A novel 3D imaging approach for quantification of GLUT4 levels across the intact myocardium.

Cellular heterogeneity is a well-accepted feature of tissues, and both transcriptional and metabolic diversity have been revealed by numerous approaches, including optical imaging. However, the high magnification objective lenses needed for high-resolution imaging provides information from only small layers of tissue, which can result in poor cell statistics. There is therefore an unmet need for an imaging modality that can provide detailed molecular and cellular insight within intact tissue samples in 3D. Using GFP-tagged GLUT4 as proof of concept, we present here a novel optical mesoscopy approach that allows precise measurement of the spatial location of GLUT4 within specific anatomical structures across the myocardium in ultrathick sections (5 mm×5 mm×3 mm) of intact mouse heart. We reveal distinct GLUT4 distribution patterns across cardiac walls and highlight specific changes in GLUT4 expression levels in response to high fat diet-feeding, and we identify sex-dependent differences in expression patterns. This method is applicable to any target that can be labelled for light microscopy, and to other complex tissues when organ structure needs to be considered simultaneously with cellular detail.

Cardiac function may be compromised in patients with elevated blood cobalt levels secondary to metal-on-metal hip implants.

Aims: Elevated blood cobalt levels secondary to metal-on-metal (MoM) hip arthroplasties are a suggested risk factor for developing cardiovascular complications including cardiomyopathy. Clinical studies assessing patients with MoM hips using left ventricular ejection fraction (LVEF) have found conflicting evidence of cobalt-induced cardiomyopathy. Global longitudinal strain (GLS) is an echocardiography measurement known to be more sensitive than LVEF when diagnosing early cardiomyopathies. The extent of cardiovascular injury, as measured by GLS, in patients with elevated blood cobalt levels has not previously been examined. Methods: A total of 16 patients with documented blood cobalt ion levels above 13 µg/l (13 ppb, 221 nmol/l) were identified from a regional arthroplasty database. They were matched with eight patients awaiting hip arthroplasty. All patients underwent echocardiography, including GLS, investigating potential signs of cardiomyopathy. Results: Patients with MoM hip arthroplasties had a mean blood cobalt level of 29 µg/l (495 nmol/l) compared to 0.01 µg/l (0.2 nmol/l) in the control group. GLS readings were available for seven of the MoM cohort, and were significantly lower when compared with controls (-15.5% vs -18% (MoM vs control); p = 0.025)). Pearson correlation demonstrated that GLS significantly correlated with blood cobalt level (r = 0.8521; p < 0.001). However, there were no differences or correlations for other echocardiography measurements, including LVEF (64.3% vs 63.7% (MoM vs control); p = 0.845). Conclusion: This study supports the hypothesis that patients with elevated blood cobalt levels above 13 µg/l in the presence of a MoM hip implant may have impaired cardiac function compared to a control group of patients awaiting hip arthroplasty. It is the first study to use the more sensitive parameter of GLS to assess for any cardiac contractile dysfunction in patients with a MoM hip implant and a normal LVEF. Larger studies should be performed to determine the potential of GLS as a predictor of cardiac complications in patients with MoM arthroplasties.

Cobalt-induced cardiomyopathy - do circulating cobalt levels matter?

Elevated levels of circulating cobalt ions have been linked with a wide range of systemic complications including neurological, endocrine, and cardiovascular symptoms. Case reports of patients with elevated blood cobalt ions have described significant cardiovascular complications including cardiomyopathy. However, correlation between the actual level of circulating cobalt and extent of cardiovascular injury has not previously been performed. This review examines evidence from the literature for a link between elevated blood cobalt levels secondary to metal-on-metal (MoM) hip arthroplasties and cardiomyopathy. Correlation between low, moderate, and high blood cobalt with cardiovascular complications has been considered. Elevated blood cobalt at levels over 250 µg/l have been shown to be a risk factor for developing systemic complications and published case reports document cardiomyopathy, cardiac transplantation, and death in patients with severely elevated blood cobalt ions. However, it is not clear that there is a hard cut-off value and cardiac dysfunction may occur at lower levels. Clinical and laboratory research has found conflicting evidence of cobalt-induced cardiomyopathy in patients with MoM hips. Further work needs to be done to clarify the link between severely elevated blood cobalt ions and cardiomyopathy. Cite this article: Bone Joint Res 2021;10(6):340-347.

Sunitinib and Imatinib Display Differential Cardiotoxicity in Adult Rat Cardiac Fibroblasts That Involves a Role for Calcium/Calmodulin Dependent Protein Kinase II.

Background: Tyrosine kinase inhibitors (TKIs) have dramatically improved cancer treatment but are known to cause cardiotoxicity. The pathophysiological consequences of TKI therapy are likely to manifest across different cell types of the heart, yet there is little understanding of the differential adverse cellular effects. Cardiac fibroblasts (CFs) play a pivotal role in the repair and remodeling of the heart following insult or injury, yet their involvement in anti-cancer drug induced cardiotoxicity has been largely overlooked. Here, we examine the direct effects of sunitinib malate and imatinib mesylate on adult rat CF viability, Ca2+ handling and mitochondrial function that may contribute to TKI-induced cardiotoxicity. In particular, we investigate whether Ca2+/calmodulin dependent protein kinase II (CaMKII), may be a mediator of TKI-induced effects. Methods: CF viability in response to chronic treatment with both drugs was assessed using MTT assays and flow cytometry analysis. Calcium mobilization was assessed in CFs loaded with Fluo4-AM and CaMKII activation via oxidation was measured via quantitative immunoblotting. Effects of both drugs on mitochondrial function was determined by live mitochondrial imaging using MitoSOX red. Results: Treatment of CFs with sunitinib (0.1-10 µM) resulted in concentration-dependent alterations in CF phenotype, with progressively significant cell loss at higher concentrations. Flow cytometry analysis and MTT assays revealed increased cell apoptosis and necrosis with increasing concentrations of sunitinib. In contrast, equivalent concentrations of imatinib resulted in no significant change in cell viability. Both sunitinib and imatinib pre-treatment increased Angiotensin II-induced intracellular Ca2+ mobilization, with only sunitinib resulting in a significant effect and also causing increased CaMKII activation via oxidation. Live cell mitochondrial imaging using MitoSOX red revealed that both sunitinib and imatinib increased mitochondrial superoxide production in a concentration-dependent manner. This effect in response to both drugs was suppressed in the presence of the CaMKII inhibitor KN-93. Conclusions: Sunitinib and imatinib showed differential effects on CFs, with sunitinib causing marked changes in cell viability at concentrations where imatinib had no effect. Sunitinib caused a significant increase in Angiotensin II-induced intracellular Ca2+ mobilization and both TKIs caused increased mitochondrial superoxide production. Targeted CaMKII inhibition reversed the TKI-induced mitochondrial damage. These findings highlight a new role for CaMKII in TKI-induced cardiotoxicity, particularly at the level of the mitochondria, and confirm differential off-target toxicity in CFs, consistent with the differential selectivity of sunitinib and imatinib.

Compromised cardiovascular function in aged rats corresponds with increased expression and activity of calcium/calmodulin dependent protein kinase IIδ in aortic endothelium.

Ageing is the greatest risk factor for cardiovascular disease. Calcium/calmodulin dependent protein kinase IIδ (CaMKIIδ) plays a fundamental role in the pathology of heart disease yet a potential role for CaMKIIδ in cardiovascular pathology associated with ageing remains unclear. Taking a combined in vivo and in vitro approach, we have for the first time investigated whether CaMKIIδ expression and CaMKII activity may be altered following age-related cardiovascular deterioration. Both cardiac contractility and aortic blood flow are compromised in aged rats and we have shown that this occurs in parallel with increased inflammation and crucially, autonomous activation of CaMKII. Endothelial cells isolated from young and aged aortae exhibit differences in cell phenotype and physiology. In line with observations in aortic tissue, aged aortic endothelial cells also show increased basal levels of pro-inflammatory markers and oxidative stress with concurrent increased basal activation of CaMKII. These results are the first to demonstrate that elevated CaMKIIδ expression and CaMKII activation occur in parallel with the pathological progression associated with ageing of the heart and vasculature. Specifically, CaMKIIδ expression is significantly increased and activated in the endothelium of aged aorta. As such, CaMKIIδ could serve as an important marker of endothelial dysfunction that accompanies the ageing process and may be an appropriate candidate for investigating targeted therapeutic intervention.

Cobalt Administration Causes Reduced Contractility with Parallel Increases in TRPC6 and TRPM7 Transporter Protein Expression in Adult Rat Hearts.

Exposure to circulating cobalt (Co2+) in patients with metal-on-metal orthopaedic hip implants has been linked to cardiotoxicity but the underlying mechanism(s) remain undefined. The aim of the current study was to examine the effects of Co2+ on the heart in vivo and specifically on cardiac fibroblasts in vitro. Adult male rats were treated with CoCl2 (1 mg/kg) for either 7 days or 28 days. Inductively coupled plasma mass spectrometry (ICP-MS) was used to measure Co2+ uptake into various organs of the body. Co2+ accumulated in the heart over time with significant levels evident after only 7 days of treatment. There was no evidence of cardiac remodelling following Co2+ treatment as assessed by heart weight:body weight and left ventricular weight:body weight. However, a decrease in fractional shortening, as measured using echocardiography, was observed after 28 days of Co2+ treatment. This was accompanied by increased protein expression of the ion transient receptor potential (TRP) channels TRPC6 and TRPM7 as assessed by quantitative immunoblotting of whole cardiac homogenates. Uptake of Co2+ specifically into rat cardiac fibroblasts was measured over 72 h and was shown to dramatically increase with increasing concentrations of applied CoCl2. Expression levels of TRPC6 and TRPM7 proteins were both significantly elevated in these cells following Co2+ treatment. In conclusion, Co2+ rapidly accumulates to significant levels in the heart causing compromised contractility in the absence of any overt cardiac remodelling. TRPC6 and TRPM7 expression levels are significantly altered in the heart following Co2+ treatment and this may contribute to the Co2+-induced cardiotoxicity observed over time.

CaMKIIδ interacts directly with IKKß and modulates NF-κB signalling in adult cardiac fibroblasts.

Calcium/calmodulin dependent protein kinase IIδ (CaMKIIδ) acts as a molecular switch regulating cardiovascular Ca2+ handling and contractility in health and disease. Activation of CaMKIIδ is also known to regulate cardiovascular inflammation and is reported to be required for pro-inflammatory NF-κB signalling. In this study the aim was to characterise how CaMKIIδ interacts with and modulates NF-κB signalling and whether this interaction exists in non-contractile cells of the heart. Recombinant or purified CaMKIIδ and the individual inhibitory -κB kinase (IKK) proteins of the NF-κB signalling pathway were used in autoradiography and Surface Plasmon Resonance (SPR) to explore potential interactions between both components. Primary adult rat cardiac fibroblasts were then used to study the effects of selective CaMKII inhibition on pharmacologically-induced NF-κB activation as well as interaction between CaMKII and specific IKK isoforms in a cardiac cellular setting. Autoradiography analysis suggested that CaMKIIδ phosphorylated IKKß but not IKKα. SPR analysis further supported a direct interaction between CaMKIIδ and IKKß but not between CaMKIIδ and IKKα or IKKγ. CaMKIIδ regulation of IκΒα degradation was explored in adult cardiac fibroblasts exposed to pharmacological stimulation. Cells were stimulated with agonist in the presence or absence of a CaMKII inhibitor, autocamtide inhibitory peptide (AIP). Selective inhibition of CaMKII resulted in reduced NF-κB activation, as measured by agonist-stimulated IκBα degradation. Importantly, and in agreement with the recombinant protein work, an interaction between CaMKII and IKKß was evident following Proximity Ligation Assays in adult cardiac fibroblasts. This study provides new evidence supporting direct interaction between CaMKIIδ and IKKß in pro-inflammatory signalling in cardiac fibroblasts and could represent a feature that may be exploited for therapeutic benefit.

Changes in IP3 Receptor Expression and Function in Aortic Smooth Muscle of Atherosclerotic Mice.

Peroxynitrite is an endothelium-independent vasodilator that induces relaxation via membrane hyperpolarization. The activation of IP3 receptors triggers the opening of potassium channels and hyperpolarization. Previously we found that relaxation to peroxynitrite was maintained during the development of atherosclerosis due to changes in the expression of calcium-regulatory proteins. In this study we investigated: (1) the mechanism of peroxynitrite-induced relaxation in the mouse aorta, (2) the effect of atherosclerosis on relaxation to peroxynitrite and other vasodilators, and (3) the effect of atherosclerosis on the expression and function of the IP3 receptor. Aortic function was studied using wire myography, and atherosclerosis was induced by fat-feeding ApoE-/- mice. The expression of IP3 receptors was studied using Western blotting and immunohistochemistry. Relaxation to peroxynitrite was attenuated by the IP3 antagonists 2-APB and xestospongin C and also the Kv channel blocker 4-aminopyridine (4-AP). Atherosclerosis attenuated vasodilation to cromakalim and the AMPK activator A769662 but not peroxynitrite. Relaxation was attenuated to a greater extent by 2-APB in atherosclerotic aortae despite the reduced expression of IP3 receptors. 4-AP was less effective in ApoE-/- mice fat-fed for 4 months. Peroxynitrite relaxation involves an IP3-induced calcium release and KV channel activation. This mechanism becomes less important as atherosclerosis develops, and relaxation to peroxynitrite may be maintained by increased calcium extrusion.

Small heat shock protein 20 (Hsp20) facilitates nuclear import of protein kinase D 1 (PKD1) during cardiac hypertrophy.

BACKGROUND: Nuclear import of protein kinase D1 (PKD1) is an important event in the transcriptional regulation of cardiac gene reprogramming leading to the hypertrophic growth response, however, little is known about the molecular events that govern this event. We have identified a novel complex between PKD1 and a heat shock protein (Hsp), Hsp20, which has been implicated as cardioprotective. This study aims to characterize the role of the complex in PKD1-mediated myocardial regulatory mechanisms that depend on PKD1 nuclear translocation. RESULTS: In mapping the Hsp20 binding sites on PKD1 within its catalytic unit using peptide array analysis, we were able to develop a cell-permeable peptide that disrupts the Hsp20-PKD1 complex. We use this peptide to show that formation of the Hsp20-PKD1 complex is essential for PKD1 nuclear translocation, signaling mechanisms leading to hypertrophy, activation of the fetal gene programme and pathological cardiac remodeling leading to cardiac fibrosis. CONCLUSIONS: These results identify a new signaling complex that is pivotal to pathological remodelling of the heart that could be targeted therapeutically.

Improving arteriovenous fistula patency: Transdermal delivery of diclofenac reduces cannulation-dependent neointimal hyperplasia via AMPK activation.

Creation of an autologous arteriovenous fistula (AVF) for vascular access in haemodialysis is the modality of choice. However neointimal hyperplasia and loss of the luminal compartment result in AVF patency rates of ~60% at 12months. The exact cause of neointimal hyperplasia in the AVF is poorly understood. Vascular trauma has long been associated with hyperplasia. With this in mind in our rabbit model of AVF we simulated cannulation autologous to that undertaken in vascular access procedures and observed significant neointimal hyperplasia as a direct consequence of cannulation. The neointimal hyperplasia was completely inhibited by topical transdermal delivery of the non-steroidal anti-inflammatory (NSAID) diclofenac. In addition to the well documented anti-inflammatory properties we have identified novel anti-proliferative mechanisms demonstrating diclofenac increases AMPK-dependent signalling and reduced expression of the cell cycle protein cyclin D1. In summary prophylactic transdermal delivery of diclofenac to the sight of AVF cannulation prevents adverse neointimal hyperplasic remodelling and potentially offers a novel treatment option that may help prolong AVF patency and flow rates.

Targeted disruption of the heat shock protein 20-phosphodiesterase 4D (PDE4D) interaction protects against pathological cardiac remodelling in a mouse model of hypertrophy.

Phosphorylated heat shock protein 20 (HSP20) is cardioprotective. Using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and a mouse model of pressure overload mediated hypertrophy, we show that peptide disruption of the HSP20-phosphodiesterase 4D (PDE4D) complex results in attenuation of action potential prolongation and protection against adverse cardiac remodelling. The later was evidenced by improved contractility, decreased heart weight to body weight ratio, and reduced interstitial and perivascular fibrosis. This study demonstrates that disruption of the specific HSP20-PDE4D interaction leads to attenuation of pathological cardiac remodelling.

Altered vascular smooth muscle function in the ApoE knockout mouse during the progression of atherosclerosis.

OBJECTIVES: Relaxation of vascular smooth muscle (VSM) requires re-uptake of cytosolic Ca(2+) into the sarcoplasmic reticulum (SR) via the Sarco/Endoplasmic Reticulum Ca(2+) ATPase (SERCA), or extrusion via the Plasma Membrane Ca(2+) ATPase (PMCA) or sodium Ca(2+) exchanger (NCX). Peroxynitrite, a reactive species formed in vascular inflammatory diseases, upregulates SERCA activity to induce relaxation but, chronically, can contribute to atherogenesis and altered vascular function by escalating endoplasmic reticulum stress. Our objectives were to determine if peroxynitrite-induced relaxation and Ca(2+) handling processes within vascular smooth muscle cells were altered as atherosclerosis develops. METHODS: Aortae from control and ApoE(-/-) mice were studied histologically, functionally and for protein expression levels of SERCA and PMCA. Ca(2+) responses were assessed in dissociated aortic smooth muscle cells in the presence and absence of extracellular Ca(2+). RESULTS: Relaxation to peroxynitrite was concentration-dependent and endothelium-independent. The abilities of the SERCA blocker thapsigargin and the PMCA inhibitor carboxyeosin to block this relaxation were altered during fat feeding and plaque progression. SERCA levels were progressively reduced, while PMCA expression was upregulated. In ApoE(-/-) VSM cells, increases in cytosolic Ca(2+) [Ca(2+)]c in response to SERCA blockade were reduced, while SERCA-independent Ca(2+) clearance was faster compared to control. CONCLUSION: As atherosclerosis develops in the ApoE(-/-) mouse, expression and function of Ca(2+) handling proteins are altered. Up-regulation of Ca(2+) removal via PMCA may offer a potential compensatory mechanism to help normalise the dysfunctional relaxation observed during disease progression.

The cardioprotective role of small heat-shock protein 20.

The small HSP (heat-shock protein) HSP20 is a molecular chaperone that is transiently up-regulated in response to cellular stress/damage. Although ubiquitously expressed in various tissues, it is most highly expressed in skeletal, cardiac and smooth muscle. Phosphorylation at Ser16 by PKA (cAMP-dependent protein kinase) is essential for HSP20 to confer its protective qualities. HSP20 and its phosphorylation have been implicated in a variety of pathophysiological processes, but most prominently cardiovascular disease. A wealth of knowledge of the importance of HSP20 in contractile function and cardioprotection has been gained over the last decade. The present mini-review highlights more recent findings illustrating the cardioprotective properties of HSP20 and its potential as a therapeutic agent.

Adult cardiac fibroblast proliferation is modulated by calcium/calmodulin-dependent protein kinase II in normal and hypertrophied hearts.

Increased adult cardiac fibroblast proliferation results in an increased collagen deposition responsible for the fibrosis accompanying pathological remodelling of the heart. The mechanisms regulating cardiac fibroblast proliferation remain poorly understood. Using a minimally invasive transverse aortic banding (MTAB) mouse model of cardiac hypertrophy, we have assessed fibrosis and cardiac fibroblast proliferation. We have investigated whether calcium/calmodulin-dependent protein kinase IIδ (CaMKIIδ) regulates proliferation in fibroblasts isolated from normal and hypertrophied hearts. It is known that CaMKIIδ plays a central role in cardiac myocyte contractility, but nothing is known of its role in adult cardiac fibroblast function. The MTAB model used here produces extensive hypertrophy and fibrosis. CaMKIIδ protein expression and activity is upregulated in MTAB hearts and, specifically, in cardiac fibroblasts isolated from hypertrophied hearts. In response to angiotensin II, cardiac fibroblasts isolated from MTAB hearts show increased proliferation rates. Inhibition of CaMKII with autocamtide inhibitory peptide inhibits proliferation in cells isolated from both sham and MTAB hearts, with a significantly greater effect evident in MTAB cells. These results are the first to show selective upregulation of CaMKIIδ in adult cardiac fibroblasts following cardiac hypertrophy and to assign a previously unrecognised role to CaMKII in regulating adult cardiac fibroblast function in normal and diseased hearts.

A novel approach for assessing cardiac fibrosis using label-free second harmonic generation.

To determine whether second harmonic generation (SHG) can be used as a novel and improved label-free technique for detection of collagen deposition in the heart. To verify whether SHG will allow accurate quantification of altered collagen deposition in diseased hearts following hypertrophic remodelling. Minimally invasive transverse aortic banding (MTAB) of mouse hearts was used to generate a reproducible model of cardiac hypertrophy. Physiological and functional assessment of hypertrophic development was performed using echocardiography and post-mortem analysis of remodelled hearts. Cardiac fibroblasts were isolated from sham-operated and hypertrophied hearts and proliferation rates compared. Multi-photon laser scanning microscopy was used to capture both two-photon excited autofluorescence (TPEF) and SHG images simultaneously in two channels. TPEF images were subtracted from SHG images and the resulting signal intensities from ventricular tissue sections were calculated. Traditional picrosirius red staining was used to verify the suitability of the SHG application. MTAB surgery induced significant hypertrophic remodelling and increased cardiac fibroblast proliferation. A significant increase in the density of collagen fibres between hypertrophic and control tissues (p < 0.05) was evident using SHG. Similar increases and patterns of staining were observed using parallel traditional picrosirius red staining of collagen. Label-free SHG microscopy provides a new alternative method for quantifying collagen deposition in fibrotic hearts.

Optimising conditions for studying the acute effects of drugs on indices of cardiac contractility and on haemodynamics in anaesthetized guinea pigs.

INTRODUCTION: Detecting adverse effects of drugs on cardiac contractility is becoming a priority in pre-clinical safety pharmacology. The aim of this work was to optimise conditions and explore the potential of using the anaesthetized guinea pig as an in vivo model. METHODS: Guinea pigs were anaesthetized with Hypnorm/Hypnovel, isoflurane, pentobarbital or fentanyl/pentobarbital. The electrocardiogram (ECG), heart rate, arterial blood pressure and indices of cardiac contractility were recorded. In further experiments in fentanyl/pentobarbital anaesthetized guinea pigs the influence of bilateral versus unilateral carotid artery occlusion on haemodynamic responses was investigated and the effects of inotropic drugs on left ventricular (LV) dP/dt(max) and the QA interval were determined. RESULTS: Pentobarbital, given alone or after fentanyl, provided suitable anaesthesia for these experiments. Bilateral carotid artery occlusion did not alter heart rate or arterial blood pressure responses to isoprenaline or angiotensin II. Isoprenaline and ouabain increased LVdP/dt(max) and decreased the QA interval whereas verapamil had opposite effects and strong inverse correlations between LVdP/dt(max) and the QA interval were found. DISCUSSION: Conditions can be optimised to allow the pentobarbital-anaesthetized guinea pig to be used for simultaneous measurement of the effects of drugs on the ECG, haemodynamics and indices of cardiac contractility. The use of this small animal model in early pre-clinical safety pharmacology should contribute to improvements in detecting unwanted actions on the heart during the drug development process.

Surgical optimization and characterization of a minimally invasive aortic banding procedure to induce cardiac hypertrophy in mice.

Left ventricular pressure overload in response to aortic banding is an invaluable model for studying progression of cardiac hypertrophy and transition to heart failure. Traditional aortic banding has recently been superceded by minimally invasive transverse aortic banding (MTAB), which does not require ventilation so is less technically challenging. Although the MTAB approach is superior, few laboratories have documented success, and minimal information on the model is available. The aim of this study was to optimize conditions for MTAB and to characterize the development and progression of cardiac hypertrophy. Isofluorane proved the most suitable anaesthetic for MTAB surgery in mice, and 1 week after surgery the MTAB animals showed significant increases in systolic blood pressure (MTAB 110 ± 6 mmHg versus sham 78 ± 3 mmHg, n = 7, P < 0.0001) and heart weight to body weight ratio (MTAB 6.2 ± 0.2 versus sham 5.1 ± 0.1, n = 12, P < 0.001), together with systolic (e.g. fractional shortening, MTAB 31.7 ± 1% versus sham 36.6 ± 1.4%, P = 0.01) and diastolic dysfunction (e.g. left ventricular end-diastolic pressure, MTAB 12.7 ± 1.0 mmHg versus sham 6.7 ± 0.8 mmHg, P < 0.001). Leucocyte infiltration to the heart was evident after 1 week in MTAB hearts, signifying an inflammatory response. More pronounced remodelling was observed 4 weeks postsurgery (heart weight to body weight ratio, MTAB 9.1 ± 0.6 versus sham 4.6 ± 0.04, n = 10, P < 0.0001) and fractional shortening was further decreased (MTAB 24.3 ± 2.5% versus sham 43.6 ± 1.7%, n = 10, P = 0.003), together with a significant increase in cardiac fibrosis and further cardiac inflammation. Our findings demonstrate that MTAB is a relevant experimental model for studying development and progression of cardiac hypertrophy, which will be highly valuable for future studies examining potential novel therapeutic interventions in this setting.

Remodelling of human atrial K+ currents but not ion channel expression by chronic ß-blockade.

Chronic ß-adrenoceptor antagonist (ß-blocker) treatment in patients is associated with a potentially anti-arrhythmic prolongation of the atrial action potential duration (APD), which may involve remodelling of repolarising K(+) currents. The aim of this study was to investigate the effects of chronic ß-blockade on transient outward, sustained and inward rectifier K(+) currents (I(TO), I(KSUS) and I(K1)) in human atrial myocytes and on the expression of underlying ion channel subunits. Ion currents were recorded from human right atrial isolated myocytes using the whole-cell-patch clamp technique. Tissue mRNA and protein levels were measured using real time RT-PCR and Western blotting. Chronic ß-blockade was associated with a 41% reduction in I(TO) density: 9.3 ± 0.8 (30 myocytes, 15 patients) vs 15.7 ± 1.1 pA/pF (32, 14), p < 0.05; without affecting its voltage-, time- or rate dependence. I(K1) was reduced by 34% at -120 mV (p < 0.05). Neither I(KSUS), nor its increase by acute ß-stimulation with isoprenaline, was affected by chronic ß-blockade. Mathematical modelling suggested that the combination of I(TO)- and I(K1)-decrease could result in a 28% increase in APD(90). Chronic ß-blockade did not alter mRNA or protein expression of the I(TO) pore-forming subunit, Kv4.3, or mRNA expression of the accessory subunits KChIP2, KChAP, Kvß1, Kvß2 or frequenin. There was no reduction in mRNA expression of Kir2.1 or TWIK to account for the reduction in I(K1). A reduction in atrial I(TO) and I(K1) associated with chronic ß-blocker treatment in patients may contribute to the associated action potential prolongation, and this cannot be explained by a reduction in expression of associated ion channel subunits.