Cardiac betaARK1 upregulation induced by chronic salt deprivation in rats

Antidiabetic and Cardioprotective Effects of Pharmacological Inhibition of GRK2 in db/db Mice

Despite the availability of several therapies for the management of blood glucose in diabetic patients, most of the treatments do not show benefits on diabetic cardiomyopathy, while others even favor the progression of the disease. New pharmacological targets are needed that might help the management of diabetes and its cardiovascular complications at the same time. GRK2 appears a promising target, given its established role in insulin resistance and in systolic heart failure. Using a custom peptide inhibitor of GRK2, we assessed in vitro in L6 myoblasts the effects of GRK2 inhibition on glucose extraction and insulin signaling. Afterwards, we treated diabetic male mice (db/db) for 2 weeks. Glucose tolerance (IGTT) and insulin sensitivity (ITT) were ameliorated, as was skeletal muscle glucose uptake and insulin signaling. In the heart, at the same time, the GRK2 inhibitor ameliorated inflammatory and cytokine responses, reduced oxidative stress, and corrected patterns of fetal gene expression, typical of diabetic cardiomyopathy. GRK2 inhibition represents a promising therapeutic target for diabetes and its cardiovascular complications.

β-Adrenergic Receptor and Insulin Resistance in the Heart

Insulin resistance is characterized by the reduced ability of insulin to stimulate tissue uptake and disposal of glucose including cardiac muscle. These conditions accelerate the progression of heart failure and increase cardiovascular morbidity and mortality in patients with cardiovascular diseases. It is noteworthy that some conditions of insulin resistance are characterized by up-regulation of the sympathetic nervous system, resulting in enhanced stimulation of β-adrenergic receptor (βAR). Overstimulation of βARs leads to the development of heart failure and is associated with the pathogenesis of insulin resistance in the heart. However, pathological consequences of the cross-talk between the βAR and the insulin sensitivity and the mechanism by which βAR overstimulation promotes insulin resistance remain unclear. This review article examines the hypothesis that βARs overstimulation leads to induction of insulin resistance in the heart.

Sustained βAR Stimulation Mediates Cardiac Insulin Resistance in a PKA-Dependent Manner

Insulin resistance is a condition in which cells are defective in response to the actions of insulin in tissue glucose uptake. Overstimulation of β-adrenergic receptors (βARs) leads to the development of heart failure and is associated with the pathogenesis of insulin resistance in the heart. However, the mechanisms by which sustained βAR stimulation affects insulin resistance in the heart are incompletely understood. In this study, we demonstrate that sustained βAR stimulation resulted in the inhibition of insulin-induced glucose uptake, and a reduction of insulin induced glucose transporter (GLUT)4 expression that were mediated by the β2AR subtype in cardiomyocytes and heart tissue. Overstimulation of β2AR inhibited the insulin-induced translocation of GLUT4 to the plasma membrane of cardiomyocytes. Additionally, βAR mediated cardiac insulin resistance by reducing glucose uptake and GLUT4 expression via the cAMP-dependent and protein kinase A-dependent pathways. Treatment with β-blockers, including propranolol and metoprolol antagonized isoproterenol-mediated insulin resistance in the heart. The data in this present study confirm a critical role for protein kinase A in βAR-mediated insulin resistance.

β2-Adrenergic receptor-dependent attenuation of hypoxic pulmonary vasoconstriction prevents progression of pulmonary arterial hypertension in intermittent hypoxic rats

In sleep apnea syndrome (SAS), intermittent hypoxia (IH) induces repeated episodes of hypoxic pulmonary vasoconstriction (HPV) during sleep, which presumably contribute to pulmonary arterial hypertension (PAH). However, the prevalence of PAH was low and severity is mostly mild in SAS patients, and mild or no right ventricular hypertrophy (RVH) was reported in IH-exposed animals. The question then arises as to why PAH is not a universal finding in SAS if repeated hypoxia of sufficient duration causes cycling HPV. In the present study, rats underwent IH at a rate of 3 min cycles of 4-21% O2 for 8 h/d for 6 w. Assessment of diameter changes in small pulmonary arteries in response to acute hypoxia and drugs were performed using synchrotron radiation microangiography on anesthetized rats. In IH-rats, neither PAH nor RVH was observed and HPV was strongly reversed. Nadolol (a hydrophilic β(1, 2)-blocker) augmented the attenuated HPV to almost the same level as that in N-rats, but atenolol (a hydrophilic β1-blocker) had no effect on the HPV in IH. These β-blockers had almost no effect on the HPV in N-rats. Chronic administration of nadolol during 6 weeks of IH exposure induced PAH and RVH in IH-rats, but did not in N-rats. Meanwhile, atenolol had no effect on morphometric and hemodynamic changes in N and IH-rats. Protein expression of the β1-adrenergic receptor (AR) was down-regulated while that of β2AR was preserved in pulmonary arteries of IH-rats. Phosphorylation of p85 (chief component of phosphoinositide 3-kinase (PI3K)), protein kinase B (Akt), and endothelial nitric oxide synthase (eNOS) were abrogated by chronic administration of nadolol in the lung tissue of IH-rats. We conclude that IH-derived activation of β2AR in the pulmonary arteries attenuates the HPV, thereby preventing progression of IH-induced PAH. This protective effect may depend on the β2AR-Gi mediated PI3K/Akt/eNOS signaling pathway.

Aldosterone augments Na+-induced reduction of cardiac norepinephrine reuptake

Impairment of the cardiac norepinephrine (NE) reuptake by the neuronal NE transporter contributes to enhanced cardiac NE net release in congestive heart failure. Elevated plasma levels of aldosterone (AL) promote sympathetic overstimulation in failing hearts by unclear mechanisms. Our aim was to evaluate if elevated AL and/or alterations in Na(+) intake regulate cardiac NE reuptake. To test the effects of AL and Na(+) on cardiac NE reuptake, Wistar rats were fed a normal-salt (NS) diet (0.2% NaCl), a low-salt (LS) diet (0.015% NaCl), or a high-salt (HS) diet (8% NaCl). Another group of animals received AL infusion alone (0.75 μg/h) or AL infusion plus HS diet. Specific cardiac [(3)H]NE uptake via the NE transporter in a Langendorff preparation and AL plasma levels were measured at different time points between 5 and 42 days of treatment. To compare these findings from healthy animals with a disease model, Dahl salt-sensitive rats were investigated as a model of congestive heart failure with endogenously elevated AL. In summary, neither exogenous nor endogenous elevations of AL alone were sufficient to reduce cardiac NE reuptake. Only the HS diet induced a reduction of NE reuptake by 26%; additional infusion of AL augmented this effect to a further reduction of NE reuptake by 36%. In concordance, Dahl salt-sensitive rats treated with a HS diet displayed elevated AL and a marked reduction of NE reuptake. We conclude that exogenous or endogenous AL elevations alone do not reduce cardiac NE reuptake, but AL serves as an additional factor that negatively regulates cardiac NE reuptake in concert with HS intake.

Design, synthesis and efficacy of novel G protein-coupled receptor kinase 2 inhibitors

G protein-coupled receptor kinase 2 (GRK2) is a relevant signaling node of the cellular transduction network, playing major roles in the physiology of various organs/tissues including the heart and blood vessels. Emerging evidence suggests that GRK2 is up regulated in pathological situations such as heart failure, hypertrophy and hypertension, and its inhibition offers a potential therapeutic solution to these diseases. We explored the GRK2 inhibitory activity of a library of cyclic peptides derived from the HJ loop of G protein-coupled receptor kinases 2 (GRK2). The design of these cyclic compounds was based on the conformation of the HJ loop within the X-ray structure of GRK2. One of these compounds, the cyclic peptide 7, inhibited potently and selectively the GRK2 activity, being more active than its linear precursor. In a cellular system, this peptide confirms the beneficial signaling properties of a potent GRK2 inhibitor. Preferred conformations of the most potent analog were investigated by NMR spectroscopy.

GRK2 levels in umbilical arteries of pregnancies complicated by gestational hypertension and preeclampsia

BACKGROUND

G-Protein coupled receptor kinase 2 (GRK2) represents a regulator of cell function in different cardiovascular conditions, including high blood pressure. The relationship between elevated GRK2 levels and impaired vasorelaxant responses is causative of hypertension through the increase in vascular resistances. The aim of this study is to ascertain if this feature is present in the fetal placental vasculature of pregnancies complicated by hypertensive disorders.

METHODS

We have assessed GRK2 levels in the umbilical arteries (UA) of 21 preeclamptic or gestational hypertensive and 23 normotensive women at time of delivery.

RESULTS

GRK2 levels were increased in the hypertensive group (0.83 ± 0.14 vs. 0.48 ± 0.06 densitometry units; P < 0.05). GRK2 levels were in correlation and in linear regression with systolic, diastolic, and mean arterial pressure (P < 0.05, r(2) = 0.12, r(2) = 0.11, r(2) = 0.12). Correlations did not reach a significant value for other clinical parameters such as gestational age at birth, umbilical artery pulsatility index, maternal proteinuria, and neonatal birth weight. Out of the 21 hypertensive women, 7 who developed a preeclampsia associated with early preterm delivery (before 34 weeks) had a significantly lower GRK2 levels compared to the remaining 14 (0.51 ± 0.12 vs. 1.08 ± 0.20 densitometry units, P < 0.05).

CONCLUSIONS

We conclude that elevated GRK2 levels in the umbilical vasculature is correlated to elevated blood pressure levels, with a likely compensatory rather than causative role since the lack of protective effect of elevated GRK2 levels may negatively affect the outcome of the hypertensive state.

Impaired neoangiogenesis in β₂-adrenoceptor gene-deficient mice: restoration by intravascular human β₂-adrenoceptor gene transfer and role of NFκB and CREB transcription factors

BACKGROUND AND PURPOSE

There is much evidence supporting the role of β₂-adrenoceptors (β₂AR) in angiogenesis but the mechanisms underlying their effects have not been elucidated. Hence, we studied post-ischaemic angiogenesis in the hindlimb (HL) of β₂AR knock-out mice (β₂AR-/-) in vivo and explored possible molecular mechanisms in vitro.

EXPERIMENTAL APPROACH

Femoral artery resection (FAR) was performed in wild-type and β₂AR-/- mice and adaptive responses to chronic HL ischaemia were explored; blood flow was measured by ultrasound and perfusion of dyed beads, bone rarefaction, muscle fibrosis and skin thickness were evaluated by immunoflourescence and morphometric analysis. Intrafemoral delivery of an adenovirus encoding the human β₂AR (ADβ₂AR) was used to reinstate β₂ARs in β₂AR-/- mice. Molecular mechanisms were investigated in mouse-derived aortic endothelial cells (EC) in vitro, focusing on NFκB activation and transcriptional activity.

RESULTS

Angiogenesis was severely impaired in β₂AR-/- mice subjected to FAR, but was restored by gene therapy with ADβ₂AR. The proangiogenic responses to a variety of stimuli were impaired in β₂AR-/- EC in vitro. Moreover, removal of β₂ARs impaired the activation of NFκB, a transcription factor that promotes angiogenesis; neither isoprenaline (stimulates βARs) nor TNFα induced NFκB activation in β₂AR(-/-) EC. Interestingly, cAMP response element binding protein (CREB), a transcription factor that counter regulates NFκB, was constitutively increased in β₂AR(-/-) ECs. ADβ₂AR administration restored β₂AR membrane density, reduced CREB activity and reinstated the NFκB response to isoprenaline and TNFα.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that β₂ARs control angiogenesis through the tight regulation of nuclear transcriptional activity.

G alpha(q)-mediated activation of GRK2 by mechanical stretch in cardiac myocytes: the role of protein kinase C

G protein-coupled receptor kinase-2 (GRK2) is a critical regulator of beta-adrenergic receptor (beta-AR) signaling and cardiac function. We studied the effects of mechanical stretch, a potent stimulus for cardiac myocyte hypertrophy, on GRK2 activity and beta-AR signaling. To eliminate neurohormonal influences, neonatal rat ventricular myocytes were subjected to cyclical equi-biaxial stretch. A hypertrophic response was confirmed by "fetal" gene up-regulation. GRK2 activity in cardiac myocytes was increased 4.2-fold at 48 h of stretch versus unstretched controls. Adenylyl cyclase activity was blunted in sarcolemmal membranes after stretch, demonstrating beta-AR desensitization. The hypertrophic response to mechanical stretch is mediated primarily through the G alpha(q)-coupled angiotensin II AT(1) receptor leading to activation of protein kinase C (PKC). PKC is known to phosphorylate GRK2 at the N-terminal serine 29 residue, leading to kinase activation. Overexpression of a mini-gene that inhibits receptor-G alpha(q) coupling blunted stretch-induced hypertrophy and GRK2 activation. Short hairpin RNA-mediated knockdown of PKC alpha also significantly attenuated stretch-induced GRK2 activation. Overexpression of a GRK2 mutant (S29A) in cardiac myocytes inhibited phosphorylation of GRK2 by PKC, abolished stretch-induced GRK2 activation, and restored adenylyl cyclase activity. Cardiac-specific activation of PKC alpha in transgenic mice led to impaired beta-agonist-stimulated ventricular function, blunted cyclase activity, and increased GRK2 phosphorylation and activity. Phosphorylation of GRK2 by PKC appears to be the primary mechanism of increased GRK2 activity and impaired beta-AR signaling after mechanical stretch. Cross-talk between hypertrophic signaling at the level of PKC and beta-AR signaling regulated by GRK2 may be an important mechanism in the transition from compensatory ventricular hypertrophy to heart failure.

G-protein-coupled receptor kinases in cardiovascular conditions: focus on G-protein-coupled receptor kinase 2, a gain in translational medicine

With increasing knowledge of the regulatory mechanisms of G-protein-coupled receptor signaling in heart physiology, many studies have focused on the role of this system in cardiovascular disease. In recent years, scientists have moved their attention from the receptors to their regulatory proteins: the G-protein-coupled receptor kinases. This class of protein is indispensable for terminating signaling of G-protein-coupled receptors through receptor desensitization and downregulation. This article attempts to assemble the currently available information regarding G-protein-coupled receptor kinases and their role in cardiovascular disease and, in particular, the potential employment of G-protein-coupled receptor kinase 2 as biomarker of cardiac dysfunction.

The G protein coupled receptor kinase 2 plays an essential role in beta-adrenergic receptor-induced insulin resistance

AIMS

Insulin (Ins) resistance (IRES) associates to increased cardiovascular risk as observed in metabolic syndrome. Chronic stimulation of beta-adrenergic receptors (betaAR) due to exaggerated sympathetic nervous system activity is involved in the pathogenesis of IRES. The cellular levels of G protein coupled receptor kinase 2 (GRK2) increase during chronic betaAR stimulation, leading to betaAR desensitization. We tested the hypothesis that GRK2 plays a role in betaAR-induced IRES.

METHODS AND RESULTS

We evaluated Ins-induced glucose uptake and signalling responses in vitro in cell overexpressing the beta(2)AR, the GRK2, or the catalytically dead mutant GRK2-DN. In a model of increased adrenergic activity, IRES and elevated cellular GRK2 levels, the spontaneously hypertensive rats (SHR) we performed the intravenous glucose tolerance test load. To inhibit GRK2, we synthesized a peptide based on the catalytical sequence of GRK2 conjugated with the antennapedia internalization sequence (Ant-124). Ins in human kidney embryonic (HEK-293) cells causes rapid accumulation of GRK2, tyrosine phosphorylation of Ins receptor substrate 1 (IRS1) and induces glucose uptake. In the same cell type, transgenic beta(2)AR overexpression causes GRK2 accumulation associated with significant deficit of IRS1 activation and glucose uptake by Ins. Similarly, transgenic GRK2 overexpression prevents Ins-induced tyrosine phosphorylation of IRS1 and glucose uptake, whereas GRK2-DN ameliorates glucose extraction. By immunoprecipitation, GRK2 binds IRS1 but not the Ins receptor in an Ins-dependent fashion, which is lost in HEK-GRK2 cells. Ant-124 improves Ins-induced glucose uptake in HEK-293 and HEK-GRK2 cells, but does not prevent GRK2/IRS1 interaction. In SHR, Ant-124 infusion for 30 days ameliorates IRES and IRS1 tyrosine phosphorylation.

CONCLUSION

Our results suggest that GRK2 mediates adrenergic IRES and that inhibition of GRK2 activity leads to increased Ins sensitivity both in cells and in animal model of IRES.

Endothelial alpha1-adrenoceptors regulate neo-angiogenesis

BACKGROUND AND PURPOSE

Intact endothelium plays a pivotal role in post-ischaemic angiogenesis. It is a phenomenon finely tuned by activation and inhibition of several endothelial receptors. The presence of alpha(1)-adrenoceptors on the endothelium suggests that these receptors may participate in regenerative phenomena by regulating the responses of endothelial cells involved in neo-angiogenesis.

EXPERIMENTAL APPROACH

We evaluated the expression of the subtypes of the alpha(1)-adrenoceptor in isolated endothelial cells harvested from Wistar-Kyoto (WKY) rats. We explored the possibility these alpha(1)-adrenoceptors may influence the pro-angiogenic phenotype of endothelial cells in vitro. In vivo, we used a model of hindlimb ischaemia in WKY rats, to assess the effects of alpha(1) adrenoceptor agonist or antagonist on angiogenesis in the ischaemic hindlimb by laser Doppler blood flow measurements, digital angiographies, hindlimb perfusion with dyed beads and histological evaluation.

KEY RESULTS

In vitro, pharmacological antagonism of alpha(1)-adrenoceptors in endothelial cells from WKY rats by doxazosin enhanced, while stimulation of these adrenoceptors with phenylephrine, inhibited endothelial cell proliferation and DNA synthesis, ERK and retinoblastoma protein (Rb) phosphorylation, cell migration and tubule formation. In vivo, we found increased alpha(1)-adrenoceptor density in the ischaemic hindlimb, compared to non-ischaemic hindlimb, suggesting an enhanced alpha(1)-adrenoceptor tone in the ischaemic tissue. Treatment with doxazosin (0.06 mg kg(-1) day(-1) for 14 days) did not alter systemic blood pressure but enhanced neo-angiogenesis in the ischaemic hindlimb, as measured by all our assays.

CONCLUSIONS

Our findings support the hypothesis that the alpha(1)-adrenoceptors in endothelial cells provide a negative regulation of angiogenesis.

Kinase inhibitors for cardiovascular disease

Over the last decade, there has been substantial progress toward understanding the pathophysiology and treatment of cardiovascular diseases (CVDs). Elucidating cellular responses to the extracellular environment and signal transduction mechanisms have provided the opportunity to explore novel molecular therapeutic approaches for the treatment of CVDs. Neurohormonal stimulation has been implicated in these diseases; blockade of the renin-angiotensin and beta-adrenergic systems are examples of therapeutic effectiveness. There are multiple cell signaling cascades, some of which are beneficial or compensatory and others deleterious. The balance between these pathways, which in large part is dictated by the cellular environment, determines the outcome as a diseased or non-diseased state. Selective targeting of signaling pathways using protein kinase inhibitors, would have a potential advantage over receptor blockers. We review potential protein kinase targets and recent evidence supporting therapeutic interventional value in CVDs.

Adenoviral gene transfer of Akt enhances myocardial contractility and intracellular calcium handling

The serine-threonine kinase Akt/PKB mediates stimuli from different classes of cardiomyocyte receptors, including the growth hormone/insulin like growth factor and the beta-adrenergic receptors. Whereas the growth-promoting and antiapoptotic properties of Akt activation are well established, little is known about the effects of Akt on myocardial contractility, intracellular calcium (Ca(2+)) handling, oxygen consumption, and beta-adrenergic pathway. To this aim, Sprague-Dawley rats were subjected to a wild-type Akt in vivo adenoviral gene transfer using a catheter-based technique combined with aortopulmonary crossclamping. Left ventricular (LV) contractility and intracellular Ca(2+) handling were evaluated in an isolated isovolumic buffer-perfused, aequorin-loaded whole heart preparations 10 days after the surgery. The Ca(2+)-force relationship was obtained under steady-state conditions in tetanized muscles. No significant hypertrophy was detected in adenovirus with wild-type Akt (Ad.Akt) versus controls rats (LV-to-body weight ratio 2.6+/-0.2 versus 2.7+/-0.1 mg/g, controls versus Ad.Akt, P, NS). LV contractility, measured as developed pressure, increased by 41% in Ad.Akt. This was accounted for by both more systolic Ca(2+) available to the contractile machinery (+19% versus controls) and by enhanced myofilament Ca(2+) responsiveness, documented by an increased maximal Ca(2+)-activated pressure (+19% versus controls) and a shift to the left of the Ca(2+)-force relationship. Such increased contractility was paralleled by a slight increase of myocardial oxygen consumption (14%), while titrated dose of dobutamine providing similar inotropic effect augmented oxygen consumption by 39% (P<0.01). Phospholamban, calsequestrin, and ryanodine receptor LV mRNA and protein content were not different among the study groups, while sarcoplasmic reticulum Ca(2+) ATPase protein levels were significantly increased in Ad.Akt rats. beta-Adrenergic receptor density, affinity, kinase-1 levels, and adenylyl cyclase activity were similar in the three animal groups. In conclusion, our results support an important role for Akt/PKB in the regulation of myocardial contractility and mechanoenergetics.

Elevated myocardial and lymphocyte GRK2 expression and activity in human heart failure

AIMS

The G protein-coupled receptor kinase-2 (GRK2 or beta-ARK1) regulates beta-adrenergic receptors (beta-ARs) in the heart, and its cardiac expression is elevated in human heart failure (HF). We sought to determine whether myocardial levels and activity of GRK2 could be monitored using white blood cells, which have been used to study cardiac beta-ARs. Moreover, we were interested in determining whether GRK2 levels in myocardium and lymphocytes may be associated with beta-AR dysfunction and HF severity.

METHODS AND RESULTS

In myocardial biopsies from explanted failing human hearts, GRK activity was inversely correlated with beta-AR-mediated cAMP production (R(2)=-0.215, P<0.05, n=24). Multiple regression analysis confirmed that GRK activity participates with beta-AR density to regulate catecholamine-sensitive cAMP responses. Importantly, there was a direct correlation between myocardial and lymphocytes GRK2 activity (R(2)=0.5686, P<0.05, n=10). Lymphocyte GRK activity was assessed in HF patients with various ejection fractions (EFs) (n=33), and kinase activity was significantly higher in patients with lower EFs and was higher with increasing NYHA class (P<0.001).

CONCLUSION

Myocardial GRK2 expression and activity are mirrored by lymphocyte levels of this kinase, and its elevation in HF is associated with the loss of beta-AR responsiveness and appears to increase with disease severity. Therefore, lymphocytes may provide a surrogate for monitoring cardiac GRK2 in human HF.

Hypertensive left ventricular hypertrophy: relation to beta-adrenergic receptor kinase-1 (betaARK1) in peripheral lymphocytes

BACKGROUND

Left ventricular hypertrophy (LVH) is associated with increased cardiovascular risk and altered sympathetic regulation in hypertension.

OBJECTIVES

To determine whether the level of beta-adrenergic receptor kinase-1 (betaARK1) in lymphocytes is related to LVH in patients with hypertension.

METHODS

Forty-nine patients with untreated essential hypertension were recruited to the study and classified into two groups: left ventricular hypertrophy (LVH: left ventricular mass index > or =134 g/m in men and > or =110 g/m in women; ages 52.4 +/- 12.8 years, n = 25) and non-LVH (NLVH: left ventricular mass index < 134 g/m in men and < 110 g/m in women; ages 50.8 +/- 13.1 years, n = 24). Lymphocytes were isolated from patients and quantitative-competitive reverse transcriptase-polymerase chain reaction (RT-PCR) and immunoblotting were used to estimate the expression of betaARK1 in the lymphocytes. G-protein-coupled receptor kinase activity was assessed by rhodopsin phosphorylation assay.

RESULTS

The expression of betaARK1 in lymphocytes was greater in the LVH group than in the NLVH group (0.0069 +/- 0.002 ng compared with 0.0048 +/- 0.0017 ng, P < 0.01) and correlated well with left ventricular mass index (r = 0.527, P < 0.001) and relative wall thickness (r = 0.627, P < 0.001). The concentration of betaARK1 protein in lymphocytes from individuals with LVH was increased two-fold compared with that in the NLVH group (both n = 7). Lymphocyte G-protein-coupled receptor kinase activity from LVH was enhanced 1.7-fold compared with NLVH (1.03 +/- 2.16 and 1.79 +/- 1.87 pmol phosphate/min per mg protein, respectively; P < 0.05, n = 7 for each group).

CONCLUSIONS

The concentration of betaARK1 in lymphocytes is greater in hypertensive individuals with LVH than in those without LVH and parallels the degree of hypertrophy. Generalized alterations in beta-adrenergic signalling, including betaARK1, could be a major contributory factor in the development of LVH in hypertension, and the concentration of betaARK1 in lymphocytes can reflect the development of LVH in a patient with hypertension.

Transgenic mice targeting the heart unveil G protein-coupled receptor kinases as therapeutic targets

GRKs critically regulate betaAR signaling via receptor phosphorylation and the triggering of desensitization. In the heart, betaARs control the chronotropic, lusitropic, and inotropic responses to the catecholamine neurotransmitters, norepinephrine and epinephrine. Signaling through cardiac betaARs is significantly impaired in many cardiovascular disorders, including congestive heart failure. betaARK1 (also known as GRK2) is the most abundant GRK in the heart, and it is increased in several cardiovascular diseases associated with impaired cardiac signaling and function, suggesting that this molecule could have pathophysiological relevance in the setting of heart failure. The ability to manipulate the mouse genome has provided a powerful tool to study the physiological implications of altering GRK activity and expression in the heart. Recent studies in several different mouse models have demonstrated that betaARK1 plays a key role not only in the regulation of myocardial signaling, but also in cardiac function and development. Moreover, studies have shown that targeting the activity of GRKs, especially betaARK1, appears to be a novel therapeutic strategy for the treatment of the failing heart. Gene therapy technology makes it possible, beyond what is possible in the mouse, to directly test in larger animals whether betaARK1 inhibition in the setting of disease will improve the function of the compromised heart, and this methodology has also lead to compelling results. These genetic approaches or the development of small molecule inhibitors of betaARK1 and GRK activity may advance therapeutic options for heart disease.

Exercise restores beta-adrenergic vasorelaxation in aged rat carotid arteries

Aging is associated with alterations in beta-adrenergic receptor (beta-AR) signaling and reduction in cardiovascular responses to beta-AR stimulation. Because exercise can attenuate age-related impairment in myocardial beta-AR signaling and function, we tested whether training could also exert favorable effects on vascular beta-AR responses. We evaluated common carotid artery responsiveness in isolated vessel ring preparations from 8 aged male Wistar-Kyoto (WKY) rats trained for 6 wk in a 5 days/wk swimming protocol, 10 untrained age-matched rats, and 10 young WKY rats. Vessels were preconstricted with phenylephrine (10-6 M), and vasodilation was assessed in response to the beta-AR agonist isoproterenol (10-10-3 x 10-8 M), the alpha2-AR agonist UK-14304 (10-9-10-6 M), the muscarinic receptor agonist ACh (10-9-10-6 M), and nitroprusside (10-8-10-5 M). beta-AR density and cytoplasmic beta-AR kinase (beta-ARK) activity were tested on pooled carotid arteries. beta-ARK expression was assessed in two endothelial cell lines from bovine aorta and aorta isolated from a 12-wk WKY rat. beta-AR, alpha2-AR, and muscarinic responses, but not that to nitroprusside, were depressed in untrained aged vs. young animals. Exercise training restored beta-AR and muscarinic responses but did not affect vasodilation induced by UK-14304 and nitroprusside. Aged carotid arteries showed reduced beta-AR number and increased beta-ARK activity. Training counterbalanced these phenomena and restored beta-AR density and beta-ARK activity to levels observed in young rat carotids. Our data indicate that age impairs beta-AR vasorelaxation in rat carotid arteries through beta-AR downregulation and desensitization. Exercise restores this response and reverts age-related modification in beta-ARs and beta-ARK. Our data support an important role for beta-ARK in vascular beta-AR vasorelaxation.