The role of cyclic AMP in the positive inotropic effect mediated by beta 1- and beta 2-adrenoceptors in isolated human right atrium

Multiplexed Optical Sensors in Arrayed Islands of Cells for multimodal recordings of cellular physiology

Cells typically respond to chemical or physical perturbations via complex signaling cascades which can simultaneously affect multiple physiological parameters, such as membrane voltage, calcium, pH, and redox potential. Protein-based fluorescent sensors can report many of these parameters, but spectral overlap prevents more than ~4 modalities from being recorded in parallel. Here we introduce the technique, MOSAIC, Multiplexed Optical Sensors in Arrayed Islands of Cells, where patterning of fluorescent sensor-encoding lentiviral vectors with a microarray printer enables parallel recording of multiple modalities. We demonstrate simultaneous recordings from 20 sensors in parallel in human embryonic kidney (HEK293) cells and in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), and we describe responses to metabolic and pharmacological perturbations. Together, these results show that MOSAIC can provide rich multi-modal data on complex physiological responses in multiple cell types.

Cardiac β3 -adrenoceptors-A role in human pathophysiology?

As β₃ -adrenoceptors were first demonstrated to be expressed in adipose tissue they have received much attention for their metabolic effects in obesity and diabetes. After the existence of this subtype had been suggested to be present in the heart, studies focused on its role in cardiac function. While the presence and functional role of β₃ -adrenoceptors in the heart has not uniformly been detected, there is a broad consensus that they become up-regulated in pathological conditions associated with increased sympathetic activity such as heart failure and diabetes. When detected, the β₃ -adrenceptor has been demonstrated to mediate negative inotropic effects in an inhibitory G protein-dependent manner through the NO-cGMP-PKG signalling pathway. Whether these negative inotropic effects provide protection from the adverse effects induced by overstimulation of β₁ /β₂ -adrenoceptors or in themselves are potentially harmful is controversial, but ongoing clinical studies in patients with congestive heart failure are testing the hypothesis that β₃ -adrenceptor agonism has a beneficial effect. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

The beat goes on: Cardiac pacemaking in extreme conditions

In order for an animal to survive, the heart beat must go on in all environmental conditions, or at least restart its beat. This review is about maintaining a rhythmic heartbeat under the extreme conditions of anoxia (or very severe hypoxia) and high temperatures. It starts by considering the primitive versions of the protein channels that are responsible for initiating the heartbeat, HCN channels, divulging recent findings from the ancestral craniate, the Pacific hagfish (Eptatretus stoutii). It then explores how a heartbeat can maintain a rhythm, albeit slower, for hours without any oxygen, and sometimes without autonomic innervation. It closes with a discussion of recent work on fishes, where the cardiac rhythm can become arrhythmic when a fish experiences extreme heat.

Are there functional β₃-adrenoceptors in the human heart?

β₃-Adrenoceptor mRNA is expressed in the human heart, but corresponding receptor protein has not yet consistently been demonstrated. Furthermore, their physiological role remains highly controversial. For example, in human atria these receptors apparently do not promote cAMP formation. Evidence presented in this issue of the BJP suggests that a previously reported β₃-adrenoceptor-mediated stimulation of Ca(2+) channels at room temperature is absent at physiological temperatures, and that β₃-adrenoceptors have no effect on atrial contraction. Drugs classified as β₃-adrenoceptor agonists cause contraction in human atria but in most cases this involves β₁- and/or β₂-adrenoceptors. In contrast, in human ventricles β₃-adrenoceptor agonists can exhibit negative inotropic effects, potentially involving Pertussis toxin-sensitive G-proteins and activation of a NO synthase. However, firmer pharmacological evidence is required that these effects indeed occur via β₃-adrenoceptors. Whether the expected future use of β₃-adrenoceptor agonists in the treatment of urinary bladder dysfunction is associated with adverse events related to cardiac function remains to be determined from clinical studies.

Tissue functions mediated by beta(3)-adrenoceptors-findings and challenges

As β3-adrenoceptor agonists metamorphose from experimental tools into therapeutic drugs, it is vital to obtain a comprehensive picture of the cell and tissue functions mediated by this receptor subtype in humans. Human tissues with proven functions and/or a high expression of β3-adrenoceptors include the urinary bladder, the gall bladder, and other parts of the gastrointestinal tract. While several other β3-adrenoceptor functions have been proposed based on results obtained in animals, their relevance to humans remains uncertain. For instance, β3-adrenoceptors perform an important role in thermogenesis and lipolysis in rodent brown and white adipose tissue, respectively, but their role in humans appears less significant. Moreover, the use of tools such as the agonist BRL 37344 and the antagonist SR59230A to demonstrate functional involvement of β3-adrenoceptors may lead in many cases to misleading conclusions as they can also interact with other β-adrenoceptor subtypes or even non-adrenoceptor targets. In conclusion, we propose that many responses attributed to β3-adrenoceptor stimulation may need re-evaluation in the light of the development of more selective tools. Moreover, findings in experimental animals need to be extended to humans in order to better understand the potential additional indications and side effects of the β3-adrenoceptor agonists that are beginning to enter clinical medicine.

Regulation of SMN protein stability

Spinal muscular atrophy (SMA) is caused by mutations of the survival of motor neuron (SMN1) gene and deficiency of full-length SMN protein (FL-SMN). All SMA patients retain one or more copies of the SMN2 gene, but the principal protein product of SMN2 lacks exon 7 (SMNDelta7) and is unable to compensate for a deficiency of FL-SMN. SMN is known to oligomerize and form a multimeric protein complex; however, the mechanisms regulating stability and degradation of FL-SMN and SMNDelta7 proteins have been largely unexplored. Using pulse-chase analysis, we characterized SMN protein turnover and confirmed that SMN was ubiquitinated and degraded by the ubiquitin proteasome system (UPS). The SMNDelta7 protein had a twofold shorter half-life than FL-SMN in cells despite similar intrinsic rates of turnover by the UPS in a cell-free assay. Mutations that inhibited SMN oligomerization and complex formation reduced the FL-SMN half-life. Furthermore, recruitment of SMN into large macromolecular complexes as well as increased association with several Gemin proteins was regulated in part by protein kinase A. Together, our data indicate that SMN protein stability is modulated by complex formation. Promotion of the SMN complex formation may be an important novel therapeutic strategy for SMA.

Evidence for a role for cyclic AMP in modulating the action of 5-HT and an excitatory neuropeptide, FLP17A, in the pharyngeal muscle of Caenorhabditis elegans

The feeding activity of the nematode Caenorhabditis elegans is regulated by an anatomically well-defined network of 20 enteric neurones that employs small molecule and neuropeptidergic signalling. Two of the most potent excitatory agents are 5-HT and the neuropeptide FLP17A. Here we have examined the role of cAMP in modulating their excitatory actions by pharmacological manipulation of the level of cAMP. Application of the membrane permeable cAMP analogue, dibutyryl-cAMP (1 microM), enhanced the excitatory response to both FLP17A and 5-HT. Furthermore, the adenylyl cyclase activator, forskolin (50 nM), significantly enhanced the excitatory response to both FLP17A and 5-HT. The phosphodiesterase inhibitor, ibudilast (10 microM), enhanced the excitatory response to FLP17A. The protein kinase inhibitor, H-9 dihydrochloride (10 microM) significantly reduced the excitatory response to 5-HT. H-9 dihydrochloride also had a direct effect on pharyngeal activity. The effect of FLP17A and 5-HT on two mutants, egl-8 (loss-of-function phospholipase-Cbeta) and egl-30 (loss-of-function Galphaq) was also investigated. Both these mutants have a lower pharyngeal pumping rate than wild-type which has to be considered when interpreting the effects of these mutations on the excitatory responses to FLP17A and 5HT. However, even taking into consideration the lower basal activity of these mutants, it is clear that the percentage increase in pharyngeal pumping rate induced by FLP17A is greatly reduced in both mutants compared to wild-type. In the case of 5-HT, the effect of the mutant backgrounds on the response was less pronounced. Overall, the data support a role for cAMP in modulating the excitatory action of both FLP17A and 5-HT on C. elegans pharyngeal pumping and furthermore implicate an EGL-30 dependent pathway in the regulation of the response to FLP17A.

Canine ventricular myocyte beta2-adrenoceptors are not functionally coupled to L-type calcium current

INTRODUCTION

To establish the functional coupling of beta adrenoceptor (betaAR) subtypes beta1AR and beta2AR to L-type calcium current (I(CaL)), we investigated the nonselective agonist isoproterenol (ISO) and the relatively selective beta2AR agonists zinterol (ZIN) and salbutamol (SAL) on I(CaL) in isolated canine ventricular myocytes in the presence and absence of CGP 20712A (CGP) and atenolol (AT), selective beta1AR antagonists, and ICI 118,551 (ICI) a selective beta2AR antagonist.

METHODS AND RESULTS

Peak I(CaL) was determined using "patch type" microelectrodes and whole cell voltage clamp. ISO (0.5 microM) increased I(CaL) maximally 3.5 +/- 0.67 fold. ZIN (10.0 microM) and SAL (10.0 microM) increased I(CaL) maximally 1.5 +/- 0.2 fold (n = 5) and 1.4 +/- 0.1 fold (n = 5), respectively. These effects were fully inhibited by CGP (0.3 microM) and AT (1.0 microM), which are inhibitors of beta1AR, but not by ICI (0.1 microM), which is a beta2AR inhibitor. ZIN at relatively lower concentrations (< or = 0.1 microM) did not increase I(CaL). CGP (0.3 microM) but not AT and ICI inhibited I(CaL) in the absence of betaAR agonists. CGP inhibition of I(CaL) was absent in the presence of forskolin (1.0 microM), which increases cAMP levels and I(CaL) by directly stimulating the adenylate cyclase. These data indicate that none of the antagonists affect I(CaL) through an action downstream of betaAR.

CONCLUSION

Beta-adrenergic agonists increase I(CaL) via beta1AR but not beta2AR in canine ventricular myocytes.

Effects of beta2-adrenergic stimulation on single-channel gating of rat cardiac L-type Ca2+ channels

Cardiac L-type Ca2+ channels can be stimulated by activation of beta2-adrenoceptors. We intended to determine how the gating behavior at the single-channel level (cell-attached configuration) is affected after selective stimulation of beta2-adrenoceptors. Rat cardiomyocytes were exposed to zinterol, a beta2-agonist (n = 7), isoproterenol (n = 6), a nonselective agonist, 8-bromo-cAMP (n = 6), and a combination of isoproterenol and ICI-118551 (n = 8), a selective beta2-receptor antagonist, or isoproterenol and CGP-20712A, a beta1-selective antagonist (n = 7). In all groups the ensemble-average current and the availability of the channels to open on depolarization were increased in a similar fashion. In addition, the open probability (Po) within active sweeps was elevated. However, zinterol exerted this effect in a unique manner. It elevated Po not by shortening closed times but solely by reducing active sweeps with very low Po and a short burst duration. All zinterol effects were abolished by ICI-118551 (n = 5) and mimicked by isoproterenol plus CGP-20712A (n = 7). We conclude that beta2-adrenoceptor activation of L-type channels differs qualitatively from the classical cAMP-dependent mechanism.

Modulation of pacemaker activity in sheep cardiac Purkinje fibers by stimulation of beta-adrenoceptor subtypes

The electrophysiological effects mediated by beta 1- and beta 2-adrenoceptors in spontaneously active sheep cardiac Purkinje fibers were investigated using the non-selective agonist (-)-isoproterenol (IPN) and the selective agonists (-)-noradrenaline (beta 1) and procaterol (beta 2) in the absence and presence of the selective antagonists bisoprolol (beta 1) and ICI 118,551 (beta 2). IPN (0.01 mumol/l) increased the spontaneous rate by 54% and the slope of diastolic depolarization by 68% of the respective control values. Further, IPN increased the action potential duration at -20 mV (APD -20 mV) from 96 to 154 ms, reduced the APD-70 mV by 17% and the duration of the diastole by 39% and slightly hyperpolarized the maximum diastolic potential. These effects were partially inhibited by ICI 118,551 (0.03 mumol/l), diminished by bisoprolol (0.1 mumol/l) and almost completely blocked by the combination of both antagonists. Concentration response curves of IPN were influenced by the selective antagonists as follows: ICI 118,551 (0.03 mumol/l) shifted the curves to the right by 0.2-0.4 log units and increased the slope factor. Bisoprolol (0.1 mumol/l) induced a greater shift to the right by 1.1-1.5 log units. Combination of bisoprolol with ICI 118,551 shifted the curves to the right by 1.5-1.7 log units. Noradrenaline (0.3 mumol/l) elicited similar actions as IPN. Bisoprolol (0.1 mumol/l) shifted the concentration response curves of noradrenaline to the right by 1.1-1.9 log units. Actions of procaterol (0.1 mumol/l) were weak, attained only 15-35% of the maximal effects of IPN and could be blocked by ICI 118,551 (0.03 mumol/l). These results show that the increase of pacemaker activity induced by catecholamines in sheep cardiac Purkinje fibers is predominantly mediated by stimulation of beta 1-receptors. However, contribution of beta 2-receptor mediated effects could be demonstrated.

Response of failing canine and human heart cells to beta 2-adrenergic stimulation

BACKGROUND

Failing human hearts lose beta 1- but not beta 2-adrenergic receptors. In canine hearts with tachypacing failure, the ratio of beta 2- to beta 1-adrenergic receptors is increased. The present study was designed to determine whether heart failure increases sensitivity to beta 2-adrenergic stimulation in isolated canine ventricular cardiomyocytes and to verify that myocytes from failing human ventricles contain functional beta 2-adrenergic receptors.

METHODS AND RESULTS

Myocytes from healthy dogs, dogs with tachypacing failure, and human transplant recipients were loaded with fura 2-AM and subjected to electric field stimulation in the presence of zinterol, a highly selective beta 2-adrenergic agonist. Zinterol significantly increased [Ca2+]i transient amplitudes in all three groups. The failing canine myocytes were significantly more responsive than normal to beta 2-adrenergic stimulation. We also measured isotonic twitches, indo-1 fluorescence transients, and L-type Ca2+ currents in healthy canine myocytes. Zinterol (10(-5) mol/L) elicited large increases in the amplitudes of simultaneously recorded twitches and [Ca2+]i transients. Zinterol also increased L-type Ca2+ currents in the normal canine myocytes; this augmentation was abolished by 10(-7) mol/L ICI 118,551. cAMP production by suspensions of healthy and failing canine myocytes was not increased by zinterol (10(-9) to 10(-5) mol/L), nor did 10(-5) mol/L zinterol elicit phospholamban phosphorylation.

CONCLUSIONS

Failing human ventricular cardiomyocytes contain functional beta 2-adrenergic receptors. Canine myocytes also contain functional beta 2-adrenergic receptors. The canine ventricular response to beta 2-agonists is increased in tachypacing failure. Positive inotropic responses to beta 2-stimulation are not mediated by increases in cAMP or cAMP-dependent phosphorylation of phospholamban.

Beta-adrenoceptors in cardiac disease

The human heart contains both beta 1 and beta 2-adrenoceptors; both mediate positive inotropic and chronotropic effects. In chronic heart failure, beta-adrenoceptor number is reduced, presumably, by down-regulation by endogenous noradrenaline which is elevated due to increased sympathetic activity. Since the human heart contains only a few spare receptors for beta-adrenoceptor-mediated positive inotropic effects and the amount of spare receptors declines in chronic heart failure, it is not surprising that the reduced beta-adrenoceptor number is accompanied by decreased contractile responses to beta-adrenoceptor agonists (including endogenous catecholamines), and the extent of decrease in maximal inotropic response is more pronounced as the disease becomes more advanced. Moreover, in chronic heart failure myocardial G(i)-protein, which inhibits cAMP formation, is increased, which might further contribute to the reduction in beta-adrenoceptor-mediated effects. It appears that, at present, the best therapy for severe heart failure is a successful heart transplant, since in the transplanted heart beta-adrenoceptor number and function seems to be normalized. Moreover, the data currently available do not suggest any development of super- or subsensitivity of postsynaptic cardiac beta-adrenoceptors in the transplanted human heart.

Beta 1-adrenoceptor stimulation and beta 2-adrenoceptor stimulation differ in their effects on contraction, cytosolic Ca2+, and Ca2+ current in single rat ventricular cells

The effects of beta 2- and beta 1-adrenoceptor (beta 2AR and beta 1AR, respectively) agonists on the cytosolic Ca2+ (Cai) transient (indexed by the transient increase in indo-1 fluorescence ratio after excitation), twitch amplitude (measured via photodiode array), membrane potential, and L-type sarcolemmal Ca2+ current (ICa, measured by whole-cell patch electrode) were assessed in single rat ventricular myocytes. The selective beta 2AR agonist Zinterol increased the amplitudes of both the Cai transient and twitch in a concentration-dependent manner. Similar results were obtained when beta 2ARs were stimulated with isoproterenol in the presence of the selective beta 1AR antagonist CGP 20712A. beta 1AR stimulation induced by norepinephrine increased twitch amplitude to about the same extent as did beta 2AR stimulation. However, several striking differences between response to beta 1AR and beta 2AR stimulation were observed. beta 1AR stimulation had the potent effect of abbreviating the time course of the contraction and Cai transient, and beta 2AR stimulation did not reduce the time course of the Cai transient and had only a minor effect on the twitch duration. For a given increase in twitch amplitude, beta 1AR stimulation caused a greater increase in Cai transient, suggesting a diminished Cai-myofilament interaction. beta 1AR, but not beta 2AR, stimulation evoked spontaneous Cai oscillations, increased the diastolic indo fluorescence level, and caused a decline in resting cell length. beta 1AR and beta 2AR also differed in their effects on ICa. Whereas both beta 1AR and beta 2AR stimulation increased the peak ICa amplitude, beta 2AR stimulation markedly prolonged the ICa inactivation time. Accordingly, beta 2AR stimulation prolonged the action potential duration to a greater extent than did beta 1AR stimulation. 8-(4-Chlorophenylthio)cAMP mimicked the effects of beta 1AR stimulation by norepinephrine but not those due to beta 2AR stimulation. These results clearly indicate that both beta 2ARs and beta 1ARs functionally coexist in rat ventricular myocytes but that stimulation of these receptor subtypes elicits qualitatively different cell responses at the levels of ionic channels, the myofilaments, and sarcoplasmic reticulum.

Uncoupling of human cardiac beta-adrenoceptors during cardiopulmonary bypass with cardioplegic cardiac arrest

BACKGROUND

It is well known that during cardiopulmonary bypass (CPB) with cardioplegic cardiac arrest, catecholamines are vigorously increased. We therefore investigated whether this might cause desensitization of human cardiac beta-adrenoceptors.

METHODS AND RESULTS

We assessed in 12 children with acyanotic congenital heart disease who underwent open-heart surgery right atrial beta-adrenoceptor number and subtype distribution [by (-)-[125I]iodocyanopindolol binding] and adenylate cyclase activation [by the beta-adrenoceptor agonist isoprenaline (100 microM) and by the non-receptor-mediated activators 10 microM GTP, 10 mM NaF, 100 microM forskolin, and 10 mM Mn2+] before and after CPB with cardiac arrest by means of St. Thomas' cardioplegic solution. CPB affected neither beta-adrenoceptor number of subtype distribution nor GTP-, NaF-, forskolin-, or Mn(2+)-induced activation of adenylate cyclase. In contrast, activation of adenylate cyclase by 100 microM isoprenaline was significantly (p = 0.0249) lower after CPB than before CPB.

CONCLUSIONS

CPB with cardioplegic cardiac arrest decreases beta-adrenoceptor-mediated adenylate cyclase activation in a manner compatible with an uncoupling of beta-adrenoceptors from the Gs-protein-adenylate cyclase complex. Such a beta-adrenoceptor desensitization may be the reason why after CPB many patients need inotropic support but do not respond sufficiently to catecholamines.

Receptor systems in the non-failing human heart

Catecholamines acting through beta 1- and beta 2-adrenoceptors cause positive inotropic and chronotropic effects in the human heart. In recent years, however, evidence has accumulated that in the human heart also other receptor systems can affect heart rate and/or contractility. Positive inotropic effects can be mediated by receptor systems acting through accumulation of intracellular cAMP (Gs-protein coupled receptors such as 5-HT4-like, histamine H2, and vasoactive intestinal peptide) or by receptor systems acting independent of cAMP possibly through the phospholipase C/diacylglycerol/inositol-1,4,5-trisphosphate pathway (such as alpha 1-adrenergic, angiotensin II, and endothelin). In the non-failing human heart, however, activation of all these receptor systems induces only submaximal positive inotropic effects when compared with those caused by beta-adrenoceptor stimulation, indicating that in humans the cardiac beta-adrenoceptor-Gs-protein-adenylate cyclase pathway is the most powerful mechanism to increase heart rate and contractility. On the other hand, at least three receptor systems acting through inhibition of cAMP formation (Gi-protein coupled receptors) exist in the human heart: muscarinic M2-, adenosine A1-, and somatostatin-receptors. Activation of M2- and A1-receptors causes negative inotropic effects in the non-failing human heart: in atria activation of both receptors causes decreases in basal as well as in isoprenaline-stimulated force of contraction, but in ventricles only isoprenaline-stimulated force of contraction is depressed.

Mechanism underlying the reduced positive inotropic effects of the phosphodiesterase III inhibitors pimobendan, adibendan and saterinone in failing as compared to nonfailing human cardiac muscle preparations

The present study was performed to compare the effects of the new positive inotropic phosphodiesterase III inhibitors pimobendan, adibendan, and saterinone on the isometric force of contraction in electrically driven ventricular trabeculae carneae isolated from explanted failing (end-stage myocardial failure) with those from nonfailing (prospective organ donors) human hearts. In preparations from nonfailing hearts the phosphodiesterase inhibitors, as well as the beta-adrenoceptor agonist isoprenaline, the cardiac glycoside dihydro-ouabain, and calcium, which were studied for comparison, revealed pronounced positive inotropic effects. The maximal effects of pimobendan, adibendan, and saterinone amounted to 56%, 36% and 45%, respectively, of the maximal effect of calcium. In contrast, in preparations from failing hearts the phosphodiesterase III inhibitors failed to significantly increase the force of contraction and the effect of isoprenaline was markedly reduced. The effects of dihydroouabain and calcium were almost unaltered. The diminished effects of isoprenaline were restored by the concomitant application of phosphodiesterase inhibitors. To elucidate the underlying mechanism of the lack of effect of the phosphodiesterase III inhibitors in the failing heart we also investigated the inhibitory effects of these compounds on the activities of the phosphodiesterase isoenzymes I-III separated by DEAE-cellulose chromatography from both kinds of myocardial tissue. Furthermore, the effects of pimobendan and isoprenaline on the content of cyclic adenosine monophosphate (determined by radioimmunoassays) of intact contracting trabeculae were studied. The lack of effect of the phosphodiesterase inhibitors in failing human hearts could not be explained by an altered phosphodiesterase inhibition, since the properties of the phosphodiesterase isoenzymes I-III and also the inhibitory effects of the phosphodiesterase inhibitors on these isoenzymes did not differ between failing and nonfailing human myocardial tissue. Instead, it may be due to a diminished formation of cyclic adenosine monophosphate in failing hearts, presumably caused mainly by a defect in receptor-adenylate cyclase coupling at least in idiopathic dilated cardiomyopathy. Both the basal and the pimobendan-stimulated or isoprenaline-stimulated contents of cyclic adenosine monophosphate of intact contracting trabeculae from failing hearts were decreased compared with the levels in nonfailing hearts. However, under the combined action of isoprenaline and pimobendan the cyclic adenosine monophosphate level reached values as high as with each compound alone in nonfailing preparations, and in addition the positive inotropic effect of isoprenaline was restored. These findings may have important clinical implications. Along with the elevated levels of circulating catecholamines the positive inotropic effects of the phosphodiesterase inhibitors may be maintained in patients with heart failure.(ABSTRACT TRUNCATED AT 400 WORDS)

Some aspects of heart beta adrenoceptor function

Heart rate and force can be increased by noradrenaline and adrenaline through an interaction with both beta 1-adrenoceptors (beta 1AR) and beta 2-adrenoceptors (beta 2 AR). Several ionic currents (I) can flow upon beta AR activation: ICa (through either beta 1AR or beta 2AR), INa, IK, and ICl. Calcium currents (ICa) can be increased directly by the alpha s unit of a GTP binding protein, Gs, or by coupling of Gs to adenylyl cyclase with subsequent formation of cyclic AMP, release of the catalytic unit of cyclic AMP-dependent protein kinase, and phosphorylation of calcium channels and other proteins. Chronic exposure (days or months), but not acute exposure (hours), to a catecholamine downregulates human heart beta 1AR. Acute desensitization partially uncouples human heart beta AR from the adenylyl cyclase. Both acute and chronic desensitization reduce positive inotropic responses to catecholamines. In human heart, catecholamine-induced activation of one beta 2AR causes the production of at least four times more cyclic AMP than activation of one beta 1AR. Chronic treatment of patients with beta 1AR-selective blockers paradoxically induces selective inotropic beta 2AR hyperresponsiveness, presumably by increasing coupling of beta 2AR to Gs. Several partial agonists with high affinity for heart beta 1AR and beta 2AR cause stimulant effects that are resistant to blockade of beta 1AR and beta 2AR. Such nonconventional partial agonists could perhaps interact with beta AR that resemble beta 3 adrenoceptors.

Chronic beta 1-adrenoceptor antagonist treatment sensitizes beta 2-adrenoceptors, but desensitizes M2-muscarinic receptors in the human right atrium

1. In 64 patients undergoing coronary artery bypass grafting the effects of chronic beta 1-adrenoceptor antagonist (metoprolol, atenolol, bisoprolol) treatment on right atrial beta-adrenoceptor and muscarinic M2-receptor number and functional responsiveness were investigated. 2. The beta 1-adrenoceptor antagonists increased right atrial beta 1-adrenoceptor number, did not affect beta 2-adrenoceptor number, and decreased muscarinic M2-receptor number. 3. Concomitantly, activation of right atrial adenylate cyclase by 10 microM GTP, 10 microM isoprenaline and 1 microM forskolin was enhanced and inhibition by 100 microM carbachol was diminished. 4. On isolated, electrically driven right atria the beta 1-adrenoceptor-mediated positive inotropic effect of noradrenaline was - even with beta 1-adrenoceptor number increased - not altered, while the beta 2-adrenoceptor-mediated effect of procaterol was markedly enhanced. However, the carbachol-induced negative inotropic effect was decreased. 5. It is concluded that chronic beta 1-adrenoceptor antagonist treatment increases beta 1-adrenoceptor number and concomitantly sensitizes beta 2-adrenoceptor function, but desensitizes muscarinic M2-receptor function in the human heart.

Pertussis toxin treatment of whole blood. A novel approach to assess G protein function in congestive heart failure

This study was designed to assess G protein function in mononuclear leukocytes (MNL) of patients with congestive heart failure (CHF). MNL membranes were ADP-ribosylated in vitro in the presence of pertussis or cholera toxin. The amount of pertussis toxin substrates did not differ significantly between CHF patients (6,100 +/- 224 fmol/mg, n = 23) and age-matched healthy control subjects (5,812 +/- 972 fmol/mg protein, n = 19). Among the CHF patients, no differences were observed between those with idiopathic and ischemic CHF. The amount of cholera toxin substrates also did not differ significantly between CHF patients (7,522 +/- 1,405 fmol/mg protein, n = 11) and control subjects (5,654 +/- 707 fmol/mg protein, n = 14). Moreover, basal and isoproterenol- and prostaglandin E1-stimulated cyclic AMP (cAMP) accumulation in MNL was similar in control subjects and patients. To detect more subtle alterations of the cAMP-generating system, we incubated anticoagulated blood with 250-400 ng/ml pertussis toxin for 4 hours at 37 degrees C. This treatment completely ADP-ribosylated the MNL pertussis toxin substrates. Incubation with pertussis toxin did not change basal or prostaglandin E1-stimulated cAMP generation in MNL of control subjects, but it significantly enhanced stimulated generation (443 +/- 44 vs. 643 +/- 93 pmol/10(7) cells, p less than 0.025) in MNL of CHF patients. This enhancement was most pronounced in the most severely ill patients (New York Heart Association class IV) and correlated with plasma norepinephrine levels, another marker of CHF severity (r = 0.798, n = 11, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

On the physiologic role of beta-2 adrenoceptors in the human heart: in vitro and in vivo studies

To study the physiologic role of human myocardial beta-2 adrenoceptors, the beta adrenoceptor subtype(s) involved in the effects of catecholamines in vitro on the force of contraction and in vivo on heart rate were characterized. In vitro on both isolated electrically driven right and left atrial and left papillary muscle preparations, isoprenaline and adrenaline caused positive inotropic effects via beta-1 and beta-2 adrenoceptor stimulation. In the atria both beta-1 and beta-2 adrenoceptor stimulation increased contractile force to a maximum; in left papillary muscle, however, only beta-1 adrenoceptor stimulation maximally increased contractile force, whereas beta-2 adrenoceptor stimulation caused only submaximal increases. Noradrenaline, on the other hand, caused a positive inotropic effect nearly exclusively via atrial and ventricular beta-1 adrenoceptor stimulation. In vivo in 10 healthy volunteers isoprenaline-induced tachycardia was antagonized with equal potency by the beta-2 adrenoceptor-selective antagonist ICI 118,551 and the beta-1 adrenoceptor-selective antagonist bisoprolol indicating that it is mediated by cardiac beta-1 and beta-2 adrenoceptor stimulation to about the same degree. In contrast, exercise-induced tachycardia (that is mediated mainly by noradrenaline released from the neurons) was antagonized only by bisoprolol but not by ICI 118,551. It is concluded that in humans under normal physiologic conditions contractility and/or heart rate is regulated only by cardiac beta-1 adrenoceptors. In situations of stress, however, when large amounts of adrenaline are released from the adrenal medulla, stimulation of cardiac beta-2 adrenoceptors could contribute to additional increases in contractility, heart rate, or both.

Mitigation of beta 1- and/or beta 2-adrenoceptor function in human heart failure

1. Patients with congestive heart failure (CHF) have an elevated activity of the sympatho-adrenal system. We have investigated several aspects of beta-adrenoceptor desensitization in such patients. 2. The positive inotropic response to isoprenaline was attenuated in CHF patients, and the pD2-values for isoprenaline's positive inotropic effect gradually decreased in more severe forms of the disease. Stimulation of adenylate cyclase by isoprenaline was also mitigated in cardiac membranes from patients with CHF. 3. We then studied the density of cardiac beta 1- and beta 2-adrenoceptors in order to understand the mechanism of beta-adrenoceptor desensitization in these patients. Our data show that cardiac beta 1-adrenoceptors are down-regulated in all forms of severe CHF, but that cardiac beta 2-adrenoceptor density decreases only in some forms of CHF including ischaemic cardiomyopathy and mitral valve disease. 4. In circulating mononuclear leucocytes (MNL) obtained from CHF patients at rest, isoprenaline- and prostaglandin E1-stimulated cAMP generation as well as cholera toxin and pertussis toxin catalyzed ADP ribosylation were similar to those in MNL from control patients. However, pretreatment of intact MNL with pertussis toxin enhanced cAMP generation in CHF patients but not in healthy control subjects, suggesting a tonic inhibitory effect of Gi in such patients. 5. We conclude that alterations of adrenoceptors and of their signal transduction might contribute to the desensitization of beta-adrenergic responses in CHF.

Do human cardiac beta-2 adrenoceptors play a (patho)physiological role in regulation of heart rate and/or contractility?

There can be no doubt that in human heart in addition to beta 1-adrenoceptors, functional beta 2-adrenoceptors exist. Their (patho)physiological role in regulating heart rate and/or contractility, however, is still an open question. Under normal physiological conditions cardiac beta 2-adrenoceptors may not be of functional importance, since heart rate and contractility seem to be under the control of noradrenaline that in the human heart acts nearly exclusively at beta 1-adrenoceptors. However, in situations of stress when large amounts of adrenaline are released from the adrenal medulla additional stimulation of cardiac beta 2-adrenoceptors may contribute to increases in heart rate and/or contractility. Moreover, in endstage congestive cardiomyopathy where cardiac beta 1-adrenoceptors are selectively down-regulated, cardiac beta 2-adrenoceptors may substitute for the loss of beta 1-adrenoceptors to maintain (at least partially) contractility; under these conditions beta 2-adrenoceptor agonists, like dopexamine, may be of beneficial therapeutic effect. While a decrease in cardiac beta-adrenoceptor function appears to be a general phenomenon in all kinds of heart failure, it is not always due to a selective reduction in cardiac beta 1-adrenoceptors: in mitral valve disease both cardiac beta 1- and beta 2-adrenoceptors decline concomitantly in relation to the degree of heart failure. It is, therefore, doubtful whether under these conditions beta 2-adrenoceptor agonists may also be useful to support the failing heart.

The functional importance of beta 1 and beta 2 adrenoceptors in the human heart

Radioligand binding studies have demonstrated convincingly the coexistence of beta 1 and beta 2 adrenoceptors in the human heart. Both subtypes are involved in the increase in tissue levels of cyclic adenosine monophosphate in isolated, electrically driven, human right atria and in the activation of adenylate cyclase in human cardiac membrane preparations. In isolated, electrically driven strips of human right atria, isoproterenol increased contractile force through stimulation of both beta 1 and beta 2 adrenoceptors, while the selective beta 2-adrenoceptor agonist, procaterol, caused its positive inotropic effect predominantly through beta 2-adrenoceptor stimulation. Norepinephrine, however, increased contractile force solely via beta 1-adrenoceptor stimulation. In this preparation, dobutamine also acted as a full agonist, producing a positive inotropic effect through stimulation of both beta-adrenoceptor subtypes. Dopexamine hydrochloride, on the other hand, having an approximately 10-fold greater affinity for right atrial beta 2 than for beta 1 adrenoceptors, acted as a partial agonist (maximal positive inotropic effect: about 30% that of isoproterenol). Similar effects have been obtained in human right and left ventricular strips; thus, there can be no doubt that cardiac beta 2 adrenoceptors can contribute to the positive inotropic effects of beta-adrenoceptor agonists in the human heart. Besides mediating positive inotropic effects, right atrial beta 2 adrenoceptors may be involved in the regulation of heart rate since, in healthy volunteers, the selective beta 2-adrenoceptor antagonist, ICI 118,551, was more potent than the selective beta 1-adrenoceptor antagonist, bisoprolol, in antagonizing isoproterenol-induced tachycardia, when both antagonists were administered in doses that selectively occupied more than 90% of beta 2 and beta 1 adrenoceptors, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)