Intrinsic sympathomimetic activity of beta-adrenoceptor antagonists: down-regulation of cardiac beta 1- and beta 2-adrenoceptors

Bucindolol, a nonselective beta 1- and beta 2-adrenergic receptor antagonist, decreases beta-adrenergic receptor density in cultured embryonic chick cardiac myocyte membranes

Bucindolol and carvedilol, nonselective beta1- and beta2-adrenergic receptor antagonists, have been widely used in clinical therapeutic trials of congestive heart failure. The aim of the current study was to investigate long-term effects of bucindolol or carvedilol on beta-adrenergic receptor protein and gene expression in cardiac myocytes. Embryonic chick cardiac myocytes were cultured and incubated with bucindolol (1 microM), carvedilol (1 microM), or norepinephrine (1 microM) for 24 h. 125I-iodocyanopindolol binding assays demonstrated that incubation with norepinephrine or bucindolol, but not carvedilol, significantly decreased beta-adrenergic receptor density in crude membranes prepared from the myocytes. Neither bucindolol nor carvedilol significantly stimulated adenylyl cyclase activity in membranes from drug-untreated cells. Unlike by norepinephrine, the receptor density reduction by bucindolol incubation was not accompanied by a change in beta1-adrenergic receptor messenger RNA abundance. A decrease in membrane beta-adrenergic receptor density without a change in cognate messenger RNA abundance was also observed in hamster DDT1 MF2 cell line incubated with bucindolol (1 microM, 24 h). We conclude that incubation with bucindolol, but not carvedilol, results in true reduction of beta-adrenergic receptor density in chick cardiac myocyte membranes by mechanisms that are distinct from those responsible for receptor density reduction by the agonist norepinephrine.

Evaluation of partial beta-adrenoceptor agonist activity

A partial beta-adrenoceptor (beta-AR) agonist will exhibit opposite agonist and antagonist activity depending on the prevailing degree of adrenergic tone or the presence of a beta-AR agonist with higher intrinsic activity. In vivo partial beta-AR agonist activity will be evident at rest with low endogenous adrenergic tone, as for example with chronotropicity (beta 1/beta 2), inotropicity (beta 1) or peripheral vasodilatation and finger tremor (beta 2). beta-AR blocking drugs which have partial agonist activity may exhibit a better therapeutic profile when used for hypertension because of maintained cardiac output without increased systemic vascular resistance, along with an improved lipid profile. In the presence of raised endogenous adrenergic tone such as exercise or an exogenous full agonist, beta-AR subtype antagonist activity will become evident in terms of effects on exercise induced heart rate (beta 1) and potassium (beta 2) responses. Reduction of exercise heart rate will occur to a lesser degree in the case of a beta-adrenoceptor blocker with partial beta 1-AR agonist activity compared with a beta-adrenoceptor blocker devoid of partial agonist activity. This may result in reduced therapeutic efficacy in the treatment of angina on effort when using beta-AR blocking drugs with partial beta 1-AR agonist activity. Effects on exercise hyperkalaemia are determined by the balance between beta 2-AR partial agonist activity and endogenous adrenergic activity. For predominantly beta 2-AR agonist such as salmeterol and salbutamol, potentiation of exercise hyperkalaemia occurs. For predominantly beta 2-AR antagonists such as carteolol, either potentiation or attenuation of exercise hyperkalaemia occurs at low and high doses respectively. beta 2-AR partial agonist activity may also be expressed as antagonism in the presence of an exogenous full agonist, as for example attenuation of fenoterol induced responses by salmeterol. Studies are required to investigate whether this phenomenon is relevant in the setting of acute severe asthma.

Chronic muscarinic cholinoceptor stimulation increases adenylyl cyclase responsiveness in rat cardiomyocytes by a decrease in the level of inhibitory G-protein alpha-subunits

Exposure of neonatal rat cardiomyocytes for 3 days to the muscarinic cholinoceptor agonist carbachol led to a concentration-dependent increase in adenylyl cyclase stimulation by the beta-adrenoceptor agonist isoproterenol by up to 115% (at 1 mmol/l carbachol). In addition, direct adenylyl cyclase stimulation by forskolin was increased in carbachol (1 mmol/l)-treated cells by 32%. Pretreatment of the rat cardiomyocytes with pertussis toxin, which enhances adenylyl cyclase activity by a functional inactivation of the inhibitory G-protein (Gi), was performed to investigate the possible role of Gi-proteins in carbachol-induced sensitization of adenylyl cyclase stimulation. After pretreatment of the cells with pertussis toxin, the carbachol-mediated increase in forskolin-stimulated adenylyl cyclase activity was lost and the carbachol-mediated increase in beta-adrenoceptor-stimulated adenylyl cyclase activity was attenuated. Labelling of the 40 kDa pertussis toxin substrates in cardiomyocyte membranes was decreased by carbachol in a concentration-dependent manner by up to 34% (at 1 mmol/l carbachol). The number and affinity of beta 1-adrenoceptors was unaltered following the chronic carbachol treatment. The specific protein synthesis inhibitor Pseudomonas exotoxin A was used to study whether the carbachol-induced decrease in the level of pertussis toxin-sensitive G-proteins and increase in adenylyl cyclase activity depend on de-novo protein synthesis. Pseudomonas exotoxin A inhibits peptide chain elongation by ADP-ribosylating elongation factor 2. Treatment of the cells with 1 ng/ml Pseudomonas exotoxin A for 3 days led to a reduction in the subsequent ADP-ribosylation of elongation factor 2 in the cytosol of the heart muscle cells by 57%.(ABSTRACT TRUNCATED AT 250 WORDS)

Reverse intrinsic activity of antagonists on G protein-coupled receptors

Biological effects observed with an antagonist are usually interpreted as the result of its ability to block receptor activation produced by an endogenous agonist. In this Principles article, Wolfgang Schütz and Michael Freissmuth show how considerable evidence has now been accumulated for G protein-coupled receptors that antagonists not only bind to the receptor, but also induce a conformational change that favours uncoupling of the receptor from its G protein. The spontaneous activity of the unliganded receptor (i.e. the receptor not occupied by any ligand) is a well-established phenomenon in reconstituted systems with purified components. However, its physiological relevance needs to be verified in a more physiological environment before biological effect of antagonists can be primarily ascribed to negative intrinsic activity.

Selectivity of antagonist and partial agonist activity of celiprolol in normal subjects

1. The aims of this study were to assess the relative beta 1/beta 2 selectivity of the antagonist and partial agonist activity (PAA) of celiprolol in man. 2. Eight normal males received single oral doses of celiprolol 200 mg (C200), 400 mg (C400) and 800 mg (C800); atenolol 50 mg (A50), 100 mg (A100) and 200 mg (A200); nadolol 40 mg (N40) and placebo (PL), administered in a single-blind, randomised crossover design. 3. At rest, in the presence of low levels of circulating adrenaline and noradrenergic tone, a low dose of celiprolol (C200) showed evidence of beta 1-PAA by significant increases in systolic blood pressure and resting heart rate. At higher doses (C400, C800), beta 2-PAA became evident by a significant increase in postural finger tremor, whereas C200 had no effect. 4. In the presence of a beta 1-adrenoceptor agonist, as assessed by reduction of exercise tachycardia, increasing doses of celiprolol produced significantly less beta 1-adrenoceptor blockade compared with atenolol. Furthermore, there was no increase in beta 1-adrenoceptor blockade beyond C400. 5. In the presence of a beta 2-adrenoceptor agonist, as assessed by blunting of terbutaline-induced chronotropic, hypokalaemic and finger tremor responses, celiprolol exhibited less beta 2-adrenoceptor blockade than comparable doses of atenolol used in clinical practice. 6. Exercise hyperkalaemia was blunted significantly by C400 and C800 in comparison with all doses of atenolol and nadolol.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of cell-surface ?-adrenergic ([3H]CGP-12177) binding in adult rat ventricular myocytes: lack of regulation by ?-agonists at physiological concentrations

The major focus of this paper is the characterization and quantification of rat cardiomyocyte, cell-surface beta-adrenergic receptors labelled with the hydrophilic radioligand [3H]CGP-12177. The ventricular cardiomyocytes used in these experiments have previously been extensively studied in our laboratory and confirmed to be functionally compatible with similar cells in vivo. Specific binding of [3H]CGP was stereospecific, saturable and of high affinity. Binding of [3H]CGP was also readily reversible, demonstrated appropriate drug specificity and positively correlated with increasing cell concentrations. The potency of the beta 1-antagonist atenolol was almost 100 times higher than that of the beta 2-antagonist ICI-118.551 in binding to the [3H]CGP binding site. This preparation appears ideal for the investigation of beta-adrenergic receptor regulation in heart cells. Indeed, our initial experiments show clearly that pharmacological concentrations of isoproterenol, and norepinephrine, can reduce (down-regulate) the number of specific [3H]CGP binding sites. This result is in agreement with many other reports on similar experiments in a variety of cell types. However, physiologically relevant concentrations of these two agonists (1-100 nM) do not induce down-regulation of the beta-adrenergic receptors in short-term (2 h) incubations. Nevertheless, the high-affinity receptors that we have described mediate a contractile response to isoproterenol in the nanomolar concentration range (EC50 = 3.6 +/- 0.3 nM). This is approximately 300 times lower than the concentration needed to produce down-regulation. Thus, our data indicate that short-term down-regulation of cardiomyocyte beta-adrenergic receptors can only be observed with high, pharmacological concentrations of isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of in vivo partial beta 1/beta 2-agonist activity: a dose-ranging study with carteolol

1. The aims of this study were to investigate the partial agonist profile of carteolol and evaluate methodology for differentiating relative beta 1 and beta 2 partial agonist activity (PAA) in vivo. 2. Eight normal subjects received single oral doses of carteolol 10 mg, 30 mg and 60 mg; nadolol 40 mg; pindolol 30 mg and placebo, given in a single-blind, randomised crossover design. 3. beta 1-PAA was demonstrated with carteolol by dose-related increases in resting heart rate and systolic blood pressure, and a plateau in the dose-response curve for attenuation of exercise tachycardia. beta 2-PAA with carteolol was evidenced by a dose-related increase in resting finger tremor and progressive attenuation of exercise-induced hyperkalaemia. beta 2-adrenoceptor antagonism was shown by attenuation of terbutaline induced hypokalaemic, chronotropic and finger tremor responses. 4. Carteolol behaved as a non-selective beta-adrenoceptor antagonist with both beta 1 and beta 2-PAA components. In the standard clinical dose range of 10-30 mg, its in vivo PAA effects were relatively beta 1-selective. Thus at low doses, there appeared to be a dissociation between selectivity of antagonist and partial agonist activity. 5. Attenuation of exercise hyperkalaemia appears to be a novel and sensitive method for the evaluation of beta 2-PAA in vivo.

Tumor necrosis factor alpha up-regulates Gi alpha and G beta proteins and adenylyl cyclase responsiveness in rat cardiomyocytes

Treatment of cultured rat cardiomyocytes in serum-free medium for 48 h with recombinant human tumor necrosis factor alpha (TNF alpha) led to a concentration-dependent increase in the level of membrane-inhibitory guanine nucleotide-binding protein (Gi) alpha-subunits and in pertussis toxin-catalyzed [32P]ADP ribosylation of 40 kDa proteins. Both Gi alpha protein subtypes present in rat cardiac myocyte membranes, Gi alpha 40 and Gi alpha 41, were up-regulated by the cytokine, with the maximal increase occurring at 10 U/ml TNF alpha. In contrast to noradrenaline exposure, which causes a similar, but apparently exclusive, increase in alpha i-subunits, treatment with TNF alpha in addition increased the level of membrane G protein beta 36-subunits. Furthermore, while noradrenaline exposure led to a decrease in receptor-dependent and -independent adenylyl cyclase activity, treatment of cardiomyocytes with TNF alpha caused a concentration-dependent increase in cyclase responsiveness to either forskolin, guanosine 5'-O-(3-thiotriphosphate) or isoproterenol, even though beta-adrenoceptor density was decreased by TNF alpha. The increase in adenylyl cyclase activity induced by TNF alpha was completely suppressed when the cells were cocultured with noradrenaline, a condition leading to an additive increase in Gi alpha level. The data indicate that the cytokine TNF alpha can potently modulate G protein-mediated signal transduction in rat cardiac myocytes. Although TNF alpha, like noradrenaline, exposure of the cells increased the level of membrane Gi alpha proteins, it did not decrease but rather caused an increase in adenylyl cyclase responsiveness.(ABSTRACT TRUNCATED AT 250 WORDS)