Effects of chronic infusion of (-)-isoprenaline on rat cardiac muscarinic (M2)-cholinoceptors and beta 1- and beta 2-adrenoceptors

Differential regulation of the β-adrenoceptor density and cyclic AMP level with age and sex in turkey cardiac chambers

Decreased responses of the heart to β-adrenoceptor stimulation with aging have been shown to occur merely in selected heart chambers in relation to increased catecholamine levels. However, there are no systematic studies that investigate all cardiac chambers with regard to receptor density and cAMP (adenosine 3', 5'-cyclic monophosphate) responses. We used meat-type turkey poults (British United Turkey (B.U.T.) Big 6) with increasing age because their heart seems to decrease in weight in relation to body weight and they are often used as an animal model for heart failure. The receptor density and distribution were quantified by radioligand binding analysis using (-)-[(125)I]-iodocyanopindolol and β-adrenoceptor subtype-specific antagonists (ICI 118.551 and CGP 20712 A) in membranes of four cardiac chambers (right and left atria and ventricles) of 6-week-, 12-week-, 16/21-week-, and 57-week-old B.U.T. BIG 6 turkeys. Receptor function was determined by measuring basal and stimulated cAMP production. In both sexes, the β-adrenoceptor density decreased significantly in all chambers with age without altered β-adrenoceptor subtype distribution. The receptor affinity (KD) to the radioligand was similar in hearts of all age groups. β-adrenoceptor-(isoproterenol and guanosine 5'-triphosphate), G-protein-(NaF) and catalytic unit of adenylate cyclase (forskolin, Mn(2+)) mediated cAMP responses were not chamber-dependent. Indeed, the cAMP level was significantly lower in 57-week-old hearts than in 6-week-, 12-week-, 16/21-week-old hearts. These data suggest that with increasing age and body weight, the β-adrenoceptor signal transduction pathway was highly blunted in all cardiac chambers, occurring by decreased receptor density and cAMP responses.

Role of the excessive amounts of circulating catecholamines and glucocorticoids in stress-induced heart disease

Various stressful stimuli are known to activate the sympathetic nervous system to release catecholamines and the hypothalamic-pituitary-adrenal axis to release glucocorticoids in the circulation. Although initial actions of both catecholamines and glucocorticoids are beneficial for the function of the cardiovascular system, their delayed effects on the heart are deleterious. Glucocorticoids not only increase plasma levels of catecholamines by inhibiting their extraneuronal uptake, but they have also been shown to induce supersensitivity to catecholamines in the heart by upregulating different components of the betta-adrenoceptor signal transduction system. Low concentrations of catecholamines stimulate the heart by promoting Ca2+ movements, whereas excessive amounts of catecholamines produce cardiac dysfunction by inducing intracellular Ca2+ overload in cardiomyocytes. Several studies have shown, however, that under stressful conditions high concentrations of catecholamines become oxidized to form aminolutins and generate oxyradicals. These oxidation products of catecholamines have been demonstrated to produce coronary spasm, arrhythmias, and cardiac dysfunction by inducing Ca2+-handling abnormalities in both sarcolemmal and sarcoplasmic reticulum, defects in energy production by mitochondria, and myocardial cell damage. In this article we have focused the discussion to highlight the interrelationship between catecholamines and glucocorticoids and to emphasize the role of oxidation products of catecholamines in the development of stress-induced heart disease.

Cardiac hypertrophy induced by sustained β-adrenoreceptor activation: pathophysiological aspects

Cardiac hypertrophy is promoted by adrenergic over-activation and represents an independent risk factor for cardiovascular morbidity and mortality. The basic knowledge about mechanisms by which sustained adrenergic activation promotes myocardial growth, as well as understanding how structural changes in hypertrophied myocardium could affect myocardial function has been acquired from studies using an animal model of chronic systemic beta-adrenoreceptor agonist administration. Sustained beta-adrenoreceptor activation was shown to enhance the synthesis of myocardial proteins, an effect mediated via stimulation of myocardial growth factors, up-regulation of nuclear proto-oncogenes, induction of cardiac oxidative stress, as well as activation of mitogen-activated protein kinases and phosphatidylinositol 3-kinase. Sustained beta-adrenoreceptor activation contributes to impaired cardiac autonomic regulation as evidenced by blunted parasympathetically-mediated cardiovascular reflexes as well as abnormal storage of myocardial catecholamines. Catecholamine-induced cardiac hypertrophy is associated with reduced contractile responses to adrenergic agonists, an effect attributed to downregulation of myocardial beta-adrenoreceptors, uncoupling of beta-adrenoreceptors and adenylate cyclase, as well as modifications of downstream cAMP-mediated signaling. In compensated cardiac hypertrophy, these changes are associated with preserved or even enhanced basal ventricular systolic function due to increased sarcoplasmic reticulum Ca(2+) content and Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. The increased availability of Ca(2+) to maintain cardiomyocyte contraction is attributed to prolongation of the action potential due to inhibition of the transient outward potassium current as well as stimulation of the reverse mode of the Na(+)-Ca(2+) exchange. Further progression of cardiac hypertrophy towards heart failure is due to abnormalities in Ca(2+) handling, necrotic myocardial injury, and increased myocardial stiffness due to interstitial fibrosis.

Cardiac implications for the use of beta2-adrenoceptor agonists for the management of muscle wasting

There are proposals for the implementation of beta(2)-adrenoceptor agonists for the management of muscle wasting diseases. The idea has been initiated by studies in animal models which show that beta(2)-adrenoceptor agonists cause hypertrophy of skeletal muscle. Their use in clinical practice will also need an understanding of possible effects of activation of human heart beta(2)-adrenoceptors. Consequences could include an increased probability of arrhythmias in susceptible patients.

Validity of (-)-[3H]-CGP 12177A as a radioligand for the 'putative beta4-adrenoceptor' in rat atrium

1. We have recently suggested the existence in the heart of a 'putative beta4-adrenoceptor' based on the cardiostimulant effects of non-conventional partial agonists, compounds that cause cardiostimulant effects at greater concentrations than those required to block beta1- and beta2-adrenoceptors. We sought to obtain further evidence by establishing and validating a radioligand binding assay for this receptor with (-)-[3H]-CGP 12177A ([-]-4-(3-tertiarybutylamino-2-hydroxypropoxy) benzimidazol-2-one) in rat atrium. We investigated (-)-[3H]-CGP 12177A for this purpose for two reasons, because it is a non-conventional partial agonist and also because it is a hydrophilic radioligand. 2. Increasing concentrations of (-)-[3H]-CGP 12177A, in the absence or presence of 20 microM (-)-CGP 12177A to define non-specific binding, resulted in a biphasic saturation isotherm. Low concentrations bound to beta1- and beta2-adrenoceptors (pKD 9.4+/-0.1, Bmax 26.9+/-3.1 fmol mg(-1) protein) and higher concentrations bound to the 'putative beta4-adrenoceptor' (pKD 7.5+/-0.1, Bmax 47.7+/-4.9 fmol mg(-1) protein). In other experiments designed to exclude beta1- and beta2-adrenoceptors, (-)-[3H]-CGP 12177A (1-200 nM) binding in the presence of 500 nM (-)-propranolol was also saturable (pKD 7.6+/-0.1, Bmax 50.8+/-7.4 fmol mg(-1) protein). 3. The non-conventional partial agonists (-)-CGP 12177A (pKi 7.3+/-0.2), (+/-)-cyanopindolol (pKi 7.6+/-0.2), [-]-pindolol (pK1 6.6+/-0.1) and (+/-)-carazolol (pKi 7.2+/-0.2) and the antagonist (-)-bupranolol (pKi 6.6+/-0.2), all competed for (-)-[3H]-CGP 12177A binding in the presence of 500 nM (-)-propranolol at the 'putative beta4-adrenoceptor', with affinities closely similar to potencies and affinities determined in organ bath studies. 4. The catecholamines competed with (-)-[3H]-CGP 12177A at the 'putative beta 4-adrenoceptor' in a stereoselective manner, (-)-noradrenaline (pKiH 6.3+/-0.3, pKiL 3.5+/-0.1), (-)-adrenaline (pKiH 6.5+/-0.2, pKiL 2.9+/-0.1), (-)-isoprenaline (pKiH 6.2+/-0.5, pKiL 3.4+/-0.1), (+)-isoprenaline (pKi< 1.7), (-)-RO363 ((-)-(1-(3,4-dimethoxyphenethylamino)-3-(3,4-dihydroxyphenoxy++ +)-2-propranol)oxalate, pKi 5.5+/-0.1). 5. The inclusion of guanosine 5-triphosphate (GTP 0.1 mM) had no effect on binding of (-)-CGP 12177A or (-)-isoprenaline to the 'putative beta4-adrenoceptor'. In competition binding studies, (-)-CGP 12177A competed with (-)-[3H]-CGP 12177A for one receptor state in the absence (pKi 7.3+/-0.2) or presence of GTP (pKi 7.3+/-0.2). (-)-Isoprenaline competed with (-)-[3H]-CGP 12177A for two states in the absence (pKiH 6.6+/-0.3, pKiL 3.5+/-0.1; % H 25+/-7) or presence of GTP (pKiH 6.2+/-0.5, pKiL 3.4+/-0.1; % H 37+/-6). In contrast, at beta1-adrenoceptors, GTP stabilized the low affinity state of the receptor for (-)-isoprenaline. 6. The specificity of binding to the 'putative beta 4-adrenoceptor' was tested with compounds active at other receptors. High concentrations of the beta 3-adrenoceptor agonists, BRL 37344 ((RR+SS)[4-[2-[[2-(3-chlorophenyl)-2-hydroxy-ethyl]amino]propyl]phenoxy]acetic acid, 6 microM), SR 58611A (ethyl{(7S)-7-[(2R)-2-(3-chlorophenyl)-2-hydroxyethylamino]-5,6,7,8-tetrahydronaphtyl2-yloxy} acetate hydrochloride, 6 microM), ZD 2079 ((+/-)-1-phenyl-2-(2-4-carboxymethylphenoxy)-ethylamino)-ethan-1-ol, 60 microM), CL 316243 (disodium (R,R)-5-[2-[2-(3-chlorophenyl)-2-hydroxyethyl-amino]propyl]- 1,3-benzodioxole-2,2-dicarboxylate, 60 microM) and antagonist SR 59230A (3-(2-ethylphenoxy)-1-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-2S-2-propanol oxalate, 6 microM) caused less than 22% inhibition of (-)[3H]-CGP 12177A binding in the presence of 500 nM (-)propranolol. Histamine (1mM), atropine (1 microM), phentolamine (10 microM), 5-HT (100 microM) and the 5-HT4 receptor antagonist SB 207710 ((1-butyl-4-piperidinyl)-methyl 8-amino-7-iodo-1,4-benzodioxan-5-carboxylate, 10nM) caused less than 26% inhibition of binding. 7.Non-conventional partial agonists, the antagonist (-)bupranolol and catecholamines all competed for (-)[3H]-CGP 12177A binding in the absence of (-)propranolol at beta1-adrenoceptors, with affinities (pKi) ranging from 1.6-3.6 log orders greater than at the 'putative beta 4-adrenoceptor'. 8.We have established and validated a radioligand binding assay in rat atrium for the 'putative beta 4-adrenoceptor' which is distinct from beta1-, beta2- and beta 3-adrenoceptors. The stereoselective interaction with the catecholamines provides further support for the classification of the receptor as 'putative beta 4-adrenoceptor'.

Proposal for the interaction of non-conventional partial agonists and catecholamines with the 'putative beta 4-adrenoceptor' in mammalian heart

1. Evidence for a 'putative beta 4-adrenoceptor' originated over 20 years ago when cardiostimulant effects were observed to non-conventional partial agonists. These agonists were originally described as beta 1- and beta 2-adrenoceptor antagonists; however, they cause cardiostimulant effects at much higher concentrations than those required to block beta 1- and beta 2-adrenoceptors. Cardiostimulant effects of non-conventional partial agonists have been observed in mouse, rat, guinea-pig, cat, ferret and human heart tissues. 2. The receptor is expressed in several heart regions, including the sinoatrial node, atrium and ventricle. 3. The receptor is resistant to blockade by most antagonists that possess high affinity for beta 1- and beta 2-adrenoceptors, but is blocked with moderate affinity by (-)-bupranolol and CGP 20712A. 4. The receptor is pharmacologically distinct from the beta 3-adrenoceptor. Micromolar concentrations of beta 3-adrenoceptor agonists have no agonist or blocking activity. The receptor is also resistant to blockade by a beta 3-adrenoceptor-selective antagonist. 5. The receptor mediates increases in cAMP levels and cAMP-dependent protein kinase (PK) A activity in cardiac tissues. Phosphodiesterase inhibition potentiates the positive chronotropic and inotropic effects of non-conventional partial agonists. 6. The receptor mediates hastening of atrial and ventricular relaxation, which is consistent with involvement of a cAMP-dependent pathway. 7. The non-conventional partial agonist (-)-[3H]-CGP 12177A labels the cardiac putative beta 4-adrenoceptor. Non-conventional partial agonists compete for binding with affinities that are closely similar to their agonist potencies. Catecholamines compete for binding in a stereoselective manner with a rank order of affinity of (-)-RO363 > (-)-isoprenaline > (-)-noradrenaline > or = (-)-adrenaline >> (+)-isoprenaline, suggesting that catecholamines can interact with the receptor. 8. The putative beta 4-adrenoceptor appears to be coupled to the Gs-adenylyl cyclase system, which could serve as a guide to its future cloning. Activation of the receptor may plausibly improve diastolic function but could also mediate arrhythmias.

Changes in cardiovascular responsiveness to dopexamine and beta 1- and beta 2-adrenoceptor function after the chronic treatment of beta-adrenoceptor antagonists and agonists in anaesthetized dogs

1. The studies were designed to investigate the changes in responsiveness to beta-adrenoceptor agonists induced by chronic administration of beta-adrenoceptor antagonists and agonists. 2. Dose-response curves for dopexamine, isoprenaline, noradrenaline and impromidine on heart rate, blood pressure and myocardial contractility (dP/dt:integrated isometric tension) were obtained in untreated dogs and compared to those measured in dogs which had been pretreated with propranolol (8 mg kg-1 day-1), atenolol (6 mg kg-1 day-1), isoprenaline (0.5 microgram kg-1 min-1) or noradrenaline (0.5 microgram kg-1 min-1) for 14 days. 3. Propranolol pretreatment significantly enhanced the inotropic and chronotropic responses to isoprenaline. There were noticeable, but non-significant increases in inotropic sensitivity to noradrenaline and dopexamine, especially at higher dose levels. In the atenolol treatment group, there were significant increases in inotropic responses to dopexamine and isoprenaline and in depressor responses to isoprenaline. 4. Thus, chronic administration of propranolol increased responses mediated by both beta 1- and beta 2-adrenoceptors, whereas, atenolol selectively enhanced the inotropic responsiveness to dopexamine, which is mediated mainly by beta 2-adrenoceptors. 5. Isoprenaline pretreatment caused a significant decrease in inotropic sensitivity to dopexamine, isoprenaline and noradrenaline and a significant reduction in chronotropic responses to dopexamine. The tendency to reduced depressor responses to isoprenaline and dopexamine failed to reach significance. Pretreatment with noradrenaline decreased only the inotropic response to noradrenaline. 6. Thus, chronic isoprenaline treatment reduced the responsiveness of both beta 1- and beta 2-adrenoceptors, while chronic noradrenaline infusion only reduced beta 1-adrenoceptor-mediated responses. 7. There was no significant change in any of the dose-response curves to impromidine after any beta-adrenoceptor antagonist or agonist treatment. This indicates that there was no non-specific alteration in responsiveness and that the observed changes were likely to be associated with specific alterations in beta-adrenoceptor number or function.

Loss of [125I]-pindolol binding to beta-adrenoceptors on rat nodose ganglion after chronic isoprenaline treatment

The nodose ganglion contains the cell bodies of afferent nerves which convey predominantly sensory information from the viscera to the central nervous system (CNS). Autoradiographic studies show binding sites for beta-adrenoceptor ligands are present on sections of the rat nodose ganglion and also on the corresponding inferior vagal ganglion in humans, indicating the presence of beta-adrenoceptors in these ganglia. Since prolonged stimulation of beta-adrenoceptors in rats with the nonselective beta-adrenoceptor agonist isoprenaline (400 micrograms kg-1 day-1 s.c.) for 14 days results in desensitisation and/or down-regulation of receptors in peripheral tissues, such as heart, kidney and blood vessels, the effects of this treatment on the beta-adrenoceptor population on the nodose ganglion have been examined. Using [125I]-pindolol as a radioligand, autoradiographic studies revealed that specific binding was reduced by 74% in ganglia from isoprenaline-pretreated rats compared to that in ganglia from vehicle-pretreated rats, demonstrating down-regulation of receptors by isoprenaline. [125I]-Pindolol binding was sensitive to inhibition by ICI 118.551 (selective beta 2-adrenoceptor antagonist) but not to atenolol (selective beta 1-adrenoceptor antagonist), indicating receptors are predominantly of the beta 2-adrenoceptor subtype. No change in binding was apparent over the vagus nerve. The nodose ganglion appears to be an additional site at which beta 2-adrenoceptors may be down-regulated in vivo, possibly interfering with normal baro-, chemo- and sensory reflexes.

Chronic (-)-isoprenaline infusion down-regulates beta 1- and beta 2-adrenoceptors but does not transregulate muscarinic cholinoceptors in rat heart

Regulation of beta-adrenoceptor (beta-ar) subtypes and transregulation of muscarinic cholinoceptors (mAchr) was examined in regions of rat heart after chronic infusion of (-)-isoprenaline (450 micrograms/kg per hour) for 14 days. Following (-)-isoprenaline infusion systolic blood pressure was reduced for 10 days but then gradually returned to control levels, whereas heart rate was increased for 7 days before declining to a level significantly above control. Heart weight to body weight ratio was increased in (-)-isoprenaline treated rats. beta-ar subtype densities were measured by quantitative autoradiography with [125I]-cyanopindolol (CYP) in sinoatrial node (SA), atrioventricular node (AV), bundle of His (BH), left (LB) and right (RB) bundle branches, interventricular (IVS) and interatrial (IAS) septa, right atria (RA), apex (AX) and mitral valve (MV). beta 1-ars were reduced by 59.1-74.2% in the AV conducting regions, 53.4% in the SA node and 43.3-53.4% in myocardial areas, beta 2-ars were markedly reduced in myocardial regions (93.2-98.5%) and in pacemaker and conducting regions (87.7-97.8%). No changes in mAchr densities measured using [3H]-N-methyl scopolamine (NMS) occurred in the AV node, BH, LB, RB, IVS and IAS following (-)-isoprenaline infusion. Densities of beta 1- and beta 2-ars and mAchrs were also measured in ventricular homogenates from control and (-)-isoprenaline treated animals. beta-ar levels were significantly reduced (P < 0.05) in treated animals and the ratio of beta 1- to beta 2-ars increased after treatment. mAchr density in ventricular homogenates measured using either [3H]-NMS or [3H]-quinuclidinyl [phenyl-4-3H]benzilate (QNB) was unchanged. Homogenates of left and right ventricle also showed no change using [3H]-NMS. Organ bath studies were used to investigate the effect of (-)-isoprenaline infusion on negative inotropic and chronotropic effects of the non-selective muscarinic receptor agonist bethanechol in left and right atria, respectively. Lower concentrations of bethanechol (3 x 10(-10) to 10(-6) M) produced a negative inotropic response in isolated electrically driven left atria from (-)-isoprenaline treated rats, but not from control rats, with the slope of the curves being significantly different between groups (ANCOVA, P = 0.037). At concentrations of bethanechol from 10(-6) to 3 x 10(-4) M the negative inotropic response was not changed between (-)-isoprenaline treated and control animals. Bethanechol also produced a negative chronotropic response at lower concentrations (10(-10) to 10(-6) M) in (-)-isoprenaline treated rats, but not in controls. A second, steeper phase of the negative chronotropic response occurred at concentrations of bethanechol greater than 10(-6) M and was also seen in control rats. Expression of M2 (cardiac) mAchrs (m2Achr) in left and right ventricular tissues measured using a quantitative non-competitive polymerase chain reaction (PCR) assay showed a significant (P = 0.001) 28.5% increase in expression in left ventricle and a significant (P = 0.003) 21.5% decrease in expression in right ventricle after (-)-isoprenaline treatment, compared to controls. There was no significant difference in total ventricular m2Achr expression between the two groups of rats. The results suggest that chronic beta-ar stimulation down-regulates both beta 1- and beta 2-ars, and appears to differentially transregulate m2Achr expression, but not mAchr protein. Following (-)-isoprenaline infusion, muscarinic receptor mediated responses were sensitised, with no change in receptor densities, suggesting changes occur in the cell signalling system beyond the level of the receptor.