Cardiac adrenoceptors: physiological and pathophysiological relevance

Impaired left and right ventricular function following prolonged exercise in young athletes: influence of exercise intensity and responses to dobutamine stress

We examined the effect of intensity during prolonged exercise (PE) on left (LV) and right ventricular (RV) function. Subjects included 18 individuals (mean ± SE: age = 28.1 ± 1.1 yr, maximal aerobic power = 55.1 ± 1.6 ml · kg−1 · min−1), who performed 150 min of exercise at 60 and 80% maximal aerobic power on two separate occasions. Transthoracic echocardiography assessed systolic and diastolic performance, and blood sampling assessed hydration status and noradrenaline levels before (pre), during (15 and 150 min), and 60 min following (post) PE. β-Adrenergic sensitivity pre- and post-PE was assessed by dobutamine stress. High-intensity PE (15 vs. 150 min) induced reductions in LV ejection fraction (69.3 ± 1.3 vs. 63.5 ± 1.3%, P = 0.000), LV strain (−23.5 ± 0.6 vs. −22.3 ± 0.6%, P = 0.034), and RV strain (−26.3 ± 0.6 vs. −23.0 ± 0.6%, P < 0.01). Both exercise intensities induced diastolic reductions (pre vs. post) in the ratio of septal early wave of annular tissue velocities to late/atrial wave of annular tissue velocities (2.15 ± 0.15 vs. 1.62 ± 0.09; 2.21 ± 0.15 vs. 1.48 ± 0.10), ratio of lateral early wave of annular tissue velocities to late/atrial wave of annular tissue velocities (3.84 ± 0.42 vs. 2.49 ± 0.20; 3.56 ± 0.32 vs. 2.08 ± 0.18), ratio of early to late LV strain rate (2.42, ± 0.16 vs. 1.97 ± 0.13; 2.30 ± 0.15 vs. 1.81 ± 0.11), and ratio of early to late RV strain rate (2.03 ± 0.17 vs. 1.51 ± 0.09; 2.16 ± 0.16 vs. 1.44 ± 0.11) ( P < 0.001). Evidence of β-adrenergic sensitivity was supported by a decreased strain, strain rate, ejection fraction, and systolic pressure-volume ratio response to dobutamine ( P < 0.05) with elevated noradrenaline ( P < 0.01). PE-induced reductions in LV and RV systolic function were related to exercise intensity and β-adrenergic desensitization. The clinical significance of exercise-induced cardiac fatigue warrants further research.

Genetic influences on cardiovascular stress reactivity

Individual differences in the cardiovascular response to stress play a central role in the reactivity hypothesis linking frequent exposure to psychosocial stress to adverse outcomes in cardiovascular health. To assess the importance of genetic factors, a meta-analysis was performed on all published twin studies that assessed heart rate (HR) or blood pressure (BP) reactivity to the cold pressor test or various mental stress tasks. For reactivity to mental stress, the pooled heritability estimate ranged from 0.26 to 0.43. Reactivity to the cold pressor test yielded heritability estimates from 0.21 to 0.55. An ensuing review of genetic association studies revealed a number of genes, mostly within the sympathoadrenal pathway, that may account for part of the heritability of cardiovascular stress reactivity. Future progress in gene finding, that should include measures of sympathetic and vagal stress reactivity, may help uncover the molecular pathways from genetic variation to stress reactivity.

Effects of diabetes mellitus, pressure-overload and their association on myocardial structure and function

BACKGROUND

Structural and functional changes involved in cardiac injury induced by diabetes mellitus, pressure-overload, or both conditions were evaluated.

METHODS

Pressure-overload was established by suprarenal aortic banding in rats. Six weeks later, diabetes was induced by streptozotocin (STZ, 65 mg/kg, intraperitoneally), resulting in four groups: SHAM, banded (BA), diabetic (DM), and diabetic-banded (DM-BA). On the 12th week, left ventricular (LV) structure and function were evaluated. LV function was assessed in vivo with pressure-volume catheters and in vitro by papillary muscles' performance at baseline and in response to isoprenaline (ISO, 10(-8) to 10(-5) M).

RESULTS

Compared to SHAM, we observed a significant increase of type-B natriuretic peptide (BA = 370 +/- 110%; DM-BA = 580 +/- 210%), LV mass (BA = 36.8 +/- 3.6%; DM-BA = 32.1 +/- 3.1%), cardiomyocyte diameter (BA = 19.5 +/- 2.3%; DM = 14.3 +/- 1.9%; DM-BA = 11.4 +/- 2.0%), fibrosis (BA = 85 +/- 14%; DM = 145 +/- 28%; DM-BA = 155 +/- 14%), advanced glycation end-product (AGE) deposition (DM = 141 +/- 29%; DM-BA = 166 +/- 46%), contraction (tAT: DM = 13.7 +/- 2.4%; DM-BA = 26.3 +/- 7.1%); a delayed relaxation (tHR: DM = 13.8 +/- 2.6%; DM-BA = 25.5 +/- 9.2%) and a decrease of collagen type-I/type-III ratio (DM = -66.1 +/- 4.6%; DM-BA = -51.9 +/- 5.5). In SHAM animals, ISO (10(-5) M) increased 86.5 +/- 26.2% active tension, 105.3 +/- 20.2% dT/dt(max), and 166.8 +/- 29.9% dT/dt(min). Similar effects were observed in BA and DM animals, whereas in DM-BA these inotropic and lusitropic responses were blunted. Moreover, at a similar resting muscle length, ISO decreased passive tension by 12 +/- 3% in SHAM and 11 +/- 3% in BA, indicating an increase in myocardial distensibility, an effect that was absent in both diabetic groups.

CONCLUSION

Long-standing pressure-overload increased LV mass, while diabetes promoted AGE and collagen deposition, which might explain the abolition of ISO-induced increased myocardial distensibility. Association of pressure-overload and diabetes completely blunted the inotropic and lusitropic responses to ISO, with no additional structural damages than in pressure-overload or diabetes alone.

Association of beta-1 and beta-2 adrenergic receptor gene polymorphisms with myocardial infarction

Both beta(1)- and beta2-adrenergic receptors (beta(1)- and beta(2)-AR) have important roles in heart function mainly in response to catecholamines. Some specific polymorphisms in the beta(1)- and beta(2)-AR genes, named ADRB1 and ADRB2, respectively, have been implicated in several cardiovascular and noncardiovascular phenotypes. In this study, we aimed to investigate the possible relationship between Ser49Gly and Arg389Gly polymorphisms of the ADRB1 and Arg16Gly and Gln27Glu polymorphisms of the ADRB2 gene with ST elevation myocardial infarction (MI) in a Turkish population. One hundred patients with ST elevation MI and 100 healthy control subjects were genotyped using the PCR-RFLP method. Although the Arg389 allele of the ADRB1 gene was associated with an elevated risk of MI, the Glu27 allele of the ADRB2 gene was associated with a decreased risk of MI. Carriers of the ADRB1 Arg389 allele (heterozygotes+homozygotes) had an approximately 3.5-fold increased risk for MI than Gly389 homozygotes (OR=3.59, 95% CI=0.96-13.47, P=0.045). For the ADRB2 Gln27Glu polymorphism, subjects having one or two copies of the Glu27 allele showed a decreased risk of MI compared with Gln27 homozygote subjects (OR=0.48, 95% CI=0.24-0.94, P=0.03). Haplotype analysis of these polymorphisms showed no significant differences between groups. These results suggest that the Arg389Gly and Gln27Glu polymorphisms may be associated with an altered risk of MI in this Turkish population.

Cardiac autonomic neural remodeling and susceptibility to sudden cardiac death: effect of endurance exercise training

Sudden cardiac death resulting from ventricular tachyarrhythmias remains the leading cause of death in industrially developed countries, accounting for between 300,000 and 500,000 deaths each year in the United States. Yet, despite the enormity of this problem, both the identification of factors contributing to ventricular fibrillation as well as the development of safe and effective antiarrhythmic agents remain elusive. Subnormal cardiac parasympathetic regulation coupled with an elevated cardiac sympathetic activation may allow for the formation of malignant ventricular arrhythmias. In particular, myocardial infarction can reduce cardiac parasympathetic regulation and alter beta-adrenoceptor subtype expression enhancing beta(2)-adrenoceptor sensitivity that can lead to intracellular calcium dysregulation and arrhythmias. As such, myocardial infarction can induce a remodeling of cardiac autonomic regulation that may be required to maintain cardiac pump function. If alterations in cardiac autonomic regulation play an important role in the genesis of life-threatening arrhythmias, then one would predict that interventions designed to either augment parasympathetic activity and/or reduce cardiac adrenergic activity would also protect against ventricular fibrillation. Recently, studies using a canine model of sudden death demonstrate that endurance exercise training (treadmill running) enhanced cardiac parasympathetic regulation (increased heart rate variability), restored a more normal beta-adrenoceptor balance (i.e., reduced beta(2)-adrenoceptor sensitivity and expression), and protected against ventricular fibrillation induced by acute myocardial ischemia. Thus exercise training may reverse the autonomic neural remodeling induced by myocardial infarction and thereby enhance the electrical stability of the heart in individuals shown to be at an increased risk for sudden cardiac death.

Cardiomyocyte contractile dysfunction in the APPswe/PS1dE9 mouse model of Alzheimer's disease

Objectives

Ample clinical and experimental evidence indicated that patients with Alzheimer's disease display a high incidence of cardiovascular events. This study was designed to examine myocardial histology, cardiomyocyte shortening, intracellular Ca²⁺ homeostasis and regulatory proteins, electrocardiogram, adrenergic response, endoplasmic reticulum (ER) stress and protein carbonyl formation in C57 wild-type (WT) mice and an APPswe/PS1dE9 transgenic (APP/PS1) model for Alzheimer's disease.

Methods

Cardiomyocyte mechanical properties were evaluated including peak shortening (PS), time-to-PS (TPS), time-to-relengthening (TR), maximal velocity of shortening and relengthening (±dL/dt), intracellular Ca²⁺ transient rise and decay.

Results

Little histological changes were observed in APP/PS1 myocardium. Cardiomyocytes from APP/PS1 but not APP or PS1 single mutation mice exhibited depressed PS, reduced±dL/dt, normal TPS and TR compared with WT mice_. Rise in intracellular Ca²⁺ was lower accompanied by unchanged resting/peak intracellular Ca²⁺ levels and intracellular Ca²⁺ decay in APP/PS1 mice. Cardiomyocytes from APP/PS1 mice exhibited a steeper decline in PS at high frequencies. The responsiveness to adrenergic agonists was dampened although β₁-adrenergic receptor expression was unchanged in APP/PS1 hearts. Expression of the Ca²⁺ regulatory protein phospholamban and protein carbonyl formation were downregulated and elevated, respectively, associated with unchanged SERCA2a, Na⁺-Ca²⁺ exchanger and ER stress markers in APP/PS1 hearts. Our further study revealed that antioxidant N-acetylcysteine attenuated the contractile dysfunction in APP/PS1 mice.

Conclusions

Our results depicted overt cardiomyocyte mechanical dysfunction in the APP/PS1 Alzheimer's disease model, possibly due to oxidative stress.

Acute methamphetamine exposure inhibits cardiac contractile function

Methamphetamine, a commonly seen substance of abuse, has been reported to exert detrimental effect on bodily function including the cardiovascular system although its mechanism of action is poorly understood. This study was designed to examine the direct impact of methamphetamine on isolated whole heart and single cardiomyocyte contractile function. Murine hearts and isolated cardiomyocytes from adult FVB mice were exposed to various concentrations of methamphetamine for 30min prior to the assessment of mechanical function using a Langendroff apparatus and an IonOptix Myocam system, respectively. Cardiac contractile properties analyzed included maximal velocity of left ventricular pressure development and decline (+/-dP/dt), peak shortening amplitude (PS), maximal velocity of shortening/relengthening (+/-dLdt), time-to-PS (TPS), time-to-90% relengthening (TR(90)), resting and electrically stimulated increase of intracellular Ca(2+) as well as intracellular Ca(2+) decay. Our results revealed that acute methamphetamine exposure depressed +/-dP/dt, PS and rise of intracellular Ca(2+) without affecting +/-dLdt, TPS, TR(90), resting intracellular Ca(2+) and intracellular Ca(2+) decay. Furthermore, methamphetamine nullified the adrenergic agonist norepinephrine-elicited positive cardiomyocyte contractile response, including elevated PS, +/-dLdt and shortened TR(90) without affecting TPS. Western blot analysis showed unchanged expression of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) and phospholamban, associated with upregulated Na(+)-Ca(2+) exchanger levels following acute methamphetamine exposure. In addition, methamphetamine promoted overt cardiomyocyte protein damage evaluated by carbonyl formation. Taken together, these results demonstrate direct cardiac depressant effect of methamphetamine in myocardium and isolated cardiomyocytes, possibly associated with protein damage and dampened adrenergic response.

Beta 1- and beta 2-adrenoceptor polymorphisms and cardiovascular diseases

Beta(1)- and beta(2)-adrenoceptors (AR) play a pivotal role in the regulation of cardiovascular function. Both beta-AR subtypes are polymorphic: two single nucleotide polymorphisms (SNPs) have been described for the beta(1)- (Ser49Gly, Arg389Gly) and four for the beta(2)-AR (Arg-19Cys, Arg16Gly, Gln27Glu, Thr164Ile), and they are possibly of functional relevance. In recombinant cell systems, Gly49-beta(1)-AR are more susceptible to agonist-promoted down-regulation than Ser49-beta(1)-AR, whereas Arg389-beta(1)-AR are three to four times more responsive to agonist-evoked stimulation than Gly389-beta(1)-AR. With respect to beta(2)-AR, the Cys-19 variant is associated with greater beta(2)-AR expression than the Arg-19 variant; Gly16-beta(2)-AR are more susceptible, whereas Glu27-beta(2)-AR are almost resistant to agonist-promoted down-regulation; Thr164-beta(2)-AR are three to four times more responsive to agonist-evoked stimulation than Ile164-beta(2)-AR. Several studies addressed potential phenotypic consequences of these SNPs in vivo by influencing and/or contributing to the pathophysiology of cardiovascular/pulmonary diseases such as hypertension, congestive heart failure, arrhythmias or asthma. At present, it appears that these beta-AR SNPs are very likely not disease-causing genes but possibly predictive for the responsiveness to agonists and antagonists. Patients carrying one or two alleles of the Gly389-beta(1)-AR are poor or non-responders to agonists and antagonists, whereas patients homozygous for the Arg389-beta(1)-AR are good responders. Subjects carrying the Ile164-beta(2)-AR exhibit blunted responses to beta(2)-AR stimulation. Asthma patients carrying the Arg16-Gln27-Thr164-beta(2)-AR haplotype who receive regularly short- or long-acting beta(2)-AR agonists are rather susceptible to agonist-induced desensitization and in consequence exhibit reduced bronchodilating and -protective effects and/or increased asthma exacerbations. The clinical relevance of these findings is still under debate.

Capturing adenylyl cyclases as potential drug targets

Cyclic AMP (cAMP) is an important intracellular signalling mediator. It is generated in mammals by nine membrane-bound and one soluble adenylyl cyclases (ACs), each with distinct regulation and expression patterns. Although many drugs inhibit or stimulate AC activity through the respective upstream G-protein coupled receptors (for example, opioid or beta-adrenergic receptors), ACs themselves have not been major drug targets. Over the past decade studies on the physiological functions of the different mammalian AC isoforms as well as advances in the development of isoform-selective AC inhibitors and activators suggest that ACs could be useful drug targets. Here we discuss the therapeutic potential of isoform-selective compounds in various clinical settings, including neuropathic pain, neurodegenerative disorders, congestive heart failure, asthma and male contraception.

Phosphodiesterases inhibition unmask a positive inotropic effect mediated by beta2-adrenoceptors in rat ventricular myocardium

The effects of salbutamol on contractility and cAMP levels were investigated in rat right ventricular myocardium. Salbutamol (1-300 microM), produced a concentration-dependent positive inotropic effect which was not affected by ICI 118551 (50 nM), a beta2-adrenoceptor antagonist but was abolished by CGP 20712A (1 microM) a beta1-adrenoceptor antagonist. However, in rats pretreated with pertussis toxin (30 microg/kg intraperitoneal injection) salbutamol increases contractility (Emax = 9.8 +/- 1.8%, - log EC50 = 6.25 +/- 0.07, n = 5). The combination of salbutamol + CGP 20712A, also produces a concentration-dependent enhancement of contractility (Emax = 43.0 +/- 7.5%, - log EC50 = 6.3 +/- 0.04, n = 6), in the presence of 30 microM of the non selective phosphodiesterase (PDE) inhibitor 3-isobutylmethylxantine (IBMX) which was prevented by ICI 118551 (50 nM). Also, salbutamol + CGP 20712A fail to increase cAMP tissue levels but enhance them in the presence of IBMX. This effect was also prevented by ICI 118551. These results indicate that PDEs blunt contractility and cAMP production mediated by beta2-adrenoceptors in rat ventricular myocardium. Gi protein, although less efficiently than PDEs, also limits inotropic effects of salbutamol mediated by beta2-adrenoceptors in this tissue.

Cardiac hypertrophy in neonatal nephrectomized rats: the role of the sympathetic nervous system

Cardiac hypertrophy is frequently encountered in patients with renal failure and represents an independent risk factor for cardiovascular morbidity and mortality. The pathogenesis of cardiac hypertrophy is related to multiple factors, including excess adrenergic activity. This study investigated how renal injury in the early stages of life affects the adrenergic system and thereby potentially influences cardiac growth. Biomarkers of cardiac hypertrophy were used to assess adrenergic function. Newborn male Sprague-Dawley rats were allocated to three groups of five rats each: 5/6 nephrectomy (Nx), pair-fed controls (PF), and sham-operated (SH). Nx animals had significantly higher plasma urea nitrogen, serum creatinine, and mean arterial blood pressure. The heart-weight/body-weight ratio of the Nx cohort was higher than SH and PF (p < 0.001) groups. Plasma norepinephrine (NE) of Nx animals was almost twofold higher than SH and PF (p < 0.01) animals. Compared with SH and PF, Nx animals had higher alpha1A-receptor protein expression, lower cardiac beta1- and beta2-receptor protein expression (p < 0.05), but higher G-protein-coupled receptor kinase-2 (GRK2) expression (p < 0.05). Norepinephrine transporter protein (NET) and renalase protein expression in cardiac tissue from Nx pups were significantly lower than SH and PF. Our data suggest that early age Nx animals have increased circulating catecholamines due to decreased NE metabolism. Enhancement of cardiac GRK2 and NE can contribute to cardiac hypertrophy seen in Nx animals. Furthermore, AKT (activated via alpha1A receptors), as well as increased alpha1A receptors and their agonist NE, might contribute to the observed hypertrophy.

Current concepts of neurohormonal activation in heart failure: mediators and mechanisms

Neurohormonal activation is a commonly cited array of phenomena in the body's physiologic response to heart failure. Although various neurohormones and pharmacologic agents that moderate their pathophysiologic effects have been reviewed in the nursing literature, both the mechanisms of neurohormonal system activation and cellular and organ system effects have been described only in brief. Accordingly, this article reviews mechanisms of neurohormonal activation and describes cellular and cardiovascular effects of the (1) sympathetic nervous system, (2) renin-angiotensin-aldosterone system, (3) kallikrein-kininogen-kinin system, (4) vasopressinergic system, (5) natriuretic peptide systems, and (6) endothelin in the context of heart failure. This article implicitly details the physiologic basis for numerous current and potential future pharmacologic agents used in the management of heart failure. It is intended that this article be used as a reference for advanced clinical nursing practice, research, and education.

Heart adrenoceptor gene expression and binding sites in the human failing heart

Adrenergic regulation of the heart function is well documented by many studies. Catecholamines act through alpha(1)-, beta(1)-, beta(2)-, and beta(3)-adrenoceptors (ARs) in the heart. There are many findings about the changes of beta(1)- and beta(2)-AR in heart failure (HF). On the other hand, the role of other AR subtypes is not clear yet. We focused on determining how HF could affect gene expression and specific ligand binding to alpha(1A)-, alpha(1B)-, alpha(1D)-, beta(1)-, beta(2)-, and beta(3)-AR. Hearts from 11 patients with HF subjected to transplantation were investigated. As a control, corresponding parts from hearts not suitable for transplantation were used. We have found significantly higher mRNA levels of alpha(1A)-, alpha(1B)-,beta(1)-, and beta(2)-AR in the left ventricle of failing hearts compared to the levels in controls. beta(3)-AR mRNA levels in the left ventricle of failing hearts were not changed. No changes in mRNA levels of all receptors studied in other cardiac areas were found. On the other hand, binding studies showed a substantial decrease in left ventricles of failing hearts in all alpha(1)-AR subtypes and in beta(1)- and beta(2)-AR. However, the binding to beta(3)-AR was not changed. Our results suggest that alpha(1)-AR changes might be part of a compensatory mechanism, by which the heart suffering from the HF tries to secure its function, and it could be hypothesized that ineffective beta(3)-AR regulation might be involved in development of HF. According to our knowledge, this is the first report about the beta(3)-AR binding in HF.

Influence of gender and sex hormones on 5alpha-dihydrotestosterone elicited effect in isolated left atria of rats: Role of beta-adrenoceptors and ornithine decarboxylase activity

Androgens elicit an acute cardiotonic effect in cardiac preparations of rats. This effect is produced via an extracellular interaction that may be coupled to pertussis-sensitive G-proteins and is associated with an increase in cAMP, polyamine synthesis and intracellular calcium. The nature of the targets and the existence of a dimorphic effect in this nongenomic effect of androgens are unknown. The purpose of this study was to characterize a possible gender and sex hormone influence on the 5alpha-dihydrotestosterone-elicited cardiotonic effect, taking into account the possible role of the beta-adrenoceptors and ornithine decarboxylase activity on this response. [Float1]Regarding this, the effect of 5alpha-dihydrotestosterone on isolated left atria from male, estrogenized female and gonadectomized male and female rats was studied. The results showed that 5alpha-dihydrotestosterone-elicited cardiotonic effect was preserved independent of gender and sex hormones, being higher in control males than in the rest of the groups. This correlated with the testosterone plasma levels, except in estrogenized females, suggesting that the androgens positively and the estrogens negatively regulated the response. In all groups, 5alpha-dihydrotestosterone produced an increase in cAMP levels, but only in control males did it produce an increase in ornithine decarboxylase activity. In the other groups, the absence of an effect on ornithine decarboxylase might limit the capability of the response to the androgen. Altogether, androgens may help to control cardiac performance by a direct interaction on the heart in both sexes. Gender and sex differences in the magnitude of inotropism being due mainly to changes in beta-adrenoceptors and cAMP production and in intracellular polyamine synthesis.

Blood pressure and heart rate QTL in mice of the B6/D2 lineage: sex differences and environmental influences

A quantitative trait locus (QTL) approach was used to define the genetic architecture underlying variation in systolic blood pressure (SBP) and heart rate (HR), measured indirectly on seven occasions by the tail cuff procedure. The tests were conducted in 395 F(2) adult mice (197 males, 198 females) derived from a cross of the C57BL/6J (B6) and DBA/2J (D2) strains and in 22 BXD recombinant-inbred (RI) strains. Interval mapping of F(2) data for the first 5 days of measurement nominated one statistically significant and one suggestive QTL for SBP on chromosomes (Chr) 4 and 14, respectively, and two statistically significant QTL for HR on Chr 1 (which was specific to female mice) and Chr 5. New suggestive QTL emerged for SBP on Chr 3 (female-specific) and 8 and for HR on Chr 11 for measurements recorded several weeks after mice had undergone stressful blood sampling procedures. The two statistically significant HR QTL were confirmed by analyses of BXD RI strain means. Male and female F(2) mice did not differ in SBP or HR but RI strain analyses showed pronounced strain-by-sex interactions and a negative genetic correlation between the two measures in both sexes. Evidence for a role for mitochondrial DNA was found for both HR and SBP. QTL for HR and SBP may differ in males and females and may be sensitive to different environmental contexts.

Role of putrescine on androgen-elicited positive inotropism in the left atrium of rats

Functional and biochemical studies were performed in isolated left atria of male Wistar rats to study whether endogenous polyamines may mediate androgen-elicited positive inotropism and their relationship with a rise in cAMP during the cardiotonic effect. 5 alpha-Dihydrotestosterone (100 microM) exposure increased intracellular putrescine as determined by HPLC, but it did not increase spermidine and spermine. This effect was antagonized by an inhibitor of ornithine decarboxylase, alpha-difluoromethylornithine (10 mM), suggesting enzyme activation. alpha-Difluoromethylornithine also antagonized androgens-elicited inotropism and the increase in intracellular cAMP. Putrescine (1 to 10 mM) elicited a concentration-dependent positive inotropism associated with the cAMP increase. The prior incubation with putrescine antagonized 5 alpha-dihydrotestosterone-elicited inotropism and did not produce sinergism on intracellular cAMP. Short-term incubation with 5 alpha-dihydrotestosterone or forskolin shifted to the left the cardiotonic effect of isoproterenol, an agonist of beta-adrenoceptors, without any increase in Emax, suggesting that a common mechanism was involved. Therefore, polyamines might modulate the cAMP production associated with the cardiotonic effect of androgens.

beta3-adrenergic receptor activation increases human atrial tissue contractility and stimulates the L-type Ca2+ current

beta3-adrenergic receptor (beta3-AR) activation produces a negative inotropic effect in human ventricles. Here we explored the role of beta3-AR in the human atrium. Unexpectedly, beta3-AR activation increased human atrial tissue contractility and stimulated the L-type Ca2+ channel current (I Ca,L) in isolated human atrial myocytes (HAMs). Right atrial tissue specimens were obtained from 57 patients undergoing heart surgery for congenital defects, coronary artery diseases, valve replacement, or heart transplantation. The I(Ca,L) and isometric contraction were recorded using a whole-cell patch-clamp technique and a mechanoelectrical force transducer. Two selective beta3-AR agonists, SR58611 and BRL37344, and a beta3-AR partial agonist, CGP12177, stimulated I(Ca,L) in HAMs with nanomolar potency and a 60%-90% efficacy compared with isoprenaline. The beta3-AR agonists also increased contractility but with a much lower efficacy (approximately 10%) than isoprenaline. The beta3-AR antagonist L-748,337, beta1-/beta2-AR antagonist nadolol, and beta1-/beta2-/beta3-AR antagonist bupranolol were used to confirm the involvement of beta3-ARs (and not beta1-/beta2-ARs) in these effects. The beta3-AR effects involved the cAMP/PKA pathway, since the PKA inhibitor H89 blocked I(Ca,L) stimulation and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) strongly increased the positive inotropic effect. Therefore, unlike in ventricular tissue, beta3-ARs are positively coupled to L-type Ca2+ channels and contractility in human atrial tissues through a cAMP-dependent pathway.

Etomidate has no effect on hypoxia reoxygenation and hypoxic preconditioning in isolated human right atrial myocardium

BACKGROUND

We examined the effects of etomidate on recovery of contractile function after hypoxia reoxygenation and hypoxic preconditioning in vitro using isolated human myocardium.

METHODS

Human right atrial myocardium were obtained at the time of cardiac surgery from 38 adults patients. We recorded isometric force of contraction (FoC) of atrial trabeculae suspended in an oxygenated Tyrode's solution (34 degrees C, stimulation frequency 1 Hz). In all groups, a 30-min hypoxic period was followed by 60 min of reoxygenation (HR). In separate groups, muscles were exposed to etomidate (10(-7), 10(-6), 10(-5) M) 10 min before and throughout the HR periods. Hypoxic preconditioning was induced by 4-min hypoxia followed by 7-min reoxygenation applied before HR periods. Etomidate 10(-5) M was administered before, throughout, and after the hypoxic preconditioning stimulus. Recovery of FoC (expressed as % of baseline value) at the end of HR was compared among groups.

RESULTS

Compared with the control group (FoC: 52%+/-10%), etomidate 10(-7) M (FoC: 57%+/-9%; P=0.24), 10(-6) M (FoC: 61%+/-11%; P=0.10), and 10(-5) M (FoC: 54%+/-9%; P=0.29) did not modify the recovery of FoC after HR. Hypoxic preconditioning-induced increase in the recovery of FoC (87%+/-5%; P<0.001 vs control group) was not modified in the presence of etomidate 10(-5) M (FoC: 86%+/-7%; P=0.74 vs hypoxic preconditioning group).

CONCLUSIONS

Etomidate did not modify the in vitro FoC of human myocardium exposed to HR. Furthermore, etomidate did not modify the protective effect of hypoxic preconditioning.

Modulation of enzymatic activities by n-3 polyunsaturated fatty acids to support cardiovascular health

Epidemiological evidence from Greenland Eskimos and Japanese fishing villages suggests that eating fish oil and marine animals can prevent coronary heart disease. Dietary studies from various laboratories have similarly indicated that regular fish oil intake affects several humoral and cellular factors involved in atherogenesis and may prevent atherosclerosis, arrhythmia, thrombosis, cardiac hypertrophy and sudden cardiac death. The beneficial effects of fish oil are attributed to their n-3 polyunsaturated fatty acid (PUFA; also known as omega-3 fatty acids) content, particularly eicosapentaenoic acid (EPA; 20:5, n-3) and docosahexaenoic acid (DHA; 22:6, n-3). Dietary supplementation of DHA and EPA influences the fatty acid composition of plasma phospholipids that, in turn, may affect cardiac cell functions in vivo. Recent studies have demonstrated that long-chain omega-3 fatty acids may exert beneficial effects by affecting a wide variety of cellular signaling mechanisms. Pathways involved in calcium homeostasis in the heart may be of particular importance. L-type calcium channels, the Na+-Ca2+ exchanger and mobilization of calcium from intracellular stores are the most obvious key signaling pathways affecting the cardiovascular system; however, recent studies now suggest that other signaling pathways involving activation of phospholipases, synthesis of eicosanoids, regulation of receptor-associated enzymes and protein kinases also play very important roles in mediating n-3 PUFA effects on cardiovascular health. This review is therefore focused on the molecular targets and signaling pathways that are regulated by n-3 PUFAs in relation to their cardioprotective effects.

Some components of the cardiac β-adrenergic system are altered in the chronic indeterminate form of experimental Trypanosoma cruzi infection

The chronic indeterminate form of Trypanosoma cruzi infection could be the key to knowing which patients will develop chagasic myocardiopathy. Infected mice present a period in which cardiac functional and structural alterations are different from those described for acute or chronic phases. We studied some components of the cardiac β-adrenergic system in mouse hearts infected with T. cruzi Tulahuen strain or SGO-Z12 isolate during the chronic indeterminate phase of infection. We determined: (i) the primary messenger (epinephrine and norepinephrine) levels in plasma by reverse-phase-HPLC; (ii) the cardiac β-adrenergic receptors' (β-AR) density and affinity by binding with tritiated dihidroalprenolol and by immunofluorescence; (iii) the cardiac concentration of the second messenger (cAMP) (by ELISA) given its importance for the phosphorylation of the proteins involved in cardiac contraction; (iv) the cardiac contractility and functional studies of the β-ARs as a response to the ligand binding to the receptor; and (v) the left ventricular ejection fraction as a measure of in vivo cardiac function. Plasma catecholamines levels remained similar to those found in uninfected controls. The β-ARs' affinity decreased in both infected groups compared with the uninfected group (P<0.05) while the receptors' density increased only in the SGO-Z12 group (P<0.01). Cyclic AMP levels were higher in both infected groups (P<0.01) relative to controls, and were higher in SGO-Z12-infected mice compared with those infected with the Tulahuen strain. However, the basal contractile force remained unchanged and the response to catecholamines only increased in the Tulahuen group (P<0.05). The left ventricular ejection fraction, on the other hand, was diminished in SGO-Z12-infected mice. Heterogeneity between T. cruzi strains determine, in the chronic indeterminate form, alterations in the signaling pathways of the β-adrenergic system at different levels: (i) between catecholamines and the β(1)-receptors; (ii) between the receptors' activation and the adenylyl-cyclase activation; and/or (iii) between cAMP and the contractile response.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.

New signaling pathways associated with increased cardiac adenylyl cyclase 6 expression: implications for possible congestive heart failure therapy

Congestive heart failure (CHF) affects more than five million people in the United States and results in considerable morbidity, mortality, and economic costs. Patients with class III and IV CHF have a 40% to 50% probability of dying 5 years after symptom onset despite optimal therapy, a prognosis worse than many cancers. A variety of drugs and devices have improved survival-the 50% survival time in 1980 was just 18 months-but the outlook for patients remains dismal and the prevalence of CHF continues to increase. This unmet medical need underscores the importance of developing new approaches for the treatment of CHF. This brief review focuses on data from preclinical experiments regarding the effects of increased adenylyl cyclase type 6 (AC6) expression on cellular and cardiac function, and possible mechanisms for the unexpected favorable effects of increased AC6 content on the failing heart.

Roles of PKA, PI3K, and cPLA2 in the NO-mediated negative inotropic effect of β2-adrenoceptor agonists in guinea pig right papillary muscles

Although β2-adrenoceptors represent 15–25% of β-adrenoceptors in the guinea pig heart, their functionality is controversial. We assessed the inotropic effects of β2-adrenoceptor partial agonists in right papillary muscles. Salbutamol induced a small but significant concentration-dependent negative inotropic effect (NIE, −5% at 60 nM) followed by a moderate positive inotropic effect (+36% at 6 μM) due to activation of β1-adrenoceptors. In the presence of 4 μM atenolol, the concentration-dependent NIE (−12% at 6 μM) was biphasic, best described by a double logistic equation with respective EC50 values of 3 and ∼420 nM, and was insensitive to SR59230A. In muscles from pertussis toxin-treated guinea pigs, the salbutamol-induced positive inotropic effect was sensitive to low concentrations of ICI-118551 in an unusual manner. Experiments in reserpinized animals revealed the importance of the phosphorylation-dephosphorylation processes. PKA inhibition reduced and suppressed the effects obtained at low and high concentrations, respectively, indicating that its activation was a prerequisite to the NIE. The effect occurring at nanomolar concentrations depended upon PKA/phosphatidylinositol 3-kinase/cytosolic phospholipase A2 (cPLA2) activations leading to nitric oxide (NO) release via the arachidonic acid/cyclooxygenase pathway. NO release via PKA-dependent phosphorylation of the receptor was responsible for the inotropic effect observed at submicromolar concentrations, which is negatively controlled by cPLA2. The possibility that these effects are due to an equilibrium between different affinity states of the receptor (Gs/Gi coupled and Gi independent with different signaling pathways) that can be displaced by ICI-118551 is discussed. We conclude that β2-adrenoceptors are functional in guinea pig heart and can modulate the inotropic state.

Effects of milrinone on contractility and cyclic adenosine monophosphate production induced by beta1- and beta2-adrenergic receptor activation in human myocardium

BACKGROUND

Because milrinone is a widely used phosphodiesterase-3 (PDE3) inhibitor, it would be of interest to know whether it interacts with beta1- and beta2-adrenergic receptor (AR) agonists in human myocardium.

OBJECTIVES

This in vitro study was conducted to test whether milrinone differentially regulates cyclic adenosine-3',5'-monophosphate (cAMP) production and to examine the effect of milrinone on the positive inotropic responses and cAMP production induced by activation of the beta1-AR with norepinephrine (NE) and activation of the beta2-AR with epinephrine (EPI) in human atrial myocardium.

METHODS

Right atrial trabeculae were obtained from patients undergoing cardiac surgery for valve repair. Concentration-response curves for inotropic responses mediated through the beta1-AR (NE in the presence of the beta2-blocker ICI 118, 551) and the beta2-AR (EPI in the presence of the beta1-blocker CGP 20712A) were obtained in the absence and presence of milrinone 1 micromol/L. This concentration of milrinone was chosen because it corresponded to its 50% inhibitory concentration as a PDE3 inhibitor and its therapeutic plasma concentration. The production of cAMP induced by exposure to selective beta1- and beta2-AR stimulation was also measured in the absence and presence of milrinone.

RESULTS

Right atrial tissue samples were obtained from 12 white patients (7 women, 5 men; mean [SE] age, 64.6 [6.3] years) undergoing cardiac surgery for valve repair (8 mitral, 4 aortic). The presence of milrinone was associated with leftward shifts in the concentration-response curves for both NE and EPI. cAMP production in myocardial tissue samples in the presence of milrinone was increased only with NE induction (mean [SEM], 745.0 [136.7] pmol/g in the absence of milrinone vs 1620.5 [372.3] pmol/g in the presence of milrinone; P < 0.05).

CONCLUSIONS

In this preliminary study in human atrial myocardium, milrinone potentiated the contractile responses to both NE and EPI. However, only the effect of NE on tissue levels of cAMP was increased in the presence of milrinone.

Treating patients with overactive bladder syndrome with antimuscarinics: heart rate considerations

In this excellent mini-review, the authors present an extensive and relevant paper on the effect of antimuscarinic agents on the heart. This is without doubt the most detailed and the most reader-friendly paper on this subject, and I am sure that it will help urologists to assist in further educating their patients when prescribing these compounds.

Adrenergic regulation of a key cardiac potassium channel can contribute to atrial fibrillation: evidence from an I Ks transgenic mouse

Inherited gain-of-function mutations of genes coding for subunits of the heart slow potassium (I Ks) channel can cause familial atrial fibrillation (AF). Here we consider a potentially more prevalent mechanism and hypothesize that beta-adrenergic receptor (beta-AR)-mediated regulation of the I Ks channel, a natural gain-of-function pathway, can also lead to AF. Using a transgenic I Ks channel mouse model, we studied the role of the channel and its regulation by beta-AR stimulation on atrial arrhythmias. In vivo administration of isoprenaline (isoproterenol) predisposes I Ks channel transgenic mice but not wild-type (WT) littermates that lack I Ks to prolonged atrial arrhythmias. Patch-clamp analysis demonstrated expression and isoprenaline-mediated regulation of I Ks in atrial myocytes from transgenic but not WT littermates. Furthermore, computational modelling revealed that beta-AR stimulation-dependent accumulation of open I Ks channels accounts for the pro-arrhythmic substrate. Our results provide evidence that beta-AR-regulated I Ks channels can play a role in AF and imply that specific I Ks deregulation, perhaps through disruption of the I Ks macromolecular complex necessary for beta-AR-mediated I Ks channel regulation, may be a novel therapeutic strategy for treating this most common arrhythmia.

Genetic variation in the cytochrome P450 epoxygenase pathway and cardiovascular disease risk

The cytochrome (CYP) P450 epoxygenase pathway catalyzes the epoxidation of arachidonic acids to epoxyeicosatrienoic acids, which are subsequently hydrolyzed to less active dihydroxyeicosatrienoic acids by soluble epoxide hydrolase. Numerous preclinical studies have demonstrated that CYP-derived epoxyeicosatrienoic acids possess potent vasodilatory and anti-inflammatory properties in the cardiovascular system. In humans, functionally relevant polymorphisms, which may significantly modulate epoxyeicosatrienoic acid levels in vivo, have been identified in the genes encoding CYP2J2, CYP2C8, CYP2C9 and soluble epoxide hydrolase. Initial epidemiologic studies have demonstrated that genetic variation in the CYP epoxygenase pathway significantly modifies cardiovascular disease risk at the population level in humans, providing support for the hypothesis that modulation of this pathway may represent a novel approach to the prevention and treatment of cardiovascular disease. Future studies in humans validating these relationships and characterizing the underlying mechanisms will be necessary to fully understand the functional role of the CYP epoxygenase pathway in cardiovascular disease.

Are [O-methyl-11C]derivatives of ICI 89,406 beta1-adrenoceptor selective radioligands suitable for PET?

PURPOSE

Radioligand binding studies show that beta(1)-adrenoceptor (beta(1)-AR) density may be reduced in heart disease without down regulation of beta(2)-ARs. Radioligands are available for measuring total beta-AR density non-invasively with clinical positron emission tomography (PET) but none are selective for beta(1)- or beta(2)-ARs. The aim was to evaluate ICI 89,406, a beta(1)-AR-selective antagonist amenable to labelling with positron emitters, for PET.

METHODS

The S-enantiomer of an [O-methyl-(11)C] derivative of ICI 89,406 ((S)-[(11)C]ICI-OMe) was synthesised. Tissue radioactivity after i.v. injection of (S)-[(11)C]ICI-OMe (< 2 nmol x kg(-1)) into adult Wistar rats was assessed by small animal PET and post mortem dissection. Metabolism was assessed by HPLC of extracts prepared from plasma and tissues and by measuring [(11)C]CO(2) in exhaled air.

RESULTS

The heart was visualised by PET after injection of (S)-[(11)C]ICI-OMe but neither unlabelled (S)-ICI-OMe nor propranolol (non-selective beta-AR antagonist) injected 15 min after (S)-[(11)C]ICI-OMe affected myocardial radioactivity. Ex vivo dissection showed that injecting unlabelled (S)-ICI-OMe, propranolol or CGP 20712A (beta(1)-selective AR antagonist) at high dose (> 2 mumol x kg(-1)) before (S)-[(11)C]ICI-OMe had a small effect on myocardial radioactivity. HPLC demonstrated that radioactivity in myocardium was due to unmetabolised (S)-[(11)C]ICI-OMe although (11)C-labelled metabolites rapidly appeared in plasma and liver and [(11)C]CO(2) was detected in exhaled air.

CONCLUSION

Myocardial uptake of (S)-[(11)C]ICI-OMe after i.v. injection was low, possibly due to rapid metabolism in other tissues. Injection of unlabelled ligand or beta-AR antagonists had little effect indicating that binding was mainly to non-specific myocardial sites, thus precluding the use of (S)-[(11)C]ICI-OMe to assess beta(1)-ARs with PET.

Catecholaminergic neurotransmitters regulate migration and repopulation of immature human CD34+ cells through Wnt signaling

Catecholamines are important regulators of homeostasis, yet their functions in hematopoiesis are poorly understood. Here we report that immature human CD34+ cells dynamically expressed dopamine and beta2-adrenergic receptors, with higher expression in the primitive CD34+CD38(lo) population. The myeloid cytokines G-CSF and GM-CSF upregulated neuronal receptor expression on immature CD34+ cells. Treatment with neurotransmitters increased the motility, proliferation and colony formation of human progenitor cells, correlating with increased polarity, expression of the metalloproteinase MT1-MMP and activity of the metalloproteinase MMP-2. Treatment with catecholamines enhanced human CD34+ cell engraftment of NOD-SCID mice through Wnt signaling activation and increased cell mobilization and bone marrow Sca-1+c-Kit+Lin- cell numbers. Our results identify new functions for neurotransmitters and myeloid cytokines in the direct regulation of human and mouse progenitor cell migration and development.

Synthesis and Pharmacological Evaluation of New 1-[3-(4-Arylpiperazin-1-yl)-2-hydroxypropyl]-pyrrolidin-2-one Derivatives with Anti-arrhythmic, Hypotensive, and α-Adrenolytic Activity

A series of novel arylpiperazines bearing a pyrrolidin-2-one fragment was synthesized and evaluated for the binding affinity of the alpha(1)- and alpha(2)-adrenoceptors (AR) and for the antiarrhythmic and hypotensive activities of the compounds. The most potent and selective compound 1-[2-hydroxy-3-[4-[(2-hydroxyphenyl)piperazin-1-yl]propyl]pyrrolidin-2-one 8 binds with pK(i) = 6.71 for alpha(1)-AR. Derivative 8 was also the most active in the prophylactic antiarrhythmic test in adrenaline-induced arrhythmia in anaesthetized rats. Its ED(50 )value equals 1.9 mg/kg (i.v.). Compounds with substituents such as a fluorine atom 4, a methyl 5, or a hydroxyl 8 group, or two substituents such as fluorine/chlorine atoms and methoxy groups in the phenyl ring, significantly decreased the systolic and diastolic pressure in normotensive anesthetized rats at a dosages of 5-10 mg/kg (i.v.). It was found that the presence of the piperazine ring and a hydroxy group in the second position of the propyl chain are critical structural features in determining the affinity of the compounds tested.

Beta-1 and beta-2 adrenoceptor polymorphisms: functional importance, impact on cardiovascular diseases and drug responses

Beta-1 and beta-2 adrenoceptors (AR) play a pivotal role in regulation of the activity of the sympathetic nervous system and agonists and antagonists at both beta AR subtypes are frequently used in treatment of cardiovascular diseases. Both beta-1 and beta-2 AR genes have several polymorphisms that encode different amino acids. This review summarizes new insights into the functional importance of these polymorphisms, as well as their relationship to cardiovascular diseases and their impact on responses to adrenergic drug treatment. At present, it seems that, for cardiovascular diseases, beta-1 and beta-2 AR polymorphisms do not play a role as disease-causing genes; they might, however, be associated with disease-related phenotypes. In addition they could influence adrenergic drug responses. Thus, the Arg389Gly beta-1 AR polymorphism might predict responsiveness to beta-1 AR agonist and blocker treatment: patients homozygous for the Arg389 beta-1 AR polymorphism should be good responders, while patients homozygous for the Gly389 beta-1 AR polymorphism should be poor or nonresponders. Furthermore, the Arg16Gln27 beta-2 AR seems to have strong impact on long-term agonist-induced beta-2 AR desensitization. Thus, patients carrying this haplotype appear to suffer from rapid loss of therapeutic efficacy of chronic agonist treatment, as has been demonstrated in asthma patients. Moreover, the Arg16Gln27 beta-2 AR haplotype might have some predictive value for poor outcome of heart failure. Future large prospective studies have to replicate these findings in order to reach the final goal of pharmacogenomic research: to optimize and individualize drug therapy based on the patient's genetic determinants of drug efficacy.

Pretreatment of male guinea pigs by 17-beta-estradiol induces hypersensitivity of beta-adrenoceptors in electrically driven left atria

BACKGROUND

It is well known that estrogen can modulate distribution and function of adrenergic receptors in the heart of different species. We reported here gender differences in adrenergic responsiveness of electrically driven guinea pig left atria.

METHODS

Experiments were performed on the guinea pigs divided in four groups: males control (MC), males treated by 17-beta-estradiol (MTE), females control (FC) and females treated by tamoxifen (FTT). After two weeks of treatment, the animals were sacrificed, the left atria were isolated and force of contraction (Fc), velocity of contraction (+dF/dt), velocity of relaxation (-dF/dt) and time to peak contraction (ttp) and relaxation time at 10% of amplitude (tt(10) ) were measured.

RESULTS

Apart from significantly lower Fc and longer tt10 in FC (0.97+/-0.12 mN, 233+/-7 ms, respectively) vs. MC (1.66+/-0.3, 176.3+/-18 ms, respectively, n=6, P<0.05), isoprenaline (ISO) and noradrenaline (NOR) (in the presence of prazosine) concentration-response curves were strongly shifted leftward in comparison with male group. Additionally, the maximal effects of. NOR was significantly lower in FC (about 40%) than in MC. Application of 17-beta-estradiol to males and tamoxifen to females guinea pigs confirmed crucial role of estrogen in observed phenomenon.

CONCLUSION

Our results indicate that estrogen not only downregulates beta1-adrenoceptors, but induces its hypersensitivity to catecholamines, at least in guinea pig left atria.

G-protein coupled receptor signaling in myocardium: not for the faint of heart

Catecholamines, endothelin-1 and angiotensin II are among a diverse group of diffusible extracellular signals that regulate pump function of the heart by binding to G-protein coupled receptors (GPCR). When the body demands a temporary boost of power output or if temporary budgeting of resources is required, these signals can adjust heart rate and contractile strength to maintain continuous perfusion of all vascular beds with nutrient- and oxygen-rich blood. Given adequate time in the face of prolonged challenges, activation of GPCRs can also promote "remodeling of the heart" by increasing cell size, organ size, and chamber dimensions, or by varying tissue composition and altering the expression of protein isoforms controlling excitability and contractility. A common feature of heart disease is the state of chronic activation of GPCR signaling systems. Paradoxically, whereas acute activation is beneficial, chronic activation often contributes to further deterioration of cardiac performance. A better understanding of how chronic GPCR activation contributes to the development of heart disease is needed so that it can be translated into better prevention and therapeutic strategies in the clinic.

Regulation and Role of the Presynaptic and Myocardial Na+/H+Exchanger NHE1: Effects on the Sympathetic Nervous System in Heart Failure

In acute myocardial ischemia and in chronic heart failure, sympathetic activation with excessive norepinephrine (NE) release from and reduced NE reuptake into sympathetic nerve endings is a prominent cause of arrhythmias and cardiac dysfunction. The Na(+)/H(+) exchanger NHE1 is the predominant isoform in the heart. It contributes to cellular acid-base balance, and electrolyte, and volume homeostasis, and is activated in response to intracellular acidosis and/or activation of guanine nucleotide binding (G) protein-coupled receptors. NHE1 mediates its signaling via protein kinases A (PKA) or C (PKC). In cardiomyocytes, NHE1 is restricted to specialized membrane domains, where it regulates the activity of pH-sensitive proteins and modulates the driving force of the Na(+)/Ca(2+) exchanger. During acute ischemia/reperfusion and in heart failure the activity/amount of NHE1 is increased, leading to intracellular Ca(2+) overload and promoting structural (apoptosis, hypertrophy) and functional (arrhythmias, hypercontraction) myocardial damage. In sympathetic nerve endings, increased NHE1 activity results in the accumulation of axoplasmic Na(+) that diminishes the inward and/or favors the outward transport of NE via the neuronal norepinephrine transporter (NET). The increased NE levels within the nerve-muscle junction facilitate the sustained stimulation of myocardial alpha- and beta-adrenoceptors (ARs), which in turn aggravate the increases in myocardial NHE1 activity and the associated deleterious effects. Furthermore, the responsiveness of the beta-AR declines overtime, which results in further release of NE, initiating a vicious cycle. Accordingly, NHE1 is a potential candidate for targeted intervention to suppress this feedback loop.

Activation of inwardly rectifying Kir2.x potassium channels by beta 3-adrenoceptors is mediated via different signaling pathways with a predominant role of PKC for Kir2.1 and of PKA for Kir2.2

beta(3)-adrenoceptors have recently been shown to induce a complex modulation of intracellular signaling pathways including cyclic guanine monophosphate, cyclic adenosine monophosphate, nitric oxide, and protein kinases A and C. They are expressed in a broad variety of tissues including the myocardium, vascular smooth muscle, and endothelium. In those tissues, resting membrane potential is controlled mainly by inwardly rectifying potassium channels of the Kir2 family namely, Kir2.1 in the vascular smooth muscle, Kir2.1-2.3 in the myocardium, and Kir2.1-2.2 in the endothelium. In the present study, we investigated the possible modulation of Kir2 channel function by beta(3)-adrenoceptors in an expression system. Human-cloned beta(3)-adrenoceptors and Kir2.1 (KCNJ2), Kir2.2 (KCNJ12), and Kir2.3 (KCNJ4) channels were coexpressed in Xenopus oocytes, and currents were measured with double-microelectrode voltage clamp. Activation of beta(3)-adrenoceptors with isoproterenol resulted in markedly increased currents in Kir2.1 and in Kir2.2 potassium channels with EC50 values of 27 and 18 nM, respectively. In contrast, Kir2.3 currents were not modulated. Coapplication of specific inhibitors of protein kinase A (KT-5720) and calmodulin kinase II (KN-93) had no effects on the observed regulation in Kir2.1. However, coapplication of protein kinase C (PKC) inhibitors staurosporine and chelerythrine suppressed the observed effect. In Kir2.2, coapplication of KT-5720 reduced the effect of beta(3)-adrenoceptor activation. No differences in current increase after application of isoproterenol were observed between mutant Kir2.2 potassium channels lacking all functional PKC phosphorylation sites and Kir2.2 wild-type channels. In heteromeric Kir2.x channels, all types of heteromers were activated. The effect was most pronounced in Kir2.1/Kir2.2 and in Kir2.2/Kir2.3 channels. In summary, homomeric and heteromeric Kir2.x channels are activated by beta(3)-adrenoceptors via different protein kinase-dependent pathways: Kir2.1 subunits are modulated by PKC, whereas Kir2.2 is modulated by protein kinase A. In heteromeric composition, a marked activation of currents can be observed particularly with involvement of Kir2.2 subunits. This regulation may contribute to the hyperpolarizing effects of beta(3)-adrenoceptors in tissues that exhibit modulation by Kir2 channel function.

Beta-adrenergic receptor antagonism preserves myocardial function after brain death in a porcine model

BACKGROUND

Cardiac dysfunction after brain death decreases the already limited number of potential donors for cardiac transplantation. Acute beta-adrenergic receptor (betaAR) desensitization after the brain death-associated catecholamine surge is an important mechanism. We hypothesized that acute betaAR antagonism could improve myocardial function after brain death by preserving betaAR signaling.

METHODS

Pigs were randomly assigned to three study groups (n = 5): sham; brain death; and brain death with betaAR antagonist (200 microg/kg/min esmolol), 30 minutes before brain death until 45 minutes after brain death. Functional data were collected for 6 hours after brain death and tissues procured.

RESULTS

Compared with baseline, pre-load recruitable stroke work (PRSW), a pre-load-independent measure of systolic function (21.4 +/- 7.5 vs 43.3 +/- 6.8, slope of regression line during vena caval occlusion, p < 0.001), diastolic function (Tau, 101 +/- 54.7 vs 36.4 +/- 5.4 ms, p = 0.03) and systemic oxygen delivery (151 +/- 79.7 vs 298 +/- 78.7 ml/min, p < 0.001) deteriorated in untreated animals at 6 hours after brain death. In contrast, betaAR antagonist maintained baseline systolic function (PRSW, 37.8 +/- 5.6 vs 38.2 +/- 4.7, slope of regression line during vena caval occlusion, p = 0.92), diastolic function (Tau, 32.6 +/- 5.1 vs 48.5 +/- 28.3 ms, p = 0.57) and oxygen delivery (427 +/- 116 vs 397 +/- 98.8 ml/min, p = 0.36) at 6 hours after brain death. betaAR antagonist preserved betaAR signaling, as demonstrated by similar left ventricular (LV) basal (55.4 +/- 32.8 vs 58.8 +/- 10.9 pmol/mg/min, p = 0.40) and isoproterenol-stimulated (125 +/- 70.5 vs 124 +/- 52.0 pmol/mg/min, p = 0.49) adenylate cyclase activity at 6 hours after brain death, upon comparing betaAR antagonist and sham treatment groups. Both LV basal and isoproterenol-stimulated adenyl cyclase activity were higher with betaAR antagonist (25.9 +/- 4.8 pmol/mg/min, p = 0.03) than with untreated brain death (55.6 +/- 17.3 pmol/mg/min, p = 0.02).

CONCLUSIONS

Beta-adrenergic receptor antagonism before brain death preserves cardiac function by preventing betaAR desensitization. This therapy in potential donors might increase the number of organs available for transplantation.

Beta3-adrenoceptor in the eel (Anguilla anguilla) heart: negative inotropy and NO-cGMP-dependent mechanism

Neuroendocrine regulation of cardiac function involves a population of three types of beta-adrenoceptors (ARs). In various mammalian species, beta1- and beta2-AR stimulation produces an increase in contractility; whereas beta3-AR activation mediates negative inotropic effects. At the moment, nothing is known about the physiological role of beta3-AR in fish. Using an isolated working heart preparation, we show that a beta3-AR selective agonist BRL(37344) (0.1-100 nmol l(-1)) elicits a dose-dependent negative inotropism in the freshwater eel Anguilla anguilla. This effect was insensitive to the beta1/beta2-AR inhibitor nadolol (10 mumol l(-1)), but was blocked by the beta3-AR-specific antagonist SR(59230) (10 nmol l(-1)). The analysis of the percentage of stroke work (SW) variations, in terms of EC(50) values, induced by BRL(37344) alone (10 nmol l(-1)), and in presence of SR(59230) (10 nmol l(-1)), indicated a competitive antagonism of SR(59230). In addition to the classic positive inotropism, the non-specific beta agonist isoproterenol (100 nmol l(-1)) induced, in 30% of the preparations, a negative inotropic effect that was abrogated by pre-treatment with SR(59230), pointing to a beta3-mediated pathway. The BRL(37344)-induced negative inotropic effect was abolished by exposure to a G(i/o) proteins inhibitor pertussis toxin (PTx; 0.01 nmol l(-1)), suggesting a G(i/o)-dependent mechanism. Using L-N5(l-imino-ethyl)ornithine (L-NIO; 10 mumol l(-1)), as a nitric oxide (NO) synthase (NOS) blocker and haemoglobin (Hb; 1 mumol l(-1)), as a NO scavenger, we demonstrated that NO signalling is involved in the BRL(37344)-induced response. Pre-treatment with either an inhibitor of soluble guanylate cyclase (GC) 1H-(1,2,4) oxadiazolo-(4,3-a)quinoxalin-1-one (ODQ; 10 mumol l(-1)), or an inhibitor of the cGMP-activated protein kinase (PKG) KT(5823) (100 nmol l(-1)), abolished the beta3-dependent negative inotropism, indicating the cGMP-PKG component as a crucial target of NO signalling. Taken together, our findings provide functional evidence for the presence of beta3-like adrenoceptors in the eel Anguilla anguilla heart identifying, for the first time in a working fish heart, the beta3-AR-dependent negative inotropy discovered in mammals.

β-adrenoceptor blocker treatment and the cardiac β-adrenoceptor-G-protein(s)-adenylyl cyclase system in chronic heart failure

Recent studies showed that chronic beta-adrenoceptor (AR) blocker treatment exerts beneficial effects in patients with chronic heart failure (CHF). In CHF, sympathetic drive to the heart is increased, and this causes pathological changes in cardiac beta-AR-G-protein(s)-adenylyl cyclase system: Cardiac beta-1 AR are decreased, and amount and activity of cardiac G(i)-protein and G-protein-coupled receptor kinase (GRK) are increased resulting in diminished cardiac beta-AR functional responsiveness. One possible mechanism of beneficial effects of beta-AR blockers could be that they prevent adverse effects of increased sympathetic activity and up-regulate cardiac (and vascular) beta-AR density, and by this, enhance beta-AR-mediated effects. Another possibility could be that chronic beta-AR blocker treatment normalizes activity of G(i)-protein and may thereby restore beta-AR functional responsiveness. Moreover, failing human heart exhibits an inverse force-frequency relationship. beta-AR blockers reduce heart rate; this may, therefore, improve force of contraction. One of the strongest stimuli to activate GRK is increased sympathetic activity (as in CHF) via beta-AR stimulation. beta-AR blockers, by blocking beta-AR, can prevent GRK activation and/or can reduce the (previously enhanced) GRK activity, and this might-at least partly-contribute to beneficial effects of beta-AR blockers in CHF treatment. Finally, the "loss-of-function" Arg389Gly beta-1 AR polymorphism seems to determine heart rate and blood pressure responses to beta-1 AR blocker administration: Arg389Arg beta-1 AR subjects exhibit stronger effects than subjects with one or two Gly389 alleles. Thus, it might be predicted that patients homozygous Arg389 beta-1 AR should be good responders, whereas patients homozygous Gly389 beta-1 AR polymorphism should be poor or non-responders.

Mechanisms of regulation and function of G-protein-coupled receptor kinases

G-protein-coupled receptor kinases (GRKs) interact with the agonist-activated form of G-protein-coupled receptor (GPCR) to affect receptor phosphorylation and to initiate profound impairment of receptor signaling, or desensitization. GPCR forms the largest family of cell surface receptors, and defects in GRK function have the potential consequence to affect GPCR-stimulated biological responses in many pathological situations.