Are there ventricle-specific postnatal maturational differences in myocardial beta-adrenoceptors?

Newborn hearts have restricted functional reserve and variable responsiveness to inotropes that could be partly due to differences in myocardial beta-adrenoceptors (beta-AR). To clarify this issue, this study documented ventricle-specific changes in myocardial beta-AR density and affinity during postnatal maturation. In vivo left and right ventricle (LV and RV, respectively) biopsies were obtained from newborn (3-day-old, n = 11), immature (14-day-old, n = 7), and adult (n = 6) pigs. Total beta-AR density (B(max), fmol/g) and dissociation constant (K(d), pmol/L) were determined by radioligand binding with I125 iodocyanopindolol. Overall, beta-AR B(max) in the LV significantly decreased with maturation. Interestingly, newborn animal hearts (LV and RV) subdivided into 2 groups: an adult-like low K(d) group with low B(max) and a fetal-like high K(d) group with high B(max), which were significantly different from one another. The high K(d) newborn group also had significantly higher K(d) and B(max) than both immature and adult hearts. Newborns had similar Bmax but higher Kd in the LV than the RV, whereas immature and adult hearts did not have ventricular differences. During maturation, beta-AR density decreased, whereas LV beta-AR binding affinity increased. Variable beta-AR maturity was also identified immediately post partum, which could potentially explain the newborn heart's variable responsiveness to inotropes. The subset of newborn hearts with lower binding affinity (reduced responsiveness) could also contribute to the newborn heart's overall reduction in functional reserve.

Agonist-independent alteration in beta-adrenoceptor-G-protein-adenylate cyclase system in an equine model of recurrent airway obstruction

We examined the inhibitory sympathetic beta-adrenergic mechanisms in peripheral lung, bronchi and trachea of an equine model of recurrent airway obstruction (RAO), to support the hypothesis that the beta-adrenergic receptor dysfunction is not only restricted to cell surface receptor density but rather encompasses a mechanistic defect apart from the receptor, to the intracellular signaling components. The non-asthmatic lung possessed 3.2-fold more beta-adrenergic receptors than bronchi (496 +/- 19.4 vs. 155.1+/- 19.6 fmol/mg protein; P < 0.01) and 6.2-fold higher than in the trachea (79.8 +/- 12.6 fmol/mg protein; P < 0.001) (assessed by radioligand binding assays using (-)-[(125)I]-iodocyanopindolol, ICYP) and in all tissues a greater proportion of the beta(2)- than the beta(1)-subtype (75-80%). The receptor density (B(max)) in lung parenchyma and bronchial membranes was 33 and 42%, respectively, lower (P < 0.001) in RAO than in control animals, attributable to a decrease in the beta(2)-subtype. This receptor down-regulation was accompanied with an attenuated coupling efficiency of the receptor to the stimulatory G(S)-protein (P < 0.05 vs. control). Concomitantly, activation of adenylate cyclase evoked by isoproterenol was significantly reduced in lung and bronchial membranes of animals with RAO, whereas effects of 10 microM GTP, 10mM NaF, 10 microM forskolin and 10 mM Mn(2+) were not altered. There was no difference in beta-adrenergic receptor density, G(S)-protein or adenylate cyclase coupling in the trachea between asthmatic and control animals. In conclusion, in stable asthma the pulmonary beta-adrenergic receptor-G(S)-protein-adenylate cyclase system is impaired, thus the pathologic process involves all signaling components, and due to its close similarity, this animal model seems to serve as a suitable model, at least partly, of chronic asthmatic patients.

Beta-adrenergic receptor density and adenylate cyclase activity in lead-exposed rat brain after cessation of lead exposure

To understanding the reversible or irreversible harm to the beta-adrenergic system in the brain of lead-exposed rats, this study sets up an animal model to estimate the change in the sympathetic nervous system of brain after lead exposure was withdrawn. We address the following topics in this study: (a) the relationship between withdrawal time of lead exposure and brain beta-adrenergic receptor, blood lead level, and brain lead level in lead-exposed rats after lead exposure was stopped; and (b) the relationship between lead level and beta-adrenergic receptor and cyclic AMP (c-AMP) in brain. Wistar rats were chronically fed with 2% lead acetate and water for 2 months. Radioligand binding was assayed by a method that fulfilled strict criteria of beta-adrenergic receptor using the ligand [125I]iodocyanopindolol. The levels of lead were determined by electrothermal atomic absorption spectrometry. The c-AMP level was determined by radioimmunoassay. The results showed a close relationship between decreasing lead levels and increasing numbers of brain beta-adrenergic receptors and brain adenylate cyclase activity after lead exposure was withdrawn. The effect of lead exposure on the beta-adrenergic system of the brain is a partly reversible condition.

Pharmacological and biochemical characterization of the beta-adrenergic signal transduction pathway in different segments of the respiratory tract

Although in the respiratory system there is great therapeutic interest in manipulating and understanding the beta-adrenoceptor-G-protein-adenylate cyclase (AC) signal transduction pathway, little is known on segmental differences among lung, bronchus, and trachea with regard to the receptor concentration and interaction to G-proteins and coupling to AC. In this study, patterns of distribution and absolute quantities of beta-adrenoceptor subtypes beta(1) and beta(2) were determined in membranes of equine lung parenchyma, bronchial and tracheal epithelium with the underlying smooth muscle by saturation and competition binding assays using the radioligand (-)-[125I]-iodocyanopindolol (ICYP). Additionally, the functional coupling of beta-adrenoceptors to G-proteins (assessed by beta-agonist competition binding in the presence and absence of GTP) as well as the coupling efficiency and biochemical activities of AC was investigated in each region. The specific ICYP binding was rapid, reversible, saturable with time and of high affinity. The radioligand binding identified more total beta-adrenoceptors in the lung than in bronchus or trachea (428+/-19, 162.4+/-4.8, 75.6+/-1.2 fmol/mg protein, respectively) with about 40% of receptors in the high affinity state. The beta(2)-adrenoceptor subtype predominated in all segments (approximately 74-80%), as the highly selective beta(2)-adrenoceptor antagonist ICI 118,551 was about 10,000 times more potent in inhibiting ICYP binding than was the beta(1)-selective adrenoceptor antagonist CGP 20712A, and beta-adrenoceptor agonists inhibited ICYP binding with an order of potency: (-)-isoprenaline>(-)-adrenaline>(-)-noradrenaline. The dissociation constant (K(d)) was higher in the trachea than in bronchus or lung (13.0+/-0.9 pM vs. 20.0+/-2.3 pM vs. 30.8+/-4.4 pM, P<0.05, respectively). The beta(2)-adrenoceptor-mediated AC response was tissue-dependent; stimulants acting on beta-adrenoceptor (isoproterenol), G-protein (GTP, NaF) and AC (forskolin, Mn(2+)) enhanced AC responses in all three regions, but the AC activity was higher in tracheal crude membranes than in bronchus or lung (trachea>>>bronchus>lung), hence, the number of beta(2)-adrenoceptors correlated inversely with the amount of AC. We conclude that (1) the stoichiometry of components within the pulmonary beta-adrenoceptor-G-protein complex is segment-dependent, and (2) the receptor number or AC activity is possibly the rate-limiting factor in the beta-adrenoceptor-G-protein-AC-mediated physiological responses. Thus, it is speculated that this could have important therapeutic consequences in beta-adrenoceptor agonist-induced receptor regulation in bronchial asthma.

Design of new beta1-selective adrenoceptor ligands as potential radioligands for in vivo imaging

In general, the failing human heart is characterized by a selective reduction in beta(1)-adrenoceptors (beta(1)-ARs) without change in beta(2)-AR density. Medical imaging techniques, either single photon emission computed tomography (SPECT) or positron emission tomography (PET) with appropriate radioligands, offer the possibility of assessing beta-adrenoceptor density non-invasively in humans. To date, neither a SPECT nor a PET radioligand is available for the selective imaging of cardiac beta(1)-ARs. The aim of this study was to develop potential high affinity beta(1)-selective AR radioligands for the non-invasive in vivo imaging of the beta(1)-AR density in the human heart using SPECT or PET. A variety of racemic N-aryl-N'-[2-[3-aryloxy-2-hydroxy-propylamino]-ethyl]-urea derivatives and chain-elongated analogues, related to the established beta(1)-AR antagonist, ICI 89,406 8i, were synthesized. Competition studies using the non-selective AR ligand, [(125)I]iodocyanopindolol ([(125)I]ICYP), and ventricular membrane preparations of wild-type mice revealed nine ligands with higher beta(1)-AR affinities (up to 76-fold) and beta(1)-AR selectivities (up to 139-fold) than 8i. Mostly, these ligands possess a 2-substituted phenoxy group and a 4-substituted phenyl residue in contrast to the lead compound 8i. The non-radioactive counterparts of the desired SPECT- and PET-radiotracers were synthesized as reference compounds [e.g., 8f, 8g, 8h and 8l as the non-radioactive analogues of the radioiodinated SPECT radioligands, 8e and 8h as the non-radioactive compounds of C-11 labelled PET-tracers (C-11 in the methoxy group)]. The established library of high affinity beta(1)-selective AR antagonists was screened for chemical precursors for the radiosynthesis of the mentioned radioligands. Furthermore, the library consists of some comparison compounds that are unsubstituted, allyl- and alkyl-substituted or chain-elongated (e.g., 8a, 8j, 8o and 8r-t). Future steps will include radiolabelling and pharmacokinetic evaluation of the beta(1)-selective target compounds, which could be applied as sympathetic innervation agents for in vivo investigations and diagnostics in patients suffering from cardiac diseases like heart failure and ventricular arrhythmias.

Relationship between hydrophobicity and beta-blocking potencies, affinities or dissociation of beta-blockers from beta-adrenoceptors

The present study was performed to assess the relationship between the hydrophobicity of drugs and (1) inhibitory strength (pA2) on chronotropic or inotropic actions, (2) displacemental potencies of 3H-CGP12177 or 125I-iodocyanopindolol binding to beta-adrenoceptors (beta-ARs) (pKi) or (3) dissociating potencies of these drugs from beta-ARs of atria strips pretreated with drugs. The beta-blockers used in the present study were bopindolol, active metabolite of bopindolol (18-502), atenolol, propranolol, pindolol, nadolol, alprenolol, oxprenolol, metoprolol, labetalol and acebutolol. The value of the partition coefficient of propranolol was the highest, and that of the beta1-selective blocker atenolol was the lowest. Although low correlation coefficients between hydrophobicity and inhibitory beta-blocking potencies determined by pharmacological experiments or displacemental potencies by the radioligand binding assay using 3H-CGP12177 and 125I-iodocyanopindolol were observed, significant relationships between hydrophobicities of these drugs and dissociating potencies from beta-ARs were observed. These results suggest that the hydrophobicity of drugs may be important for the slow dissociation from beta-ARs, but not for the beta-blocking action.

c-Src tyrosine kinase binds the beta 2-adrenergic receptor via phospho-Tyr-350, phosphorylates G-protein-linked receptor kinase 2, and mediates agonist-induced receptor desensitization

The nonreceptor tyrosine kinase Src has been implicated in the switching of signaling of beta2-adrenergic receptors from adenylylcyclase coupling to the mitogen-activated protein kinase pathway. In the current work, we demonstrate that Src plays an active role in the agonist-induced desensitization of beta2-adrenergic receptors. Both the expression of dominant-negative Src and treatment with the 4-amine-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) inhibitor of Src kinase activity blocks agonist-induced desensitization. Agonist triggers tyrosine phosphorylation of the beta2-adrenergic receptor and recruitment and activation of Src. Because phosphorylation of the Tyr-350 residue of the beta2-adrenergic receptor creates a conditional, canonical SH2-binding site on the receptor, we examined the effect of the Y350F mutation on Src phosphorylation, Src recruitment, and desensitization. Mutant beta2-adrenergic receptors with a Tyr-to-Phe substitution at Tyr-350 do not display agonist-induced desensitization, Src recruitment, or Src activation. Downstream of binding to the receptor, Src phosphorylates and activates G-protein-linked receptor kinase 2 (GRK2), a response obligate for agonist-induced desensitization. Constitutively active Src increases GRK phosphorylation, whereas either expression of dominant-negative Src or treatment with the PP2 inhibitor abolishes tyrosine phosphorylation of GRK and desensitization. Thus, in addition to its role in signal switching to the mitogen-activated protein kinase pathway, Src recruitment to the beta2-adrenergic receptor and activation are obligate for normal agonist-induced desensitization.

Renin-angiotensin system and sympathetic nervous system in cardiac pressure-overload hypertrophy

Angiotensin II and norepinephrine (NE) have been implicated in the neurohumoral response to pressure overload and the development of left ventricular hypertrophy. The purpose of this study was to determine the temporal sequence for activation of the renin-angiotensin and sympathetic nervous systems in the rat after 3-60 days of pressure overload induced by aortic constriction. Initially on pressure overload, there was transient activation of the systemic renin-angiotensin system coinciding with the appearance of left ventricular hypertrophy (day 3). At day 10, there was a marked increase in AT(1) receptor density in the left ventricle, increased plasma NE concentration, and elevated cardiac epinephrine content. Moreover, the inotropic response to isoproterenol was reduced in the isolated, perfused heart at 10 days of pressure overload. The affinity of the beta(2)-adrenergic receptor in the left ventricle was decreased at 60 days. Despite these alterations, there was no decline in resting left ventricular function, beta-adrenergic receptor density, or the relative distribution of beta(1)- and beta(2)-receptor sites in the left ventricle over 60 days of pressure overload. Thus activation of the renin-angiotensin system is an early response to pressure overload and may contribute to the initial development of cardiac hypertrophy and sympathetic activation in the compensated heart.

Characterization of beta-adrenoceptors responsible for venom production in the venom gland of the snake Bothrops jararaca

We have shown that the stimulation of beta-adrenoceptors is an important step in venom production in the Bothrops jararaca venom gland. In the present study, the pharmacological profile of the beta-adrenoceptor present in Bothrops jararaca venom gland was characterized by radioligand binding assay and by the ability of isoprenaline to promote accumulation of cyclic AMP in dispersed secretory cells. In both cases, the venom glands were obtained from non-extracted snakes (quiescent stage) or from snakes which venom was extracted 4 days before sacrifice (venom production stimulated stage). [125I]-iodocyanopindolol ([125I]-ICYP) bound to extracted gland membranes in a concentration-dependent and saturable manner, but with low affinity. Propranolol, beta1- or beta2-selective adrenoceptors ligands displaced the [125I]-ICYP binding with low affinity, while selective beta3-adrenoceptor ligands did not displace the [125I]-ICYP binding. The displacement of [125I]-ICYP by propranolol was similar in non-extracted and extracted glands, showing the presence of beta-adrenoceptors in both stages. In dispersed secretory cells of non-extracted glands, isoprenaline (1 microM) increased the cyclic AMP production and propranolol (10 microM) was able to block this effect. On the other hand, in extracted glands, isoprenaline had no effect. The results suggest that the beta-adrenoceptors present in the Bothrops jararaca venom glands are different from those (beta1, beta2 or beta3) described in mammals, but are coupled to the Gs protein, like the known beta-adrenoceptor subtypes. Moreover, previous in vivo stimulation of venom production desensitizes the beta-adrenoceptors system and, although the receptors could be detected by binding studies, they are not coupled to the Gs protein, indicating that beta-adrenoceptors stimulation contributes to the initial steps of venom synthesis.

Probing the selectivity of allosteric modulators of muscarinic receptors at other G-protein-coupled receptors

1. The aim of the present investigation was to analyse whether three prototype allosteric modulators of ligand binding to muscarinic receptors, i.e. alcuronium, gallamine, and the alkane-bis-ammonium compound W84 (hexane-1,6-bis[dimethyl-3'-phthalimidopropylammonium bromide]), may have allosteric effects on radioligand-binding characteristics at other G-protein-coupled receptors, such as cerebral A1 adenosine receptors (Gi-coupled), cardiac left ventricular alpha1-adrenoceptors (Gq), and beta-adrenoceptors (Gs). 2. The modulators were applied at concentrations known to be high with regard to the allosteric delay of the dissociation of the antagonist [3H]-N-methylscopolamine (NMS) from muscarinic M2-receptors: 30 micromol l(-1) W84, 30 micromol l(-1) alcuronium, 1000 micromol l(-1) gallamine. As radioligands, we used the adenosine A1-receptor ligand [3H]-cyclopentyl-dipropylxanthine (CPX), the alpha1-adrenoceptor ligand [3H]-prazosin (PRAZ), and the beta-adrenoceptor ligand (-)-[125I]-iodocyanopindolol (ICYP). Allosteric actions on ligand dissociation and the equilibrium binding were measured in the membrane fractions of rat whole forebrain (CPX) and of rat cardiac left ventricle (PRAZ, ICYP, NMS), respectively. 3. CPX and PRAZ showed a monophasic dissociation with half-lives of 5.88+/-0.15 and 12.27+/-0.46 min, respectively. In the case of CPX, neither the binding at equilibrium nor the dissociation characteristics were influenced by the allosteric agents. With PRAZ, the binding at equilibrium remained almost unaltered in the presence of W84, whereas it was reduced to 36+/-2% of the control value with alcuronium and to 42+/-2% with gallamine. The dissociation of PRAZ was not affected by W84, whereas it was moderately accelerated by alcuronium and gallamine. In the case of ICYP, the binding at equilibrium was not affected by the allosteric modulators. The dissociation of ICYP was slow, and after 3 h, more than 50% of the radioligand was still bound, so that a reliable half-life could not be calculated. ICYP dissociation was not affected by W84. In the presence of alcuronium and gallamine, the dissociation curve of ICYP revealed an initial drop from the starting level, followed by the major phase of dissociation being parallel to the control curve. 4. In summary, the allosteric action of the applied agents is not a common feature of G-protein-coupled receptors and appears to be specific for muscarinic receptors.

Myocardial beta-adrenoceptor down-regulation by norepinephrine is linked to reduced norepinephrine uptake activity

Chronic administration of norepinephrine for 8 weeks has been shown to reduce neuronal norepinephrine uptake activity and increase interstitial norepinephrine concentration in the heart. To determine whether the changes could lead to myocardial beta-adrenoceptor down-regulation or beta-adrenergic subsensitivity, we measured left ventricular contractile responses to dobutamine, myocardial beta-adrenoceptor density, beta subtype distribution, competitive inhibition agonist binding, and adenylyl cyclase activity activation by isoproterenol, 5'-guanylylimidodiphosphate, and forskolin in dogs after a norepinephrine or saline infusion for 8 weeks. We found that norepinephrine infusion reduced myocardial beta-adrenoceptor density, beta(1)-adrenoceptor subtype density, and high-affinity site for isoproterenol. Left ventricular contractile responses to dobutamine were reduced in the norepinephrine-infused animals. In addition, norepinephrine infusion decreased the basal adenylyl cyclase activity and the adenylyl cyclase responses to isoproterenol, 5'-guanylylimidodiphosphate, and forskolin. The findings indicate that a decrease in cardiac norepinephrine uptake predisposes the heart to norepinephrine-induced myocardial beta-adrenoceptor down-regulation, and that norepinephrine, when present in a sufficient amount over a long period as it is in chronic heart failure, can reduce myocardial beta-adrenergic responsiveness by both homologous and heterologous desensitization.

(+) 3,4-methylenedioxymethamphetamine ('ecstasy') transiently increases striatal 5-HT1B binding sites without altering 5-HT1B mRNA in rat brain

(+) 3,4-Methylenedioxymethamphetamine (MDMA) is a psychedelic drug of abuse that causes selective degeneration of serotonergic fibers of dorsal raphe neurons that project throughout the forebrain. Previous studies using pharmacological and behavioral approaches suggested that MDMA treatment leads to desensitization of 5-HT1B receptors. We proposed to test whether this occurs by downregulation of 5-HT1B messenger RNA in dorsal raphe, striatum or CA1 hippocampal neurons and/or 5-HT1B binding site density in hippocampus and basal ganglia. In Experiment I, rats were treated with MDMA using several dosing protocols (2.5 or 10 mg kg-1 day-1 s.c. given a single time or twice daily for 4 days). The animals were killed 24 h after the last dose. [3H]-citalopram binding to serotonin transporters in hippocampus was reduced in the high dose protocol, indicating degeneration of forebrain serotonergic fibers. Despite the extensive reduction in serotonergic content, 5-HT1B mRNA did not change from control levels in any region when measured by in situ hybridization. [125I]-Iodocyanopindolol binding to 5-HT1B sites in hippocampus was also not changed. In Experiment II, high dose MDMA had no effect on 5-HT1B mRNA in any brain region either 1 or 14 days after treatment. However, [125I]-iodocyanopindolol binding more than doubled in striatum 1 day after MDMA treatment but returned to control levels by 14 days. This may have been a transient compensation to early neuronal damage caused by MDMA exposure. These results suggest that previously described changes in 5-HT1B function following MDMA treatment involve only posttranscriptional changes in receptor regulation and do not alter 5-HT1B mRNA levels.

Beta 2-adrenoceptor agonist-induced down-regulation after short-term exposure

We examined the effect of duration of beta 2-adrenergic receptor (beta 2AR) occupancy by isoproterenol on specific binding of 125I-lodocyanopindolol (125I-ICYP) in membranes from rat L6 myoblasts. Ten minute exposure caused a time-and concentration-dependent maximal decrease in 125I-ICYP binding 24 hours after exposure equal to that following continuous exposure (p < 0.05). Low temperature, concanavalin A, H89 and ICl 118,551 blocked the decline in 125I-ICYP binding during the first hour following exposure probably representing receptor sequestration to a compartment or change to a form incapable of ligand binding. Compared to controls, receptor binding 4 and 24 hours following exposure was reduced 56 +/- 8.7% and 72 +/- 8.8%, respectively (p < 0.05), and was blocked by ICl 118,551 but not CGP12177. Isoproterenol-induced, but not forskolin-stimulated, cAMP accumulation was reduced 35% 24 hours following exposure (p < 0.05). 125I-ICYP binding in intact L6 cells 4 and 24 hours after exposure were respectively 56 +/- 8.9 and 61 +/- 13% of controls (p < 0.05). Following agonist exposure, CHO cell membranes expressing human beta 2ARs exhibited 125I-ICYP binding 85 +/- 2.0% and 6 +/- 2.8% of control values 4 and 24 hours, respectively (p < 0.05). A model predicting that full occupation of the beta 2AR activates receptor degradation explains our results that agonist-induced down-regulation of beta 2AR does not require continuous presence of the agonist.

Targeted disruption of the beta2 adrenergic receptor gene

beta-Adrenergic receptors (beta-ARs) are members of the superfamily of G-protein-coupled receptors that mediate the effects of catecholamines in the sympathetic nervous system. Three distinct beta-AR subtypes have been identified (beta1-AR, beta2-AR, and beta3-AR). In order to define further the role of the different beta-AR subtypes, we have used gene targeting to inactivate selectively the beta2-AR gene in mice. Based on intercrosses of heterozygous knockout (beta2-AR +/-) mice, there is no prenatal lethality associated with this mutation. Adult knockout mice (beta2-AR -/-) appear grossly normal and are fertile. Their resting heart rate and blood pressure are normal, and they have a normal chronotropic response to the beta-AR agonist isoproterenol. The hypotensive response to isoproterenol, however, is significantly blunted compared with wild type mice. Despite this defect in vasodilation, beta2-AR -/- mice can still exercise normally and actually have a greater total exercise capacity than wild type mice. At comparable workloads, beta2-AR -/- mice had a lower respiratory exchange ratio than wild type mice suggesting a difference in energy metabolism. beta2-AR -/- mice become hypertensive during exercise and exhibit a greater hypertensive response to epinephrine compared with wild type mice. In summary, the primary physiologic consequences of the beta2-AR gene disruption are observed only during the stress of exercise and are the result of alterations in both vascular tone and energy metabolism.

Dihydropyridine and beta adrenergic receptor binding in dogs with tachycardia-induced atrial fibrillation

BACKGROUND

We have shown that rapid atrial activation, as occurs during atrial fibrillation (AF), reduces L-type Ca2+ current (ICa) and that this is the principal mechanism of the action potential duration and refractoriness changes that characterize tachycardia-induced atrial remodeling. The present study was designed to determine whether atrial tachycardia alters biochemical indices of the number of L-type Ca2+ channels and/or of the number and binding affinity of beta-adrenergic receptors.

METHODS

In canine atrial sarcolemmal preparations, the number and binding affinity of dihydropyridine receptors were determined with the use of 3H-nitrendipine and that of beta-adrenergic receptors with 125I-iodocyanopindolol. Results were obtained with preparations from dogs paced at 400/min for 1 (P1, n = 20), 7 (P7, n = 9), and 42 (P42, n = 9) days, and compared with observations in sham-operated controls (P0, n = 14).

RESULTS

Pacing reduced the Bmax of dihydropyridine receptors, from 157 +/- 18 fmol/mg (P0) to 116 +/- 9 fmol/mg (P1, P < 0.05), 100 +/- 14 fmol/mg (P7, P < 0.05) and 94 +/- 9 fmol/mg (P42, P < 0.01). The affinity of dihydropyridine receptors was unchanged, with the Kd averaging 711 +/- 102 pM. 656 +/- 74 pM, 633 +/- 155 pM and 585 +/- 92 pM in P0, P1, P7 and P42 dogs. Neither Bmax nor Kd of beta-adrenergic receptors was altered by rapid pacing. Values of Bmax of dihydropyridine receptors correlated with atrial ICa current density (r2 = 0.95) and ERP (r2 = 0.99).

CONCLUSIONS

Rapid atrial activation results in downregulation in the number of dihydropyridine receptors without altering the number or affinity of beta-adrenergic receptors. The reductions in ICa that play an important role in the atrial electrical remodeling by which 'AF begets AF' appear to be due at least in part to a decrease in the number of L-type Ca2+ channels in cardiac cell membranes.

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.

Lack of an effect of age on beta-adrenoceptor-mediated lipolysis in isolated human adipocytes

We examined the effect of age on the beta-adrenoceptor response in adipocytes that contain both beta 1- and beta 2-adrenoceptors. Twelve healthy young and 12 old subjects on a 150 mEq/24 h sodium diet underwent gluteal fat biopsies. Isolated adipocytes from all the subjects were stimulated with increasing concentrations of isoproterenol for glycerol release. In 13 of the subjects (7 young and 6 old), we also performed beta-adrenoceptor binding studies on adipocyte membranes. In addition in eight subjects (four young and four old), we also utilized a competitive binding assay to calculate the percent of beta-adrenoceptors that were of the beta 1 subtype. Our data showed that old subjects, when treated under identical conditions as the young subjects prior to fat biopsy, did not demonstrate any differences in the beta-adrenoceptor stimulated lipolysis. The Vmax of lipolysis was 10.6 +/- 1.4 nmoles glycerol/mg lipid/2 h in the young group and 9.9 +/- 1.1 in the old group. The concentrations of isoproterenol that resulted in Vmax and 1/2 Vmax were also the same in the two age groups. The addition of 8-cyclopentyl-1,3-dimethylxanthine (CPT), a specific A1-adenosine receptor antagonist, resulted in mild but equivalent enhancement of glycerol release in both age groups. The beta-adrenoceptor numbers and affinities from adipocyte membranes did not demonstrate age-related differences either (Bmax 106 +/- 17 fmol/mg of protein in the young, and 137 +/- 27 in the old; Kd 79.6 +/- 21.3 pM in the young and 81.9 +/- 16.6 in the old). The percent of beta 1-adrenoceptors on the adipocyte membranes was also similar in the two age groups (35.2 +/- 2.6% in the young; 37.1 +/- 4.5% in the old). In conclusion, we could not demonstrate any differences in the beta-adrenoceptor responses from peripheral adipocytes that contain both beta 1- and beta 2-adrenoceptors, in a group of healthy elderly and young subjects who were subjected to identical dietary and orthostatic conditions prior to the biopsy. These data suggest that neither beta 1- nor beta 2-adrenoceptor responses in human adipocytes show significant changes due to aging.

The combined effects of chronic ethanol/desipramine treatment on beta-adrenoceptor density and coupling efficiency in rat brain

Both ethanol and desipramine influence beta-adrenoceptor regulation. We reported previously that ethanol partially counteracted desipramine's effects on beta-adrenoceptor. Previous studies utilized beta-adrenoceptor radioligands that also bind to 5-HT1B receptors, thus, changes in 5-HT1B receptors could have confounded the results. The effects of chronic ethanol, desipramine and ethanol/desipramine treatment on beta-adrenoceptor coupling efficiency to Gs protein in rat brain were examined using 125I-iodocyanopindolol after blocking binding to 5-HT1B receptors. In the frontal cortex, ethanol uncoupled beta-adrenoceptor from GS. Desipramine decreased beta-adrenoceptor density, particularly in the high-conformational state, with no effect on coupling. In combined treatment, desipramine prevented ethanol-induced uncoupling. In the hippocampus, desipramine enhanced beta-adrenoceptor coupling, but ethanol had no effect. In combination with desipramine, ethanol enhanced desipramine-induced decrease in beta-adrenoceptor density in the high-conformational state, but uncoupled beta-adrenoceptors, an effect not observed with ethanol alone. These results suggest a complex interplay between ethanol and antidepressants in modulating beta-adrenoceptor function.

Myocardial beta-adrenergic reactivity in pressure overload-induced cardiac hypertrophy in the rat

The purpose of this study was to evaluate the changes in myocardial beta-adrenergic reactivity in animals undergoing a 4 week cardiac pressure-overload. Abdominal aortic constriction (AAC) or sham operation (sham) were performed in male Wistar rats, and 4 weeks later, isoprenaline dose-effects (chronotropic, inotropic and lusitropic properties) were studied after pithing. Noradrenaline (NA) and adrenaline (A) concentrations and NA turn-over index (DHPG /NE ratio) were evaluated in heart ventricles, while beta-adrenoceptor characteristics in ventricle homogenates and slices with [125I]iodocyanopindolol and the beta (1)/beta (2)-adrenoceptor ratio were estimated. Four weeks of cardiac pressure overload resulted in a 70% increase in ventricle weight/body weight ratio (from 2.5 +/- 0.1 to 4.2 +/- 0.3 mg/g in sham and AAC rats, respectively) and a 24% increase in protein contents (from 11.3 +/- 0.7 to 14.0 +/- 1.1 mg/100 mg ventricle in sham and AAC rats respectively). The ventricle NA content was similar in AAC and sham, while the ventricle A content and NA turn-over index were significantly increased in AAC rats (35 and 80% vs sham, respectively). Dose response of isoprenaline was significantly shifted to the right for all studied effects in AAC rats. However, maximal response (in relative values) was similar in AAC and sham rats only for heart rate but not for parameters depending on left ventricle contractile response. The beta-adrenoceptor density was significantly decreased in AAC by 30% without apparent affinity change and due to decreases in beta (1)-sites in septum and to beta (1)- and beta (2)-adrenoceptors in left ventricle endocardium. Decreases in isoprenaline-induced cardiac responses in AAC rats are associated with beta (1)-adrenoceptor density reduction and modification of beta (1)- and beta (2)-adrenoceptor ratio. These modifications are not the only reason for such dose response changes, at least for contractile response.

Myocardial beta-adrenergic reactivity in volume overload-induced cardiac hypertrophy in the rat

The purpose of this study was to evaluate the changes in myocardial beta-adrenergic reactivity in animals undergoing a 4 week cardiac volume overload. Aortocaval shunt (ACS) or sham operation (sham) were performed in male Wistar rats, and 4 weeks later, isoproterenol dose-effects (chronotropic, inotropic and lusitropic properties) were studied after pithing. Noradrenaline (NA) and adrenaline (A) concentrations and NA turn-over index were evaluated in plasma and heart ventricles, while beta-adrenoceptor characteristics in ventricle homogenates and slices with [125I]iodocyanopindolol, and the beta(1)/beta(2)-adrenoceptor ratio were estimated. Four weeks of cardiac volume overload resulted in a 55% increase in ventricle weight/body weight ratio (from 2.5 +/- 0.1 to 3.9 +/- 0.1 mg/g in sham and ACS rats, respectively) and a 20% increase in protein contents (from 11.3 +/- 0.7 to 13.8 +/- 1.1 mg/100 mg ventricles in sham and ACS rats, respectively). Furthermore, NA and A concentrations and NA turn-over index were increased in ACS rats (14, 40 and 80% versus sham, respectively). A shift to the right of the responses in heart rate, left ventricular systolic pressure, +dP/dtmax and -dP/dtmax responses following increasing doses of isoproterenol was observed, without change in the dose inducing maximum effect. Total beta-adrenoceptor characteristics and beta(1)/beta(2) ratio were unchanged. However, beta(1)-adrenoceptor density increased in epicardium while decreasing in endocardium of left ventricle from ACS rats. Rightward shift at lower doses of isoproterenol-induced cardiac responses in volume-overloaded rats are not likely due to overall beta-adrenoceptor density changes.