Function of myocardial alpha-adrenoceptors

The PI3K-AKT-mTOR pathway activates recovery from general anesthesia

We investigated roles of PI3K-AKT-mTOR pathway in recovery from general anesthesia. Sprague-Dawley rats divided into five groups: saline+artificial cerebrospinal fluid (ACSF; Group A), ketamine+ACSF (Group B), ketamine+IGF-1 (Group C), ketamine+PI3K inhibitor (Group D), and PI3K/Akt agonists (Group E). Proportion of δ waves on ECoGs was recorded. Rats were tested for duration of loss of righting reflex (LORR), ataxic period and behavior in Morris water maze. mRNA and protein expression of members of PI3K-AKT-mTOR pathway were measured by RT-qPCR and Western blots. Histopathologic changes in hippocampal tissues observed by HE staining. We found that the proportion of δ waves decreased in Group C, while increased in Group D compared with Group B; the durations of LORR and ataxic period were shorter in Group C, but longer in Group D. In Morris water maze, escape latency (EL) and duration and frequency of staying on platform was shorter in Group C and longer in Group D than in Group B. Group A exhibited low expression of proteins in PI3K-AKT-mTOR pathway, while p-AKT, p-mTOR and p-P70S6K expression increased in cerebral cortex, brain stem, and thalamus in Group C. By contrast, expression of those proteins was lower in Group D than Group B. Those proteins expressions were higher in Group E than in Group A. HE staining showed that anesthesia may induce cell apoptosis in rat hippocampal CA1 areas, and PI3K/Akt agonists could inhibit apoptosis. Our results suggest that activation of PI3K-AKT-mTOR pathway may promote recovery from general anesthesia and enhance spatial learning and memory.

β-Adrenergic Receptor-Stimulated Cardiac Myocyte Apoptosis: Role of β1 Integrins

Increased sympathetic nerve activity to the myocardium is a central feature in patients with heart failure. Accumulation of catecholamines plays an important role in the pathogenesis of heart disease. Acting via β-adrenergic receptors (β-AR), catecholamines (norepinephrine and isoproterenol) increase cardiac myocyte apoptosis in vitro and in vivo. Specifically, β₁-AR and β₂-AR coupled to Gαs exert a proapoptotic action, while β₂-AR coupled to Gi exerts an antiapoptotic action. β1 integrin signaling protects cardiac myocytes against β-AR-stimulated apoptosis in vitro and in vivo. Interaction of matrix metalloproteinase-2 (MMP-2) with β1 integrins interferes with the survival signals initiated by β1 integrins. This paper will discuss background information on β-AR and integrin signaling and summarize the role of β1 integrins in β-AR-stimulated cardiac myocyte apoptosis.

Alpha-1-adrenergic receptors: targets for agonist drugs to treat heart failure

Evidence from cell, animal, and human studies demonstrates that α1-adrenergic receptors mediate adaptive and protective effects in the heart. These effects may be particularly important in chronic heart failure, when catecholamine levels are elevated and β-adrenergic receptors are down-regulated and dysfunctional. This review summarizes these data and proposes that selectively activating α1-adrenergic receptors in the heart might represent a novel and effective way to treat heart failure. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Role of beta 1- and beta 2-adrenoceptors in hypertrophic and apoptotic effects of noradrenaline and adrenaline in adult rat ventricular cardiomyocytes

In adult rat ventricular cardiomyocytes alpha1-adrenoceptor (AR) stimulation causes increases in protein synthesis. On the other hand beta1-AR stimulation inhibits protein synthesis, and evokes apoptotic cell death. We studied, in adult rat ventricular cardiomyocytes, effects of noradrenaline (NA), adrenaline (ADR) and phenylephrine (PE) on protein synthesis (assessed by [3H]-phenylalanine incorporation into the cardiomyocytes) in relation to effects on early apoptosis (measured by Annexin V/propidium iodide staining). PE (10(-9)-10(-5) M) induced protein synthesis was not affected by the beta1-AR blocker CGP 20712A (CGP, 300 nM) or beta2-AR blocker ICI 118,551 (ICI, 55 nM). ADR (10(-9)-10(-5) M) induced protein synthesis was enhanced by CGP and decreased by ICI. Pretreatment of the cardiomyocytes with pertussis toxin (PTX) decreased NA- and ADR- induced protein synthesis, but did not affect PE-effects. NA (10(-5) M) and ADR (10(-5) M) caused a significant increase in the number of apoptotic cells; these effects were enhanced by PTX-treatment, abolished by CGP, but not significantly affected by ICI. Furthermore, there was a significant negative correlation between catecholamine-evoked apoptosis and catecholamine-induced hypertrophic effects. We conclude that, in ventricular cardiomyocytes of adult rats, growth-promoting effects of NA and ADR are composed of alpha1A-AR mediated increase in protein synthesis and beta1-AR mediated apoptosis that counteracts increases in protein synthesis. The role of beta2-adrenoceptor appears to be a balance of antiapoptotic effects via a PTX-sensitive pathway and proapoptotic effects via a GS-adenylyl cyclase pathway.

Alpha-2 adrenergic receptors stimulate actin organization in developing fetal rat cardiac myocytes

Expression of alpha(2)-adrenergic receptors (alpha(2)-AR) is very high in fetal rat heart although numbers decline with increasing gestational age. The current experiments were designed to identify the subtypes of alpha(2)-AR expressed and the function of these receptors in fetal cardiac myocytes. Expression of alpha(2)A and alpha(2)C, but not alpha(2)B, was confirmed in the myocyte population by indirect immunofluorescence microscopy with subtype-specific antibodies and by Western blot. Both dexmedetomidine, an alpha(2)-selective agonist, and norepinephrine, increased actin cytoskeleton organization and this increase was blocked by the alpha(2)-selective antagonist, atipamezole. Furthermore, dexmedetomidine inhibited isoproterenol-stimulated cAMP accumulation in isolated fetal rat heart and this was blocked by rauwolscine. Therefore, functional alpha(2)A and alpha(2)B subtypes are present in the fetal rat heart where they may have a role in cardiac development.

Effect of intermittent hypoxia on cardiovascular function, adrenoceptors and muscarinic receptors in Wistar rats

The usual model of intermittent hypoxia (sleep apnoea) corresponds to repeated episodes of hypoxia from a few seconds to a few hours interspersed with episodes of normoxia. The aim of this study was to evaluate in rats the effect of two periods of intermittent exposure for 2 months to hypoxia (IHX1, 24 h in hypoxia (428 Torr), 24 h in normoxia; IHX2, 48 h in hypoxia (428 Torr), 24 h in normoxia) as a new model of hypoxia simulating intermittent exposure to high altitude experienced by Andean miners. We assessed the haematological parameters, time course of resting heart rate and systolic blood pressure. We also evaluated the expression of adrenergic and muscarinic receptors. IHX1 and IHX2 produced an increase in haematocrit, haemoglobin concentration and mean corpuscular volume as previously seen in most hypoxic models. IHX1 and IHX2 induced a similar sustained elevation of systolic blood pressure (132 +/- 2 and 135 +/- 3 mmHg, respectively, vs. the control level of 121 +/- 16 mmHg) after 10 days of exposure without change in heart rate. Right ventricular (RV) hypertrophy (225 +/- 13 and 268 +/- 15 mg g(-1), vs. 178 +/- 7 mg g(-1) and downregulation of alpha1-adrenoceptor (RV: 127 +/- 21 and 94 +/- 16 fmol mg(-1) vs. 157 +/- 8 fmol mg(-1); left ventricle (LV): 141 +/- 5 and 126 +/- 9 fmol mg(-1) vs. 152 +/- 5 fmol mg(-1)) have been found in both groups, with right ventricular hypertrophy being greater and alpha1-adrenoceptor density being lower in IHX2 than in HX1 groups. These data indicate that both parameters are related to the time of exposure to hypoxia. IHX1 and IHX2 produced the same magnitude of upregulation of muscarinic receptors (LV, 60%; RV, 40%), and no change in beta-adrenoceptors. In conclusion, exposure to intermittent hypoxia led to polycythaemia and RV hypertrophy as observed in other types of hypoxia. A specific cardiovascular response was seen, that is an increase in blood pressure without change in heart rate, which was different from the one observed in episodic and chronic hypoxia. Furthermore, this model involved specific modifications of alpha1-adrenergic and muscarinic expression.

Adrenergic regulation of cardiac myocyte apoptosis

The direct effects of catecholamines on cardiac myocytes may contribute to both normal physiologic adaptation and pathologic remodeling, and may be associated with cellular hypertrophy, apoptosis, and alterations in contractile function. Norepinephrine (NE) signals via alpha- and beta-adrenergic receptors (AR) that are coupled to G-proteins. Pharmacologic studies of cardiac myocytes in vitro demonstrate that stimulation of beta1-AR induces apoptosis which is cAMP-dependent and involves the voltage-dependent calcium influx channel. In contrast, stimulation of beta2-AR exerts an anti-apoptotic effect which appears to be mediated by a pertussis toxin-sensitive G protein. Stimulation of alpha1-AR causes myocyte hypertrophy and may exert an anti-apoptotic action. In transgenic mice, myocardial overexpression of either beta1-AR or G(alpha)s is associated with myocyte apoptosis and the development of dilated cardiomyopathy. Myocardial overexpression of beta2-AR at low levels results in improved cardiac function, whereas expression at high levels leads to dilated cardiomyopathy. Overexpression of wildtype alpha1B-AR does not result in apoptosis, whereas overexpression of G(alpha)q results in myocyte hypertrophy and/or apoptosis depending on the level of expression. Differential activation of the members of the mitogen-activated protein kinase (MAPK) superfamily and production of reactive oxygen species appear to play a key role in mediating the actions of adrenergic pathways on myocyte apoptosis and hypertrophy. This review summarizes current knowledge about the molecular and cellular mechanisms involved in the regulation of cardiac myocyte apoptosis via stimulation of adrenergic receptors and their coupled effector pathways.

Chronic alpha-adrenergic receptor stimulation modulates the contractile phenotype of cardiac myocytes in vitro

BACKGROUND

Heart failure is characterized by contractile dysfunction of the myocardium and elevated sympathetic activity. We tested the hypothesis that chronic alpha-adrenergic (alpha-ADR) stimulation modifies the molecular and contractile phenotype of cardiac myocytes.

METHODS AND RESULTS

Adult rat ventricular myocytes in culture were exposed to alpha-ADR stimulation (norepinephrine + propranolol) for 48 hours. alpha-ADR stimulation decreased the mRNAs for sarcoplasmic reticulum Ca(2+)-ATPase and Ca(2+) release channel by 56% and 52%, respectively, and increased mRNA and protein for the Na(+)-Ca(2+) exchanger by 70% and 39%, respectively. After washout of the alpha-ADR agonist, simultaneous measurement of [Ca(2+)](i) transients with fura 2 and myocyte shortening by video edge-detection showed that [Ca(2+)](i) amplitude and myocyte shortening were decreased in alpha-ADR-treated myocytes, and the time to peak and time from peak to 80% decline of both [Ca(2+)](i) and myocyte shortening were increased. The concentration-response curve for myocyte shortening by the Na(+) channel activator veratridine was shifted leftward in alpha-ADR-stimulated myocytes (EC(50), 21.6+/-4.6 versus 105.8+/-10.5 nmol/L, P:<0.001).

CONCLUSIONS

Chronic alpha-ADR stimulation of cardiac myocytes causes decreases in the expression of sarcoplasmic reticulum Ca(2+)-ATPase and the Ca(2+) release channel that are associated with decreases in [Ca(2+)](i) and contractility. alpha-ADR stimulation simultaneously increases Na(+)-Ca(2+) exchanger expression, thereby increasing sensitivity to intracellular Na(+).

Canine dilated cardiomyopathy: lymphocyte and cardiac alpha(1)- and beta-adrenoceptor concentrations in normal and affected great danes

Serum catecholamine levels and myocardial and lymphocyte adrenergic receptor (AR) concentrations were measured in adult great danes affected by canine dilated cardiomyopathy (DCM) and compared to those of healthy animals. A non-homogeneous population of beta -AR, consisting of beta(1)-AR and beta(2)-AR, was observed in healthy (41 and 59%, respectively) and affected (17 and 83%, respectively) dog lymphocytes. Binding assays revealed that total beta -AR, beta(1)-AR and alpha(1)-AR were significantly downregulated (P<0.05;P<0.01;P<0. 001), both in lymphocyte and myocardial cell membranes of affected dogs. beta(2)-Adrenergic receptor concentrations were significantly reduced only in lymphocyte and right atrium cell membranes (P<0.05). Downregulation was not associated with alterations in receptor binding characteristics, as no significant differences in K(d)values were found. Mean plasma catecholamine levels were significantly higher (P<0.01) in DCM dogs (939+/-41) than in normal subjects (348+/-32), thus suggesting a sympathetic activation. The present study indicates a condition similar to that observed in human patients affected by DCM and that adrenergic receptors in canine lymphocytes reflect the fluctuation of adrenergic receptor concentrations in the myocardium.

Effects of class III antiarrhythmic agents in an in vitro rabbit model of spontaneous torsades de pointe

Acquired long QT syndrome develops as a result of pharmacological interventions that prolong action potential duration. Excessive action potential prolongation may lead to torsade de pointes, a potentially fatal arrhythmia. To study this arrhythmia, in vivo models have been developed, but are difficult to interpret due to the complex nature of the intact metabolic, nervous and humoral systems. To more clearly examine the propensity of various Class III agents to elicit torsades de pointe, an in vitro model of spontaneous torsades de pointe was used in isolated perfused rabbit hearts. Male New Zealand white rabbits were anesthetized with sodium pentobarbital, and hearts isolated and perfused in a Langendorff apparatus. Electrocardiogram and epicardial monophasic action potentials were continuously recorded, and methoxamine (30 nM) and acetylcholine (0.3 microM) were given throughout the experiment. After 10 min of methoxamine and acetylcholine perfusion, Class III agents, dofetilide (0.1 to 0.7 microM), E-4031 (0.1 to 0.5 microM), D-sotalol (10 to 30 microM), or clofilium (0.1 to 0.3 microM), were given. All agents, except D-sotalol, induced torsades de pointe in 100% of hearts tested. D-Sotalol (30 microM) elicited a low incidence of torsades de pointe (25%). This could be explained by the limited prolongation of action potential duration with D-sotalol as compared to other Class III agents under these conditions. Dofetilide, a selective Class III agent, alone did not induce torsades de pointe. Nadolol (3 microM), a beta-adrenoceptor antagonist, increased the propensity of dofetilide to elicit torsades de pointe. In conclusion, increases in action potential duration (i.e., using Class III agents) in combination with a low heart rate (muscarinic receptor stimulation) and increases in intracellular Ca2+ (alpha-adrenoceptor stimulation) are needed to develop torsades de pointe in this model. Modulating these systems may provide us with new insights into preventing the initiation or maintenance of this arrhythmia.

In vivo demonstration of alpha-adrenoceptor-mediated positive inotropy in pithed rats: evidence that noradrenaline does not stimulate myocardial alpha-adrenoceptors

1. This study examines whether positive inotropy via alpha-adrenoceptors could be observed in vivo in pithed rats. Cardiac contractility was measured as the maximum rate of rise of left ventricular pressure (dP/dt(max)). Heart rate and aortic blood pressure were also recorded. 2. The selective alpha1-adrenoceptor agonists, methoxamine, cirazoline, amidephrine and phenylephrine caused dose-related increases in dP/dt(max). This response was progressively reduced by increasing doses of the alpha1-adrenoceptor antagonist prazosin. However, since the concomitant increase in diastolic blood pressure (DBP) was also blocked, the changes in dP/dt(max) may have been a consequence of increased after load. 3. Adrenaline and noradrenaline also increased dP/dt(max), accompanied by pressor responses. Propranolol (1 mg kg(-1)) antagonized the increase in dP/dt(max) in response to noradrenaline, suggesting beta-adrenoceptor involvement, but not that to adrenaline. The additional presence of prazosin (1 mg kg(-1)) further shifted the dose-response curves for both noradrenaline and adrenaline to the right. 4. Analysis of the increases in dP/dt(max) at predetermined increases in DBP by each agonist revealed three groups of regression lines. Adrenaline in the presence of propranolol and the four selective alpha1-adrenoceptor agonists occupied a common central position. Above this group were adrenaline and noradrenaline in the absence of antagonists; their additional effects on contractility were beta-adrenoceptor-mediated since the regression lines were lowered by propranolol. Clearly below the main group of agonists was noradrenaline in the presence of propranolol. 5. Thus, for a given increase in DBP, adrenaline (in the presence of beta-blockade) and the alpha1-adrenoceptor agonists exert an additional inotropic effect to noradrenaline (also in the presence of beta-blockade). This is concluded to be an alpha-adrenoceptor-mediated increase in cardiac contractility which is not shared by noradrenaline.

Combined overdose with verapamil and atenolol: treatment with high doses of adrenergic agonists

A 55-year-old man was admitted following an overdose of sustained-release verapamil (calcium channel blocker) and ordinary-release atenolol (beta-1 blocker). At admission, there was extreme bradycardia (20-25 beats min-1) and hypotension (systolic arterial pressure 40-50 mm Hg). To counteract the cardiovascular depression, prenalterol, dopamine, dobutamine, isoprenaline, adrenaline and noradrenaline were used. A satisfactory state was obtained with adrenaline, noradrenaline and dopamine infused at high rates. Cardiac output was then more than 101 min-1, with a very low total peripheral resistance. The infusion of the adrenergic agonists could be interrupted on day 3. Prolonged ventilator treatment was necessary but the patient recovered without sequelae. Treatment options for similar cases are outlined.

Interaction between Na+/phosphate-cotransporter and the adrenoceptors in myocardial depression

The primary objective of this study was to test the hypothesis that an increase in the alpha1-adrenoceptor tone would potentiate the myocardial biphasic contractile response to inorganic phosphate [Pi, the substrate of Na/Pi-cotransporter (NP)]. A second aim was to determine whether activation of alpha1-adrenoceptor is necessary for the NP-mediated increase in myocardial contractility (+dP/dt). Earlier study from this laboratory showed that high concentration of Pi (10 mM) produces a biphasic contractile response: initial increase in +dP/dt was followed by decline. In another study, Pi (3.5 mM) potentiated phenylephrine (PHE)-induced increase in +dP/dt. The alpha1-adrenoceptor was not blocked in these studies, and it can still be activated by the electrical stimulation of the sympathetic nerve terminals to the heart. Additionally, alpha1-adrenoceptor-activated increases in the activity of NP have been reported in numerous studies in noncardiac tissues and cell lines; therefore it is not clear whether Pi-induced increase in +dP/dt occurs only in the presence of alpha1-adrenoceptor activation. This study was performed by using isolated perfused rat heart in the condition of controlled extracellular calcium activity (0.72 mM); fixed preload (15 mm Hg); and constant heart rate (280 beats/min) and coronary flow (8 ml/min). The electronically differentiated value of the left ventricular pressure (LVP) signal was used as an index of myocardial contractility. The data show that activation of the alpha1-adrenoceptor is not necessary for the Pi-induced increase in +dP/dt (i.e., NP-mediated increase in +dP/dt has both the alpha1-adrenoceptor-dependent and alpha1-adrenoceptor-independent components. The interaction between alpha1-adrenoceptor agonist (PHE) and Pi (10 mM) did not produce a biphasic myocardial contractile response in the presence of propranolol. Because our earlier data on myocardial biphasic contractile response to 10 mM Pi was obtained when neither the beta- nor the alpha-adrenoceptor was blocked, we carried out more studies to see whether beta-adrenoceptor plays a role in this Pi-induced biphasic response. When both the alpha- and beta-adrenoceptors were activated with norepinephrine (NE), myocardial depression by high Pi concentration was markedly potentiated. This myocardial depression did not occur in the presence of phosphonoformate, a selective inhibitor of NP. It also did not occur when alpha1-adrenoceptor was blocked. Our data suggest that alpha1- and beta-adrenoceptors do not interact with the cardiac NP to potentiate the Pi-induced biphasic contractile response, but they interact in a manner that potentiates Pi-induced myocardial depression.

Influences of changes in calcium concentrations, cocaine and clonidine on the cardiac effect of acetaldehyde in rat isolated atria

1. Cardiovascular responses to acetaldehyde (AChO) were investigated in rat isolated atria. 2. Results show that two different doses of AChO (0.29 and 0.88 mM) induce positive inotropic effects on rat atria, and express Ca2+ dependence when analyzed at three external Ca2+ concentrations (0.6, 1.1, 2.2 mM). 3. Cocaine (3.5 and 35 microM) produced significant potentiation of the AChO-positive (0.29 and 0.88 mM) inotropic effect in 1.1 mM Ca2+ medium. 4. Clonidine (40 microM) increased the peak tension developed (PTD) induced by AChO (both 0.29 and 0.88 mM) only 1.1 mM Ca2+ medium. 5. It is suggested that the positive inotropic effect produced by AChO involves a Ca2(+)-dependent mechanism, which can be potentiated by the additional stimulation of alpha-adrenergic receptors.

Resuscitating effect of melanocortin peptides after prolonged respiratory arrest

1. The resuscitating activity of melanocortin peptides (MSH-ACTH peptides) was tested in an experimental model of prolonged respiratory arrest. 2. Anaesthetized, endotracheally intubated rats subjected to a 5 min period of ventilation interruption, invariably died from cardiac arrest within 6-9 min of resumption of ventilation. 3. When resumption of ventilation was associated with the simultaneous intravenous (i.v.) injection of a melanocortin peptide (alpha-MSH or ACTH-(1-24)) (160 microg kg(-1) there was an almost immediate (within 1 min), impressive increase in cardiac output, heart rate, mean arterial pressure (+ 560% of the before-treatment value) and pulse pressure (+356% of the before-treatment value), with full recovery of electroencephalogram after 30-45 min. Blood gases and pH were normalized within 15-60 min after treatment, and all treated animals eventually recovered completely and survived indefinitely (= more than 15 days). 4. The same response was observed in adrenalectomized animals, as well as in animals pretreated with a beta1-adrenoceptor blocking agent (atenolol, 3 mg kg(-1), i.v.), or with an alpha1-adrenoceptor blocking agent (prazosin, 0.1 mg kg(-1), i.v.), or with an adrenergic neurone blocking agent (guanethidine, 10 mg kg(-1), intraperitoneally). 5. An effect quite similar to that produced by melanocortins was obtained with ouabain (0.1 mg kg(-1), i.v.); the antioxidant drug, glutathione (75 mg kg(-1), i.v.) also produced 100% resuscitation, but the effect was slower in onset. On the other hand, adrenaline (0.005 mg kg(-1), i.v.) was able to resuscitate only 1 out of 8 rats and dobutamine (0.02 mg kg(-1), i.v.) resuscitated 4 out of 8 rats; moreover, the effect of both catecholamines was much slower in onset than that of melanocortins and the initial, impressive stimulation of cardiovascular function was absent. 6. These results show that melanocortin peptides have a resuscitating effect in a pre-terminal condition produced in rats by prolonged asphyxia. This effect seems primarily due to the restoration of cardiac function, not mediated by catecholamines. These data also suggest that these peptides may have potential therapeutic value in conditions of transient cardiac hypoxia and re-oxygenation such as occur in coronary artery disease.

Myocardial alpha1-adrenoceptor: inotropic effect and physiologic and pathologic implications

Alpha1-adrenergic receptors have been found in myocardium of all mammalian species. Although the exact underlying mechanisms have not been conclusively determined, it would appear that the myocardial effects of alpha1-adrenoceptors may vary in importance according to the pathophysiologic process involved. In physiological conditions, this receptor system plays a role in cardiac growth, cardiac contraction, and has both an antiarrhythmic function as well as a role in cardiac adaptation to various situations. This system is also involved in some pathological processes such as ischemia/reperfusion, ischemic preconditioning, and cardiac hypertrophy. The role of alpha1-adrenoceptors in heart failure is somewhat controversial. Experimental evidence suggests that myocardial alpha1-adrenoceptors can have either beneficial or deleterious effects on the heart. It thus seems possible that the development of agents specific to certain subtypes of alpha1-adrenoceptor and a better understanding of their role in pathophysiologic states could be clinically relevant.

Thyroid hormone modulates inotropic responses, alpha-adrenoceptor density and catecholamine concentrations in the rat heart

We investigated the influence of hyper- and hypothyroidism on basal parameters of isolated perfused hearts of rats. In addition the effects of different extracellular calcium concentrations ([Ca2+]o), the calcium entry promoter Bay K8644 and the alpha 1-adrenoceptor agonist methoxamine were investigated. Since alterations in alpha-adrenoceptor density could explain the increased sensitivity to methoxamine in hearts from hypothyroid rats, alpha 1-adrenoceptor density in the left ventricle was also established. Different time-schedules of exposure to hyper- and hypothyroidism were used to investigate whether the influence of chronic dysthyroid states on alpha 1-adrenoceptor density is transient and time-dependent. Simultaneously myocardial noradrenaline and adrenaline tissue concentrations were determined, since they might correlate with the observed changes. Hyperthyroidism was induced by feeding rats for 1, 4 and 8 weeks with 5 mg/kg L-thyroxine (T4)-containing rat chow. Hypothyroid rats were obtained by adding 0.05% propylthiouracil (PTU) to the drinking water during 1, 4 and 8 weeks. For the functional experiments animals were treated during 4 weeks, to mimic the clinical situation of a chronic endocrine disease. Langendorff hearts from hyperthyroid hearts showed an increased maximally developed relaxation velocity, whereas Langendorff hearts from hypothyroid rats showed an increased left ventricular pressure (LVP). We observed an increased maximal inotropic response to [Ca2+]o in hearts from both hyperthyroid and hypothyroid rats, indicating that both dysthyroid states interfere with the handling of calcium ions by the contractile apparatus. Unchanged responses to Bay K8644 in hearts from hyperthyroid and depressed responses in hearts from hypothyroid rats suggest that the involvement of L-type calcium channels is rather unlikely. Furthermore, the reflex increase in coronary flow in response to enhanced contractile force appeared to fail in hearts from hypothyroid rats. Sensitivity of the response to methoxamine was increased in hearts from hypothyroid rats, which was accompanied by a decrease in the number of myocardial alpha 1-adrenoceptors. Both T4 and PTU treatment resulted in a non-transient decrease of alpha 1-adrenoceptor density in left ventricular tissue. Furthermore, hypothyroidism increased the percentage of alpha 1A-binding sites, whereas in hyperthyroidism the distribution of the alpha 1-adrenoceptor subtypes was not affected. Myocardial tissue concentrations of noradrenaline and adrenaline were unchanged in hyperthyroid rats and decreased in hypothyroid rats. The present study indicates that thyroid hormones have a direct rather than a sympathetically mediated effect on alpha 1-adrenoceptor mediated myocardial functions.

Pharmacological studies on a new antihypertensive agent, S-2150, a benzothiazepine derivative: 1. Antinecrotic and antiarrhythmic effects in reperfused rat hearts

S-2150 is a new 1,5-benzothiazepine derivative that inhibits [3H]diltiazem and [3H]WB4101 bindings to the membrane of rat tissue. The effects of S-2150 on ischemia/ reperfusion injury were studied in anesthetized rats. S-2150 reduced the myocardial infarct size (IS) induced by 20-min coronary artery occlusion followed by reperfusion. To evaluate reperfusion-induced ventricular tachycardia and fibrillation (VT, VF), we occluded the coronary artery for 4 min and then reperfused it. The incidence of arrhythmia was blocked by S-2150, and this effect offered protection against cardiac death. Prazosin did not modify the IS or incidence of reperfusion arrhythmias, but combined treatment with a noneffective dose of diltiazem showed significant cardioprotective effects. We also compared the direct effects of S-2150 and diltiazem on cardiac function and coronary perfusion flow using isolated rat hearts. Both drugs decreased mechanical function and increased coronary flow, with S-2150 being less cardiodepressive and more vasodilatory. S-2150 is cardioprotective at doses comparable to hypotensive doses even though its cardiodepressant effect is much weaker than that of diltiazem. This effectiveness may be partly explained by its dual characteristics: blocking the Ca channel and the alpha 1-adrenoceptor.

The positive inotropic effect of alpha 1A-adrenoceptor stimulation is inhibited by 4-aminopyridine

This study was designed to determine if 4-aminopyridine, a reported inhibitor of the transient outward K+ current (Ito), alters the inotropic actions elicited via stimulation of WB4101- or chloroethylclonidine-sensitive receptors in rat myocardium. WB4101 (N-[2-(2, 6-dimethoxyphenoxy)ethyl]-2,3-dihydro-1,4-benzodioxin-2-m ethanamine) is a competitive antagonist that is selective for alpha 1A- and alpha 1C-adrenoceptors, while chloroethylclonidine is an irreversible blocker that is reported to antagonize alpha 1B-, alpha 1C-, and alpha 1D-adrenoceptor binding. Inotropic effects of the alpha 1-adrenoceptor agonist phenylephrine were examined in isolated left atrial and papillary muscle before and after addition of 4-aminopyridine, and before and after addition of 4-aminopyridine in preparations pretreated with chloroethylclonidine or WB4101. In addition, effects of phenylephrine were examined before and after treatment with staurosporine (an inhibitor of protein kinase C) in chloroethylclonidine-pretreated preparations. Phenylephrine (10 microM) elicited a sustained positive inotropic response in left atria and a triphasic inotropic action in papillary muscle (transient positive and negative inotropic components preceding a sustained positive inotropic response). 4-Aminopyridine (1.0, 1.7, 3.0 mM) reduced the sustained positive inotropic responses in the absence of antagonists and in chloroethylclonidine-pretreated preparations. However, in the presence of 10 nM WB4101, 4-aminopyridine had no effect on the remaining inotropic actions of phenylephrine. The sustained positive inotropic response to the alpha 1-agonist in chloroethylclonidine-pretreated preparations was not inhibited by 100 nM staurosporine. These data suggest that the sustained positive inotropic actions of alpha 1A-adrenoceptor stimulation in rat atrial and ventricular myocardium are mediated via non-protein kinase C-associated reductions in Ito.

Sustained increase in rat myocardial alpha 1A-adrenoceptors induced by 6-hydroxydopamine treatment involves a decelerated receptor turnover

The biochemical mechanisms involved in the alpha 1-adrenoceptor up-regulation and possible changes in subtypes of adrenoceptors in the rat heart after chemical denervation were investigated. The effects of acute 6-hydroxydopamine treatment (two increasing doses 24 h apart) on the pseudo-steady state densities and turnover rates of alpha 1-adrenoceptors were studied in ventricular myocardium of the rat. We have assessed the repopulation kinetics of [3H]prazosin binding sites after irreversible inactivation of alpha 1-adrenoceptors induced by a single dose of phenoxybenzamine (1 mg/kg i.p.) in rats acutely treated either with 6-hydroxy-dopamine or with vehicle (control animals). Seven days after the last administration of 6-hydroxydopamine an enhanced density of [3H]prazosin binding sites (Bmax 58.7 +/- 3.6 fmol/mg protein vehicle-treated rats versus 82.6 +/- 5.3 fmol/mg protein 6-hydroxydopamine-treated rats) was observed. This was not accompanied by changes in the dissociation constant value. Furthermore, the proportion of high affinity sites for WB-4101 was altered (21 +/- 2% versus 72 +/- 3% for animals treated with vehicle and 6-hydroxydopamine, respectively). In rat myocardium, alpha 1-adrenoceptor turnover, evaluated during the 6-hydroxydopamine-induced up-regulation (7-19 days after the completion of treatment with 6-hydroxydopamine) revealed an increase in the half-life of the alpha 1-adrenoceptor (t1/2 of 67.2 h versus 38.7 h in control animals). The present study confirms an increase in alpha 1-adrenoceptors in rat myocardium after chemical denervation and reveals that the effect is almost completely confined to the alpha 1A-adrenoceptor subtype. Furthermore, the up-regulation of alpha 1A-adrenoceptors is the result of a decrease in the cellular processes that control the rate of receptor degradation.

Characterization of the adrenergic activity of arbutamine, a novel agent for pharmacological stress testing

In this study, we characterized the interactions of arbutamine, a novel catecholamine developed for use as a cardiac stress testing agent, with different adrenergic receptor subtypes in vitro. These effects were compared with those of isoproterenol. In the electrically stimulated left atria of rats, arbutamine increased contractile force. The pD2 values (- log of the dose that produces 50% of the maximal responses) for arbutamine and isoproterenol were 8.45 +/- 0.15 and 8.55 +/- 0.02, respectively. Metoprolol shifted the concentration-effect curves for both isoproterenol and arbutamine to the right with a pA2 value (- log of the dose of the antagonist that reduces the maximal responses of an agonist to 50%) of 7.22-7.5. Both arbutamine and isoproterenol increased the rate of spontaneously beating rat right atria with pD2 values of 9.0 +/- 0.19 and 8.82 +/- 0.18, respectively. The affinity constants (KA) of arbutamine and isoproterenol for cardiac beta1-adrenergic receptors, as determined by competition binding assays, were found to be 7.32 and 6.04, respectively. In guinea pig trachea, arbutamine and isoproterenol produced a concentration-dependent relaxation that was blocked by propranolol. Their pD2 values were 7.9 +/- 0.1 and 8.2 +/- 0.1, respectively. Arbutamine contracted isolated rat aortic rings with a maximal increase of 38.1 +/- 6.7% that of 10 microM of norepinephrine. In rat white adipocytes, arbutamine, isoproterenol, and BRL-37344 stimulated glycerol release, with the order of potency being BRL-37344 > arbutamine > isoproterenol. In hamster brown adipocytes, the order was arbutamine > isoproterenol > BRL-37344. Moreover, arbutamine stimulated beta3-adrenergic receptors in guinea pig ileum. In conclusion, arbutamine is a novel catecholamine with similar potency and efficacy to that of isoproterenol. It stimulates cardiac beta1-, tracheal beta2-, and adiopocyte beta3-adrenergic receptors. Arbutamine does not stimulate alpha-adrenergic receptors at concentrations that were high enough to maximally activate the beta-adrenergic receptors.

alpha 2-Adrenergic receptor binding in canine Purkinje fibers

Autoradiograms were developed from free running strands of Purkinje fibers and from left ventricular myocardium of dog hearts exposed to [7-methoxy-3H]prazosin or [O-methyl-3H]yohimbine in the presence or absence of excessive non-radioactive ligands. Both Purkinje fiber and ventricular myocardium show high density, tissue-specific binding to [3H]prazosin. In contrast, high density, tissue-specific binding to [3H]yohimbine was present in Purkinje fibers but not in ventricular myocardium. Membrane fractions showed high affinity, saturable, and displaceable binding with [3H]yohimbine in preparations from canine cardiac Purkinje fibers but not those from canine cardiac ventricular myocardium. Scatchard analysis of the canine Purkinje membrane alpha 2-adrenergic receptor binding showed a Bmax of 54.9 fmol/mg protein with a Kd of 6.25 nM. These results confirm the electrophysiological findings that post-junctional alpha 2-adrenergic receptors are present in Purkinje fibers.

Alpha-adrenergic control of volume-regulated Cl- currents in rabbit atrial myocytes. Characterization of a novel ionic regulatory mechanism

alpha-Adrenergic stimulation is known to play a role in cardiac arrhythmogenesis and to modulate a variety of cardiac K+ currents. The effects of alpha-adrenergic stimulation on Cl- currents are largely unknown. Many cardiac cell types show a volume-sensitive Cl- current induced by cell swelling (ICl.swell). The present experiments were designed to assess the potential alpha-adrenergic modulation of ICl.swell in rabbit atrial myocytes. ICl.swell was induced with the use of a hypotonic superfusate, under conditions designed to prevent currents carried by K+, Na+, and Ca2+ ions. A basal Cl- current (ICl.b) was observed under isotonic conditions in 128 of 150 cells (85%), had the same dependency on [Cl-]o as ICl.swell, and was reduced by cell shrinkage induced by hypertonic superfusion, suggesting that ICl.b is carried by the same volume-sensitive Cl- conductance as ICl.swell. Phenylephrine produced a concentration-dependent and near-complete inhibition of ICl.b and ICl.swell, with EC50 values of 86 +/- 5 and 72 +/- 7 (mean +/- SEM) mumol/L, respectively, at +20 mV. Norepinephrine (administered in the presence of 1 mumol/L propranolol) also inhibited ICl.b and ICl.swell, with EC50 values of 2.6 +/- 0.1 and 2.8 +/- 0.4 mumol/L, respectively. The concentration-response curve for phenylephrine was shifted significantly (P < .001) to the right by the alpha 1-adrenoceptor antagonist prazosin and by the alpha 1A-receptor antagonists (+)-niguldipine and 5-methylurapidil but was unaltered by the alpha 1B-receptor antagonist chloroethylclonidine (100 mumol/L). Inhibition of protein kinase C (PKC) with staurosporine, H-7, or 18-hour preincubation with the phorbol ester 4 beta-phorbol 12-myristate 13-acetate (PMA, 500 nmol/L) blocked the effects of phenylephrine on ICl.swell, and the highly selective PKC inhibitor bisindolylmaleimide blocked the effects of norepinephrine on ICl.swell and ICl.b. Both PMA and 1-oleoyl-2-acetylglycerol inhibited ICl.swell in a concentration-dependent fashion. In blinded studies, the phorbol ester phorbol 12,13-didecanoate (PDD) reduced ICl.swell by 91 +/- 3%; its inactive analogue 4 alpha-PDD had no effect (mean change, 3 +/- 1%). Preincubation with pertussis toxin (PTX) prevented the actions of phenylephrine on ICl.swell, indicating a role for a PTX-sensitive guanine nucleotide-binding (G) protein. We conclude that alpha-adrenergic agonists inhibit volume-sensitive Cl- currents in rabbit atrial cells by interacting with an alpha 1A-adrenoceptor mechanism that is coupled to PKC via a PTX-sensitive G protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Corynanthine inhibits, while idazoxan potentiates, cardiotoxic effects of ouabain

1. Ouabain, infused intravenously to anaesthetized guinea-pigs induced ventricular premature beats, ventricular tachyarrhythmias and lethality. 2. Corynanthine (1, 2 and 4 mg kg-1), an alpha 1-adrenoceptor antagonist and idazoxan (100, 200 and 400 micrograms kg-1), an alpha 2-adrenoceptor antagonist were administered 10 min prior to ouabain. Corynanthine (2 and 4 micrograms kg-1) showed significant increase in the amount of ouabain required to cause arrhythmia and lethality, whereas idazoxan (200 and 400 micrograms kg-1) decreased it. 3. Corynanthine inhibited the ouabain-induced pressor response while idazoxan potentiated it. 4. Effects of these agents on the sympathetic nervous system appear to have played a significant role in its anti- and proarrhythmic actions.

Effects of alpha adrenergic stimulation on time independent potassium current of isolated ventricular myocytes

The role of alpha adrenergic receptor stimulation on ventricular electrical activity is controversial. The aim of the present paper was to study a wide range of concentrations of alpha adrenergic agonists on the electrical properties of guinea pig's heart isolated ventricular myocytes. The experiments were performed according to the single electrode voltage clamp technique. Phenylephrine and epinephrine (in the presence of propranolol) were used at concentrations from 1 x 10(-9) to 10(-5) M. It was observed that both agonists induce an increase in the time independent inward rectifying potassium current (IK1), that could explain the shortening of the action potential. All the observed effects were dose-dependent and disappeared during washout. These results could explain, at least partially, some of the electrical changes observed during ischemia.

Inositol phosphate release and metabolism during myocardial ischemia and reperfusion in rat heart

A detailed study of the effects of global myocardial ischemia and reperfusion on inositol phosphate release and metabolism has been undertaken by using isolated perfused rat hearts. Ischemia for longer than 5 minutes caused a cessation of inositol phosphate production, with inositol phosphates initially present accumulating as isomers of inositol monophosphate. This inhibition was independent of norepinephrine. In contrast, 2-minute reperfusion following 20-minute ischemia produced a rapid and transient release of inositol phosphates that was dependent on the release of norepinephrine and mediated by alpha 1-adrenergic receptors. By a number of criteria, this reperfusion response was different from the norepinephrine response in normoxic tissue. First, total release of inositol phosphates was greater (466 +/- 37 compared with 345 +/- 29 cpm/mg protein, P < .05). Second, inositol 1,4,5-trisphosphate was released with postischemic reperfusion (103 +/- 18 to 207 +/- 11 pmol/mg protein), whereas release was not detected in normoxic myocardium. In agreement with this, neomycin (0.5 and 5 mmol/L) inhibited inositol phosphate release only under reperfusion conditions. Third, the reperfusion response, unlike the response in nonischemic tissue, required extracellular Ca2+. Longer periods of reperfusion resulted in a return to a pattern of inositol phosphate release that was not different from that seen in normoxic tissue. The rapid and transient release of inositol 1,4,5-trisphosphate at 2-minute postischemic reperfusion provides an explanation for the enhanced role of alpha 1-adrenergic receptors under these conditions and suggests an important role for this compound in initiating reperfusion-induced pathological events.

Ion channel modulators as potential positive inotropic compound for treatment of heart failure

1. Current positive inotropy therapy of heart failure is associated with major problems: digoxin and the phosphodiesterase inhibitors can cause life-threatening toxicity while beta-adrenoceptor agonists become less effective inotropic compounds as heart failure progresses. A new approach to positive inotropy is ion channel modulation. 2. An increased influx of Na+ during the cardiac action potential, as measured with DPI 201-106 and BDF 9148 which increase the probability of the open state of the Na+ channel, will increase force of contraction. 3. Activation of L-type Ca2+ channels with Bay K 8644 will increase influx of Ca2+ and increase the force of contraction. However the Ca2+ channel activators developed to date have little potential for the treatment of heart failure as they are vasoconstrictors. 4. Blocking cardiac K+ channels is a possible mechanism of positive inotropy. Terikalant inhibits the inward rectifying K+ channel, tedisamil inhibits the transient outward K+ channel and dofetilide is one of the newly developed inhibitors of the slow delayed outward rectifying K+ channel. All these drugs prolong the cardiac action potential to increase Ca2+ entry and force of contraction. 5. Thus drugs which increase Na+ influx or block K+ channels represent exciting possibilities for positive inotropy and the potential of these compounds for the treatment of heart failure needs to be fully evaluated.

Disease-induced changes in alpha-adrenoceptor-mediated cardiac and vascular responses in rats

1. The physiological relevance of cardiac and vascular alpha-adrenoceptors may increase in disease states in which beta-adrenoceptors are altered. To test this, positive inotropic and vasoconstrictor responses to phenylephrine were measured in isolated tissues from rats with experimentally-induced hyperthyroidism, hypothyroidism and diabetes as well as in genetically spontaneous hypertensive rats (SHR). 2. In left atria, positive inotropic responses to phenylephrine were increased in hypothyroid and diabetic rats and abolished in hyperthyroid and SHR. 3. In contrast, phenylephrine produced increased positive inotropy in left ventricular papillary muscles from hyperthyroid rats, increased potency in diabetic rats and negative inotropic responses in hypothyroid rats. 4. The potency of phenylephrine as a vasoconstrictor in thoracic aortic rings was increased in hyperthyroid and SHR and decreased in hypothyroid rats. 5. Thus, disease states which alter beta-adrenoceptor responsiveness can independently regulate atrial, ventricular and vascular responses to the alpha 1-adrenoceptor agonist, phenylephrine. Therefore, these disease states may alter the physiological control of the cardiovascular system by noradrenaline and adrenaline as well as the responsiveness in disease states to therapeutic agents acting via alpha-adrenoceptors.

Effects of extracellular Cl- on the inotropic response to alpha-adrenoceptor stimulation

This study was designed to determine if the sustained positive inotropic action of alpha-adrenergic stimulation is affected by the absence of extracellular chloride ion (Clo-). Atrial and papillary muscle were isolated from adult male rats, bathed in Krebs-Henseleit solution (30 degrees C) with and without Cl- (methane-sulfonate substitution), and stimulated at 0.5 Hz. Isometric developed tension was monitored during cumulative addition of phenylephrine, isoproterenol and Ca2+. The dose-dependent positive inotropic effects of isoproterenol and Ca2+ were not altered by the absence of Clo-. However, the magnitude of the response to phenylephrine was diminished in both tissues. In atrial muscle, the maximum positive inotropic effect of phenylephrine was reduced from 2.05 +/- 0.17 g in the presence of Clo- to 0.39 +/- 0.06 g in the absence of Clo-; control developed tension was 0.60 +/- 0.08 and 0.47 +/- 0.10 g in these two groups before exposure to the alpha-adrenoceptor agonist. In papillary muscle, control developed tension was 1.40 +/- 0.11 and 1.17 +/- 0.18 g in the presence and absence of Clo-, respectively; and the maximum inotropic responses to phenylephrine were 0.71 +/- 0.12 and 0.27 +/- 0.13 g. EC50 values for phenylephrine were not significantly affected by substitution for Cl-. Similar results were observed in a Hepes-buffered bathing solution without bicarbonate (HCO3-). These results indicate that the positive inotropic action of alpha-adrenergic stimulation is mediated in part by a mechanism requiring Cl-. Furthermore, data suggest that the antagonistic effect of Clo- removal is not mediated via Cl-/HCO3- exchange.

Alpha 1-adrenergic receptor modulation of repolarization in canine Purkinje fibers

INTRODUCTION

Alpha 1-adrenergic receptor stimulation increases contractility and prolongs repolarization. These effects are modulated by alpha 1-adrenergic receptor-mediated inhibition of transsarcolemmal potassium currents.

METHODS AND RESULTS

We used standard microelectrode techniques to study the actions of 4-aminopyridine (4-AP), which blocks the transient outward current, I(to), and WAY-123,398, which blocks the delayed rectifier, Ik, on canine Purkinje fiber action potential prolongation induced by phenylephrine. At a basic cycle length of 1 second, phenylephrine (0.1 to 10 microM) dose-dependently prolonged action potential duration at 90% repolarization (APD90) from 331 +/- 10 msec to 400 +/- 12 msec (P < 0.05) at phenylephrine, 10 microM. Phenylephrine did not change phase 1 or plateau height. 4-AP (0.1 mM) decreased phase 1 magnitude, shifted plateau height to more positive potentials (from 0.1 +/- 1.8 mV to 14.3 +/- 1.1 mV [P < 0.05]), and shortened APD90 from 318 +/- 9 msec to 294 +/- 8 msec (P < 0.05). 4-AP did not block phenylephrine effects on APD90, which increased, at 10 microM phenylephrine, from 294 +/- 8 msec to 342 +/- 6 msec (P < 0.05). In contrast, WAY-123,398 (0.1 microM) prolonged APD90 from 360 +/- 6 msec to 452 +/- 6 msec (P < 0.05), and had no effect on plateau height. In the presence of WAY-123,398, phenylephrine no longer increased APD90.

CONCLUSION

(1) Agents that block I(to) shorten APD in Purkinje fibers; and (2) the alpha-agonist mediated increase of APD in canine Purkinje fibers can be explained by inhibition of Ik.

Functional studies on alpha 1-adrenoceptor subtypes mediating inotropic effects in rat right ventricle

1. We have studied the alpha 1-adrenoceptor subtypes mediating inotropic effects of adrenaline in rat right ventricle and the Ca2+ sources used to elicit these effects. alpha 1A-Adrenoceptor-mediated contractile effects in rat vas deferens were studied for comparison in some cases. 2. Treatment with chloroethylclonidine did not affect the maximal beta-adrenoceptor-mediated inotropic effects in rat right ventricle or the maximal alpha 1A-adrenoceptor-mediated contractile effects in rat vas deferens; it did not alter the potency of isoprenaline in the ventricle and reduced the potency of the alpha-adrenoceptor antagonists in vas deferens only slightly. Treatment of right ventricular strips with CdCl2 markedly reduced resting tension and enhanced maximal inotropic effects of isoprenaline but did not affect its potency. 3. Inactivation of cardiac alpha 1B-adrenoceptors by treatment with chloroethylclonidine slightly enhanced the maximal inotropic effects of the full agonist, adrenaline and of several partial agonists. 4. Schild analysis of inhibition experiments with the alpha 1A-adrenoceptor-selective antagonists, 5-methyl-urapidil and (+/-)-tamsulosin, demonstrated that adrenaline causes its inotropic effects mainly via the alpha 1B-adrenoceptor subtype. Schild analysis of 5-methyl-urapidil inhibition experiments in chloroethylclonidine-treated ventricles indicated that only alpha 1A-adrenoceptors mediate the inotropic effects of adrenaline following inactivation of the alpha 1B-adrenoceptors. 5. In control ventricles the organic Ca2+ entry blocker, nitrendipine and treatment with the inorganic Ca2+ entry blocker, CdCl2 did not reduce inotropic effects of adrenaline whereas ryanodine treatment inhibited them. In contrast, nitrendipine and CdCl2 treatment had major inhibitory effects in chloroethylclonidine-treated but lacked inhibitory effects in phenoxybenzamine-treated ventricular strips. 6. We conclude that inotropic effects of adrenaline in rat heart are mediated mainly by alpha 1B-adrenoceptors via release of Ca2+ from an intracellular pool. Following inactivation of alpha 1B-adrenoceptors by chloroethylclonidine treatment, alpha lA-adrenoceptors can fully compensate and mediate inotropic effects by promoting influx of extracellular Ca2+ at least partly via voltage-operated channels.Therefore, we speculate that alpha 1B-adrenoceptors exert a tonic inhibitory effect on alpha 1A-adrenoceptors.

Radioligand binding studies of alpha 1-adrenoceptor subtypes in rat heart

1. In order to characterize the alpha 1-adrenoceptor subtypes mediating positive inotropic effects of adrenaline (in the presence of propranolol) in rat right ventricular strips and the Ca2+ sources used to elicit them, we have used radioligand binding to identify the alpha 1-adrenoceptor subtypes present in rat heart and the alpha 1-adrenoceptor affinity and subtype-selectivity of various pharmacological tools. 2. Amitryptiline, mianserin, trimipramine, oxaprotiline, clonidine, chloroethylclonidine, phenoxybenzamine, BE 2254 and 8-OH-DPAT competed for [3H]-prazosin binding in rat heart, vas deferens, liver, spleen, cerebral cortex and hippocampus but none of them displayed detectable alpha 1-adrenoceptor subtype-selectivity; nitrendipine did not compete for [3H]-prazosin binding in concentrations up to 5 mumol 1(-1). 3. The alpha 1 A-adrenoceptor-selective, 5-methyl-urapidil, (+)-niguldipine, and to a lesser extent (-)-niguldipine competed for [3H]-prazosin binding in rat heart, vas deferens, cerebral cortex and hippocampus with shallow and biphasic curves; analysis of these curves demonstrated that rat heart contains alpha 1A-and alpha 1B-adrenoceptors in a 20:80 ratio. 4. Treatment of rat right ventricular strips with 100 mumol l-1 chloroethylclonidine for 30 min at 30 degrees C followed by 60 min washout reduced the number of alpha 1-adrenoceptors, as assessed by [3H]-prazosin saturation experiments, by 74%. Treatment with 100 micromol l(-1) CdCl2 did not affect number or affinity of cardiac alpha 1-adrenoceptors and combined treatment with chlorethylclonidine and CdCl2 reduced alpha 1-adrenoceptor number by 90%. 5. Treatment of rat right ventricular strips with chloroethylclonidine steepened 5-methyl-urapidil competition curves and increased the relative contribution of alpha 1A-adrenoceptors from 26 to 89%. Treatment with CdCl2 did not affect 5-methyl-urapidil competition curves and combined treatment with chloroethylclonidineand CdCl2 increased the relative contribution of alpha lA-adrenoceptors to 66%.6. We conclude that rat heart contains alpha 1A- and alpha 1B-adrenoceptors in a 20:80% ratio. Treatment withchloroethylclonidine reduces alpha 1B-adrenoceptor number by 96% but has only minor effects on alpha 1A-adrenoceptor density. Treatment with CdCl2 does not affect the number of either alpha 1-adrenoceptor subtype.

Adrenoceptor-mediated cardiac and vascular responses in hypothyroid rats

This study has investigated adrenoceptor-mediated responses and beta-adrenoceptors in neonatal-onset hypothyroidism in rats. Four groups of adult rats were studied: controls, neonatal-onset uncorrected hypothyroidism (continuous oral methimazole treatment) and after chronic triiodothyronine (T3) replacement of these rats at either 25 or 100 micrograms/kg/day for 8 weeks beginning at 12 weeks of age. Hypothyroid rats were 61% smaller with an 18% decrease in heart rate; food and water intake were reduced to 43% and 52%, respectively; O2 consumption was reduced to 20% and rectal temperature was 2.9 degrees lower. T3 administration increased body weight to 60-62% of controls; metabolic changes were reversed; but tachycardia and cardiac hypertrophy (60-120% increases) resulted. The positive inotropic responses to the selective alpha 1-adrenoceptor agonist, phenylephrine, in left ventricular papillary muscles were abolished; the beta 1-adrenoceptor agonist, noradrenaline, was significantly less potent as an inotropic compound in isolated cardiac tissues from hypothyroid rats. The potency of phenylephrine to contract thoracic aortic rings was reduced in hypothyroid rats. These changes in alpha- and beta-adrenoceptor mediated responses were reversed by T3 administration. Both beta 1- and beta 2-adrenoceptor densities were increased in the hypothyroid left ventricle; T3 administration further increased beta 1-adrenoceptor density. We conclude that neonatal hypothyroidism produces pronounced physiological responses, changes in adrenoceptor-mediated responses and an increased ventricular beta 1-adrenoceptor density. T3 replacement reversed the changes in cardiac responses and metabolic parameters, except body weight, but produced cardiac symptoms of hyperthyroidism (tachycardia, hypertrophy as well as an increased beta 1-adrenoceptor density).

In vivo recovery of alpha 1-adrenoceptors in rat myocardial tissue after alkylation with phenoxybenzamine

The rate of recovery of rat myocardial alpha 1-adrenoceptor density and responsiveness after in vivo block with phenoxybenzamine (1 mg/kg, i.p.) have been investigated by measuring [3H]prazosin binding, and noradrenaline-stimulated [3H]inositol phosphate production. Repopulation of alpha 1-adrenoceptors was monoexponential, with a t1/2 of 33 h; functional recovery was also monoexponential, with t1/2 of 28 h. Furthermore, our results clearly demonstrate the absence of a receptor reserve for alpha 1-adrenoceptors mediating noradrenaline-stimulated phosphoinositide breakdown in rat myocardial tissue. These observations indicate a close relationship between the density of [3H]prazosin binding sites and the ability of alpha 1-adrenoceptors to respond to noradrenaline. Moreover, based on competition curves for inhibition of specific [3H]prazosin by WB-4101 to rat myocardial membranes 48 h and 7 days after the administration of phenoxybenzamine, the results suggest that rat myocardial membranes contain both alpha 1-adrenoceptors subtypes, i.e., alpha 1A and alpha 1B, in an approximate ratio of 20:80, and this relative ratio does not seem to be altered during the recovery process.

Propranolol unmasks class III like electrophysiological properties of norepinephrine

Isolated perfused spontaneously beating rabbit hearts were treated with increasing concentrations of norepinephrine (0.01, 0.1, 0.5 mumol/l) either alone or in presence of propranolol (0.1 mumol/l). For analysis of the epicardial activation and repolarization process and epicardial mapping (256 unipolar leads) was performed. For each electrode the activation and repolarization time was determined. From these data the "breakthrough-points" (BTP) of epicardial activation were determined. At each electrode an activation vector (VEC) was calculated giving direction and velocity of the local excitation wave. The beat similarity of various heart beats (under NE) compared to control was evaluated by determination of the percentage of identical BTP and of similar VEC (deviation < or = 5 degrees). Moreover at each electrode the local activation recovery interval (ARI) and its standard deviation (of 256 leads, dispersion, DISP) were determined. Norepinephrine alone (0.01, 0.1, 0.5 mumol/l) led to an increase in left ventricular pressure, heart rate and DISP with concomitant frequency dependent reduction in ARI, and to changes in the epicardial activation pattern (reduction in BTP, VEC). We found that in the presence of propranolol (0.1 mumol/l) norepinephrine prolonged ARI and reduced ARI-dispersion. This effect was not due to changes in heart rate. The disturbing effects on the activation pattern were diminished. These effects could be prevented by pretreatment with 1 mumol/l prazosin. From these results we conclude, that norepinephrine prolongs the relative action potential duration via stimulation of alpha 1-adrenoceptor and enhances cellular coupling.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of alpha 1-adrenoceptors which mediate chronotropy in neonatal rat cardiac myocytes

1. In the present study, we investigated the effect of culture on alpha 1-adrenoceptors that mediate chronotropy and on alpha 1-adrenergic signal transduction in neonatal rat cardiac myocytes. 2. The spontaneous beating rate of neonatal rat myocytes after 3 or 7 days in culture was 37.4 +/- 4.2 or 102.0 +/- 4.3 beats min-1, respectively. The alpha 1-adrenoceptor-mediated chronotropic effect of norepinephrine was positive at day 3 of culture. In contrast to day 3 of culture, the neonatal myocytes exhibited a negative chronotropic response to norepinephrine on day 7 of culture. Both of these effects of norepinephrine were completely abolished by prazosin. 3. The affinity (Kd) and/or density (Bmax) of alpha 1-adrenoceptors labeled with [3H]prazosin in membranes from cultured myocytes were not significantly different between day 3 and day 7 of culture. 4. The expression of Gs, Gi, Gq and Go alpha-subunits in membranes from cultured myocytes was found to be significantly increased with the passage of culture time by immunoblot analysis. In contrast, no significant differences in G beta-subunit expression were observed between day 3 and day 7 of culture. 5. Norepinephrine-stimulated inositol 1,4,5-trisphosphate production by radio-binding protein in neonatal myocytes after 7 days of culture was significantly higher than that of the day 3 counterpart. 6. No significant changes in phospholipid and cholesterol contents in membranes from neonatal myocytes were observed with longer culture times.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of amosulalol, a combined alpha 1- and beta-adrenoceptor-blocking agent, on ischemic myocardial energy metabolism in dogs

The effect of amosulalol, a combined alpha 1- and beta-adrenoceptor-blocking agent, on changes in myocardial energy and carbohydrate metabolism during ischemia was studied. Ischemia was induced by ligating the left anterior descending coronary artery for 3 or 30 min during open-chest surgery in anesthetized dogs. The myocardial energy stores were depleted, and the levels of glycolytic intermediates were altered by 3 and 30 min of ischemia, indicating that anaerobic myocardial metabolism had occurred. Amosulalol (0.3 or 1 mg.kg-1) was injected intravenously 5 min before ischemia. Pretreatment with amosulalol, particularly at a dose of 0.3 mg.kg-1, reduced the myocardial energy depletion and the alteration of carbohydrate metabolism induced by ischemia. This result indicates that amosulalol can reduce ischemic influences on the myocardium.

Occurrence and functions of the phosphatidylinositol cycle in the myocardium

In the last decade a great deal of attention was awarded to a signal transduction pathway which is utilized primarily by 'Ca2+ mobilizing' signal molecules and which involves the hydrolysis of a quantitatively minor phospholipid, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) by a PtdIns-specific phospholipase C (PLC). The evidence for the existence of receptor-mediated GTP binding protein-coupled PLC in myocardium and its possible functions are briefly summarized. The minireview is concentrated on the following aspects: 1) cellular localization and synthesis of polyphospho-PtdIns from PtdIns, 2) desensitization of the alpha 1-adrenergic agonist and endothelin-1 mediated PtdIns responses, 3) oscillatory Ca2+ transients initiated by PtdIns(4,5)P2 hydrolysis, 4) polyunsaturated fatty acids as constituents of polyphospho-PtdIns and of the protein kinase C activator 1,2-diacylglycerol (DAG), 5) source other than PtdIns(4,5)P2 contributing to the stimulated DAG, 6) role of the PtdIns pathway in cardiomyocyte growth and gene expression during the hypertrophic response.

Mechanisms for the positive inotropic effect of alpha 1-adrenoceptor stimulation in rat cardiac myocytes

alpha 1-Adrenoceptor activation can enhance myocardial contractility, and two possible inotropic mechanisms are an increase in myofilament Ca2+ sensitivity and action potential prolongation, which can increase net Ca2+ entry into cells. In adult rat ventricular myocytes (bath Ca2+, 1 mM; stimulated at 0.2-0.5 Hz), the drug 4-aminopyridine and the whole-cell voltage clamp have been used to control Ca2+ entry and differentiate between the two mechanisms. At 22-23 degrees C the specific alpha 1-adrenoceptor agonist methoxamine (100 microM) prolonged action potential duration at 50% repolarization from 55 +/- 2 to 81 +/- 5 msec, delayed time to peak contraction, and increased shortening amplitude from 5.3 +/- 0.6 to 7.8 +/- 1 microns (n = 18). Reduction of the transient outward current and other K+ currents by methoxamine was the major cause of action potential prolongation in rat myocytes with little change in the L-type calcium current. Block of the transient outward current with 2 mM 4-aminopyridine prolonged action potential duration from 52 +/- 6 to 98 +/- 12 msec and increased unloaded cell shortening from 2.9 +/- 0.4 to 6.6 +/- 0.6 microns (n = 4). Subsequently, methoxamine no longer increased cell shortening, although significant potentiation of twitch amplitude was still seen after a brief rest interval. In voltage-clamp experiments, with 70-500-msec pulses, although membrane currents were reduced, methoxamine had no positive inotropic effect and reduced cell shortening from 5.3 +/- 0.7 to 4.97 +/- 0.8 microns at pulse potentials positive to -40 mV. Similar alpha 1-adrenoceptor responses were observed at 35 degrees C during action potential and voltage-clamp experiments, which could be blocked by 10 microM prazosin. In myocytes loaded with the Ca2+ indicator indo-1, alpha 1-adrenoceptor stimulation or 4-aminopyridine both increased cell contraction and intracellular Ca2+ transients by similar amounts. As in unloaded cells, prior exposure to 4-aminopyridine prevented any inotropic effect of methoxamine without changing the systolic intracellular Ca2+ transient. The results indicated that under our experimental conditions positive inotropy in rat cardiomyocytes on exposure to alpha 1-adrenoceptor agonists was strongly correlated with the action potential prolongation that accompanied K+ current reduction. In addition, modulation of K+ channels could occur independent of changes in contractility and/or [Ca2+]i.

Cardiac alpha 1-adrenoceptor densities in different mammalian species

1. alpha 1-Adrenoceptor densities were studied in cardiac membrane preparations from several mammalian species including human failing hearts under identical experiment conditions; the alpha 1-adrenoceptor antagonist, [3H]-prazosin, was used as radioligand. End-stage heart failure (NYHA IV) in human hearts was due to idiopathic dilated cardiomyopathy. 2. The ventricular alpha 1-adrenoceptor densities were not significantly different in guinea-pig, mouse, pig, calf, and man (11 to 18 fmol mg-1 protein) but about 5 to 8 fold smaller than in rat (about 90 fmol mg-1 protein). Right and left ventricular receptor densities were similar in these species. 3. A sufficient amount of right and left atrial tissue was obtained from rabbit, pig, calf, and man only. The alpha 1-adrenoceptor densities in both atria of these species were found to be at the detection limit of the method used (less than 8 fmol mg-1 protein). 4. The equilibrium dissociation constant (KD) was similar in all species studied ranging from 0.047 +/- 0.006 to 0.063 +/- 0.007 nmol l-1. 5. It is concluded that differences in alpha 1-adrenoceptor density between atria and ventricles may exist in mammalian species. The exceptionally high density of these receptors in rat ventricles seem to be a particular feature in these animals.

Effects of myocardial alpha 1-adrenergic receptor stimulation and blockade on contractility in humans

BACKGROUND

Although alpha-adrenergic receptors are present in both normal and failing human left ventricular myocardium and mediate a positive inotropic effect in several other species, it is not known whether stimulation of myocardial alpha-adrenergic receptors exerts a positive inotropic effect or contributes to basal contractile state in vivo in humans.

METHODS AND RESULTS

We studied 15 patients with angiographically normal coronary arteries (seven with normal left ventricular function and eight with left ventricular failure). To avoid the confounding effects of changes in ventricular loading conditions and systemic reflex mechanisms, the alpha-adrenergic receptor-selective antagonist phentolamine and agonist phenylephrine were infused directly into the left main coronary artery, and the change in contractile state was assessed by measuring left ventricular peak (+)dP/dt. Phentolamine alone had no effect on left ventricular contractility. Phenylephrine exerted a concentration-related positive inotropic effect in patients with normal as well as those with failing ventricles. The alpha-adrenergic effect of phenylephrine, defined as the component blocked by phentolamine, was significantly less in patients with ventricular failure (108 +/- 28 mm Hg/sec) than in normal subjects (248 +/- 54 mm Hg/sec; p less than 0.03).

CONCLUSIONS

Myocardial alpha-adrenergic receptors do not contribute to the maintenance of basal left ventricular contractile state in humans. However, stimulation of myocardial alpha-adrenergic receptors exerts a positive inotropic effect, the magnitude of which may be attenuated in patients with heart failure.