Effect of the microtubule polymerizing agent taxol on contraction, Ca2+ transient and L-type Ca2+ current in rat ventricular myocytes

ZBTB20 Regulates SERCA2a Activity and Myocardial Contractility Through Phospholamban

BACKGROUND

Intracellular Ca2+ cycling determines myocardial contraction and relaxation in response to physiological demands. SERCA2a (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 2a) is responsible for the sequestration of cytosolic Ca2+ into intracellular stores during cardiac relaxation, and its activity is reversibly inhibited by PLN (phospholamban). However, the regulatory hierarchy of SERCA2a activity remains unclear.

METHODS

Cardiomyocyte-specific ZBTB20 knockout mice were generated by crossing ZBTB20flox mice with Myh6-Cre mice. Echocardiography, blood pressure measurements, Langendorff perfusion, histological analysis and immunohistochemistry, quantitative reverse transcription-PCR, Western blot analysis, electrophysiological measurements, and chromatin immunoprecipitation assay were performed to clarify the phenotype and elucidate the molecular mechanisms.

RESULTS

Specific ablation of ZBTB20 in cardiomyocyte led to a significant increase in basal myocardial contractile parameters both in vivo and in vitro, accompanied by an impairment in cardiac reserve and exercise capacity. Moreover, the cardiomyocytes lacking ZBTB20 showed an increase in sarcoplasmic reticular Ca2+ content and exhibited a remarkable enhancement in both SERCA2a activity and electrically stimulated contraction. Mechanistically, PLN expression was dramatically reduced in cardiomyocytes at the mRNA and protein levels by ZBTB20 deletion or silencing, and PLN overexpression could largely restore the basal contractility in ZBTB20-deficient cardiomyocytes.

CONCLUSIONS

These data point to ZBTB20 as a fine-tuning modulator of PLN expression and SERCA2a activity, thereby offering new perspective on the regulation of basal contractility in the mammalian heart.

What Causes Death in Esophageal Cancer Patients Other Than the Cancer Itself: A Large Population-Based Analysis

Background: Researches on noncancer causes of death in patients with esophageal cancer (EC) are not in-depth. The objective of this paper is to broadly and deeply explore the causes of death in patients with EC, especially noncancer causes.

Methods: Information about the demographics, tumor-related characteristics, and causes of death of patients with EC who met the inclusion criteria were extracted from the Surveillance, Epidemiology, and End Results (SEER) database. Calculated standardized mortality ratio (SMR) for all causes of death at different follow-up times and performed subgroup analyses.

Results: In total, 63,560 patients with EC were retrieved from the public database. And 52,503 died during the follow-up period. Most deaths were due to EC itself within 5 years after diagnosis, but over 10 years, 59% EC patients died from noncancer causes. Cardiovascular disease was the major noncancer cause of death in patients with EC, accounting for 43%. Suicide and self-injury (2%) of EC patients should not be ignored. During the 1-year follow-up period, patients with EC had statistically highest risk of death from septicemia (SMR: 7.61; 95% CI: 6.38-9.00). Within more than 10 years after EC diagnosis, more and more patients died from chronic obstructive pulmonary disease (SMR: 2.38; 95% CI: 1.79-3.10).

Conclusions: Although most patients with EC still died from the cancer itself, the role of noncancer causes of death should not be underestimated. These prompt clinicians to pay more attention to the risk of death caused by these noncancer causes, which can provide relevant measures in advance to intervene.

Cancer Chemotherapy-Induced Sinus Bradycardia: A Narrative Review of a Forgotten Adverse Effect of Cardiotoxicity

Cardiotoxicity is a common adverse effect of anticancer drugs (ACDs), including the so-called targeted drugs, and increases morbidity and mortality in patients with cancer. Attention has focused mainly on ACD-induced heart failure, myocardial ischemia, hypertension, thromboembolism, QT prolongation, and tachyarrhythmias. Yet, although an increasing number of ACDs can produce sinus bradycardia (SB), this proarrhythmic effect remains an underappreciated complication, probably because of its low incidence and severity since most patients are asymptomatic. However, SB merits our interest because its incidence increases with the aging of the population and cancer is an age-related disease and because SB represents a risk factor for QT prolongation. Indeed, several ACDs that produce SB also prolong the QT interval. We reviewed published reports on ACD-induced SB from January 1971 to November 2020 using the PubMed and EMBASE databases. Published reports from clinical trials, case reports, and recent reviews were considered. This review describes the associations between ACDs and SB, their clinical relevance, risk factors, and possible mechanisms of onset and treatment.

Oxidative stress pathogenically remodels the cardiac myocyte cytoskeleton via structural alterations to the microtubule lattice

In the failing heart, the cardiac myocyte microtubule network is remodeled, which contributes to cellular contractile failure and patient death. However, the origins of this deleterious cytoskeletal reorganization are unknown. We now find that oxidative stress, a condition characteristic of heart failure, leads to cysteine oxidation of microtubules. Our electron and fluorescence microscopy experiments revealed regions of structural damage within the microtubule lattice that occurred at locations of oxidized tubulin. The incorporation of GTP-tubulin into these damaged, oxidized regions led to stabilized "hot spots" within the microtubule lattice, which suppressed the shortening of dynamic microtubules. Thus, oxidative stress may act inside of cardiac myocytes to facilitate a pathogenic shift from a sparse microtubule network into a dense, aligned network. Our results demonstrate how a disease condition characterized by oxidative stress can trigger a molecular oxidation event, which likely contributes to a toxic cellular-scale transformation of the cardiac myocyte microtubule network.

Development and progression of cancer cachexia: Perspectives from bench to bedside

Cancer cachexia (CC) is a devastating syndrome characterized by weight loss, reduced fat mass and muscle mass that affects approximately 80% of cancer patients and is responsible for 22%-30% of cancer-associated deaths. Understanding underlying mechanisms for the development of CC are crucial to advance therapies to treat CC and improve cancer outcomes. CC is a multi-organ syndrome that results in extensive skeletal muscle and adipose tissue wasting; however, CC can impair other organs such as the liver, heart, brain, and bone as well. A considerable amount of CC research focuses on changes that occur within the muscle, but cancer-related impairments in other organ systems are understudied. Furthermore, metabolic changes in organ systems other than muscle may contribute to CC. Therefore, the purpose of this review is to address degenerative mechanisms which occur during CC from a whole-body perspective. Outlining the information known about metabolic changes that occur in response to cancer is necessary to develop and enhance therapies to treat CC. As much of the current evidences in CC are from pre-clinical models we should note the majority of the data reviewed here are from preclinical models.

Pathophysiological mechanisms of cardiotoxicity in chemotherapeutic agents

Certain success has been achieved in the treatment of cancer due to the development of various effective chemotherapeutic drugs. However, an increase in their effectiveness (aggressiveness) was associated with a growth of undesirable effects on the entire human body, in particular, on the cardiovascular system. The damage to the cardiovascular system from chemotherapy in many cases is more significant than from the underlying disease. In recent years, a new direction of medicine has been formed - cardio-oncology. The major groups of cardiotoxic chemotherapeutic agents are anthracyclines, inhibitors of epidermal growth factor receptor type 2 (anti-HER2), antimetabolites, microtubule inhibitors, proteasome inhibitors, platinum-based chemotherapeutic drugs, and angiogenesis inhibitors (inhibitors of vascular endothelial growth factor). This review discusses principal pathophysiological mechanisms of the cardiotoxicity of these chemotherapeutic drugs.

Mechanobiology Assays with Applications in Cardiomyocyte Biology and Cardiotoxicity

Cardiomyocytes are the motor units that drive the contraction and relaxation of the heart. Traditionally, testing of drugs for cardiotoxic effects has relied on primary cardiomyocytes from animal models and focused on short-term, electrophysiological, and arrhythmogenic effects. However, primary cardiomyocytes present challenges arising from their limited viability in culture, and tissue from animal models suffers from a mismatch in their physiology to that of human heart muscle. Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) can address these challenges. They also offer the potential to study not only electrophysiological effects but also changes in cardiomyocyte contractile and mechanical function in response to cardiotoxic drugs. With growing recognition of the long-term cardiotoxic effects of some drugs on subcellular structure and function, there is increasing interest in using hiPSC-CMs for in vitro cardiotoxicity studies. This review provides a brief overview of techniques that can be used to quantify changes in the active force that cardiomyocytes generate and variations in their inherent stiffness in response to cardiotoxic drugs. It concludes by discussing the application of these tools in understanding how cardiotoxic drugs directly impact the mechanobiology of cardiomyocytes and how cardiomyocytes sense and respond to mechanical load at the cellular level.

Altered skeletal muscle microtubule-mitochondrial VDAC2 binding is related to bioenergetic impairments after paclitaxel but not vinblastine chemotherapies

Microtubule-targeting chemotherapies are linked to impaired cellular metabolism, which may contribute to skeletal muscle dysfunction. However, the mechanisms by which metabolic homeostasis is perturbed remains unknown. Tubulin, the fundamental unit of microtubules, has been implicated in the regulation of mitochondrial-cytosolic ADP/ATP exchange through its interaction with the outer membrane voltage-dependent anion channel (VDAC). Based on this model, we predicted that disrupting microtubule architecture with the stabilizer paclitaxel and destabilizer vinblastine would impair skeletal muscle mitochondrial bioenergetics. Here, we provide in vitro evidence of a direct interaction between both α-tubulin and βII-tubulin with VDAC2 in untreated single extensor digitorum longus (EDL) fibers. Paclitaxel increased both α- and βII-tubulin-VDAC2 interactions, whereas vinblastine had no effect. Utilizing a permeabilized muscle fiber bundle preparation that retains the cytoskeleton, paclitaxel treatment impaired the ability of ADP to attenuate H₂O₂ emission, resulting in greater H₂O₂ emission kinetics. Despite no effect on tubulin-VDAC2 binding, vinblastine still altered mitochondrial bioenergetics through a surprising increase in ADP-stimulated respiration while also impairing ADP suppression of H₂O₂ and increasing mitochondrial susceptibility to calcium-induced formation of the proapoptotic permeability transition pore. Collectively, these results demonstrate that altering microtubule architecture with chemotherapeutics disrupts mitochondrial bioenergetics in EDL skeletal muscle. Specifically, microtubule stabilization increases H₂O₂ emission by impairing ADP sensitivity in association with greater tubulin-VDAC binding. In contrast, decreasing microtubule abundance triggers a broad impairment of ADP's governance of respiration and H₂O₂ emission as well as calcium retention capacity, albeit through an unknown mechanism.

Liposomal paclitaxel induces fewer hematopoietic and cardiovascular complications than bioequivalent doses of Taxol

Paclitaxel (PTX) exhibits potent antineoplastic activity against various human malignancies; however, clinical application must overcome the inherent hydrophobicity of this molecule. The commercialized Taxol formulation utilizes Cremophor EL (CrEL)/ethanol as a solvent to stabilize and dispense PTX in an aqueous solution. However, adverse CrEL-induced hypersensitivity reactions have been reported in ~30% of recipients, and 40% of patients receiving premedication may also experience this adverse effect. Therefore, the development of a CrEL-free delivery system is crucial, in order to fully exploit the therapeutic efficacy of PTX. In the present study, a novel liposomal PTX (lipo-PTX) formulation was optimized with regards to encapsulation rate and long-term stability, arriving at a molar constituent ratio of soybean phosp hatidylcholine:cholesterol:N-(carbonyl-methoxy-poly-ethylene glycol 2000)-1,2-distearoyl-sn-glycero-3-phosphoethanolamine, sodium salt:PTX at 95:2:1:2. Comparable doses of lipo-PTX and Taxol were bioequivalent in terms of therapeutic efficacy in xenograft tumor models. However, the systemic side effects, including hematopoietic toxicity, acute hypersensitivity reactions and cardiac irregularities, were significantly reduced in lipo-PTX-treated mice compared with those infused with reference formulations of PTX. In conclusion, the present study reported that lipo-PTX exhibited a higher therapeutic index than clinical PTX formulations.

Pharmacogenetics of Chemotherapy-Induced Cardiotoxicity

PURPOSE OF REVIEW

The goal of this review is to summarize current understanding of pharmacogenetics and pharmacogenomics in chemotherapy-induced cardiotoxicity.

RECENT FINDINGS

Most of the studies rely on in vitro cytotoxic assays. There have been several smaller scale candidate gene approaches and a handful of genome-wide studies linking genetic variation to susceptibility to chemotherapy-induced cardiotoxicity. Currently, pharmacogenomic testing of all childhood cancer patients with an indication for doxorubicin or daunorubicin therapy for RARG rs2229774, SLC28A3 rs7853758, and UGT1A6*4 rs17863783 variants is recommended. There is no recommendation regarding testing in adults. There is clear evidence pointing to the role of pharmacogenetics and pharmacogenomics in cardiotoxicity susceptibility to chemotherapeutic agents. Larger scale studies are needed to further identify susceptibility markers and to develop pharmacogenomics-based risk profiling to improve quality of life and life expectancy in cancer survivors.

Effects of pioglitazone on ventricular myocyte shortening and Ca(2+) transport in the Goto-Kakizaki type 2 diabetic rat

Pioglitazone (PIO) is a thiazolidindione antidiabetic agent which improves insulin sensitivity and reduces blood glucose in experimental animals and treated patients. At the cellular level the actions of PIO in diabetic heart are poorly understood. A previous study has demonstrated shortened action potential duration and inhibition of a variety of transmembrane currents including L-type Ca(2+) current in normal canine ventricular myocytes. The effects of PIO on shortening and calcium transport in ventricular myocytes from the Goto-Kakizaki (GK) type 2 diabetic rat have been investigated. 10 min exposure to PIO (0.1-10 microM) reduced the amplitude of shortening to similar extents in ventricular myocytes from GK and control rats. 1 microM PIO reduced the amplitude of the Ca(2+) transients to similar extents in ventricular myocytes from GK and control rats. Caffeine-induced Ca(2+) release from the sarcoplasmic reticulum and recovery of Ca(2+) transients following application of caffeine and myofilament sensitivity to Ca(2+) were not significantly altered in ventricular myocytes from GK and control rats. Amplitude of L-type Ca(2+) current was not significantly decreased in myocytes from GK compared to control rats and by PIO treatment. The negative inotropic effects of PIO may be attributed to a reduction in the amplitude of the Ca(2+) transient however, the mechanisms remain to be resolved.

Out of the frying pan and into the fire: damage-associated molecular patterns and cardiovascular toxicity following cancer therapy

Cardio-oncology is a new and rapidly expanding field that merges cancer and cardiovascular disease. Cardiovascular disease is an omnipresent side effect of cancer therapy; in fact, it is the second leading cause of death in cancer survivors after recurrent cancer. It has been well documented that many cancer chemotherapeutic agents cause cardiovascular toxicity. Nonetheless, the underlying cause of cancer therapy-induced cardiovascular toxicity is largely unknown. In this review, we discuss the potential role of damage-associated molecular patterns (DAMPs) as an underlying contributor to cancer therapy-induced cardiovascular toxicity. With an increasing number of cancer patients, as well as extended life expectancy, understanding the mechanisms underlying cancer therapy-induced cardiovascular disease is of the utmost importance to ensure that cancer is the only disease burden that cancer survivors have to endure.

Cardio-Oncology: An Update on Cardiotoxicity of Cancer-Related Treatment

Through the success of basic and disease-specific research, cancer survivors are one of the largest growing subsets of individuals accessing the healthcare system. Interestingly, cardiovascular disease is the second leading cause of morbidity and mortality in cancer survivors after recurrent malignancy. This recognition has helped stimulate a collaboration between oncology and cardiology practitioners and researchers, and the portmanteau cardio-oncology (also known as onco-cardiology) can now be found in many medical centers. This collaboration promises new insights into how cancer therapies impact cardiovascular homeostasis and long-term effects on cancer survivors. In this review, we will discuss the most recent views on the cardiotoxicity related to various classes of chemotherapy agents and radiation. We will also discuss broadly the current strategies for treating and preventing cardiovascular effects of cancer therapy.

Reduction in the amplitude of shortening and Ca(2+) transient by phlorizin and quercetin-3-O-glucoside in ventricular myocytes from streptozotocin-induced diabetic rats

Diabetes mellitus is the leading cause of cardiovascular morbidity and mortality. Phlorizin (PHLOR) and quercetin-3-O-glucoside (QUER-3-G) are two natural compounds reported to have antidiabetic properties by inhibiting sodium/glucose transporters. Their effects on ventricular myocyte shortening and intracellular Ca(2+) in streptozotocin (STZ)-induced diabetic rats were investigated. Video edge detection and fluorescence photometry were used to measure ventricular myocyte shortening and intracellular Ca(2+), respectively. Blood glucose in STZ rats was 4-fold higher (469.64+/-22.23 mg/dl, n=14) than in Controls (104.06+/-3.36 mg/dl, n=16). The amplitude of shortening was reduced by PHLOR in STZ (84.76+/-2.91 %, n=20) and Control (83.72+/-2.65 %, n=23) myocytes, and by QUER-3-G in STZ (79.12+/-2.28 %, n=20) and Control (76.69+/-1.92 %, n=30) myocytes. The amplitude of intracellular Ca(2+) was also reduced by PHLOR in STZ (82.37+/-3.16 %, n=16) and Control (73.94+/-5.22 %, n=21) myocytes, and by QUER-3-G in STZ (73.62+/-5.83 %, n=18) and Control (78.32+/-3.54 %, n=41) myocytes. Myofilament sensitivity to Ca(2+) was not significantly altered by PHLOR; however, it was reduced by QUER-3-G modestly in STZ myocytes and significantly in Controls. PHLOR and QUER-3-G did not significantly alter sarcoplasmic reticulum Ca(2+) in STZ or Control myocytes. Altered mechanisms of Ca(2+) transport partly underlie PHLOR and QUER-3-G negative inotropic effects in ventricular myocytes from STZ and Control rats.

THE INHIBITORY EFFECT OF PACLITAXEL ON (Kv2.1) K+ CURRENT IN H9c2 CELLS

Using the whole-cell voltage clamp technique, we investigated the effect of paclitaxel, an anticancer agent which promotes microtubule formation, on K(+) current in H9c2 cells originated from rat embryonic cardiac myocytes. Paclitaxel inhibited Kv2.1 voltage-dependent K(+) current (IKur) with ultra-rapidly activating and slowly inactivating kinetics in a concentration-dependent manner. The inhibitory effect of paclitaxel on IKur was time-dependent and more marked at 200 ms after the onset than at the beginning of the depolarizing pulse. The IC50 value of paclitaxel was 1.1 µM at 200 ms. The time-dependent inhibition suggests that paclitaxel might be an open channel blocker of Kv2.1. This inhibition of Kv2.1 may be involved in the adverse effects of paclitaxel on cardiac and neuronal cells.

Dapagliflozin reduces the amplitude of shortening and Ca(2+) transient in ventricular myocytes from streptozotocin-induced diabetic rats

In the management of type 2 diabetes mellitus, Dapagliflozin (DAPA) is a newly introduced selective sodium-glucose co-transporter 2 inhibitor which promotes renal glucose excretion. Little is known about the effects of DAPA on the electromechanical function of the heart. This study investigated the effects of DAPA on ventricular myocyte shortening and intracellular Ca(2+) transport in streptozotocin (STZ)-induced diabetic rats. Shortening, Ca(2+) transients, myofilament sensitivity to Ca(2+) and sarcoplasmic reticulum Ca(2+), and intracellular Ca(2+) current were measured in isolated rats ventricular myocytes by video edge detection, fluorescence photometry, and whole-cell patch-clamp techniques. Diabetes was characterized in STZ-treated rats by a fourfold increase in blood glucose (440 ± 25 mg/dl, n = 21) compared to Controls (98 ± 2 mg/dl, n = 19). DAPA reduced the amplitude of shortening in Control (76.68 ± 2.28 %, n = 37) and STZ (76.58 ± 1.89 %, n = 42) ventricular myocytes, and reduced the amplitude of the Ca(2+) transients in Control and STZ ventricular myocytes with greater effects in STZ (71.45 ± 5.35 %, n = 16) myocytes compared to Controls (92.01 ± 2.72 %, n = 17). Myofilament sensitivity to Ca(2+) and sarcoplasmic reticulum Ca(2+) were not significantly altered by DAPA in either STZ or Control myocytes. L-type Ca(2+) current was reduced in STZ myocytes compared to Controls and was further reduced by DAPA. In conclusion, alterations in the mechanism(s) of Ca(2+) transport may partly underlie the negative inotropic effects of DAPA in ventricular myocytes from STZ-treated and Control rats.

Contractility of ventricular myocytes is well preserved despite altered mechanisms of Ca2+ transport and a changing pattern of mRNA in aged type 2 Zucker diabetic fatty rat heart

There has been a spectacular rise in the global prevalence of type 2 diabetes mellitus and cardiovascular complications are the major cause of morbidity and mortality in diabetic patients. The objective of the study was to investigate ventricular myocyte shortening, intracellular Ca(2+) signalling and expression of genes encoding cardiac muscle proteins in the aged Zucker diabetic fatty (ZDF) rat. There was a fourfold elevation in non-fasting blood glucose in ZDF rats (478.43 ± 29.22 mg/dl) compared to controls (108.22 ± 2.52 mg/dl). Amplitude of shortening, time to peak (TPK) and time to half (THALF) relaxation of shortening were unaltered in ZDF myocytes compared to age-matched controls. Amplitude and THALF decay of the Ca(2+) transient were unaltered; however, TPK Ca(2+) transient was prolonged in ZDF myocytes (70.0 ± 3.2 ms) compared to controls (58.4 ± 2.3 ms). Amplitude of the L-type Ca(2+) current was reduced across a wide range of test potentials (-30 to +40 mV) in ZDF myocytes compared to controls. Sarcoplasmic reticulum Ca(2+) content was unaltered in ZDF myocytes compared to controls. Expression of genes encoding cardiac muscle proteins, membrane Ca(2+) channels, and cell membrane ion transport and intracellular Ca(2+) transport proteins were variously altered. Myh6, Tnnt2, Cacna2d3, Slc9a1, and Atp2a2 were downregulated while Myl2, Cacna1g, Cacna1h, and Atp2a1 were upregulated in ZDF ventricle compared to controls. The results of this study have demonstrated that preserved ventricular myocyte shortening is associated with altered mechanisms of Ca(2+) transport and a changing pattern of genes encoding a variety of Ca(2+) signalling and cardiac muscle proteins in aged ZDF rat.

Structural lesions and changing pattern of expression of genes encoding cardiac muscle proteins are associated with ventricular myocyte dysfunction in type 2 diabetic Goto-Kakizaki rats fed a high-fat diet

Given the clinical prevalence of type 2 diabetes and obesity and their association with high mortality linked to cardiovascular disease, the aim of the study was to investigate the effects of feeding type 2 diabetic Goto-Kakizaki (GK) rats either high- or low-fat diets on cardiomyocyte structure and function. The GK rats were fed either a high-fat diet (HFD) or a low-fat diet (LFD) from the age of 2 months for a period of 7 months. The GK-HFD rats gained more weight, ate less food and drank less water compared with GK-LFD rats. At 7 months, non-fasting blood glucose was higher in GK-LFD (334 ± 35 mg dl(-1)) compared with GK-HFD rats (235 ± 26 mg dl(-1)). Feeding GK rats with a HFD had no significant effect on glucose clearance following a glucose challenge. Time-to-peak (t(peak)) shortening was reduced in myocytes from GK-HFD (131.8 ± 2.1 ms) compared with GK-LFD rats (144.5 ± 3.0 ms), and time-to-half (t(1/2)) relaxation of shortening was also reduced in myocytes from GK-HFD (71.7 ± 6.9 ms) compared with GK-LFD rats (86.1 ± 3.6 ms). The HFD had no significant effect on the amplitude of shortening. The HFD had no significant effect on t(peak), t(1/2) decay, amplitude of the Ca(2+) transient, myofilament sensitivity to Ca(2+), sarcoplasmic reticulum Ca(2+) content, fractional release of Ca(2+) and the rate of Ca(2+) uptake. Structurally, ventricular myocytes from GK-HFD rats showed extensive mitochondrial lesions, including swelling, loss of cristae, and loss of inner and outer membranes, resulting in gross vacuolarization and deformation of ventricular mitochondria with a subsequent reduction in mitochondrial density. Expression of genes encoding various L-type Ca(2+) channel proteins (Cacnb2) and cardiac muscle proteins (Myl2 and Atp2a1) were downregulated in GK-HFD compared with GK-LFD rats. Structural lesions and changed expression of genes encoding various cardiac muscle proteins might partly underlie the altered time course of myocyte shortening and relaxation in myocytes from GK-HFD compared with GK-LFD rats.

Taxol, a microtubule stabilizer, prevents ischemic ventricular arrhythmias in rats

Microtubule integrity is important in cardio-protection, and microtubule disruption has been implicated in the response to ischemia in cardiac myocytes. However, the effects of Taxol, a common microtubule stabilizer, are still unknown in ischemic ventricular arrhythmias. The arrhythmia model was established in isolated rat hearts by regional ischemia, and myocardial infarction model by ischemia/reperfusion. Microtubule structure was immunohistochemically measured. The potential mechanisms were studied by measuring reactive oxygen species (ROS), activities of oxidative enzymes, intracellular calcium concentration ([Ca(2+) ](i) ) and Ca(2+) transients by using fluorometric determination, spectrophotometric assays and Fura-2-AM and Fluo-3-AM, respectively. The expression and activity of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) was also examined using real-time polymerase chain reaction, Western blot and pyruvate/Nicotinamide adenine dinucleotide-coupled reaction. Our data showed that Taxol (0.1, 0.3 and 1 μM) effectively reduced the number of ventricular premature beats and the incidence and duration of ventricular tachycardia. The infarct size was also significantly reduced by Taxol (1 μM). At the same time, Taxol preserved the microtubule structure, increased the activity of mitochondrial electron transport chain complexes I and III, reduced ROS levels, decreased the rise in [Ca(2+)](i) and preserved the amplitude and decay times of Ca(2+) transients during ischemia. In addition, SERCA2a activity was preserved by Taxol during ischemia. In summary, Taxol prevents ischemic ventricular arrhythmias likely through ameliorating abnormal calcium homeostasis and decreasing the level of ROS. This study presents evidence that Taxol may be a potential novel therapy for ischemic ventricular arrhythmias.

Changing pattern of gene expression is associated with ventricular myocyte dysfunction and altered mechanisms of Ca2+ signalling in young type 2 Zucker diabetic fatty rat heart

The association between type 2 diabetes and obesity is very strong, and cardiovascular complications are the major cause of morbidity and mortality in diabetic patients. The aim of this study was to investigate early changes in the pattern of genes encoding cardiac muscle regulatory proteins and associated changes in ventricular myocyte contraction and Ca(2+) transport in young (9- to 13-week-old) type 2 Zucker diabetic fatty (ZDF) rats. The amplitude of myocyte shortening was unaltered; however, time-to-peak shortening and time to half-relaxation of shortening were prolonged in ZDF myocytes (163 ± 5 and 127 ± 7 ms, respectively) compared with age-matched control rats (136 ± 5 and 103 ± 4 ms, respectively). The amplitude of the Ca(2+) transient was unaltered; however, time-to-peak Ca(2+) transient was prolonged in ZDF myocytes (66.9 ± 2.6 ms) compared with control myocytes (57.6 ± 2.3 ms). The L-type Ca(2+) current was reduced, and inactivation was prolonged over a range of test potentials in ZDF myocytes. At 0 mV, the density of L-type Ca(2+) current was 1.19 ± 0.28 pA pF(-1) in ZDF myocytes compared with 2.42 ± 0.40 pA pF(-1) in control myocytes. Sarcoplasmic reticulum Ca(2+) content, release and uptake and myofilament sensitivity to Ca(2+) were unaltered in ZDF myocytes compared with control myocytes. Expression of genes encoding various L-type Ca(2+) channel proteins (Cacna1c, Cacna1g, Cacna1h and Cacna2d1) and cardiac muscle proteins (Myh7) were upregulated, and genes encoding intracellular Ca(2+) transport regulatory proteins (Atp2a2 and Calm1) and some cardiac muscle proteins (Myh6, Myl2, Actc1, Tnni3, Tnn2, and Tnnc1) were downregulated in ZDF heart compared with control heart. A change in the expression of genes encoding myosin heavy chain and L-type Ca(2+) channel proteins might partly underlie alterations in the time course of contraction and Ca(2+) transients in ventricular myocytes from ZDF rats.

Model of ionic currents through microtubule nanopores and the lumen

It has been suggested that microtubules and other cytoskeletal filaments may act as electrical transmission lines. An electrical circuit model of the microtubule is constructed incorporating features of its cylindrical structure with nanopores in its walls. This model is used to study how ionic conductance along the lumen is affected by flux through the nanopores, both with and without an external potential applied across its two ends. Based on the results of Brownian dynamics simulations, the nanopores were found to have asymmetric inner and outer conductances, manifested as nonlinear IV curves. Our simulations indicate that a combination of this asymmetry and an internal voltage source arising from the motion of the C-terminal tails causes cations to be pumped across the microtubule wall and propagate in both directions down the microtubule through the lumen, returning to the bulk solution through its open ends. This effect is demonstrated to add directly to the longitudinal current through the lumen resulting from an external voltage source applied across the two ends of the microtubule. The predicted persistent currents directed through the microtubule wall and along the lumen could be significant in directing the dissipation of weak, endogenous potential gradients toward one end of the microtubule within the cellular environment.

Tubulin polymerization modifies cardiac sodium channel expression and gating

AIMS

Treatment with the anticancer drug taxol (TXL), which polymerizes the cytoskeleton protein tubulin, may evoke cardiac arrhythmias based on reduced human cardiac sodium channel (Na(v)1.5) function. Therefore, we investigated whether enhanced tubulin polymerization by TXL affects Na(v)1.5 function and expression and whether these effects are beta1-subunit-mediated.

METHODS AND RESULTS

Human embryonic kidney (HEK293) cells, transfected with SCN5A cDNA alone (Na(v)1.5) or together with SCN1B cDNA (Na(v)1.5 + beta1), and neonatal rat cardiomyocytes (NRCs) were incubated in the presence and in the absence of 100 microM TXL. Sodium current (I(Na)) characteristics were studied using patch-clamp techniques. Na(v)1.5 membrane expression was determined by immunocytochemistry and confocal microscopy. Pre-treatment with TXL reduced peak I(Na) amplitude nearly two-fold in both Na(v)1.5 and Na(v)1.5 + beta1, as well as in NRCs, compared with untreated cells. Accordingly, HEK293 cells and NRCs stained with anti-Na(v)1.5 antibody revealed a reduced membrane-labelling intensity in the TXL-treated groups. In addition, TXL accelerated I(Na) decay of Na(v)1.5 + beta1, whereas I(Na) decay of Na(v)1.5 remained unaltered. Finally, TXL reduced the fraction of channels that slow inactivated from 31% to 18%, and increased the time constant of slow inactivation by two-fold in Na(v)1.5. Conversely, slow inactivation properties of Na(v)1.5 + beta1 were unchanged by TXL.

CONCLUSION

Enhanced tubulin polymerization reduces sarcolemmal Na(v)1.5 expression and I(Na) amplitude in a beta1-subunit-independent fashion and causes I(Na) fast and slow inactivation impairment in a beta1-subunit-dependent way. These changes may underlie conduction-slowing-dependent cardiac arrhythmias under conditions of enhanced tubulin polymerization, e.g. TXL treatment and heart failure.

Colchicine, a microtubule depolymerizing agent, inhibits myocardial apoptosis in rats

Heart failure is the most common cardiovascular disease with high mortality and morbidity. Both enhanced microtubule polymerization and cardiomyocyte apoptosis are involved in the pathogenesis of heart failure. However, the link between the two mechanisms remains to be elucidated. In this study, we thus address this important issue in cultured cardiomyocytes from Wistar rats in vitro and in angiotensin II (ATII)-infused rats in vivo. Confocal microscopy examination showed that in cultured rat cardiomyocytes, micrographic density of microtubules was increased by paclitaxel, a microtubule-polymerizing agent, and decreased by colchicine, a microtubule-depolymerizing agent, but not affected by ATII, isoproterenol, or tumor necrosis factor-alpha alone. Immunoblotting analysis showed that Bax/Bcl-2 ratio, which is associated with the activation of caspase-3, was significantly increased in ATII-stimulated cultured cardiomyocytes in vitro and in ATII-infused rats in vivo, both of which were inhibited by co-treatment with colchicine. Caspase-3 and TUNEL assay to detect apoptosis in vitro demonstrated that paclitaxel or ATII alone significantly enhanced and their combination further accelerated cardiomyocyte apoptosis, which was again significantly inhibited by colchicine. Caspase-3 and TUNEL assay in vivo also demonstrated that ATII infusion significantly increased myocardial apoptosis and that co-treatment with colchicine significantly suppressed the apoptosis. In conclusion, these results indicate that a microtubule-depolymerizing agent could be a potential therapeutic strategy for treatment of heart failure.

Effects of brain natriuretic peptide on contraction and intracellular Ca2+ in ventricular myocytes from the streptozotocin-induced diabetic rat

The streptozotocin (STZ)-treated rat is a widely studied experimental model of diabetes mellitus (DM). Its pathophysiology includes hypoinsulinemia, hyperglycemia, cardiac hypertrophy, and a cardiomyopathy that is characterized by the presence of diastolic and/or systolic contractile dysfunction. As part of their endocrine function cardiomyocytes in the heart produce and secrete a family of related peptide hormones called the natriuretic peptides that include A-type natriuretic peptide (ANP) and B-type natriuretic peptide (BNP). ANP and BNP levels are variously augmented in patients with hypertension, cardiac overload, in the ventricles of failing or hypertrophied heart, in cardiac heart failure, in acute myocardial infarction (MI), and in some circumstances in DM. In this article, the effects of BNP on ventricular myocyte contraction and Ca2+ transport in STZ-induced diabetic rats have been investigated. BNP concentration was significantly increased in blood plasma and in atrial muscle in STZ-induced diabetic rats compared to age-matched controls. BNP was 11.9 +/- 0.9 ng/mL in plasma from diabetic rats compared to 6.7 +/- 1.6 ng/mL in controls and 15.8 +/- 2.0 ng/mg protein in diabetic atrial muscle compared to 8.5 +/- 1.0 ng/mg protein in controls. The heart weight to body weight ratio, an indicator of hypertrophy, was significantly increased in diabetic rat heart (4.3 +/- 0.1 mg/g) compared to controls (3.7 +/- 0.04 mg/g). The amplitude of shortening was not significantly altered in diabetic myocytes (10.3 +/- 0.4%) compared to controls (10.9 +/- 0.4%). BNP reduced the amplitude of shortening to a greater extent in diabetic myocytes (8.1 +/- 0.6%) compared to controls (10.1 +/- 0.4%). The time to peak (TPK) shortening was significantly prolonged in diabetic myocytes (254 +/- 8 ms) compared to controls (212 +/- 5 ms) and was not additionally altered by BNP. The time to half relaxation of shortening was also significantly prolonged in diabetic myocytes (131 +/- 8 ms) compared to controls (111 +/- 5 ms). BNP (10(-8) to 10(-6) M) normalized the time to half relaxation of shortening in diabetic myocytes to that of controls. Time to peak (TPK) shortening of Ca2+ was not different between diabetic and control rats. However, BNP (10(-7) M) increases TPK of Ca2+ significantly. The amplitude of the Ca2+ transient was significantly increased in diabetic myocytes (0.42 +/- 0.02 Ratio units [RU]) compared to controls (0.36 +/- 0.02 RU) and was not additionally altered by BNP. BNP may have a protective role in STZ-induced diabetic rat heart.

Effects of streptozotocin-induced diabetes on contraction and calcium transport in rat ventricular cardiomyocytes

Cardiovascular diseases are the major cause of morbidity and mortality in diabetic patients. Contractile function of the heart is frequently compromised in the clinical setting and in experimental models of diabetes mellitus (DM). This article investigated the effect of streptozotocin (STZ)-induced type 1 DM on contraction, L-type calcium (Ca2+) current (I(Ca(2+)L)), and on cytosolic calcium concentrations [Ca2+]i in ventricular myocytes of the rat heart. After 4-10 weeks of STZ treatment, blood glucose levels in diabetic animals were significantly (P < 0.05) higher compared to age-matched controls. Diabetic rats have significantly (P < 0.05) reduced body, reduced heart weight, and reduced viability of ventricular myocytes compared to controls. The amplitude of I(Ca(2+)L) and amplitude of contraction were significantly reduced (P < 0.05) at test potentials in the range -10 mV to +20 mV and -30 mV to +40 mV, respectively, in myocytes from diabetic animals compared to age-matched controls. Moreover, there was a significant (P < 0.05) delay in electrically stimulated and caffeine-evoked time to half relaxation of the Ca2+ transient in myocytes from diabetic animals compared to controls. A similar effect was obtained in myocytes treated with a combination of caffeine and nickel chloride (NiCl2). It is concluded that the diabetes-induced voltage-dependent decrease in contraction is associated with reduced Ca2+ channel activities and prolonged diastolic cytosolic Ca2+ compared to age-matched control. Taken together, the results suggest that Ca2+ homeostasis is deranged during DM and this may be expressed at the level of the Na+/Ca2+ exchanger.

Inhibition of ErbB2/neuregulin signaling augments paclitaxel-induced cardiotoxicity in adult ventricular myocytes

Paclitaxel (Taxol) has been successfully combined with the monoclonal antibody trastuzumab (Herceptin) in the treatment of ErbB2 overexpressing cancers. However, this combination therapy showed an unexpected synergistic increase in cardiac dysfunction. We have studied the mechanisms of paclitaxel/anti-ErbB2 cardiotoxicity in adult rat ventricular myocytes (ARVM). Myofibrillar organization was assessed by immunofluorescence microscopy and cell viability was tested by the TUNEL-, LDH- and MTT-assay. Oxidative stress was measured by DCF-fluorescence and myocyte contractile function by video edge-detection and fura-2 fluorescence. Treatment of ARVM with paclitaxel or antibodies to ErbB2 caused a significant increase in myofilament degradation, similarly as observed with an inhibitor of MAPK-signaling, but not apoptosis, necrosis or changes in mitochondrial activity. Paclitaxel-treatment and anti-ErbB2 reduced Erk1/2 phosphorylation. Paclitaxel increased diastolic calcium, shortened relaxation time and reduced fractional shortening in combination with anti-ErbB2. A minor increase in oxidative stress by paclitaxel or anti-ErbB2 was found. We conclude, that concomitant inhibition of ErbB2 receptors and paclitaxel treatment has an additive worsening effect on adult cardiomyocytes, mainly discernible in changes of myofibrillar structure and function, but in the absence of cell death. A potential mechanism is the modulation of the MAPK/Erk1/2 signaling by both drugs.

Developmental regulation of cardiac MAP4 protein expression

It has been shown that the level of expression of microtubule-associated protein 4 (MAP4) mRNAs changes throughout neonatal heart development [Chapin SJ, et al. 1995. Biochemistry 34:2289]. In the present study, both immunofluorescence and western blotting methods were used to monitor MAP4 protein expression levels in the developing heart. By both methods, it was shown that the levels of total MAP4 protein were maximal during the first postnatal week, and then declined progressively to adulthood. In addition, four major electrophoretic species that reacted with MAP4-specific antibodies (called bands 1-4) were observed in all heart tissue samples. Three of the four bands decreased in abundance throughout postnatal development, but at different rates. The fourth band remained relatively constant in abundance with increasing postnatal age. To determine if phosphorylation events might contribute to this heterogeneity, western blotting experiments using phospho-specific antibodies and phosphatase digestion of extract samples were performed. No phosphorylation-specific antibody staining was observed and no significant changes were demonstrated in the bands after phosphatase treatment, implying that the observed complexity was due mainly to alternative start site or differential isoform expression. Finally, it was discovered that cardiomyocyte MAP4 associated with drug- and cold-stable microtubules in early neonatal myocytes. Thus, the complex regulation of MAP4 protein expression may play a key role in the functional differentiation of myocyte microtubules during heart development.

Polycystin-2 cation channel function is under the control of microtubular structures in primary cilia of renal epithelial cells

Mutations in the gene encoding polycystin-2 (PC2) result in autosomal dominant polycystic kidney disease and defects in left-right asymmetry during embryogenesis. PC2 is a TRP-type Ca(2+)-permeable non-selective cation channel, which is expressed in kidney and other organs. PC2 is present and functional in microtubule-containing primary cilia of renal epithelial cells. However, no information is yet available as to whether PC2 interacts with microtubules. Here, we assessed the role of microtubular dynamics in regulating PC2 channel function in primary cilia. Isolated ciliary membranes from LLC-PK1 epithelial cells were reconstituted in a lipid bilayer system. The acute addition of the microtubular disrupter colchicine (15 mum) rapidly abolished, whereas the addition of the microtubular stabilizer paclitaxel (taxol, 15 mum) increased ciliary PC2 channel activity. The further addition of alpha-tubulin plus GTP also stimulated PC2 channel activity in ciliary membranes. However, alpha-tubulin and GTP had no effect on in vitro translated PC2. Using the yeast two-hybrid assay, we found that PC2 interacts with the microtubule-dependent motor kinesin-2 subunit KIF3A, a protein involved in polycystic kidney disease. The interaction occurred through the carboxyl termini domain of both proteins, which was further confirmed by in vitro glutathione S-transferase pull-down and dot blot overlay assays. Co-immunoprecipitation experiments showed that PC2 and KIF3A are in the same complex in native HEK293, Madin-Darby canine kidney cells (MDCK), and LLC-PK1 cells. Immunofluorescent staining also showed substantial PC2 and KIF3A co-localization in primary cilia of renal epithelial cells. The data indicate that microtubular organization regulates PC2 function, which may explain, among others, the regulatory role of PC2 in the sensory function of primary cilia.

Stable microtubules contribute to cardiac dysfunction in the streptozotocin-induced model of type 1 diabetes in the rat

Cardiac microtubule stability is increased in the streptozotocin (STZ) model of type 1 diabetes. Here, we investigate the reason for increased microtubule stability, and the functional consequences of stable microtubule disruption. Ventricular myocytes were isolated from rats at 8-12 weeks after injection of STZ. A 10% increase in microtubule density, but no difference in the ratio of microtubule-associated protein 4 (MAP4) to tubulin was seen in myocytes from STZ rats. Functionally, STZ myocytes showed a tendency for reduced shortening and intracellular Ca2+ ([Ca2+]i) transient amplitude, and a significant prolongation of time to peak (ttp) shortening and [Ca2+]i. Although microtubules in STZ myocytes were less sensitive to the microtubule disruptor nocodazole (NOC; 33 microM) than control myocytes, we only saw marked functional consequences of microtubule disruption by NOC in myocytes from diabetic animals. NOC increased shortening and [Ca2+]i transient amplitude in STZ myocytes by 45 and 24%, respectively (compared with 4 and 6% in controls). Likewise, NOC decreased ttp shortening and [Ca2+]i only in STZ myocytes, such that these parameters were no longer different between the two groups. In conclusion, stable microtubules in diabetes are not associated with an increase in MAP4, but are functionally relevant to cardiac dysfunction in diabetes, regulating both [Ca2+]i and shortening.

Microtubules and actin microfilaments regulate lipid raft/caveolae localization of adenylyl cyclase signaling components

Microtubules and actin filaments regulate plasma membrane topography, but their role in compartmentation of caveolae-resident signaling components, in particular G protein-coupled receptors (GPCR) and their stimulation of cAMP production, has not been defined. We hypothesized that the microtubular and actin cytoskeletons influence the expression and function of lipid rafts/caveolae, thereby regulating the distribution of GPCR signaling components that promote cAMP formation. Depolymerization of microtubules with colchicine (Colch) or actin microfilaments with cytochalasin D (CD) dramatically reduced the amount of caveolin-3 in buoyant (sucrose density) fractions of adult rat cardiac myocytes. Colch or CD treatment led to the exclusion of caveolin-1, caveolin-2, beta1-adrenergic receptors (beta1-AR), beta2-AR, Galpha(s), and adenylyl cyclase (AC)5/6 from buoyant fractions, decreasing AC5/6 and tyrosine-phosphorylated caveolin-1 in caveolin-1 immunoprecipitates but in parallel increased isoproterenol (beta-AR agonist)-stimulated cAMP production. Incubation with Colch decreased co-localization (by immunofluorescence microscopy) of caveolin-3 and alpha-tubulin; both Colch and CD decreased co-localization of caveolin-3 and filamin (an F-actin cross-linking protein), decreased phosphorylation of caveolin-1, Src, and p38 MAPK, and reduced the number of caveolae/mum of sarcolemma (determined by electron microscopy). Treatment of S49 T-lymphoma cells (which possess lipid rafts but lack caveolae) with CD or Colch redistributed a lipid raft marker (linker for activation of T cells (LAT)) and Galpha(s) from lipid raft domains. We conclude that microtubules and actin filaments restrict cAMP formation by regulating the localization and interaction of GPCR-G(s)-AC in lipid rafts/caveolae.

Effect of cytoskeleton disruptors on L-type Ca channel distribution in rat ventricular myocytes

The cytoskeleton plays an important role in many aspects of cardiac cell function, including protein trafficking. However, the role of the cytoskeleton in determining Ca channel location in cardiac myocytes is unknown. In the present study we therefore investigated the effect of the cytoskeletal disruptors cytochalasin D, latrunculin, nocadazole and colchicine on the distribution of Ca channels in rat ventricular myocytes during culture for up to 96 h. During culture in the absence of these agents, cell edges became rounded, t-tubule density decreased, and the normal transverse distribution of the alpha1 (pore-forming) subunit of the L-type Ca channel became more punctate and peri-nuclear; these changes were associated with loss of synchronous Ca release in response to electrical stimulation. Disruption of tubulin using nocadazole or colchicine or sequestration of monomeric actin by latrunculin had no effect on these changes. In contrast, cytochalasin D inhibited these changes: cell shape, t-tubule density, transverse Ca channel staining and synchronous Ca release were maintained during culture. The protein synthesis inhibitor cycloheximide had similar effects to cytochalasin. These data suggest that cytochalasin stabilizes actin in adult ventricular myocytes in culture, thus stabilizing cell structure and function, and that actin is important in trafficking L-type Ca channels from the peri-nuclear region to the t-tubules, where they are normally located and provide the trigger for Ca release.

Distribution of atrial natriuretic peptide and its effects on contraction and intracellular calcium in ventricular myocytes from streptozotocin-induced diabetic rat

The distribution of atrial natriuretic peptide (ANP) in blood plasma and cardiac muscle and its effects on ventricular myocyte contraction and intracellular free calcium concentration [Ca2+]i in the streptozotocin (STZ)-induced diabetic rat have been investigated. Blood plasma concentration and heart atrial and ventricular contents of ANP were significantly increased in STZ-treated rats compared to age-matched controls. STZ treatment increased the number of ventricular myocytes immunolabeled with antibodies against ANP. In control myocytes the percentage of cells that labeled positively and negatively were 17% versus 83%, respectively. However, in myocytes from STZ-treated rat the percentages were 52% versus 53%. Time to peak (TPK) shortening was significantly and characteristically prolonged in myocytes from STZ-treated rats (360+/-5 ms) compared to controls (305+/-5 ms). Amplitude of the Ca2+ transient was significantly increased in myocytes from STZ-treated rats compared to controls (0.39+/-0.02 versus 0.29+/-0.02 fura-2 RU in controls) and treatment with ANP reduced the amplitude of the Ca2+ transient to control levels. ANP may have a protective role in STZ-induced diabetic rat heart.

Post-translational modifications of tubulin and microtubule stability in adult rat ventricular myocytes and immortalized HL-1 cardiomyocytes

Little is known about the subcellular distribution and the dynamics of tubulins in adult cardiac myocytes although both are modified during cardiac hypertrophy and heart failure. Using confocal microscopy, we examined post-translational modifications of tubulin in fully differentiated ventricular myocytes isolated from adult rat hearts, as well as in immortalized and dividing HL-1 cardiomyocytes. Detyrosinated Glu-alpha-tubulin was the most abundant post-translationally modified tubulin found in ventricular myocytes, while acetylated- and delta2-alpha-tubulins were found in lower amounts or absent. In contrast, dividing HL-1 cardiomyocytes exhibited high levels of tyrosinated or acetylated alpha-tubulins. A mild nocodazole treatment (0.1 microM, 1 h) disrupted microtubules in HL-1 myocytes, but not in adult ventricular myocytes. A stronger treatment (10 microM, 2 h) was required to disassemble tubulins in adult myocytes. Glu-alpha-tubulin containing microtubules were more resistant to nocodazole treatment in HL-1 cardiomyocytes than in ventricular myocytes. Endogenous activation of the cAMP pathway with the forskolin analog L858051 (20 microM) or the beta-adrenergic agonist isoprenaline (10 microM) disrupted the most labile microtubules in HL-1 cardiomyocytes. In contrast, isoprenaline (10 microM), cholera toxin (200 ng/ml, a G(S)-protein activator), L858051 (20 microM) or forskolin (10 microM) had no effect on the microtubule network in ventricular myocytes. In addition, intracellular Ca2+ accumulation induced either by thapsigargin (2 microM) or caffeine (10 mM) did not modify microtubule stability in ventricular myocytes. Our data demonstrate the unique stability of the microtubule network in adult cardiac myocytes. We speculate that microtubule stability is required to support cellular integrity during cardiac contraction.

Effects of halothane on contraction and intracellular calcium in ventricular myocytes from streptozotocin-induced diabetic rats

BACKGROUND

Some of the cellular targets affected by volatile anaesthetics (e.g. halothane) which contribute to the negative inotropic effects of these agents are also affected during the progression of diabetic cardiomyopathy. A previous report suggested that halothane inhibited contraction to a lesser extent in papillary muscle from diabetic animals and so the aim of this study was to investigate possible mechanisms underlying this effect.

METHODS

Contractility and cytosolic calcium ion (Ca(2+)) transients were measured (fura-2) in ventricular myocytes isolated from control and streptozotocin (STZ)-induced diabetic rats in the absence and presence of halothane 0.6 mmol litre(-1) at 1 Hz stimulation. Sarcoplasmic reticulum (SR) Ca(2+) content was assessed by rapid application of caffeine. All experiments were carried out at 36-37 degrees C.

RESULTS

The amplitude of shortening, the electrically evoked Ca(2+) transient, SR Ca(2+) content and myofilament Ca(2+) sensitivity, though not altered by STZ treatment, were significantly reduced by halothane to a similar extent in control and STZ myocytes. The time course of contraction and Ca(2+) transient were prolonged in myocytes from STZ-treated rats compared with controls but this was not altered further by halothane. STZ treatment appeared to reduce Ca(2+) efflux from the cell, an effect reversed by halothane.

CONCLUSIONS

In contrast to a previous report, we could find no evidence of amelioration of the negative inotropic effect of halothane in myocytes from the STZ-induced diabetic rat. Contractility, the cytosolic Ca(2+) transient, SR Ca(2+) content and myofilament Ca(2+) sensitivity were qualitatively similar in control and STZ myocytes and were all depressed to the same extent by halothane.

Autonomic regulation of calcium and potassium channels is oppositely modulated by microtubules in cardiac myocytes

We recently showed that colchicine treatment of rat ventricular myocytes increases the L-type Ca2+ current (I(Ca)) and intracellular Ca2+ concentration ([Ca2+](i)) transients and interferes with adrenergic signaling. These actions were ascribed to adenylyl cyclase (AC) stimulation after G(s) activation by alpha,beta-tubulin. Colchicine depolymerizes microtubules into alpha,beta-tubulin dimers. This study analyzed muscarinic signals in myocytes with intact or depolymerized microtubules. Myocytes were loaded with the Ca2+ indicator fluo 3 and were field stimulated at 1 Hz or voltage clamped. In untreated cells, carbachol (CCh; 1 microM) induced ACh-activated K(+) current [I(K(ACh))], which happens via betagamma-subunits from the activation of G(i). Carbachol also reduced [Ca2+](i) transients and contractions. Once G(i) is activated by muscarinic agonist, the alpha(i)-subunit is released from the betagamma-subunits, but it is silent, and its inhibition of the AC/cAMP cascade, manifested by I(Ca) reduction, is not seen unless AC has been previously activated. In colchicine-treated cells, CCh caused greater reductions of [Ca2+](i) transients and contractions than in untreated cells. The alpha(i)-subunit became effective in signaling through the AC/cAMP cascade and reduced I(Ca) without changing its voltage-dependence. Isoproterenol (Iso) regained its efficacy and reversed I(Ca) inhibition by CCh. Stimulation of I(Ca) by forskolin persisted in colchicine-treated cells when Iso was ineffective. The effect of CCh on I(K(ACh)) was occluded in colchicine-treated cells. Colchicine treatment, per se, may increase I(K(ACh)) by betagamma-subunits released from G(s) to mask this effect of CCh. Microtubules suppress I(Ca) regulation by alpha(i); their disruption releases restraints that unmask muscarinic inhibition of I(Ca). Summarily, colchicine treatment reverses regulation of ventricular excitation-contraction coupling by autonomic agents.

Cytoskeletal modulation of electrical and mechanical activity in cardiac myocytes

The cardiac myocyte has an intracellular scaffold, the cytoskeleton, which has been implicated in several cardiac pathologies including hypertrophy and failure. In this review we describe the role that the cytoskeleton plays in modulating both the electrical activity (through ion channels and exchangers) and mechanical (or contractile) activity of the adult heart. We focus on the 3 components of the cytoskeleton, actin microfilaments, microtubules, and desmin filaments. The limited visual data available suggest that the subsarcolemmal actin cytoskeleton is sparse in the adult myocyte. Selective disruption of cytoskeletal actin by pharmacological tools has yet to be verified in the adult cell, yet evidence exists for modulation of several ionic currents, including I(CaL), I(Na), I(KATP), I(SAC) by actin microfilaments. Microtubules exist as a dense network throughout the adult cardiac cell, and their structure, architecture, kinetics and pharmacological manipulation are well described. Both polymerised and free tubulin are functionally significant. Microtubule proliferation reduces contraction by impeding sarcomeric motion; modulation of sarcoplasmic reticulum Ca(2+) release may also be involved in this effect. The lack of effect of microtubule disruption on cardiac contractility in adult myocytes, and the concentration-dependent modulation of the rate of contraction by the disruptor nocodazole in neonatal myocytes, support the existence of functionally distinct microtubule populations. We address the controversy regarding the stimulation of the beta-adrenergic signalling pathway by free tubulin. Work with mice lacking desmin has demonstrated the importance of intermediate filaments to normal cardiac function, but the precise role that desmin plays in the electrical and mechanical activity of cardiac muscle has yet to be determined.

Validation of formamide as a detubulation agent in isolated rat cardiac cells

Kawai M, Hussain M, and Orchard CH. Am J Heart Circ Physiol 277: H603-H609, 1999 developed a technique to detubulate rat ventricular myocytes using formamide and showed that detubulation results in a decrease in cell capacitance, Ca(2+) current density, and Ca(2+) transient amplitude. We have investigated the mechanism of this detubulation and possible direct effects of formamide. Staining ventricular cells with di-8-ANEPPS showed that the t tubule membranes remain inside the cell after detubulation; trapping of FITC-labeled dextran within the t tubules showed that detubulation occurs during formamide washout and that the t tubules appear to reseal within the cell. Detubulation had no effect on the microtubule network but resulted in loss of synchronous Ca(2+) release on electrical stimulation. In contrast, formamide treatment of atrial cells did not significantly change cell capacitance, Ca(2+) current amplitude, action potential configuration, the Ca(2+) transient or the response of the Ca(2+) transient to isoprenaline. We conclude that formamide washout induces detubulation of single rat ventricular myocytes, leaving the t tubules within the cell, but without direct effects on cell proteins that might alter cell function.

Microtubule disruption by colchicine reversibly enhances calcium signaling in intact rat cardiac myocytes

Using the whole-cell patch-clamp configuration in rat ventricular myocytes, we recently reported that microtubule disruption increases calcium current (I(Ca)) and [Ca(2+)](i) transient and accelerates their kinetics by adenylyl cyclase activation. In the present report, we further analyzed the effects of microtubule disruption by 1 micromol/L colchicine on Ca(2+) signaling in cardiac myocytes with intact sarcolemma. In quiescent intact cells, it is possible to investigate ryanodine receptor (RyR) activity by analyzing the characteristics of spontaneous Ca(2+) sparks. Colchicine treatment decreased Ca(2+) spark amplitude (F/F(0): 1.78+/-0.01, n=983, versus 1.64+/-0.01, n=1660, recorded in control versus colchicine-treated cells; P<0.0001) without modifying the sarcoplasmic reticulum Ca(2+) load and enhanced their time to peak (in ms: 6.85+/-0.09, n=1185, versus 7.33+/-0.13, n=1647; P<0.0001). Microtubule disruption also induced the appearance of Ca(2+) sparks in doublets. These alterations may reflect RyR phosphorylation. To further investigate Ca(2+) signaling in cardiac myocytes with intact sarcolemma, we analyzed [Ca(2+)](i) transient evoked by field stimulation. Cells were loaded with the fluorescence Ca(2+) indicator, Fluo-3 cell permeant, and stimulated at 1 HZ: [Ca(2+)](i) transient amplitude was greater and its decay was accelerated in colchicine-treated, field-stimulated myocytes. This effect is reversible. When colchicine-treated myocytes were placed in a colchicine-free solution for 30 minutes, tubulin was repolymerized into microtubules, as shown by immunofluorescence, and the increase in [Ca(2+)](i) transient was reversed. In summary, we demonstrate that microtubule disruption by colchicine reversibly modulates Ca(2+) signaling in cardiac cells with intact sarcolemma.

Modulation of Ca(2+) signaling by microtubule disruption in rat ventricular myocytes and its dependence on the ruptured patch-clamp configuration

In the absence of hypertrophic proliferation of microtubules, microtubule disruption by colchicine does not modulate contraction of adult cardiac myocytes. However, Gomez et al (Circ Res. 2000;86:30-36) recently reported that disruption of microtubules by colchicine in ruptured patch-clamped myocytes increased I(Ca,L) density and [Ca(2+)](i) transient amplitude and depressed the response of these parameters to the beta-adrenoceptor agonist isoproterenol. These effects were ascribed to stimulation of adenylyl cyclase by increased intracellular free tubulin. In the present study, we show that in intact rat ventricular myocytes, 2 to 4 hours of exposure to 10 micromol/L colchicine had no effect on shortening or [Ca(2+)](i) transient amplitude or on the amplitude of I(Ca,L) in perforated patch-clamped cells, under basal conditions and after stimulation with 1 micromol/L isoproterenol. However, in ruptured patch-clamped myocytes, basal I(Ca,L) was 2-fold higher after treatment with colchicine compared with vehicle and, in contrast to vehicle-treated cells, I(Ca,L) did not increase in response to isoproterenol. Cell width decreased during ruptured patch-clamp experiments in colchicine-treated but not vehicle-treated myocytes. We conclude that in cells with intact sarcolemma, colchicine does not modulate Ca(2+) signaling or the response to beta stimulation. However, the combination of microtubule disruption by colchicine and the ruptured patch configuration activates I(Ca,L) and attenuates the response to beta stimulation. We propose that these effects may be due to loss of free tubulin by intracellular dialysis or to increased sensitivity to mechanical stimulation as a result of microtubule disruption. These findings have important implications for cardiomyopathies associated with decreased free tubulin or a diminished microtubular network. The full text of this article is available at http://www.circresaha.org.

Beating rate of isolated neonatal cardiomyocytes is regulated by the stable microtubule subset

We investigated the roles of microtubule (MT) dynamics (growth and shrinkage), the stable, nongrowing MT subset, the posttranslationally detyrosinated MT subset, and artificially elevated tubulin levels in the negative regulation of heart cell beating rate. We manipulated the MT populations in isolated, neonatal cardiomyocytes obtained from normal animals in several ways and then measured heart cell beating rate directly. We found that the stabilized population of MTs was sufficient to maintain a normal beating rate, whereas MT dynamics and detyrosination made no observable contribution. Furthermore, by directly and acutely increasing the level of tubulin within otherwise normally beating cells, we found that the increased tubulin (and MT) levels further depressed the beating rate. In conclusion, the stabilized MT subset is sufficient to maintain the normal beating rate in these cells, whereas increasing the MT density depresses it.