduced the percentage of open arm entries and the percentage of time spent on the open arms displayed by rats on an elevated plus-maze indicating anxiogenic activity. These effects were reversed by the alpha 2-adrenoceptor agonist clonidine (0.01 mg/kg) and by the dopamine receptor agonist apomorphine (0.57 mg/kg). The following failed to reverse the effects of yohimbine: the selective alpha 2-adrenoceptor agonists, guanfacine (0.25 and 1 mg/kg), B-HT920 (0.025 and 0.1 mg/kg), B-HT933 (1 and 10 mg/kg); the beta-blocker propranolol (2.5 and 10 mg/kg); the alpha 1-adrenoceptor agonist phenylephrine; the D1 agonist SK&F 38393 (5 and 10 mg/kg) and the D2 agonist LY 171555 (0.5 and 1 mg/kg). Therefore, it is unlikely that activity at only the alpha 1, alpha 2, beta, D1 or D2 sites can entirely account for the anxiogenic actions of yohimbine in the elevated plus-maze. Evidence that clonidine affects the dopaminergic system and that apomorphine affects the noradrenergic system suggest that yohimbine may produce its anxiogenic response by activity on both the noradrenergic and dopaminergic systems.

Alpha 2-adrenoceptor blocking properties of idazoxan stereoisomers: stereoselectivity for presynaptic alpha 2-adrenoceptors

The alpha 2-adrenoceptor blocking properties of idazoxan enantiomers were evaluated at pre- and postsynaptic level. The antagonism of the two idazoxan stereoisomers was assessed, at presynaptic level, by their ability to antagonize clonidine at the alpha 2-adrenoceptors regulating noradrenaline release. The antagonist (+)-idazoxan showed an affinity towards the alpha 2-autoreceptors 40 times higher than that showed by (-)-idazoxan. Binding studies revealed (+)-idazoxan to be 7-8 times more potent than (-)-idazoxan in inhibiting the p-[3H]aminoclonidine binding. These results indicate a different affinity of alpha 2-adrenoceptors for the two idazoxan stereoisomers, thus suggesting that the alpha 2-adrenoceptors located pre- and postsynaptically may be of two stereochemically different subtypes.

Cardiovascular and central nervous system effects of rilmenidine (S 3341) in rats

Rilmenidine (S 3341) is a new alpha 2 agonist, with antihypertensive properties. Pharmacologic data concerning its hemodynamic and central nervous system effects in the rat are described in this report. In the anesthetized or conscious spontaneously hypertensive rat, rilmenidine was found effective and potent as an antihypertensive agent, lowering blood pressure in a dose-dependent manner after intravenous and oral administration. These effects are related to a reduction in sympathetic tone as seen by the decrease in plasma catecholamines induced by rilmenidine in the spontaneously hypertensive rat. Studies in the normotensive pithed rat (electrical stimulation and adrenalectomization) confirmed the presynaptic alpha 2-stimulating properties of rilmenidine and suggested that a component of the antihypertensive activity of rilmenidine could be exerted through these peripheral receptors. A study of the central effects of rilmenidine was performed using classic neuropharmacologic tests. No effect was observed on the pentobarbitone-induced sleeping time in the rat. Rilmenidine caused only a minimal and non-dose-dependent inhibition of the righting reflex in the chick. In the rat, rilmenidine did not decrease the motor activity at concentrations up to 50 times higher than the antihypertensive dose. These results confirmed the contrast between rilmenidine and clonidine and suggest that a dissociation between sedative and antihypertensive effects could occur with rilmenidine.

Pharmacology of the alpha 2-adrenoceptor agonist rilmenidine

Most alpha 2-adrenoceptor agonists developed so far will penetrate into the brain, thus causing central hypotensive activity, mediated by the stimulation of alpha 2 adrenoceptors in the region of the nucleus tractus solitarii, the vasomotor center and the nucleus of the vagus nerve. The central alpha 2 adrenoceptors are probably located at postjunctional (postsynaptic) sites. Their stimulation causes sympathoinhibition and thus a decrease in blood pressure and heart rate. The central hypotensive effect is the dominating activity of all alpha 2-adrenoceptor agonists developed so far, of which clonidine, guanfacine and alpha-methyl-DOPA (which is converted into alpha-methyl-noradrenaline) are the prototypes. Peripheral postsynaptic effects probably do not greatly contribute to the hypotensive activity of these drugs. Sedation, also mediated by central alpha 2 adrenoceptors is the major adverse reaction to these antihypertensive agents. More selective alpha 2-adrenoceptor agonists (B-HT 920, azepexole, UK 14,304) appear to display the same pattern of hypotensive and sedative activities as the nonselective compounds like clonidine. After the general survey on centrally acting alpha 2-adrenoceptor agonistic drugs, the pharmacologic profile of the new oxazoline derivative, rilmenidine, (S 3341) was compared with that of the classic compound, clonidine. In all current animal and in vitro models, rilmenidine was characterized as a clonidine-like, centrally acting antihypertensive drug. Thus, its central hypotensive activity proved mediated by the stimulation of central alpha 2 adrenoceptors. In radioligand binding studies, rilmenidine proved somewhat more selective for alpha 2 adrenoceptors, but this selectivity was not reflected by a clearly different pharmacologic profile of the drug.(ABSTRACT TRUNCATED AT 250 WORDS)