Beta 1- and beta 2-adrenergic-receptor subpopulations in nonfailing and failing human ventricular myocardium: coupling of both receptor subtypes to muscle contraction and selective beta 1-receptor down-regulation in heart failure

We used radioligand binding techniques and measurement of beta-agonist-mediated positive inotropic responses in isolated cardiac tissue to examine beta-adrenergic-receptor subpopulations in nonfailing and failing human left and right ventricular myocardium. In tissue derived from 48 human hearts the receptor subtypes identified in nonfailing ventricle by radioligand binding were beta 1 (77%) and beta 2 (23%), with no evidence of an "atypical" beta-adrenergic receptor. In failing left ventricle the beta 1:beta 2 ratio was markedly different, i.e., 60:38. This decrease in the beta 1 proportion and increase in the beta 2 proportion in the failing ventricles were due to a 62%, "selective" down-regulation of the beta 1 subpopulation, with little or no change in beta 2 receptors. In muscle bath experiments in isolated trabeculae derived from nonfailing and failing right ventricles, both beta 1- and beta 2-adrenergic receptors were coupled to a positive inotropic response. In nonfailing myocardium, beta 1 responses predominated, as the selective beta 1 agonist denopamine produced a response that was 66% of the total contractile response of isoproterenol. In heart failure the beta 1 component was markedly decreased, while the beta 2 component was not significantly diminished. Moreover, in heart failure the beta 2 component increased in prominence, as the contractile response to the selective beta 2 agonist zinterol increased from a minority (39%) to a majority (60%) of the total response generated by isoproterenol. We conclude that failing human ventricular myocardium contains a relatively high proportion of beta 2 receptors, due to selective down-regulation of beta 1 receptors. As a result, in the failing human heart the beta 2-receptor subpopulation is a relatively important mediator of inotropic support in response to nonselective beta-agonist stimulation and is available for inotropic stimulation by selective beta 2 agonists.     

Myocardial beta-adrenoceptor changes in heart failure: concomitant reduction in beta 1- and beta 2-adrenoceptor function related to the degree of heart failure in patients with mitral valve disease

In patients suffering from end-stage congestive cardiomyopathy, cardiac beta 1-adrenoceptor function is markedly reduced, whereas cardiac beta 2-adrenoceptor function is nearly normal. To determine whether beta 1-adrenoceptor function is impaired in heart failure selectively, beta 1- and beta 2-adrenoceptor density and functional responsiveness in the right and left atria and the left papillary muscles from patients with mitral valve disease (functional class III to IV) were studied. In all three tissues concomitantly beta 1- and beta 2-adrenoceptor density gradually declined when the degree of heart failure increased from functional class III to IV. This decrease in beta 1- and beta 2-adrenoceptor density was accompanied by similar decreases in the contractile response of isolated electrically driven right atrial and left ventricular papillary muscles to beta-adrenergic agonists. It is concluded that a decrease in cardiac beta-adrenoceptor function is a general phenomenon in heart failure, and its extent is related to the degree of heart failure. However, in contrast to congestive cardiomyopathy, in mitral valve disease the decrease in cardiac beta-adrenoceptor function is due to a concomitant decrease in beta 1- and beta 2-adrenoceptors.     

L-type calcium current and contractility in ventricular myocytes from mice overexpressing the cardiac beta 2-adrenoceptor.

OBJECTIVES: The reported increase in basal activity of hearts from transgenic mice (TG4) overexpressing the human beta 2-adrenoceptor (beta 2-AR) was explained by spontaneously active beta 2-ARs that stimulate the beta-adrenergic cascade in the absence of an agonist. In order to examine altered myocardial function on a cellular level, we have investigated L-type calcium current (ICa,L) and cell shortening in ventricular myocytes from TG4 hearts. Myocytes from littermates (LM) and wild type animals (WT) served as controls. METHODS: Cardiac beta-AR density was measured by [125I]-iodocyanopindolol binding to ventricular membranes. ICa,L was assessed by standard whole-cell voltage clamp technique. Contractility was measured as cell shortening in ventricular myocytes and as force of contraction in electrically stimulated left atria. RESULTS: Overexpression of beta 2-ARs was confirmed by an almost 400-fold increase in beta-AR density. The beta 1:beta 2-AR ratio in WT mice was 71:29. Myocytes from TG4 and LM mice were similar in size as judged by membrane capacitance and two dimensional cell area. ICa,L amplitude was significantly lower in TG4 than in LM myocytes (with 2 mM [Ca2+]o -4.82 +/- 0.48 vs. -6.56 +/- 0.38 pA/pF, respectively). In TG4 myocytes, the ICa,L response to isoproterenol (1 microM) was almost abolished. Cell shortening was not different in physiological [Ca2+]o, but smaller in maximum [Ca2+]o when comparing TG4 to control myocytes. Basal force of contraction in left atria did not differ between TG4 and LM at any age investigated. In TG4 left atria the inotropic response to isoproterenol was also absent, whereas responses to high [Ca2+]o or dibutyryl-cAMP (1 mM) were present but reduced. The rate of spontaneous beating of right atria was elevated in TG4 mice. CONCLUSIONS: Since only spontaneous beating rate but neither basal ICa,L amplitude nor basal contractile activity were elevated, our data fail to reveal evidence for spontaneously active, stimulating beta 2-ARs in left atrium and ventricle. A contractile deficit unrelated to the beta-adrenoceptor pathway is evident in TG4 myocytes and left atria.     

Relative significance of dopamine receptors, beta adrenoceptors and norepinephrine uptake inhibition in the cardiovascular actions of dopexamine hydrochloride

Studies in our laboratory have confirmed that dopexamine hydrochloride is a potent beta 2-adrenoceptor and DA1-dopamine receptor agonist. We examined the effects of dopexamine hydrochloride on both cardiac contractile force, determined by use of a Walton-Brodie strain-gauge arch sutured to the right ventricle, and heart rate in anesthetized dogs. Dopexamine hydrochloride increased cardiac contractile force and heart rate and decreased blood pressure. After administration of the ganglionic blocking agents, hexamethonium and atropine, or the selective beta 1-adrenoceptor antagonist, atenolol, the positive inotropic and chronotropic effects of dopexamine hydrochloride were reduced or eliminated, demonstrating that the drug had little or no direct beta 1-adrenoceptor action and that a myocardial beta 2-adrenoceptor action was not involved in its cardiac effects. During these investigations, dopexamine hydrochloride was found to be an inhibitor of norepinephrine uptake, potentiating the cardiac effects of both exogenously administered norepinephrine and norepinephrine released from sympathetic nerves. These results led to the following conclusions: Dopexamine hydrochloride stimulates the heart by 2 mechanisms--(1) baroreceptor-mediated release of norepinephrine resulting from hypotension produced by beta 2 adrenoceptor and DA1 dopamine receptor-mediated vasodilatation, and (2) potentiation of the released norepinephrine due to prevention of norepinephrine uptake by sympathetic nerves. The latter mechanism may be involved in the cardiac stimulation observed in patients with congestive heart failure and shock, since excessive sympathetic activity and elevated catecholamine levels are present in these conditions.     

The effect of xamoterol in failing human myocardium

As the dual pharmacological action of partial beta 1-adrenoceptor agonists should improve left ventricular function while also protecting the myocardium against excessive sympathetic stimulation they may be useful in the treatment of heart failure. Therefore, we studied the pharmacological effects of xamoterol (Corwin, ICI 118, 587), a compound with mixed agonist and antagonistic properties at cardiac beta-adrenoceptors in electrically driven human papillary muscle strips from failing human hearts. Specimens were obtained from patients with different grades of myocardial failure who underwent mitral valve replacement (NYHA II-III) or heart transplantation (NYHA IV). Xamoterol (0.0001-100 mumol l-1) produced only negative inotropic effects, as measured by changes in isometric force of contraction in diseased human papillary muscle strips. However, isoprenaline (0.0001-10 mumol l-1) and ouabain (0.01-0.3 mumol l-1) enhanced force of contraction in the same hearts. Prestimulation with noradrenaline (3 mumol l-1) augmented the negative inotropic effect of xamoterol. It is concluded that xamoterol exerts primarily beta-adrenoceptor antagonistic activity in the failing human myocardium.     

Cardiac beta receptors--experimental aspects

Myocardial beta-adrenoceptors mediate physiological effects of the agonists noradrenaline und adrenaline and stimulant effects of related amines and beta-blockers. At least 2 beta-adrenoceptor subtypes appear to contribute to the mediation of physiological effects. beta-adrenoceptor subtypes have recently been found with selective beta-blockers and nonphysiological agonists in several heart regions. However, no evidence has yet been reported that the affinity of a physiological agonist for cardiac beta-adrenoceptor subtypes differs. The possibility is discussed that an agonist may cause greater conformational change of one subtype than of another while binding to the same extent to both beta-adrenoceptor populations. The affinity of catecholamines for beta-adrenoceptors is low. Marked positive inotropic and chronotropic effects occur with catecholamine concentrations causing low beta-adrenoceptor occupancy and barely suprathreshold stimulation of the adenylyl cyclase. The large beta-adrenoceptor reserve and adenylyl cyclase reserve enable the regulation of the sinoatrial pacemaker activity and of ventricular and atrial contractility. Thus exposure to high catecholamine concentrations causes a pronounced decrease of Vmax for the adenylyl cyclase with a concurrent decrease in inotropic and chronotropic potencies, but without decreasing maximum inotropic and chronotropic effects of catecholamines. Data from isolated human myocardium suggest that the intropic potency of catecholamines is lower and the adenylyl cyclase reserve smaller than in other species. Pindolol, its derivatives, and some other partial agonists only cause positive chronotropic effects at concentrations considerably higher than those required to half-saturate cardiac beta-receptors.     

Selective beta 1-adrenoceptor blockade enhances positive inotropic responses to endogenous catecholamines mediated through beta 2-adrenoceptors in human atrial myocardium

We determined the relative contribution of beta 1- and beta 2-adrenoceptor stimulation to the positive inotropic responses of human atrial myocardium to catecholamines. (-)Norepinephrine produced stimulation predominantly through beta 1-receptors and (-)epinephrine through both beta 1- and beta 2-receptors. However, there were marked differences in the responses of tissues from patients treated with the beta 1-selective antagonist atenolol compared with non-beta-blocker-treated patients; surprisingly, beta 2-mediated responses were enhanced, and beta 1-mediated responses were unaltered. There was an enhanced responsiveness to (-)epinephrine (atenolol treated: -log M EC50, 7.57 +/- 0.07; non-beta-blocker treated: -log M EC50, 6.77 +/- 0.17; p less than 0.001), and the relative importance of beta 2-adrenoceptor stimulation was increased for both (-)norepinephrine and (-)epinephrine. In tissues from atenolol-treated patients, salbutamol, a beta 2-selective partial agonist, had an enhanced potency and a greater intrinsic activity (atenolol treated: -log M EC50, 7.13 +/- 0.09; intrinsic activity, 0.86 +/- 0.04; non-beta-blocker treated: -log M EC50, 5.76 +/- 0.44; intrinsic activity, 0.39 +/- 0.13). We investigated possible mechanisms underlying the enhanced responsiveness to beta 2 stimulation. Determination of beta 2-adrenoceptor affinity for salbutamol showed no change of affinity in atenolol-treated patients. Responses of the tissues to the cyclic AMP analogue dibutyryl cyclic AMP were not different between atenolol-treated and non-beta-blocker-treated patients. The results suggest that chronic blockade of beta 1-adrenoceptors causes enhanced coupling of beta 2-adrenoceptors to adenylate cyclase or to other mechanisms leading to increased contractile force.     

Place of beta 3-adrenoceptors among other beta-adrenoceptor subtypes in the regulation of the cardiovascular system

Knowledge of the sympathetic system is a basic element in the understanding of numerous physiological and physiopathological phenomena. In the two last decades, new pharmacological, biochemical and molecular tools have changed our approach to the roles of beta-adrenoceptors in the cardiovascular system. In the heart, the positive inotropic effect of predominant beta 1-adrenoceptor stimulation is classically recognised. Several studies reveal a significant physiological relevance of the beta 2-adrenoceptor which could activate different signalling pathways in addition to that of cAMP. Moreover, the detection of a third beta-adrenoceptor subtype, beta 3, in human heart, responsible for a negative inotropic effect through a NO signalling pathway, has changed the classically admitted paradigm on the regulation of heart function by the beta-adrenergic system. The identification of atypical beta-adrenoceptors, based on pharmacological tools, led to the discovery of "putative" beta 4-adrenoceptors, which constituted a low affinity state of the beta 1-adrenoceptors. In vessels, all beta-adrenoceptors subtypes, beta 1, beta 2 and beta 3, mediated a vasodilation, but the signalling pathway involved in this effect was variable according to their localization (endothelial or smooth muscle cells), the species and the vascular bed. beta-adrenoceptors are involved in several cardiovascular disease and could constitute a determinant therapeutic target. The efficiency of some beta-blockers used in the treatment of heart failure could result from action on beta 3-adrenoceptors. Moreover, a mutation of the beta-adrenoceptor subtype suggested a role in hypertension and diabetes mellitus.     

Decrease in beta 1- and increase in beta 2-adrenoceptors in long-term follow-up after orthotopic cardiac transplantation.

Total beta 1- and beta 2- subtype distribution were examined in right ventricular biopsies taken from 100 patients 1-60 months after orthotopic cardiac transplantation and from eight prospective transplant donor hearts serving as controls. The patients were classified into eight groups depending on the time after transplantation that the biopsies were taken: 1-3 (n = 15), 4-7 (n = 15), 8-11 (n = 6), 12 (n = 15), 24 (n = 15), 36 (n = 12), 48 (n = 12) and 60 months (n = 10). The non-selective beta-adrenoceptor antagonist (-)-[125I]-iodocyanopindolol (ICYP) was used as a radioligand to assess total beta-adrenoceptor density. The beta 1- and beta 2-subtype distribution was determined with a beta 1-adrenoceptor saturating concentration of the selective beta 1-adrenoceptor antagonist CGP 20712A (300 nmol/l). In transplant donor hearts the total beta-adrenoceptor density was found to be 70.8 +/- 7.1 fmol/mg protein including a beta 1:beta 2-adrenoceptor ratio of about 80:20%. Until 36 months after cardiac transplantation the total number of beta-adrenoceptors showed no significant change. A slight but insignificant decrease was observed after 48 (16.2%) and 60 (21.2%) months. In contrast, from 12 to 60 months after cardiac transplantation the beta 1:beta 2-adrenoceptor ratio was shifted significantly (66:33% to 61:39%) as compared with transplant donor hearts which was due to an increase in beta 2- and a decrease in beta 1-adrenoceptor number. Thus, the surgically denervated, transplanted human heart exhibits a beta 2-adrenoceptor up-regulation during long-term follow-up. It is suggested that this up-regulation of the beta 2-adrenoceptor subtype could be owing to an increased importance of circulating catecholamines in modulating positive chronotropic and inotropic effects.     

Beta-adrenergic control of motility in the rat colon. I. Evidence for functional separation of the beta 1- and beta 2-adrenoceptor-mediated inhibition of colon activity

The beta-adrenoceptor-mediated inhibitory response in isolated rat colon strips of beta-adrenoceptor agonists (isoproterenol, terbutaline, prenalterol) in the absence and presence of the selective beta-adrenoceptor antagonists metoprolol (beta 1) and IPS 339 (beta 2) demonstrates that both beta 1- and beta 2-adrenoceptors are involved in the inhibition of colonic motility. Neuronal blockade induced by tetrodotoxin suppressed the relatively high (68%) maximal response of prenalterol (partial beta-adrenoceptor agonist) to 23%. The concentration-response curves for terbutaline (beta 2-selective agonist) and isoproterenol (nonselective agonist) were not influenced by tetrodotoxin. The results thus indicate that the beta-adrenergic inhibition of spontaneous activity in the rat colon strip may be mediated at two functional levels within the colon wall: either by beta 2-adrenoceptors in the smooth muscle layer or by beta 1-adrenoceptors in the intramural ganglionic plexuses, which by neuronal elements are coupled to the effector smooth muscle.     

Pharmacological analysis of positive chronotropic and inotropic responses to etilefrine in isolated dog heart preparations

Summary Positive chronotropic and inotropic responses to etilefrine (-[(ethylamino)methyl]-m-hydroxybenzyl alcohol), an orally active cardiovascular agent, were investigated in isolated dog right atrial and left ventricular preparations. Intravenous administration of etilefrine to a support dog increased heart rate and mean systemic blood pressure, and increased sinus rate and atrial contractile force in the isolated right atrium perfused with blood from the support dog. Etilefrine injected intra-arterially to isolated atria and ventricles induced dose-dependent positive chronotropic and inotropic effects. Etilefrine was about 100 times less potent than isoproterenol. The effects of etilefrine in isolated atria were significantly inhibited by treatment with atenolol, but were not significantly inhibited by ICI 118,551. The effects of etilefrine were partially inhibited by imipramine. These results indicate that etilefrine is a highly selective -1 adrenoceptor agonist and suggest a moderate catecholamine-releasing activity by tyramine-like action in the blood-perfused dog heart.     

An introduction to the pharmacologic properties of Dopacard (dopexamine hydrochloride)

Dopexamine hydrochloride (Dopacard) has been developed as a peripherally acting dopamine receptor agonist with afterload reducing properties for use in the acute management of low cardiac output states. Dopexamine hydrochloride is one-third as potent as dopamine in stimulating DA1 receptors but 60 times as potent as a beta 2-adrenoceptor agonist. Unlike dopamine, it is a weak beta 1-adrenoceptor agonist and does not stimulate vascular alpha adrenoceptors. Its stimulant properties at vascular DA1 receptors and at vascular beta 2 adrenoceptors endow it with the ability to improve renal blood flow and to increase cardiac output secondary to afterload reduction. In addition, mild positive inotropic activity arises from stimulation of cardiac beta 2 adrenoceptors, potentiation of endogenous norepinephrine due to uptake-1 blockade, and activation of the baroreceptor reflex. Other features of dopexamine hydrochloride that should enhance its clinical use are lack of arrhythmogenicity and rapid responsiveness to alterations in infusion rate.     

Beta-adrenergic control of motility in the rat colon. II. Proportions of beta 1- and beta 2-adrenoceptors identified with 125I-(-)pindolol binding

The specific binding of 125I-(-)pindolol to membranes prepared from rat colon displayed high affinity (Kd = 75 +/- 3 pM) and relatively low capacity (Bmax = 48 +/- 3.8 fmol/mg protein). Furthermore, the binding was reversible and possessed properties expected of a beta-adrenoceptor. Thus, for example, the potency of agonists in displacing 125I-(-)pindolol was, in decreasing order, isoproterenol greater than epinephrine greater than norepinephrine, and the negative isomer of propranolol was two orders of magnitude more potent than the positive isomer. Displacement of specific binding by the beta 1-adrenoceptor antagonist pafenolol and the beta 2-adrenoceptor antagonist ICI 118.551 revealed that the colon preparation possessed a heterogenous beta-adrenoceptor population. Analysis of the inhibition curves using computer-assisted curve fitting suggested that the sites consisted of a small (14%-21%) beta 1-adrenoceptor population and a large (79%-86%) beta 2-adrenoceptor population. The coexistence of beta 1- and beta 2-adrenoceptors is discussed in relation to evidence of a functional linkage of both subtypes with colon motility.     

Desensitization of cardiac beta-adrenoceptor signaling with heart failure produced by myocardial infarction in the rat. Evidence for the role of Gi but not Gs or phosphorylating proteins.

This study examined mechanisms of beta-adrenergic (AR) desensitization in a myocardial infarction (MI) model of heart failure in the rat. Inotropic responses to isoproterenol (non-selective beta-AR agonist) and RO 363 (selective beta1-AR agonist), in left atria and left papillary muscle, were reduced by up to 65%, while chronotropic responses in right atria were unaffected. beta1- and beta2-AR density did not change after MI, suggesting that changes in beta-AR responsiveness are due to changes occurring downstream of the receptor. Inotropic and chronotropic responses to forskolin were not altered in right and left atria and left papillary muscle after MI, suggesting changes at the level of the G-proteins. Pertussis toxin treatment of animals restored inotropic responses to isoproterenol in left atria and left papillary muscle to levels seen in the sham group, indicating that inactivation of Gi-proteins improves inotropic function in MI rats, and that beta-ARs couple to Gi in cardiac failure. Expression of G-protein receptor kinase 2 (GRK2), beta-arrestin1 and the regulatory subunits of cAMPdPK (RI alpha and RII alpha), showed no change after MI. However the expression of Gi alpha2 was significantly increased in left ventricle (sham 0.888+/-0.140, MI 1. 759+/-0.352 P=0.026), right ventricle (sham 0.031+/-0.004, MI 0. 037+/-0.002 P=0.006) and atria (sham 0.107+/-0.006, MI 0.138+/-0.006 P=0.004), with no changes observed in the expression of Gs alpha. These results suggest that increases in Gi play an important role in the decreased beta-AR responsiveness in the rat model of MI.     

Key role of myosin light chain (MLC) kinase-mediated MLC2a phosphorylation in the alpha 1-adrenergic positive inotropic effect in human atrium

OBJECTIVE: Mechanisms of the positive inotropic response to alpha(1)-adrenergic stimulation in the heart remain poorly understood, but recent evidence in rat papillary muscle suggests an important role of regulatory myosin light chain (MLC2) phosphorylation. This study investigated alpha(1)-adrenergic contractile effects and the role of MLC kinase (MLCK)-dependent phosphorylation of MLC2 in human atrial muscle strips. METHODS: Force measurement was performed on electrically stimulated atrial muscle strips (n=140; 20 hearts) in the presence of the beta-blocker nadolol. MLC2a phosphorylation was determined by 2D-polyacrylamide gel electrophoresis and Western blotting of muscle strips that were immediately freeze-clamped following force measurements. RESULTS: The alpha(1)-agonist phenylephrine (PE; 0.3-100 microM) exerted a concentration-dependent, monophasic, sustained positive inotropic effect (86% above basal) that was accompanied by an 80% increase in MLC2a phosphorylation. Desinhibition of MLC phosphatase by the Rho kinase inhibitor Y-27632 (10 microM) reduced the effect of PE by 16%. The MLCK inhibitor wortmannin (10 microM) completely abolished both the PE-induced increase in force and MLC2a phosphorylation. The structurally unrelated MLCK inhibitor ML-7 (10 microM) had similar effects. Neither Y-27632 nor wortmannin or ML-7 affected beta-adrenergic force stimulation. In contrast to our findings in atrial muscle strips, we observed no increase in MLC2v phosphorylation after PE in human ventricular muscle strips and wortmannin failed to inhibit PE-induced force of contraction. CONCLUSION: alpha(1)-Adrenergic receptors mediate a prominent increase in contractile force in human atria that depends on MLCK activity and is accompanied by an increase in MLC2 phosphorylation.     

Positive inotropic effects of the beta2-adrenoceptor agonist terbutaline in the human heart: Effects of long-term beta1-adrenoceptor antagonist treatment

Objectives. This study was conducted to determine whether activation of cardiac beta₂-adrenoceptors increases contractility in humans and whether this is affected by long-term beta₁-adrenoceptor antagonist treatment.Background. Coexistence of beta₁- and beta₂-adrenoceptors in the human heart is generally accepted. The functional importance of cardiac beta₂-adrenoceptors for increases in contractility in humans, however, has not been completely established.Methods. We studied 1) the beta-adrenoceptor subtype mediating positive inotropic effects of the beta₂-adrenoceptor agonist terbutaline in vitro (on right atrial and left ventricular preparations from nonfailing human hearts) and increases in contractility (by measurement of systolic time intervals) in vivo in seven healthy male volunteers; and 2) in vivo whether long-term treatment of volunteers with the beta₁-adrenoceptor antagonist bisoprolol affects terbutaline-induced increases in contractility.Results. In vitro terbutaline caused a concentration-dependent increase in atrial and ventricular adenylate cyclase activity and force of contraction. Terbutaline effects were antagonized only by the beta₂-adrenoceptor antagonist ICI 118,551, indicating that they were mediated by beta₂-adrenoceptor stimulation. In vivo intravenous infusions of terbutaline (dose range 25 to 300 ng/kg body weight per min for 15 min) dose dependently increased heart rate and shortened the pre-ejection period and heart rate-corrected electromechanical systole (QS₂) time. These effects are mediated predominantly by beta₂-adrenoceptor stimulation because they were only marginally affected by the beta₁-adrenoceptor antagonist bisoprolol (1 × 10 mg orally), either given 2 h before infusion or long term for 3 weeks.Conclusions. Stimulation of cardiac beta₂-adrenoceptors in humans causes not only in vitro but also in vivo positve inotropic effects. Long-term beta₁-adrenoceptor antagonist treatment does not considerably affect beta₂-adrenoceptor-mediated in vivo increases in contractility. Thus, it may be possible to treat patients with chronic heart failure and long-term beta₁-adrenoceptor antagonist therapy with beta₂-adrenoceptor agonists if immediate inotropic support is needed.     

Densitometric analysis of beta 1- and beta 2-adrenoceptors in guinea-pig atrioventricular conducting system.

Quantitative autoradiography was used to determine the densities of beta 1- and beta 2-adrenoceptors in the atrioventricular conducting system in guinea-pig. (-)[125I]Cyanopindolol (CYP) was used to label beta 1- and beta 2-adrenoceptors in the absence or presence of the beta 1-adrenoceptor selective antagonist CGP 20712A (100 nM) or the beta 2-adrenoceptor selective antagonist ICI 118,551 (70 nM) or the non-selective beta-adrenoceptor antagonist (-)-propranolol (1 microM). Protein in discrete anatomical regions was determined using a densitometric method based on the dye Coomassie brilliant blue G. In the atrioventricular conducting system the proportion of beta 2-adrenoceptors determined by inhibition of total (-)[125I]-CYP binding by CGP 20712A (100 nM) ranged from 32.5% (atrioventricular node) to 48.7% (left bundle branch). In the atrioventricular node (16.8 fmol/mg protein), bundle of His (12.1 fmol/mg protein), right (17.4 fmol/mg protein) and left (21.1 fmol/mg protein) bundle branches and Purkinje cells there was a higher density of beta 2-adrenoceptors than in the interventricular septum (8.4 fmol/mg protein) and right atria (8.3 fmol/mg protein). The medial smooth muscle of the aorta, aortic valve, adventitia of the aorta, nerve tissue, tricuspid and mitral valves contained only beta 2-adrenoceptors. It is speculated that the use of beta-adrenoceptor antagonists to control cardiac arrhythmias involving a defect in conduction in the atrioventricular node should take into consideration both beta 1- and beta 2-adrenoceptors.     

Agmatine facilitates sympathetic neurotransmission in frog myocardium via an action on alfa 2-adrenergic receptors.

OBJECTIVE: In this study, the effect of agmatine was studied on sympathetic neurotransmission in the frog isolated ventricular strips. METHODS: Ventricular strips were prepared from the heart of the pitched frog. Each strip was mounted vertically in an organ bath. Muscle contractions were recorded isometrically by a force displacement transducer and displayed on a polygraph. RESULTS: Concentration-response relationships to noradrenaline were obtained on contractility of frog ventricular strips evoked by electrical stimulation. The responses of noradrenaline were re-obtained in presence of agmatine (3X10(-4) M). Agmatine was found to be ineffective on contractile responses of noradrenaline in electrically driven ventricular strips of frog heart. Transient additional stimulations (TAS) induced contractions. The contractions induced by TAS were re-obtained in presence of agmatine, idazoxan + agmatine and yohimbine + agmatine. Agmatine significantly increased the positive inotropic responses of TAS. The effect of agmatine on contractile responses of TAS was not changed by idazoxan, indicating that imidazoline receptors have not functions in this response. The effect of agmatine on the contractile responses to TAS was reversed by yohimbine, indicating involvement of alpha2 adrenoceptors in this response. Agmatine did not change the contractile responses of ventricular strips to exogenous noradrenaline, indicating that agmatine does not affect postjunctional adrenoceptors. CONCLUSION: These results suggest that agmatine facilitates sympathetic neurotransmission in frog myocardium via an action on prejunctional alpha2 adrenergic receptors located on sympathetic nerve terminals.     

In vivo demonstration of cardiac beta 2-adrenoreceptor sensitization by beta 1-antagonist treatment.

Treatment with beta 1-selective antagonists causes selective sensitization of isolated strips of human atrial myocardium to the inotropic action of epinephrine and beta 2-agonists but not of norepinephrine. To determine whether beta 1-selective antagonist treatment alters the responsiveness of cardiac beta 2-adrenoreceptors in vivo, we measured the positive chronotropic responses to salbutamol injected into the right coronary artery. Ten patients treated with atenolol (50-100 mg daily) were compared with 10 patients not treated with beta-blockers. The mean dose required to cause an increase in heart rate of 30 beats/min was 2.29 micrograms (log dose 0.36 +/- 0.12 micrograms [mean +/- SEM]) in the atenolol-treated patients. In the non-beta-blocker-treated patients, the dose required to cause an increase in heart rate of 30 beats/min was significantly greater, 8.91 micrograms (log dose 0.95 +/- 0.11 micrograms) (p less than 0.005). We conclude that treatment with beta 1-selective beta-blockers leads to increased cardiac responsiveness to beta 2-adrenoreceptor stimulation. This may be the underlying mechanism of the beta-blocker withdrawal syndrome and may make the heart more susceptible to the adverse effects of epinephrine in situations of stress (e.g., myocardial infarction).     

Functional assessment of beta adrenoceptor subtypes in human colonic circular and longitudinal (taenia coli) smooth muscle

BACKGROUND AND AIMS: The subtype and species related heterogeneity of beta adrenoceptors prompted a functional reappraisal of these molecular targets of motility inhibition in the human colon. METHODS: Relaxation of muscle strips was measured in vitro. RESULTS: The following agonists had decreasing relaxing potency (effective concentration range 10(-8)-10(-4) mol/l): (-)isoprenaline (non-selective), terbutaline (beta(2) selective), CGP 12177 (beta(3) selective, also beta(1), beta(2) antagonist), and SR 58611A (beta(3) selective). Isoprenaline and terbutaline were more potent on circular than taenia strips; CGP 12177 and SR 58611A weakly and partially relaxed taenia but had little effect on circular strips. The potency of isoprenaline on circular strips was greatly reduced by the beta(1) selective antagonist CGP 20712 (10(-7) mol/l), and less so by ICI 118551 (10(-7) mol/l, beta(2) selective). CGP 20712 and ICI 118551 together (both 3 x 10(-6) mol/l) had no effect on taenia relaxation by SR 58611A and rendered isoprenaline and terbutaline virtually inactive on circular strips, although not on taenia, which was relaxed at higher than control concentrations and maximally by isoprenaline. Propranolol, a beta(1), beta(2) non-selective antagonist, at high concentrations (10(-5) mol/l) prevented taenia relaxation by CGP 12177 and SR 58611A; its quantitative antagonism of isoprenaline (in common with that of CGP 12177 used as an antagonist) was competitive in circular strips but not on taenia. CONCLUSIONS: beta(1), beta(2), and beta(3) adrenoceptors are functionally detectable in the human colon; agonist stimulation of any one type relaxed taenia but only isoprenaline was fully effective at the beta(3) subtype.