Alpha-adrenoceptor Stimulation Research Articles

0038 Genioglossus muscle (GG) activity evoked by pharmacological stimulation of alpha1-adrenoceptors in pre-hypoglossal region (PHR)

Abstract Introduction The sleep-related withdrawal of noradrenergic excitation and increased cholinergic inhibition of hypoglossal motoneurons (HM), which control the tone of the GG, the largest protruder muscle of the tongue, play a critical role in obstructive sleep apnea pathophysiology. In an animal model of REM sleep, antagonism of alpha1 noradrenergic signaling by microinjections of prazosin into the hypoglossal nucleus (HN) strongly decreased hypoglossal nerve activity. However, a significant delay of this decrease prompted a hypothesis that prazosin must diffuse outside the HN to block alpha1-adrenoceptors that are located on interneurons, which mediate noradrenergic drive to HM. Here, we aimed to map the PHR by microinjecting alpha1 drugs for possible location of these interneurons. Methods Adult C57bl/6J mice (n=28) of both sexes were anesthetized with either ketamine/xylazine (120/20 mg/kg, i.p.) or 1.5-2.0% isoflurane inhaled through a nose mask. Electromyogram electrodes were implanted into GG and diaphragm to record their activity. We microinjected phenylephrine (alpha1-adrenoceptor agonist, 5-10 nl, 1 mM) in both ketamine/xylazine- and isoflurane-anesthetized mice, while prazosin (10 nl, 0.2 mM) was injected only in mice under isoflurane anesthesia. The injection sites were marked with Pontamine Skye Blue. After the completion of experiments, all animals were perfused, and brains removed for histological analysis. Results The activity of GG showed spontaneous phasic inspiratory modulation with minimal tonic component under both anesthesia conditions although its amplitude was markedly lager in animals anesthetized by isoflurane. Effective microinjections of phenylephrine elicited short-lasting (approximately 15 minutes) excitatory GG responses. When injected into the HN, phenylephrine strongly excited GG as compared to injections to the PHR in ketamine/xylazine-anesthetized mice. However, in isoflurane-anesthetized animals, microinjections of phenylephrine into the PHR induced greater activation of GG than injections onto the HN. Microinjections of prazosin in isoflurane-anesthetized mice produced long-lasting disfacilitation of GG activity. However, prazosin injections into the PHR elicited responses with the shortest latencies (0-120 s). Conclusion These results suggest that the putative interneurons that mediate noradrenergic drive to HM are located in the PHR. This finding is important for deciphering the neuronal network that controls the state-dependent activity of GG, which may provide new therapeutic targets for OSA treatment. Support (if any) NIHR01HL133847, NIAR01AG065233.

Stimulation of alpha-1 adrenoceptors may intensify cutaneous inflammation in complex regional pain syndrome.

Alpha-1 adrenoceptors are overexpressed in the epidermis of a subgroup of patients with complex regional pain syndrome (CRPS). Activating α 1 -adrenoceptors in epidermal cells increases production of the proinflammatory cytokine interleukin-6 (IL-6), a mediator of inflammation. To investigate whether this might exacerbate inflammation in CRPS, primary keratinocytes or dermal fibroblasts were cultured from skin biopsies obtained from the affected limb of 25 patients and a similar site in 28 controls. The fundamental proinflammatory cytokine, tumor necrosis factor alpha, was administered for 24 hours to initiate inflammation. After this, cells were incubated for 6 hours with the α 1 -adrenoceptor agonist phenylephrine. Exposure to tumor necrosis factor alpha induced proinflammatory cytokine mRNA production and protein secretion in keratinocytes and fibroblasts and enhanced α 1B -adrenoceptor mRNA expression in keratinocytes. Additional stimulation of α 1 adrenoceptors with phenylephrine increased the production of IL-6 mRNA and protein secretion in both cell types. Under all conditions, gene and protein α 1 -adrenoceptor levels and cytokine gene expression and protein secretion were similar, overall, in patients and controls, except for abnormally high α 1 -adrenoceptor protein levels in the keratinocytes of 3 of 17 patients. These findings suggest that persistent inflammation in CRPS is not due to dysfunction of skin cells but is a normal response to extrinsic signals. After α 1 -adrenoceptor stimulation of keratinocytes, increases in IL-6 mRNA but not protein were proportional to basal α 1 -adrenoceptor protein levels. Skin cells play an important role in persistent inflammation in CRPS. Potentially, a positive feedback loop between α 1 -adrenoceptors and IL-6 production in skin cells contributes to this inflammatory state.

Comparative pharmacodynamic analysis of imidazoline compounds using rat model of ocular mydriasis with a test of quantitative structure–activity relationships

Imidazol(in)e derivatives, having the chemical structure similar to clonidine, exert diverse pharmacological activities connected with their interactions with alpha2-adrenergic receptors, e.g. hypotension, bradycardia, sedation as well as antinociceptive, anxiolytic, antiarrhythmic, muscle relaxant and mydriatic effects. The mechanism of pupillary dilation observed after systemic administration of imidazol(in)es to rats, mice and cats depends on the stimulation of postsynaptic alpha2-adrenoceptors within the brain. It was proved that the central nervous system (CNS)-localized I1-imidazoline receptors are not engaged in those effects. It appeared interesting to analyze the CNS-mediated pharmacodynamics of imidazole(in)e agents in terms of their chromatographic and calculation chemistry-derived parameters.In the present study a systematic determination and comparative pharmacometric analysis of mydriatic effects in rats were performed on a series of 20 imidazol(in)e agents, composed of the well-known drugs and of the substances used in experimental pharmacology. The eye pupil dilatory activities of the compounds were assessed in anesthetized Wistar rats according to the established Koss method. Among twenty imidazol(in)e derivatives studied, 18 produced diverse dose-dependent mydriatic effects. In the quantitative structure–activity relationships (QSAR) analysis, the pharmacological data (half maximum mydriatic effect – ED50 in μmol/kg) were considered along with the structural parameters of the agents from molecular modeling. The theoretically calculated lipophilicity parameters, CLOGP, of imidazol(in)es, as well as their lipophilicity parameters from HPLC, logkw, were also considered. The attempts to derive statistically significant QSAR equations for a full series of the agents under study were unsuccessful. However, for a subgroup of eight apparently structurally related imidazol(in)es a significant relationship between log(1/ED50) and logkw values was obtained. The lack of “predictive” QSAR for the whole series of the structurally diverse agents is probably due to a complex mechanism of the ligand–alpha2-adrenergic receptor interactions, which are predominantly of a highly structurally specific polar nature. Such interactions are difficult to quantify with the established chemical structural descriptors, contrary to the less specific, molecular bulkiness-related interactions.

Stress Alone or associated with Ethanol Induces Prostanoid Release in Rat Aorta via α2-Adrenoceptor

BackgroundStress and ethanol are both, independently, important cardiovascular risk factors.ObjectiveTo evaluate the cardiovascular risk of ethanol consumption and stress exposure, isolated and in association, in male adult rats.MethodsRats were separated into 4 groups: Control, ethanol (20% in drinking water for 6 weeks), stress (immobilization 1h day/5 days a week for 6 weeks) and stress/ethanol. Concentration-responses curves to noradrenaline - in the absence and presence of yohimbine, L-NAME or indomethacin - or to phenylephrine were determined in thoracic aortas with and without endothelium. EC50 and maximum response (n=8-12) were compared using two-way ANOVA/Bonferroni method.ResultsEither stress or stress in association with ethanol consumption increased the noradrenaline maximum responses in intact aortas. This hyper-reactivity was eliminated by endothelium removal or by the presence of either indomethacin or yohimbine, but was not altered by the presence of L-NAME. Meanwhile, ethanol consumption did not alter the reactivity to noradrenaline. The phenylephrine responses in aortas both with and without endothelium also remained unaffected regardless of protocol.ConclusionChronic stress increased rat aortic responses to noradrenaline. This effect is dependent upon the vascular endothelium and involves the release of vasoconstrictor prostanoids via stimulation of endothelial alpha-2 adrenoceptors. Moreover, chronic ethanol consumption appeared to neither influence noradrenaline responses in rat thoracic aorta, nor did it modify the increase of such responses observed as a consequence of stress exposure.

Key differences in hypertrophic signalling between hESC- and hIPSC-derived cardiomyocytes

The potential of stem cell-based disease modelling is enhanced by the realisation that cardiomyocytes from human embryonic stem cells (hESC-CM) and induced pluripotent stem cells (hiPSC-CM) can be obtained with disease-specificity. Hypertrophy is a high priority target because of its central role in the transition to heart failure. Strikingly, here we found that hiPSC-CM are relatively unresponsive to major hypertrophic signals compared to hESC-CM. We show that the normal alpha-adrenergic receptor 1A subtype (ADRA1A) is not expressed robustly in either cell type. ADRA1A is reversibly silenced during differentiation, accompanied by up-regulation of ADRA1B, resulting in a distinct gene profile from that in adult human cardiomyocytes. Loss of ADRA1A is more pronounced in hiPSC-CM, due to greater epigenetic silencing and more marked up-regulation of HIF-1α, but ultimately both cell types differ from adult in their reliance on active ADRA1B rather than ADRA1A. ADRA1B up-regulation is sufficient in hESC-CM for hypertrophic changes such as cell size, cell volume and ANF. However, in hiPSC-CM, additional decreased G-protein signalling and tonically inhibitory pathway networks suppress the effect of alpha-adrenoceptor stimulation on growth. Superficial similarities between hESC-CM, hiPSC-CM and adult cardiomyocytes may mask complex differences in signalling. These data raise serious questions regarding the hiPSC-CM as a valid model system for certain aspects of cardiac disease.

Cocaine synergism with alpha agonists in rat aorta: Computational analysis reveals an action beyond reuptake inhibition

BackgroundCocaine has long been known to increase blood pressure, but the degree and mechanism of vasoconstricting action remain poorly understood. Here we examine the interaction between cocaine and alpha-adrenoceptor agonists, with the action of reuptake inhibition minimized. MethodsCocaine was administered to isolated rings of rat thoracic aorta, alone and in combination with three different adrenoceptor agonists: phenylephrine, methoxamine, and norepinephrine. Synergy analysis begins with the predicted additive effect of the combination of two agonists, based upon dose equivalence theory. This case where one agonist (cocaine) has no effect when administered alone requires only a t-test to demonstrate that a departure from additivity has occurred. ResultsAt doses where cocaine alone produced no vasoconstriction, it potentiated the vasoconstriction produced by all three alpha agonists, a clear indication of synergism between cocaine and these agents. Higher doses of cocaine in combination with alpha adrenoceptor agents gave an inverted-U shaped (hormetic) dose–effect curve, i.e., dose-related relaxation at higher doses. The hormetic dose–effect relation was analyzed using computational methodology based on dose equivalence to derive the unknown second component of action that causes relaxation. ConclusionsCocaine exhibits both vasoconstricting and vasorelaxant effects. This relaxing component, possibly related to activation of myosin light chain phosphatase, was quantified as a dose–effect curve. Most important is the synergism between cocaine and alpha-adrenoceptor stimulation which cannot be explained as an action due to reuptake inhibition, and has not been previously described.

55 Adverse Events Following the Combination of Dexmedetomidine with Therapeutic Hypothermia in a Piglet Asphyxia Model

BackgroundDexmedetomidine (DXM) is a potent, selective a2 adrenoceptor agonist which exerts sedative, neuroprotective, analgesic and anti-inflammatory properties that may be beneficial for neonatal asphyxia. The safety of DXM combined with...

Brain pathways involved in the modulatory effects of noradrenaline in lateral septal area on cardiovascular responses.

We have previously reported that stimulation of alpha-1 adrenoceptors by noradrenaline (NA) injected into the lateral septal area (LSA) of anaesthetized rats causes pressor and bradycardic responses that are mediated by acute vasopressin release into the circulation through activation of the paraventricular nucleus (PVN). Although the PVN is the final structure of this pathway, the LSA has no direct connections with the PVN, suggesting that other structures may connect these areas. To address this issue, the present study employed c-Fos immunohistochemistry to investigate changes caused by NA microinjection into the LSA in neuronal activation in brain structures related to systemic vasopressin release. NA microinjected in the LSA caused pressor and bradycardic responses, which were blocked by intraseptal administration of α-1 adrenoceptor antagonist (WB4101, 10 nmol/200 nL) or systemic V-1 receptor antagonist (dTyr(CH2)5(Me)AVP, 50 μg/kg). NA also increased c-Fos immunoreactivity in the prelimbic cortex (PL), infralimbic cortex (IL), dorsomedial periaqueductal gray (dmPAG), bed nucleus of the stria terminalis (BNST), PVN, and medial amygdala (MeA). No differences in the diagonal band of Broca, cingulate cortex, and dorsolateral periaqueductal gray (dlPAG) were found. Systemic administration of the vasopressin receptor antagonist dTyr AVP (CH2)5(Me) did not change the increase in c-Fos expression induced by intra-septal NA. This latter effect, however, was prevented by local injection of the alpha-1 adrenoceptor antagonist WB4101. These results suggest that areas such as the PL, IL, dmPAG, BNST, MeA, and PVN could be part of a circuit responsible for vasopressin release after activation of alpha-1 adrenoceptors in the LSA.

The effects of bromocriptine on pre-synaptic and post-synaptic alpha-adrenoceptors in the mouse vas deferens.

Noradrenaline (NA) and dopamine (DA) contracted the mouse vas deferens and reduced the responses to low frequency nerve stimulation (0.1 Hz). The relative potencies of antagonists suggested that these effects were due to stimulation of post-synaptic and pre-synaptic alpha-adrenoceptors respectively. Bromocriptine produced a non-competitive antagonism of contractile responses to NA (pD2' = 7.6) and DA (pD2' = 8.0) but had no effect on responses to carbachol. Bromocriptine also reduced single twitch responses of the vas to low frequency field stimulation (0.1 Hz), but did not affect stimulation at higher frequencies (1--20 Hz). Yohimbine selectively and rapidly reversed the inhibiting effects of bromocriptine on single twitches, although they could not easily be reversed by washing. Bromocriptine produced a yohimbine-reversible reduction in the stimulated overflow of tritium from vasa previously loaded with 3H--NA. Thus the mouse vas deferens does not appear to contain specific DA receptors and the results suggest that bromocriptine acts as a pre-synaptic alpha-adrenoceptor agonist and post-synaptic alpha-adrenoceptor antagonist in this tissue.

Evidence for a central α-sympathomimetic action of Clonidine in the rat

The antihypertensive effects of clonidine (0.15 mg kg-1, i.p.) were studied in conscious DOCA/saline hypertensive rats having chronically implanted arterial cannulae. The response to clonidine was markedly reduced by simultaneously administered desipramine (3 mg kg-1, i.p.), antagonized dose-dependently by piperoxan (2-10 mg kg-1, i.v.) and prevented by pretreatment with phentolamine (0.2 mg, i.c.v.). Pretreatment with 6-hydroxydopamine (3 x 250 mu-g, i.c.v.), haloperidol (1 mg kg-1, i.p.), p-chloro-N-methylamphetamine (3.5 mg kg-1, i.p.) or 5,6-dihydroxytryptamine (50 mu-g and 25 mu-g, i.c.v.) did not significantly modify the antihypertensive response. It is concluded that the antihypertensive response to clonidine is mediated via stimulation of central alpha-adrenoceptors and is independent of central dopaminergic receptors and intact central serotoninergic neurons. The necessity for intact central noradrenergic neurons remains uncertain.

Influence of the Endothelium on Vascular Responses of Aortae from Endotoxic Rats

Intraperitoneal injection of endotoxin diminished the in-vitro contractile response of rat aorta to phenylephrine or clonidine, whether the intimal layer was disrupted or not. The relaxing responses to acetylcholine and sodium nitroprusside in aorta precontracted with 10(-6) M phenylephrine were similar between control and endotoxic groups. However, when the precontractile force following phenylephrine was adjusted to an equivalent level, the relaxing responses to acetylcholine and sodium nitroprusside were diminished in the endotoxic aorta compared with the controls. There was no significant difference between the two groups in the increase in cyclic GMP levels induced by acetylcholine or by sodium nitroprusside. These results suggest that aortae from endotoxic rats show decreased responsiveness to alpha-adrenoceptor stimulation not because of enhancement of the endothelium-derived relaxing factor but because of abnormality in the vascular smooth muscle which is not specific for subtypes of the alpha-adrenoceptor.

The role of adrenoceptors in the mechanism of reserpine-induced stimulation of gastric acid secretion in the rat.

The role of alpha- and beta-adrenoceptors in the mechanism of reserpine-induced stimulation of gastric acid secretion in the rat has been examined. After 6 h reserpine (0.1 mg kg-1 i.p.) significantly stimulated acid secretion relative to control values (176 +/- 4 vs 60 +/- 3 mumol, mean +/- s.e.m., n = 10, P less than 0.001). Neither coeliac ganglionectomy nor propranolol (5-15 mg kg-1) influenced this action. Vagotomy prevented acid stimulation by reserpine and was associated with H+ output similar to that of vagotomy controls (13 +/- 1 vs 14 +/- 1 mumol, mean +/- s.e.m., n = 10). Dose-dependent inhibition of the reserpine-induced acid secretion was produced by phenoxybenzamine or phentolamine; an inhibition similar to that achieved by vagotomy was noted with the 15 mg kg-1 dose (13 +/- 1 and 15 +/- 1 mumol, respectively, vs 176 +/- 4 mumol, mean +/- s.e.m., n = 10, P less than 0.001). The similarity in action between vagotomy and large doses of phenoxybenzamine or phentolamine suggests that, in the rat, vagal alpha-adrenoceptor stimulation is directly involved in the mechanism of reserpine-induced stimulation of gastric acid secretion.

Inhibition of basal and insulin-stimulated gastric acid secretion by phenoxybenzamine or phentolamine.

The present study in the rat was undertaken to investigate the possible involvement of alpha- and beta-adrenoceptors in mediating gastric acid secretion in response to insulin-hypoglycaemia. Phenoxybenzamine, 15 mg kg-1, depressed the basal acid secretion to a level similar to that associated with vagotomy; phentolamine, 15 mg kg-1, had a similar effect. Insulin (1 unit kg-1) significantly stimulated gastric acid secretion. Vagotomy or phenoxybenzamine or phentolamine, 15 mg kg-1 prevented this action. The beta-adrenoceptor blocking drug, propranolol, 5-15 mg kg-1, had no significant effect on the basal acid secretion or its stimulation by insulin. The similarity in action between vagotomy and large doses of phenoxybenzamine and phentolamine suggests that, in the rat, vagal alpha-adrenoceptor stimulation is involved in the mechanisms of basal and insulin-stimulated gastric acid secretion.

Alpha2‐Adrenoceptor Blockade Accelerates the Neurogenic, Neurotrophic, and Behavioral Effects of Chronic Antidepressant Treatment

Enhanced adult hippocampal neurogenesis and increased trophic factor expression mediate the behavioral effects of antidepressants. We investigated the influence of alpha2‐adrenoceptors on adult hippocampal neurogenesis. We found that alpha2 agonists selectively decrease adult hippocampal progenitor proliferation. These effects persist in Dbh−/− mice lacking norepinephrine, suggesting an action through alpha2‐heteroceptors. Adult hippocampal progenitors express all the alpha2‐adrenoceptor subtypes, and decreased neurosphere frequency and BrdU incorporation indicate direct effects of Alpha2‐adrenoceptor stimulation on progenitors. Furthermore, coadministration of the alpha2 antagonist yohimbine with the antidepressant imipramine accelerates effects on hippocampal progenitor proliferation, the maturation of newborn neurons, and the increase in expression of BDNF and VEGF implicated in the neurogenic and behavioral effects of antidepressants. Finally, short‐duration (7 d) yohimbine and imipramine treatment results in robust behavioral responses in the novelty suppressed feeding test, which normally requires 3 weeks of treatment with classical antidepressants. Our results highlight importance of alpha2‐adrenoceptors as targets for faster acting antidepressants.

Spinal α2-Adrenoceptor-mediated Analgesia in Neuropathic Pain Reflects Brain-derived Nerve Growth Factor and Changes in Spinal Cholinergic Neuronal Function

Spinal alpha2-adrenoceptor stimulation produces analgesia in neuropathic pain states, and this effect in animals is blocked by the inhibitors of brain-derived neurotrophic factor (BDNF) function. In rats, alpha2-adrenoceptor stimulation normally inhibits acetylcholine release, but it excites release after nerve injury. The authors examined the roles of BDNF and excitatory Gs-protein in this change. Male rats underwent L5-L6 spinal nerve ligation (SNL), and their lumbar spinal dorsal horns with or without spinal BDNF infusion were used for either synaptosome preparation for acetylcholine release or immunostaining for choline acetyltransferase. SNL did not alter spontaneous release from synaptosomes or choline acetyltransferase immunoreactivity in the spinal dorsal horn, but it reduced KCl-evoked acetylcholine release. Dexmedetomidine inhibited KCl-evoked acetylcholine release in synaptosomes from normal rats, but it excited KCl-evoked release in synaptosomes from SNL rats, and both effects were blocked by the alpha2-adrenoceptor antagonist idazoxan. Spinal infusion of an antibody to BDNF reduced choline acetyltransferase immunoreactivity in the spinal dorsal horn in both normal and SNL rats and abolished facilitation of KCl-evoked acetylcholine release by dexmedetomidine in SNL rats. Dexmedetomidine facilitation of acetylcholine release was also blocked by the inhibitors of Gs function. The increased reliance of spinal alpha2 adrenoceptors on cholinergic stimulation to cause analgesia after nerve injury reflects in part a shift from direct inhibition to direct excitation of spinal cholinergic neurons. The authors' results suggest that this shift relies on an interaction with Gs-proteins and BDNF.

In Vivo Biomechanical Assessment of Anterior Rabbit Urethra After Repair of Surgically Created Hypospadias

We studied the in vivo biomechanical properties of the anterior urethra in a rabbit hypospadias simulation model at long-term followup after urethroplasty. A total of 38, 9-week-old white New Zealand male rabbits were randomized into active treatment and control groups. The active treatment group included hypospadias-like simulation and acute repair (sham, mobilization and advancement, tubularized incised posterior urethral plate). After 23 weeks all groups underwent biomechanical investigation using impedance planimetry. Cross-sectional area pressure measurements during bag distention were conducted at 3 different positions along the urethra, and biomechanical (tension-strain) relations were computed. The biomechanical parameters studied were urethral luminal dimension (cross-sectional area pressure relation) and its deformability to luminal pressure (tension-strain relation). The urethral cross-sectional area was significantly larger in the mobilization and advancement group compared to the tubularized incised plate group (p <0.001), shams and controls at the distal distention site (p <0.05), and other groups at the intermediate distention site (p <0.05). The strain-tension curves were not significantly different between the groups. All groups showed a nonlinear cross-sectional area pressure and circumferential wall tension-strain relationships in the 3 distention sites. The urethras exhibited a decreasing ability to be distended with increasing pressures, indicating self-protection against damage. At long-term followup no differences in tension-strain relations were found between groups, despite the fact that the mobilization and advancement group had enlarged urethras. Our hypospadias simulation animal model could be useful to test commonly used or new urethroplasty techniques and their functional (biomechanical) long-term results before clinical use.

Alpha-adrenergic responsiveness in rat isolated perfused heart after abdominal aortic coarctation.

Chronic isoproterenol pre-treatment, a well-known model of compensatory hypertrophy associated with cardiac beta-adrenoceptor desensitization, enhances the inotropic response to phenylephrine in rat isolated perfused hearts, supporting the hypothesis that myocardial alpha-adrenoceptor stimulation contributes to the maintenance of myocardial performance in situations in which cardiac beta-adrenoceptor function is compromised. To further corroborate this hypothesis, the effects of abdominal aortic coarctation on cardiac alpha-adrenergic responsiveness were investigated in Langendorff heart preparations. Abdominal aortic coarctation causes cardiac hypertrophy (21%) as shown by a significant increase in the ratio of ventricular dry weight to bodyweight. In preparations from hypertrophied rats, both maximum increases in left ventricular systolic pressure and heart rate elicited by isoproterenol (10(-12) to 10(-4) M) were significantly reduced (the isoproterenol concentration producing 50% of the maximum positive inotropic and chronotropic responses was enhanced almost 21- and 2-fold, respectively). However, the positive inotropic response to phenylephrine (10(-12) to 10(-4) M) remained unaffected following abdominal aortic coarctation, when compared with sham-operated rats. In preparations from both groups, phenylephrine infusion did not induce significant changes in heart rate. These results show that although abdominal aortic stenosis induced desensitization of cardiac beta-adrenoceptors, it did not enhance cardiac alpha-adrenoceptor responsiveness. This suggests that such an enhancement depends on the experimental model used to induce cardiac hypertrophy associated with desensitization of cardiac beta-adrenoceptors.

Effects of long-term pretreatment with isoproterenol on inotropic responsiveness to alpha-adrenoceptor stimulation: study in isolated perfused rat hearts.

The effects of chronic pretreatment with isoproterenol (5 mg kg(-1)) daily for 10 days on cardiac alpha-adrenergic responsiveness in Langendorff heart preparations were investigated. Isoproterenol pretreatment caused cardiac hypertrophy (29%) as shown by a significant increase in the ratio of ventricular dry weight to body weight. In preparations from isoproterenol-pretreated rats, both maximum increases in left ventricular systolic pressure and heart rate elicited by isoproterenol (10(-12) to 10(-4) M) were significantly reduced (the isoproterenol concentration producing 50% of the maximum positive inotropic and chronotropic responses was enhanced almost 32- and 4-fold, respectively), while the positive inotropic response to phenylephrine (10(-12) to 10(-4) M) was significantly enhanced (the phenylephrine concentration producing 50% of the maximum positive inotropic effect was reduced almost 100-fold), compared with saline-pretreated rats. In preparations from both groups, phenylephrine infusion induced non-significant changes in heart rate and its positive inotropic response was reduced in the presence of propranolol (10(-7) M) in the perfusion medium. Even under beta-adrenoceptor blockade, the curve for the phenylephrine-induced positive inotropic effect remained shifted upward after isoproterenol pretreatment. Chronic isoproterenol pretreatment induces the expected cardiac beta-adrenoceptor desensitization while simultaneously enhancing the positive inotropic responsiveness to phenylephrine in Langendorff heart preparations. These findings support the hypothesis that cardiac alpha1-adrenoceptor stimulation may contribute to the maintenance of myocardial function under conditions in which beta-adrenoceptor function is compromised.

Guanfacine Potentiates the Activation of Prefrontal Cortex Evoked by Warning Signals

Warning signals evoke an alert state of readiness that prepares for a rapid response by priming a thalamo-frontal-striatal network that includes the dorsolateral prefrontal cortex (DLPFC). Animal models indicate that noradrenergic input is essential for this stimulus-driven activation of DLPFC, but the precise mechanisms involved have not been determined. We tested the role that postsynaptic alpha(2A) adrenoceptors play in the activation of DLPFC evoked by warning cues using a placebo-controlled challenge with the alpha(2A) agonist guanfacine. Sixteen healthy young adults were scanned twice with event-related functional magnetic resonance imaging (fMRI), while performing a simple cued reaction time (RT) task following administration of a single dose of oral guanfacine (1 mg) and placebo in counterbalanced order. The RT task temporally segregates the neural effects of warning cues and motor responses and minimizes mnemonic demands. Warning cues produced a marked reduction in RT accompanied by significant activation in a distributed thalamo-frontal-striatal network, including bilateral DLPFC. Guanfacine selectively increased the cue-evoked activation of the left DLPFC and right anterior cerebellum, although this increase was not accompanied by further reductions in RT. The effects of guanfacine on DLPFC activation were specifically associated with the warning cue and were not seen for visual- or target-related activation. Guanfacine produced marked increases in the cue-evoked activation of DLPFC that correspond to the well-described actions of postsynaptic alpha(2) adrenoceptor stimulation. The current procedures provide an opportunity to test postsynaptic alpha(2A) adrenoceptor function in the prefrontal cortex in the pathophysiology of several psychiatric disorders.

CHANGES IN PLATELET SIZE AND SPLEEN VOLUME IN RESPONSE TO SELECTIVE AND NON‐SELECTIVE β‐ADRENOCEPTOR BLOCKADE IN HYPERTENSIVE PATIENTS

1. The spleen contains approximately one-third of all the body's platelets. These platelets are relatively larger and haemostatically more active than platelets in the systemic circulation and can be released into the systemic circulation by stimulation of alpha-adrenoceptors or inhibition of beta-adrenoceptors. In the present study, we evaluated the effects of selective (bisoprol) and non-selective (carvedilol) beta-blockers agents on mean platelet volume (MPV) and spleen size in hypertensive patients at rest and after exercise. 2. Blood pressure, heart rate, platelet count, MPV and spleen volume were measured in 18 newly diagnosed hypertensive patients, as well as in nine healthy control subjects, subjected to treadmill exercise test at their first visit and, for the hypertensive group, after 15 and 30 days of treatment with the selective beta(1)-adrenoceptor antagonist bisoprolol 5 mg/day (n = 9) or the non-selective alpha(1)-, beta(1)- and beta(2)-adrenoceptor antagonist carvedilol 25 mg/day (n = 9). 3. Increases in resting MPV values with concomitant decreases in spleen volume were found after 15 and 30 days treatment with either bisoprolol or carvedilol. The pronounced decrease in splenic volume after exercise and the increased MPV and platelet counts seen at first visit were halved after 15 and 30 days of treatment with either drug. 4. We conclude that in hypertensive patients treated with either selective or non-selective beta-blockers, the spleen contracts and MPV increases, which may increase the risk of atherothrombosis.