Impaired modulation of sympathetic alpha-adrenergic vasoconstriction in contracting forearm muscle of ageing men

Recent evidence indicates that older healthy humans demonstrate greater vasoconstrictor tone in their active muscles during exercise compared with young adults. Therefore, we tested the hypothesis that the normal ability of muscle contractions to blunt sympathetic alpha-adrenergic vasoconstriction (functional sympatholysis) is impaired with age in healthy humans. We measured forearm blood flow (FBF; Doppler ultrasound) and calculated the forearm vascular conductance (FVC) responses to alpha-adrenergic receptor stimulation during rhythmic handgrip exercise (15% maximum voluntary contraction) and during a control non-exercise vasodilator condition (intra-arterial adenosine infusion) in seven young (25 +/- 2 years) and eight healthy older men (65 +/- 2 year). FVC responses to intra-arterial tyramine (evokes endogenous noradrenaline release), phenylephrine (alpha1-agonist) and clonidine (alpha2-agonist) were assessed. In young men, the vasoconstrictor responses to tyramine (-25 +/- 1 versus -56 +/- 6%), phenylephrine (-11 +/- 4 versus -39 +/- 4%) and clonidine (-12 +/- 4 versus -38 +/- 5%; all P < 0.005) were blunted during exercise compared with adenosine. In contrast, exercise did not significantly blunt the response to tyramine (-30 +/- 2 versus -36 +/- 7%; P = 0.4) or phenylephrine (-16 +/- 2 versus -19 +/- 3%; P = 0.3) in older men, but did attenuate the response to clonidine (-22 +/- 3 versus -37 +/- 6%; P < 0.05). The magnitude of functional sympatholysis, calculated as the difference in the vasoconstrictor responses during adenosine infusion and exercise, was significantly lower in older compared with young men in the presence of tyramine (-6 +/- 7 versus -31 +/- 6%), phenylephrine (-3 +/- 3 versus -28 +/- 4%) and clonidine (-15 +/- 4 versus -26 +/- 3%; all P < 0.05). We conclude that ageing is associated with impaired functional sympatholysis in the vascular beds of contracting forearm muscle in healthy men. These findings might help explain the greater skeletal muscle vasoconstrictor tone and reduced blood flow during large muscle dynamic exercise in older adults.

Non-adrenergic inhibition at prejunctional sites by agmatine of purinergic vasoconstriction in rabbit saphenous artery

We investigated the effects of agmatine, clonidine, xylazine and moxonidine on the purinergic vasoconstriction induced by electrical stimulation in the rabbit isolated saphenous artery without endothelium. Transmural electrical stimulations induced reproducible responses in the arterial preparations, which were abolished by tetrodotoxin at 0.1 microM or pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium salt (PPADS, 30 microM), but were not affected by 1 microM prazosin. Clonidine, xylazine and moxonidine induced transient and concentration-independent vasoconstriction, with threshold concentrations of 1, 3 and 30 microM, respectively. Agmatine, in contrast, did not produce any vascular response even at 1 mM. Lower concentrations of clonidine, xylazine and moxonidine (0.01-0.3 microM) concentration-dependently decreased vasoconstrictor responses to electrical stimulation, whereas agmatine (0.1-1 mM) induced an inhibitory followed by a facilitatory effect on electrically evoked responses. Agmatine, clonidine and moxonidine but not xylazine significantly enhanced the vasoconstriction elicited by 1 mM ATP. The concentration-response curve for NA was shifted to the left slightly by 1 mM agmatine, but not affected by 0.3 microM of other three agonists. Phenoxybenzamine did not affect the vasoconstrictive responses to 1mM ATP and to electrical stimulations, but abolished those to NA. Agmatine at 1mM evoked only an inhibitory effect on electrical stimulation-induced vasoconstriction in the preparation pretreated with phenoxybenzamine, and the inhibitory action was enhanced to 38.6% from the control value (without treatment with phenoxybenzamine) of 22.5%. The non-imidazoline compound xylazine at 0.3 microM lost its inhibitory effect on the neurogenic vasoconstriction in the presence of phenoxybenzamine. In conclusion, agmatine produces a biphasic effect on the purinergic vasoconstriction induced by sympathetic nerve stimulation in the rabbit isolated saphenous artery. The monophasic inhibition of agmatine in the artery treated with phenoxybenzamine is due to an alpha-adrenoceptor-independent mechanism at prejunctional sites, and the potentiation effect of agmatine is mainly dependent on its enhancement of vasoconstriction at postjunctional sites.

Two G protein-coupled receptors activate Na+/H+ exchanger isoform 1 in Chinese hamster lung fibroblasts through an ERK-dependent pathway

The sodium hydrogen exchanger isoform 1 (NHE1) is present in nearly all cells. Regulation of proton flux via the exchanger is a permissive step in cell growth and tumorgenesis and is vital in control of cell volume. The regulation of NHE1 by growth factors involves the Ras-extracellular signal regulated kinase (ERK) pathway, however, the mechanism for G protein-coupled receptor (GPCR) activation of NHE1 is not well established. In this report, the relationship between GPCRs, ERK, and NHE1 in CCL39 cells is investigated. We give evidence that two agonists, the specific alpha(1)-adrenergic agonist, phenylephrine and the water-soluble lipid mitogen, lysophosphatidic acid (LPA) activate NHE1 in CCL39 cells. Activation of ERK by phenylephrine and LPA occurs in a dose- and time-dependent manner. Optimal ERK activation was observed at 10 min and displayed a maximum stimulation at 100 microM phenylephrine and 10 microM LPA. alpha(1)-Adrenergic stimulation also led to a rise in steady-state pH(i) of 0.16+/-0.02 pH units, and incubation with LPA induced a 0.43+/-0.06 pH unit increase in pH(i). Phenylephrine-induced activation of NHE1 transport and ERK activity was inhibited by pretreating the cells with the MEK inhibitor PD98059. While only half of the LPA activatable exchange activity was abolished by PD98059 and U0126. To further demonstrate the specificity of the phenylephrine and LPA regulation of NHE1 and ERK, CCL39 cells were transfected with a kinase inactive MEK. The data indicate that ERK activation is essential for phenylephrine stimulation of NHE1, and that ERK and RhoA are involved in LPA stimulation of NHE1 by more than one mechanism. In addition, evidence of the convergence of these two pathways is shown by the loss of NHE1 activity when both pathways are inhibited and by the partial additivity of the two agonists on ERK and NHE1 activity. These studies indicate a direct involvement of ERK in the alpha(1)-adrenergic activation of NHE1 and a significant role for both ERK and RhoA in LPA stimulation of NHE1 in CCL39 fibroblasts.

Sympathetic alpha-adrenergic regulation of blood flow and volume in hamsters arousing from hibernation

Mammals arousing from hibernation display pronounced regional heterothermy, where the thoracic and head regions warm faster than the abdominal and hindlimb regions. We used laser-Doppler flowmetry to measure peripheral hind foot blood flow during hibernation and arousal and gamma imaging of technetium-labeled albumin to measure whole blood volume distribution in hamsters arousing from hibernation. It was discovered that the hibernating hamster responds to physical but not to sound or hypercapnic stimulation with rapid, 73% reduction of hind foot blood flow. Hind foot blood flow vasoconstriction was maintained from the onset of arousal until late in arousal when rectal temperature was rapidly increased. alpha-Adrenergic blockade early in arousal increased hind foot blood flow by 700%, suggesting that vasoconstriction was mediated by activation of sympathetic tone. Gamma imaging revealed that, by the early phase of arousal from hibernation, the blood volume of the body below the liver is greatly reduced, whereas blood volumes of the thorax and head are much greater than corresponding volumes in anesthetized hamsters. As arousal progresses and cardiac activity increases and regional heterothermy develops, this regional blood volume distribution is largely maintained; however, blood volume slowly decreases in the thoracic region and slowly increases in the shoulder and head regions. The rapid increase in rectal temperature, characteristic of mid- to late- arousal phases, is probably mediated, in part, by reduction of adrenergic tone on abdominal and hindlimb vasculature. Warm blood then moves into the hind body, produces an increase in temperature, blood flow, and blood volume in the hind body and compensatory reductions of blood volume in the neck, head, and thoracic regions.

Role of endothelin in α-adrenoceptor coronary vasoconstriction

It has been proposed that α-adrenoceptor vasoconstriction in coronary resistance vessels results not from α-adrenoceptors on coronary smooth muscle but from α-adrenoceptors on cardiac myocytes that stimulate endothelin (ET) release. The present experiments tested the hypothesis that the α-adrenoceptor-mediated coronary vasoconstriction that normally occurs during exercise is due to endothelin. In conscious dogs ( n = 10), the endothelin ETA/ETB receptor antagonist tezosentan (1 mg/kg iv) increased coronary venous oxygen tension at rest but not during treadmill exercise. This result indicates that basal endothelin levels produce a coronary vasoconstriction at rest that is not observed during the coronary vasodilation during exercise. In contrast, the α-adrenoceptor antagonist phentolamine increased coronary venous oxygen tension during exercise but not at rest. The difference between the endothelin blockade and α-adrenoceptor blockade results indicates that α-adrenoceptor coronary vasoconstriction during exercise is not due to endothelin. However, in anesthetized dogs, bolus intracoronary injections of the α-adrenoceptor agonist phenylephrine produced reductions in coronary blood flow that were partially antagonized by endothelin receptor blockade with tezosentan. These results are best explained if α-adrenoceptor-induced endothelin release requires high pharmacological concentrations of catecholamines that are not reached during exercise.

Mitogen-activated protein kinase phosphatase-1 (MKP-1): >100-fold nocturnal and norepinephrine-induced changes in the rat pineal gland

The norepinephrine-driven increase in mitogen-activated protein kinase (MAPK) activity is part of the mechanism that regulates arylalkylamine N-acetyltransferase (AA-NAT) activity in the rat pineal gland. We now report a marked nocturnal increase in the expression of a MAPK phosphatase, MAP kinase phosphatase-1 (MKP-1), that was blocked by maintaining animals in constant light or treatment with propranolol. MKP-1 expression was regulated by norepinephrine acting through both alpha- and beta-adrenergic receptors. These results establish a nocturnal increase in pineal MKP-1 expression that is under the control of a photoneural system. Because substrates of MKP-1 can influence AA-NAT activity, our findings suggest the involvement of MKP-1 in the regulation of the nocturnal AA-NAT signal.

Role of alpha and beta adrenoceptors in locus coeruleus stimulation-induced reduction in rapid eye movement sleep in freely moving rats

Based on the results of independent studies the involvement of norepinephrine in REM sleep regulation was known. Isolated studies showed that the effect could be mediated through either one or more subtypes of adrenoceptors. Earlier we have reported that REM-OFF neurons continue firing during REM sleep deprivation and mild but continuous stimulation of locus coeruleus (LC) or picrotoxin injection into the LC, that did not allow the REM-OFF neurons in the LC to stop firing, reduced REM sleep. However, the mechanism of action and type of adrenoreceptors involved in REM sleep regulation were unknown. The possible mechanism of action has been investigated in this study. It was proposed that if LC stimulation-induced decrease in REM sleep was due to norepinephrine, adrenergic antagonist must prevent the effect. Therefore, in this study, the effects of alpha1, alpha2 and beta-antagonists, viz. prazosin, yohimbine and propranolol, respectively, and alpha2 agonist, clonidine, on LC stimulation-induced reduction in REM sleep were investigated. The results showed that stimulation of LC inhibited REM sleep by reducing the frequency of generation of REM sleep, although the duration per episode remained unaffected. This decrease in the frequency of REM sleep was blocked by beta-antagonist propranolol while the duration of REM sleep per episode was blocked by alpha1-antagonist, prazosin. Also, a critical level of norepinephrine in the system was required for the generation of REM sleep, however, a higher level may be inhibitory. Based on the results of this study and our earlier studies, an interaction between neurons, containing different neurotransmitters and their subtypes of receptors for LC-mediated regulation of REM sleep has been proposed.

Alpha-adrenergic receptor-stimulated hypertrophy in adult rat ventricular myocytes is mediated via thioredoxin-1-sensitive oxidative modification of thiols on Ras

BACKGROUND

Alpha-adrenergic receptor (alphaAR)-stimulated hypertrophy in adult rat ventricular myocytes is mediated by reactive oxygen species-dependent activation of the Ras-Raf-MEK1/2-ERK1/2 signaling pathway. Because Ras is known to have redox-sensitive cysteine residues, we tested the hypothesis that alphaAR-stimulated hypertrophic signaling is mediated via oxidative modification of Ras thiols.

METHODS AND RESULTS

The effect of alphaAR stimulation on the number of free thiols on Ras was measured with biotinylated iodoacetamide labeling. alphaAR stimulation caused a 48% decrease in biotinylated iodoacetamide-labeled Ras that was reversed by dithiothreitol (10 mmol/L), indicating a decrease in the availability of free thiols on Ras as a result of an oxidative posttranslational modification. This effect was abolished by adenoviral overexpression of thioredoxin-1 (TRX1) and potentiated by the TRX reductase inhibitor azelaic acid. Likewise, alphaAR-stimulated Ras activation was abolished by TRX1 overexpression and potentiated by azelaic acid. TRX1 overexpression inhibited the alphaAR-stimulated phosphorylation of MEK1/2, ERK1/2, and p90RSK and prevented cellular hypertrophy, sarcomere reorganization, and protein synthesis (versus beta-galactosidase). Azelaic acid potentiated alphaAR-stimulated protein synthesis. Although TRX1 can directly reduce thiols, it also can scavenge ROS by increasing peroxidase activity. To examine this possibility, peroxidase activity was increased by transfection with catalase, and intracellular reactive oxygen species were measured with dichlorofluorescein diacetate fluorescence. Although catalase increased peroxidase activity approximately 20-fold, TRX1 had no effect. Likewise, the alphaAR-stimulated increase in dichlorofluorescein diacetate fluorescence was abolished with catalase but retained with TRX1.

CONCLUSIONS

AlphaAR-stimulated hypertrophic signaling in adult rat ventricular myocytes is mediated via a TRX1-sensitive posttranslational oxidative modification of thiols on Ras.

A study on alpha-adrenoceptor mediated contractile responses of high fat diet fed rat thoracic aorta

Feeding rats with high fat diet (HFD) leads to the various conditions of syndrome-X. These are associated with hypertension through a variety of mechanisms. Vascular abnormalities probably contribute to the etiology of many diabetic complications. There is an increase in maximal responses to various agonists with blood vessels of streptozotocin induced diabetic animals. The purpose of this study was to evaluate the development in HFD fed rats for altered biochemical parameters, to assess the vascular responses to phenylephrine (PE), to estimate the KA values and to observe the receptor occupancy. Body weights, plasma triglycerides, cholesterol, and glucose levels were measured every week in male Sprague-Dawley rats. Glucose tolerance test was performed after 4 weeks of feeding. At the end of the fourth week of feeding, concentration-response curves of PE were recorded. Altered KA values of PE (NPD fed rats 2.0 +/- 0.4 microM and HFD fed rats 0.3 +/- 0.1 microM) and receptor occupancy response (NPD fed rats 92.1 +/- 1.7% and HFD fed rats 77.5 +/- 5.6%) strongly suggest that hypertension in HFD fed rats is associated with altered alpha-adrenoceptor function.

alpha- and beta-adrenergic receptor mechanisms in spontaneous contractile activity of rat ileal longitudinal smooth muscle

Gastrointestinal motility is influenced by adrenergic modulation. Our aim was to identify specific subtypes of adrenergic receptors involved in inhibitory mechanisms that modulate gut smooth muscle contractile activity. Muscle strips of rat ileal longitudinal muscle were evaluated for spontaneous contractile activity and for equimolar dose-responses (10(-7) to 3 x 10(-5) M) to the adrenergic agents norepinephrine (nonselective agonist), phenylephrine (alpha(1)-agonist), clonidine (alpha(2)-agonist), prenalterol (beta(1)-agonist), ritodrine (beta(2)-agonist), and ZD7114 (beta(3)-agonist) in the presence and absence of tetrodotoxin (nonselective nerve blocker). Norepinephrine (3 x 10(-5) M) inhibited 65 +/- 6% (mean +/- SEM) of spontaneous contractile activity. The same molar dose of ritodrine, phenylephrine, or ZD7114 resulted in less inhibition (46 +/- 7%, 31 +/- 5%, and 39 +/- 3%, respectively; P < 0.05). The calculated molar concentration of ZD7114 needed to induce 50% inhibition was similar to that of norepinephrine, whereas higher concentrations of phenylephrine or ritodrine were required. Clonidine and prenalterol had no effect on contractile activity. Blockade of intramural neural transmission by tetrodotoxin affected the responses to ritodrine and phenylephrine (but not to norepinephrine or ZD7114), suggesting that these agents exert part of their effects via neurally mediated enteric pathways. Our results suggest that adrenergic modulation of contractile activity in the rat ileum is mediated primarily by muscular beta(3)-, beta(2)-, and alpha(1)-receptor mechanisms; the latter two also involve neural pathways.

Assessment of anti-migraine potential of a novel alpha-adrenoceptor agonist S19014: effects on porcine carotid and regional haemodynamics and human coronary artery

Taking into account the drawbacks associated with the use of triptans, attempts are being made to explore other avenues for the treatment of migraine. Recently, it has been shown that both alpha1- and alpha2-adrenoceptors mediate the constriction of porcine carotid arteriovenous anastomoses, which has effectively served as an experimental model predictive of anti-migraine activity. The present study investigated the effects of a novel alpha-adrenoceptor agonist S19014 (spiro[(1,3-diazacyclopent-1-ene)-5 : 2'-(4',5'-dimethylindane)] fumarate) on carotid and systemic haemodynamics in anaesthetized pigs, and on human isolated coronary arteries. Increasing doses of S19014 (1-30 micro g/kg, i.v.) produced a dose-dependent initial short-lasting vasopressor response and a decrease of total carotid blood flow and conductance. The carotid blood flow and conductance changes were exclusively due to constriction of carotid arteriovenous anastomoses (capillary blood flow increased) and were accompanied by an increase in arterio-jugular venous oxygen saturation difference. Whereas prazosin (100 micro g/kg, i.v.) was ineffective, rauwolscine (300 micro g/kg, i.v.) attenuated the responses to S19014. The compound did not much affect the distribution of cardiac output to peripheral organs when compared with the vehicle group. Furthermore, S19014 only slightly contracted the human isolated coronary artery and its contractions, contrary to those of sumatriptan, were not increased in blood vessels pre-contracted with U46619. These results suggest that (i) the systemic and carotid vascular effects of S19014 are mainly mediated by alpha2-adrenoceptors, and (ii) S19014 could be effective in the treatment of migraine with an improved cardiovascular tolerance.

[Influence of adrenoreceptors on functions of the body]

The remarkably diverse effects of the catecholamines and similar sympathomimetic agents are directly related to an understanding of the classification and different types of adrenoreceptors. Characteristics and physiological regulatory mechanisms of the receptor result in variable response of organ systems to catecholamines stimulation. Different adrenoreceptors regulate distinct physiological processes by controlling the synthesis or release of a variety of second messengers. The goal of this review was to turn one's attention to the below mentioned aspects. There are three known subtypes of each alpha1-, alpha2- and beta-adrenoreceptor types. Structure of the adrenoreceptors, which belong to subtypes of the same receptor type, is similar and structure of the adrenoreceptors of the separate types is very different. Genetic peculiarities of the receptors may influence liability to some diseases. Acting on the adrenoreceptors may change function of many organs and may serve for the treatment of cardiovascular, respiratory tract diseases and allergic reactions. Selective acting on the adrenoreceptors of the separate subtypes may have the different effect on the organs. Great consideration is given for that property in the development of new drugs: substitution by different chemical radicals leads to increasing selectivity for the separate subtypes of the adrenoreceptors. The prolonged use of the adrenomimetics may lead to refractoriness.

Effects of social stress on blood leukocyte distribution: the role of α- and β-adrenergic mechanisms

Social stress in mammals has repeatedly been shown to cause substantial alterations in the distribution pattern of immune cells in the peripheral blood. The studies described here investigated the role of adrenergic mechanisms in mediating stressor-induced changes in blood leukocyte numbers using a social confrontation procedure in the rat. Experimental manipulations were carried out to eliminate the stress-associated release of adrenal hormones or to block the binding of endogenous catecholamines to alpha- and beta-adrenergic receptors. Adrenalectomy completely abolished the stressor-induced decreases in circulating numbers of T helper cells (CD4+), cytotoxic T cells (CD8+) and B cells but was ineffective in preventing neutrophil granulocytosis, monocytosis and an increase in natural killer (NK) cells. Treatment with the alpha-adrenergic antagonist phentolamine (PHE) was highly effective in preventing granulocytosis and partially blocked lymphopenia, but failed to abolish monocytosis and an increase in NK cells. Treatment with the beta-adrenergic antagonists propranolol (PROP) or nadolol (NAD) entirely blocked the increases in monocytes and NK cells. In addition, beta-adrenergic blockade also significantly reduced neutrophilia, with PROP being more effective than NAD. The results presented here provide evidence that catecholamines play an important role in the redistribution of blood leukocytes during social stress. In particular, the mobilization of cells of the innate immune response seems to be regulated by adrenergic mechanisms.

Inhibitory effects of halothane on the thermogenic pathway in brown adipocytes: localization to adenylyl cyclase and mitochondrial fatty acid oxidation

Volatile anesthetics such as halothane efficiently inhibit nonshivering thermogenesis as well as the cellular manifestation of that phenomenon: norepinephrine-induced respiration in brown adipocytes. To identify the molecular site(s) of action of such anesthetics, we have examined the effect of halothane on the sequential intracellular steps from the interaction of norepinephrine with isolated brown adipocytes to the stimulation of mitochondrial respiration (=thermogenesis). We did not identify an inhibition at the level of the adrenergic receptors, but a first site of inhibition was identified as the generation of cAMP by adenylyl cyclase; this led to inhibition of norepinephrine-induced expression of the uncoupling protein-1 (UCP1) gene and reduced norepinephrine-induced lipolysis as secondary effects. Although an inhibition of lipolysis in itself would inhibit thermogenesis, circumvention of this inhibition revealed that a second, postlipolytic, site of inhibition existed: halothane also inhibited the stimulatory effect of exogenous fatty acids on cellular respiration. This inhibition was independent of the presence of UCP1 in the mitochondria of the cells and was thus not directly on the thermogenic uncoupling mechanism. Since not only fatty acid oxidation but also pyruvate oxidation were inhibited by halothane in isolated mitochondria, whereas glycerol-3-phosphate oxidation was not, the second site of action of halothane, evident when cyclase/lipolytic inhibition was circumvented, was located to the respiratory chain, complex I. The results thus explain the inhibition of nonshivering thermogenesis by identifying two sites of action of halothane in brown adipocytes. In addition, the results may open for new formulations of the molecular background to anesthesia.

Ouabain-induced hypertension alters the participation of endothelial factors in alpha-adrenergic responses differently in rat resistance and conductance mesenteric arteries

1. This study compares the role of endothelial factors in alpha-adrenoceptor contractile responses in mesenteric resistance (MRA) and superior (SMA) mesenteric arteries from ouabain-treated (8.0 microg day(-1), 5 weeks) and untreated rats. The role of the renin-angiotensin system was also evaluated. 2. Ouabain treatment increased systolic blood pressure. In addition, ouabain reduced the phenylephrine response in SMA but did not alter noradrenaline responses in MRA. 3. Endothelium removal or the nitric oxide synthase (NOS) inhibitor (l-NAME, 100 microm) increased the responses to alpha-adrenergic agonists in both vessels. After ouabain treatment, both endothelial modulation and the l-NAME effect were increased in SMA, while only the l-NAME effect was increased in MRA. Endothelial NOS expression remained unaltered after ouabain treatment. 4. Indomethacin (10 microm) similarly reduced the noradrenaline contraction in MRA from both groups; in contrast, in SMA, indomethacin only reduced phenylephrine-induced contractions in segments from untreated rats. Co-incubation of l-NAME and indomethacin leftward shifted the concentration-response curves for noradrenaline more in MRA from ouabain-treated rats; tetraethylammonium (2 mm) shifted the noradrenaline curves further leftward only in MRA from untreated rats. 5.Losartan treatment prevents the development of hypertension but not all vascular changes observed after ouabain treatment. 6. In conclusion, a rise in endothelial NO and impaired prostanoid participation might explain the reduction in phenylephrine-induced contraction in SMA after ouabain treatment. An increase in the modulatory effect of endothelial NO and impairment of endothelium-dependent hyperpolarizing factor effect might explain why the ouabain treatment had no effect on noradrenaline responses in MRA.

Regulation of phagocytic process of macrophages by noradrenaline and its end metabolite 4-hydroxy-3-metoxyphenyl-glycol. Role of alpha- and beta-adrenoreceptors

The regulatory capacity of noradrenaline and its end metabolite 4-hydroxy-3-metoxyphenylglycol (HMPG) on the complete phagocytic process of macrophages were investigated. Either noradrenaline or HMPG did not modify adherence. However, 10(-12) M of noradrenaline stimulated the chemotaxis of macrophages, mainly mediated by alpha-adrenergic receptors. In contrast, 10(-12) M of HMPG induced an opposed effect on this stage of the phagocytic process. To stimulate phagocytosis, it is necessary to employ a higher concentration (10(-5) M) of noradrenaline and this effect was blocked with either 10(-6) M propranolol or 10(-6) M phentolamine, and maintained by HMPG. Noradrenaline and HMPG did not modify the microbicide capacity of macrophages (measured by O2- production after phagocytosis). In conclusion, noradrenaline modulates the phagocytic process of macrophages, and this modulation is completed by HMPG, maintaining the phagocytic functions at physiologically optimal levels. Modulation of chemotaxis is mainly mediated by alpha-receptors and phagocytosis needs both alpha- and beta-receptor-stimulation.

Noradrenaline and its end metabolite 3-methoxy-4-hydroxyphenylglycol inhibit lymphocyte chemotaxis: role of alpha- and beta-adrenoreceptors

The capacity of noradrenaline (NA) and its end metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) to modulate the chemotaxis of lymphocytes from a primary immunocompetent organ (thymus) and a secondary one (spleen) was investigated over a range of concentrations from 10(-12) M to 10(-5) M. Lymphocyte chemotaxis was evaluated in a Boyden chamber. The results indicated that 10(-5) M of NA inhibits the chemotaxis of lymphocytes from both the immunocompetent organs studied, and that this effect is blocked by either propranolol (10(-6) M) or phentolamine (10(-5) M). Similarly, 10(-5) M of MHPG induced a decrease in the chemotaxis capacity of the lymphocytes. In conclusion, high physiological concentrations of NA and its end metabolite modulate the mobility of lymphocytes, and the participation of both alpha and beta adrenoreceptors is necessary, showing a new aspect of neuroimmune interactions.

Cardiac sympathetic nerve stimulation does not attenuate dynamic vagal control of heart rate via α-adrenergic mechanism

Complex sympathovagal interactions govern heart rate (HR). Activation of the postjunctional beta-adrenergic receptors on the sinus nodal cells augments the HR response to vagal stimulation, whereas exogenous activation of the presynaptic alpha-adrenergic receptors on the vagal nerve terminals attenuates vagal control of HR. Whether the alpha-adrenergic mechanism associated with cardiac postganglionic sympathetic nerve activation plays a significant role in modulation of the dynamic vagal control of HR remains unknown. The right vagal nerve was stimulated in seven anesthetized rabbits that had undergone sinoaortic denervation and vagotomy according to a binary white-noise signal (0-10 Hz) for 10 min; subsequently, the transfer function from vagal stimulation to HR was estimated. The effects of beta-adrenergic blockade with propranolol (1 mg/kg i.v.) and the combined effects of beta-adrenergic blockade and tonic cardiac sympathetic nerve stimulation at 5 Hz were examined. The transfer function from vagal stimulation to HR approximated a first-order, low-pass filter with pure delay. beta-Adrenergic blockade decreased the dynamic gain from 6.0 +/- 0.4 to 3.7 +/- 0.6 beats x min(-1) x Hz(-1) (P < 0.01) with no alteration of the corner frequency or pure delay. Under beta-adrenergic blockade conditions, tonic sympathetic stimulation did not further change the dynamic gain (3.8 +/- 0.5 beats x min(-1) x Hz(-1)). In conclusion, cardiac postganglionic sympathetic nerve stimulation did not affect the dynamic HR response to vagal stimulation via the alpha-adrenergic mechanism.

Enhanced arteriolar α-adrenergic constriction impairs dilator responses and skeletal muscle perfusion in obese Zucker rats

The present study tested the hypothesis that enhanced vascular α-adrenergic constriction in obese Zucker rats (OZR) impairs arteriolar dilation and perfusion of skeletal muscle at rest and with increased metabolic demand. In lean Zucker rats (LZR) and OZR, isolated gracilis arterioles were viewed via television microscopy, and the contralateral cremaster muscle or gastrocnemius muscle was prepared for study in situ. Gracilis and cremasteric arterioles were challenged with dilator stimuli under control conditions and after blockade of α-adrenoreceptors with prazosin, phentolamine, or yohimbine. Gastrocnemius muscles performed isometric twitch contractions of increasing frequency, and perfusion was continuously monitored. In OZR, dilator responses of arterioles to hypoxia (gracilis), wall shear rate (cremaster), acetylcholine, and iloprost (both) were impaired vs. LZR. Treatment with prazosin and phentolamine (and in cremasteric arterioles only, yohimbine) improved arteriolar reactivity to these stimuli in OZR, although responses remained impaired vs. LZR. Gastrocnemius muscle blood flow was reduced at rest in OZR; this was corrected with intravenous infusion of phentolamine or prazosin. At all contraction frequencies, blood flow was reduced in OZR vs. LZR; this was improved by infusion of phentolamine or prazosin at low-moderate metabolic demand only (1 and 3 Hz). At 5 Hz, adrenoreceptor blockade did not alter blood flow in OZR from levels in untreated rats. These results suggest that enhanced α-adrenergic constriction of arterioles of OZR contributes to impaired dilator responses and reduced muscle blood flow at rest and with mild-moderate (although not with large) elevations in metabolic demand.

Combined NO and PG inhibition augments alpha-adrenergic vasoconstriction in contracting human skeletal muscle

Sympathetic alpha-adrenergic vasoconstrictor responses are blunted in the vascular beds of contracting muscle (functional sympatholysis). We tested the hypothesis that combined inhibition of nitric oxide (NO) and prostaglandins (PGs) restores sympathetic vasoconstriction in contracting human muscle. We measured forearm blood flow via Doppler ultrasound and calculated the reduction in forearm vascular conductance in response to alpha-adrenergic receptor stimulation during rhythmic handgrip exercise (6.4 kg) and during a control nonexercise vasodilator condition (using intra-arterial adenosine) before and after combined local inhibition of NO synthase (NOS; via N(G)-nitro-L-arginine methyl ester) and cyclooxygenase (via ketorolac) in healthy men. Before combined inhibition of NO and PGs, the forearm vasoconstrictor responses to intra-arterial tyramine (which evoked endogenous noradrenaline release), phenylephrine (a selective alpha1-agonist), and clonidine (an alpha2-agonist) were significantly blunted during exercise compared with adenosine treatment. After combined inhibition of NO and PGs, the vasoconstrictor responses to all alpha-adrenergic receptor stimuli were augmented by approximately 10% in contracting muscle (P <0.05), whereas the responses to phenylephrine and clonidine were also augmented by approximately 10% during passive vasodilation in resting muscle (P <0.05). In six additional subjects, PG inhibition alone did not alter the vasoconstrictor responses in resting or contracting muscles. Thus in light of our previous findings, it appears that inhibition of either NO or PGs alone does not affect functional sympatholysis in healthy humans. However, the results from the present study indicate that combined inhibition of NO and PGs augments alpha-adrenergic vasoconstriction in contracting muscle but does not completely restore the vasoconstrictor responses compared with those observed during passive vasodilation in resting muscle.

Left ventricular ejection time: a potential determinant of pulse wave velocity in young, healthy males

OBJECTIVE

Pulse wave velocity (PWV) is a classic marker of vascular stiffness. Recent studies showed that heart rate is an important determinant of PWV. The purpose of this study was to evaluate the role of myocardial function in determining PWV under resting conditions and under adrenergic stimulation.

DESIGN AND METHODS

Hemodynamic parameters were investigated under resting conditions in 102 young, healthy males and under stimulation of either beta- or alpha(2)-adrenoceptors in six young, healthy males. PWV was determined from pressure tracing over the carotid and femoral artery. Central hemodynamics were assessed by impedance cardiography and systolic time intervals. Simple (r) and multiple (beta) regression analyses were used to assess the relationships between PWV and hemodynamic parameters.

RESULTS

Under resting conditions, PWV was correlated to age (beta = 0.259, P = 0.0052), diastolic blood pressure (beta = 0.279, P = 0.0072) and left ventricular ejection time (beta = -0.314, P = 0.0277). Under alpha(2)-adrenergic stimulation PWV was only correlated to diastolic blood pressure (DBP) (beta = 0.806, P = 0.0020). Under beta-adrenergic stimulation PWV was only correlated to left ventricular ejection time index (r = -0.52, P = 0.0325).

CONCLUSIONS

Left ventricular ejection time may be an important determinant of pulse wave velocity under resting and adrenergic conditions in young, healthy males. Further studies are needed to evaluate this relationship in other populations including females and patients with cardiovascular disease.

α-Adrenoceptor-mediated coronary vasoconstriction is augmented during exercise in experimental diabetes mellitus

This study tested whether α-adrenoceptor-mediated coronary vasoconstriction is augmented during exercise in diabetes mellitus. Experiments were conducted in dogs instrumented with catheters in the aorta and coronary sinus and with a flow transducer around the circumflex coronary artery. Diabetes was induced with alloxan monohydrate ( n = 8, 40 mg/kg iv). Arterial plasma glucose concentration increased from 4.7 ± 0.2 mM in nondiabetic, control dogs ( n = 8) to 21.4 ± 1.9 mM 1 wk after alloxan injection. Coronary blood flow, myocardial oxygen consumption (MV̇o2), aortic pressure, and heart rate were measured at rest and during graded treadmill exercise before and after infusion of the α-adrenoceptor antagonist phentolamine (1 mg/kg iv). In untreated diabetic dogs, exercise increased MV̇o2 2.7-fold, coronary blood flow 2.2-fold, and heart rate 2.3-fold. Coronary venous Po2 fell as MV̇o2 increased during exercise. After α-adrenoceptor blockade, exercise increased MV̇o2 3.1-fold, coronary blood flow 2.7-fold, and heart rate 2.1-fold. Relative to untreated diabetic dogs, α-adrenoceptor blockade significantly decreased the slope of the relationship between coronary venous Po2 and MV̇o2. The difference between the untreated and phentolamine-treated slopes was greater in the diabetic dogs than in the nondiabetic dogs. In addition, the decrease in coronary blood flow to intracoronary norepinephrine infusion was significantly augmented in anesthetized, open-chest, β-adrenoceptor-blocked diabetic dogs compared with the nondiabetic dogs. These findings demonstrate that α-adrenoceptor-mediated coronary vasoconstriction is augmented in alloxan-induced diabetic dogs during physiological increases in MV̇o2.

5-HT1B receptors, alpha2A/2C- and, to a lesser extent, alpha1-adrenoceptors mediate the external carotid vasoconstriction to ergotamine in vagosympathectomised dogs

It has previously been suggested that ergotamine produces external carotid vasoconstriction in vagosympathectomised dogs via 5-HT1B/1D receptors and alpha2-adrenoceptors. The present study has reanalysed this suggestion by using more selective antagonists alone and in combination. Fifty-two anaesthetised dogs were prepared for ultrasonic measurements of external carotid blood flow. The animals were divided into thirteen groups (n=4 each) receiving an i.v. bolus injection of, either physiological saline (0.3 ml/kg; control), or the antagonists SB224289 (300 microg/kg; 5-HT1B), BRL15572 (300 microg/kg; 5-HT1D), rauwolscine (300 microg/kg; alpha2), SB224289 + BRL15572 (300 microg/kg each), SB224289 + rauwolscine (300 microg/kg each), BRL15572 + rauwolscine (300 microg/kg each), rauwolscine (300 microg/kg) + prazosin (100 microg/kg; alpha1), SB224289 (300 microg/kg) + prazosin (100 microg/kg), SB224289 (300 microg/kg) + rauwolscine (300 microg/kg) + prazosin (100 microg/kg), SB224289 (300 microg/kg) + prazosin (100 microg/kg) + BRL44408 (1,000 microg/kg; alpha2A), SB224289 (300 microg/kg) + prazosin (100 microg/kg)+ imiloxan (1,000 microg/kg; alpha2B), or SB224289 (300 microg/kg) + prazosin (100 microg/kg) + MK912 (300 microg/kg; alpha2C). Each group received consecutive 1-min intracarotid infusions of ergotamine (0.56, 1, 1.8, 3.1, 5.6, 10 and 18 microg/min), following a cumulative schedule. In saline-pretreated animals, ergotamine induced dose-dependent decreases in external carotid blood flow without affecting arterial blood pressure or heart rate. These control responses were: unaffected by SB224289, BRL15572, rauwolscine or the combinations of SB224289 + BRL15572, BRL15572 + rauwolscine, rauwolscine + prazosin, SB224289 + prazosin, or SB224289 + prazosin + imiloxan; slightly blocked by SB224289 + rauwolscine; and markedly blocked by SB224289 + rauwolscine + prazosin, SB224289 + prazosin + BRL44408 or SB224289 + prazosin + MK912. Thus, the cranio-selective vasoconstriction elicited by ergotamine in dogs is predominantly mediated by 5-HT1B receptors as well as alpha2A/2C-adrenoceptor subtypes and, to a lesser extent, by alpha1-adrenoceptors.

[Update on medical treatment of female stress urinary incontinence]

A reasonable assumption is that incontinence would be relieved by increasing urethral resistance through stimulating alpha-adrenergic receptors in urethral smooth muscle. A review of available medical treatment of stress urinary incontinence is done. Alpha-receptor agonists are not in common use because of systemic side-effects. Estrogens do not seem to have beneficial effect on stress urinary incontinence. Currently, new molecules such as duloxetine, are uptake inhibitor of serotonin and noradrenaline could provide a noninvasive therapy for patients with urinary incontinence. Further studies to identify clinical applications are required.

Adrenergic receptor knockout mice: distinct functions of 9 receptor subtypes

The biological effects of epinephrine and norepinephrine are mediated via 9 different adrenergic receptor subtypes, which all belong to the superfamily of G protein-coupled receptors. Although pharmacological ligands for adrenergic receptors have an important place in medical therapy, the full therapeutic potential of the 9 adrenergic receptor subtypes has not been explored yet. To dissect the physiological relevance of adrenergic receptor subtype diversity, gene-targeted mouse models carrying deletions in these receptor genes ("knockout mice") have been generated. This review gives an overview of the phenotypes observed in mice deficient in adrenergic receptors and discusses the therapeutic relevance of subtype-specific drug therapy.