Thyroid hormone represses protein kinase C isoform expression and activity in rat cardiac myocytes

2,4,6-Tribromophenol Exposure Decreases P-Glycoprotein Transport at the Blood-Brain Barrier

2,4,6-Tribromophenol (TBP, CAS No. 118-79-6) is a brominated chemical used in the production of flame-retardant epoxy resins and as a wood preservative. In marine environments, TBP is incorporated into shellfish and consumed by predatory fish. Food processing and water treatment facilities produce TBP as a byproduct. 2,4,6-Tribromophenol has been detected in human blood and breast milk. Biologically, TBP interferes with estrogen and thyroid hormone signaling, which regulate important transporters of the blood-brain barrier (BBB). The BBB is a selectively permeable barrier characterized by brain microvessels which are composed of endothelial cells mortared by tight-junction proteins. ATP-binding cassette (ABC) efflux transporters on the luminal membrane facilitate the removal of unwanted endobiotics and xenobiotics from the brain. In this study, we examined the in vivo and ex vivo effects of TBP on two important transporters of the BBB: P-glycoprotein (P-gp, ABCB1) and Multidrug Resistance-associated Protein 2 (MRP2, ABCC2), using male and female rats and mice. 2,4,6-Tribromophenol exposure ex vivo resulted in a time- (1-3 h) and dose- (1-100 nM) dependent decrease in P-gp transport activity. MRP2 transport activity was unchanged under identical conditions. Immunofluorescence and western blotting measured decreases in P-gp expression after TBP treatment. ATPase assays indicate that TBP is not a substrate and does not directly interact with P-gp. In vivo dosing with TBP (0.4 µmol/kg) produced decreases in P-gp transport. Co-treatment with selective protein kinase C (PKC) inhibitors prevented the TBP-mediated decreases in P-gp transport activity.

Cardiac Cx43 and ECM Responses to Altered Thyroid Status Are Blunted in Spontaneously Hypertensive versus Normotensive Rats

Heart function and its susceptibility to arrhythmias are modulated by thyroid hormones (THs) but the responsiveness of hypertensive individuals to thyroid dysfunction is elusive. We aimed to explore the effect of altered thyroid status on crucial factors affecting synchronized heart function, i.e., connexin-43 (Cx43) and extracellular matrix proteins (ECM), in spontaneously hypertensive rats (SHRs) compared to normotensive Wistar Kyoto rats (WKRs). Basal levels of circulating THs were similar in both strains. Hyperthyroid state (HT) was induced by injection of T3 (0.15 mg/kg b.w. for eight weeks) and hypothyroid state (HY) by the administration of methimazol (0.05% for eight weeks). The possible benefit of omega-3 polyunsaturated fatty acids (Omacor, 200 mg/kg for eight weeks) intake was examined as well. Reduced levels of Cx43 in SHRs were unaffected by alterations in THs, unlike WKRs, in which levels of Cx43 and its phosphorylated form at serine368 were decreased in the HT state and increased in the HY state. This specific Cx43 phosphorylation, attributed to enhanced protein kinase C-epsilon signaling, was also increased in HY SHRs. Altered thyroid status did not show significant differences in markers of ECM or collagen deposition in SHRs. WKRs exhibited a decrease in levels of profibrotic transforming growth factor β1 and SMAD2/3 in HT and an increase in HY, along with enhanced interstitial collagen. Short-term intake of omega-3 polyunsaturated fatty acids did not affect any targeted proteins significantly. Key findings suggest that myocardial Cx43 and ECM responses to altered thyroid status are blunted in SHRs compared to WKRs. However, enhanced phosphorylation of Cx43 at serine368 in hypothyroid SHRs might be associated with preservation of intercellular coupling and alleviation of the propensity of the heart to malignant arrhythmias.

Fluoride Exposure Induces Inhibition of Sodium-and Potassium-Activated Adenosine Triphosphatase (Na+, K+-ATPase) Enzyme Activity: Molecular Mechanisms and Implications for Public Health

In this study, several lines of evidence are provided to show that Na + , K + -ATPase activity exerts vital roles in normal brain development and function and that loss of enzyme activity is implicated in neurodevelopmental, neuropsychiatric and neurodegenerative disorders, as well as increased risk of cancer, metabolic, pulmonary and cardiovascular disease. Evidence is presented to show that fluoride (F) inhibits Na + , K + -ATPase activity by altering biological pathways through modifying the expression of genes and the activity of glycolytic enzymes, metalloenzymes, hormones, proteins, neuropeptides and cytokines, as well as biological interface interactions that rely on the bioavailability of chemical elements magnesium and manganese to modulate ATP and Na + , K + -ATPase enzyme activity. Taken together, the findings of this study provide unprecedented insights into the molecular mechanisms and biological pathways by which F inhibits Na + , K + -ATPase activity and contributes to the etiology and pathophysiology of diseases associated with impairment of this essential enzyme. Moreover, the findings of this study further suggest that there are windows of susceptibility over the life course where chronic F exposure in pregnancy and early infancy may impair Na + , K + -ATPase activity with both short- and long-term implications for disease and inequalities in health. These findings would warrant considerable attention and potential intervention, not to mention additional research on the potential effects of F intake in contributing to chronic disease.

Pituitary-testicular Axis Dysfunction in Methimazole-induced Hypothyroidism in Rats

Introduction

Thyroid hormones play a major role in the regulation of testicular maturation and growth and in the control of Sertoli and Leydig cell functions in adulthood. When naturally occurring, hypothyroidism causes male hypogonadotropic hypogonadism and Sertoli cell function disorders, but when iatrogenic and methimazole-induced its influence on the pituitary-testicular axis function with respect to Sertoli cells is poorly known.

Material and Methods

Male adult Wistar rats (n = 14) were divided into two groups: E – taking methimazole orally for 60 days, and C – control animals. After 60 d, the concentrations in serum of testosterone, follicle-stimulating and luteinising hormones, and inhibins A and B were measured. Testicles were examined morphologically: the apoptotic Sertoli cell percentage (ASC%) and number of these cells functional per tubular mm² (FSCN/Tmm²) were calculated.

Results

In group E, inhibin A was higher while inhibin B was lower than in group C. ASC% was higher and FSCN/Tmm² lower in group E than in group C.

Conclusion

A specific modulation of Sertoli cell function in the course of methimazole-induced hypothyroidism leads to a simultaneous concentration increase in inhibin A and decrease in B. Inhibin A might share responsibility for pituitary-testicular axis dysfunction and hypogonadotropic hypogonadism in this model of hypothyroidism.

Thyroid hormones and cardiac remodeling

Thyroid hormones have many cardioprotective actions expressed mainly through the action of T3 on thyroid receptors α1 and β1. They are procontractile anti-apoptotic, anti-inflammatory, and anti-fibrotic, promote angiogenesis and regeneration, and have beneficial effects on microRNA profiles. They have proven to be anti-remodeling in numerous animal studies, mostly in rodents; a specific action on the border zone has been described. Studies in humans with DIPTA have been in conclusion. Remodeling can be defined as an increase of ≥20 % of the end-diastolic or end-systolic volume, together with a return to the fetal phenotype. An overview of animal and clinical studies is given.

Cardioprotection and thyroid hormones

The evolution of cardiac disease after an acute ischemic event depends on a complex and dynamic network of mechanisms alternating from ischemic damage due to acute coronary occlusion to reperfusion injury due to the adverse effects of coronary revascularization till post-ischemic remodeling. Cardioprotection is a new purpose of the therapeutic interventions in cardiology with the goal to reduce infarct size and thus prevent the progression toward heart failure after an acute ischemic event. In a complex biological system such as the human one, an effective cardioprotective strategy should diachronically target the network of cross-talking pathways underlying the disease progression. Thyroid system is strictly interconnected with heart homeostasis, and recent studies highlighted its role in cardioprotection, in particular through the preservation of mitochondrial function and morphology, the antifibrotic and proangiogenetic effect and also to the potential induction of cell regeneration and growth. The objective of this review was to highlight the cardioprotective role of triiodothyronine in the complexity of post-ischemic disease evolution.

Cardiac connexin-43 and PKC signaling in rats with altered thyroid status without and with omega-3 fatty acids intake

Thyroid hormones are powerful modulators of heart function and susceptibility to arrhythmias via both genomic and non-genomic actions. We aimed to explore expression of electrical coupling protein connexin-43 (Cx43) in the heart of rats with altered thyroid status and impact of omega-3 polyunsaturated fatty acids (omega-3) supplementation. Adult male Lewis rats were divided into following six groups: euthyroid controls, hyperthyroid (treated with T(3)) and hypothyroid (treated with methimazol) with or without six-weeks lasting supplementation with omega-3 (20 mg/100 g/day). Left and right ventricles, septum and atria were used for immunoblotting of Cx43 and protein kinase C (PKC). Total expression of Cx43 and its phosphorylated forms were significantly increased in all heart regions of hypothyroid rats compared to euthyroid controls. In contrast, the total levels of Cx43 and its functional phosphorylated forms were decreased in atria and left ventricle of hyperthyroid rats. In parallel, the expression of PKC epsilon that phosphorylates Cx43, at serine 368, was increased in hypothyroid but decreased in hyperthyroid rat hearts. Omega-3 intake did not significantly affect either Cx43 or PKC epsilon alterations. In conclusion, there is an inverse relationship between expression of cardiac Cx43 and the levels of circulating thyroid hormones. It appears that increased propensity of hyperthyroid while decreased of hypothyroid individuals to malignant arrhythmias may be in part attributed to the changes in myocardial Cx43.

Thyroid hormones and cardiovascular disease

Myocardial and vascular endothelial tissues have receptors for thyroid hormones and are sensitive to changes in the concentrations of circulating thyroid hormones. The importance of thyroid hormones in maintaining cardiovascular homeostasis can be deduced from clinical and experimental data showing that even subtle changes in thyroid hormone concentrations - such as those observed in subclinical hypothyroidism or hyperthyroidism, and low triiodothyronine syndrome - adversely influence the cardiovascular system. Some potential mechanisms linking the two conditions are dyslipidaemia, endothelial dysfunction, blood pressure changes, and direct effects of thyroid hormones on the myocardium. Several interventional trials showed that treatment of subclinical thyroid diseases improves cardiovascular risk factors, which implies potential benefits for reducing cardiovascular events. Over the past 2 decades, accumulating evidence supports the association between abnormal thyroid function at the time of an acute myocardial infarction (MI) and subsequent adverse cardiovascular outcomes. Furthermore, experimental studies showed that thyroid hormones can have an important therapeutic role in reducing infarct size and improving myocardial function after acute MI. In this Review, we summarize the literature on thyroid function in cardiovascular diseases, both as a risk factor as well as in the setting of cardiovascular diseases such as heart failure or acute MI, and outline the effect of thyroid hormone replacement therapy for reducing the risk of cardiovascular disease.

Role of the Thyroid System in the Dynamic Complex Network of Cardioprotection

Cardioprotection is a common goal of new therapeutic strategies in patients with coronary artery disease and/or left ventricular dysfunction. Myocardial damage following ischaemia/reperfusion injury lead to left ventricular adverse remodelling through many mechanisms arising from different cell types in different myocardial districts, namely the border and remote zone. Cardioprotection must face this complex, dynamic network of cooperating units. In this scenario, thyroid hormones can represent an effective therapeutic strategy due to the numerous actions and regulating mechanisms carried out at the level of the myocytes, interstitium and the vasculature, as well as to the activation of different pro-survival intracellular pathways involved in cardioprotection.

PKCε and allopregnanolone: functional cross-talk at the GABAA receptor level

Changes in GABAergic inhibition occur during physiological processes, during response to drugs and in various pathologies. These changes can be achieved through direct allosteric modifications at the γ-amino butyric acid (GABA) type A (GABAA) receptor protein level, or by altering the synthesis, trafficking and stability of the receptor. Neurosteroids (NSs) and protein kinase C (PKC) are potent modulators of GABAA receptors and their effects are presumably intermingled, even though evidence for this hypothesis is only partially explored. However, several PKC isoforms are able to phosphorylate the GABAA receptor, producing different functional effects. We focused on the ε isoform, that has been correlated to the sensitivity of the GABAA receptor to allosteric modulators and whose expression may be regulated in peripheral sensory neurons by NSs. The cross-talk between PKC-ε and NSs, leading to changes in GABAA receptor functionality, is considered and discussed in this perspective.

The emerging role of TRα1 in cardiac repair: potential therapeutic implications

Thyroid hormone (TH) is critical for adapting living organisms to environmental stress. Plasma circulating tri-iodothyronine (T3) levels drop in most disease states and are associated with increased oxidative stress. In this context, T3 levels in plasma appear to be an independent determinant for the recovery of cardiac function after myocardial infarction in patients. Thyroid hormone receptor α1 (TRα1) seems to be crucial in this response; TRα1 accumulates to cell nucleus upon activation of stress induced growth kinase signaling. Furthermore, overexpression of nuclear TRα1 in cardiomyocytes can result in pathological or physiological growth (dual action) in absence or presence of its ligand, respectively. Accordingly, inactivation of TRα1 receptor prevents reactive hypertrophy after myocardial infarction and results in heart failure with increased phospholamban (PLB) expression and marked activation of p38MAPK. In line with this evidence, TH is shown to limit ischemia/reperfusion injury and convert pathologic to physiologic growth after myocardial infarction via TRα1 receptor. TRα1 receptor may prove to be a novel pharmacological target for cardiac repair/regeneration therapies.

Protein kinase Cα is involved in impaired perinatal hypothyroid rat brain development

Protein kinase Cα (PKCα) has been implicated in the regulation of a variety of cellular functions, such as proliferation, differentiation, and apoptosis, in response to a diverse range of stimuli. Activated PKCα mediates oxidative stress, apoptosis, and inflammatory reaction. Thyroid hormone (TH) is essential for the proper development of the mammalian central nervous system. TH deficiency during critical periods of brain development results in permanent cognitive and neurological impairments. In the present study, we attempted to explore whether PKCα is involved in impaired brain function in developing hypothyroid rat brain. Severe perinatal hypothyroidism was obtained by administration of 30 mg/day propylthiouracil to dams. Brain PKC activity in hypothyroid pups was increased significantly in cytosol and membrane fractions. The change of membrane PKC activity was more marked than that of cytosol, and hypothyroidism led to a higher ratio of membrane PKC activity to that in cytosol, which means abnormal activation of PKC in developing hypothyroid rat brain. Thyroxine replacement partially corrected these changes. After being treated with bisindolmaleimide XI, a mainly selective inhibitor for PKCα, the hypothyroid pups showed improved place navigation test results, and further Western blot analysis showed that PKCα expression in cytosol fractions was increased in hypothyroid rat brain with or without bisindolmaleimide XI treatment, but, after treatment with bisindolmaleimide XI, PKCα content in membrane fractions decreased almost to normal. Therefore, we conclude that PKCα appears to be involved in the impaired brain development observed in perinatal hypothyroid rat brain.

Thyroid hormone and cardiac repair/regeneration: from Prometheus myth to reality?

Nature's models of repair and (or) regeneration provide substantial evidence that a natural healing process may exist in the heart. The potential for repair and (or) regeneration has been evolutionarily conserved in mammals, and seems to be restricted to the early developmental stages. This window of regeneration is reactivated during the disease state in which fetal gene reprogramming occurs in response to stress. Analogies exist between the damaged and developing heart, indicating that a regulatory network that drives embryonic heart development may control aspects of heart repair and (or) regeneration. In this context, thyroid hormone (TH), which is a critical regulator of the maturation of the myocardium, appears to have a reparative role later in adult life. Changes in TH - thyroid hormone receptor (TR) homeostasis govern the return of the injured myocardium to the fetal phenotype. Accordingly, TH can induce cardiac repair and (or) regeneration by reactivating developmental gene programming. As a proof of concept in humans, TH is found to be an independent determinant of functional recovery and mortality after myocardial infarction. The potential of TH to regenerate and (or) repair the ischemic myocardium is now awaited to be tested in clinical trials.

Thyroid hormones are involved in 5'-nucleotidase modulation in soluble fraction of cardiac tissue

AIMS

To investigate the role of TH (thyroid hormones) in 5'-nucleotidase activity and expression in cardiac soluble fraction (SF).

MAIN METHODS

Male Wistar rats received daily injections of T4 (10, 25 or 50 μg T4/100g body weight) for 14 days to develop a hyperthyroidism condition. Thyroidectomy was performed in other animals to mimic hypothyroidism, and 14 days after surgery they were submitted to TH replacement therapy.

KEY FINDINGS

T4 reduced the 5'-nucleotidase activity (T4-25, P<0.05 and T4-50, P<0.01) in the SF. Conversely, hypothyroidism significantly increased the 5'-nucleotidase activity in this fraction (P<0.001) and TH replacement therapy reversed the latter result (P<0.001 compared to hypothyroid group). The analysis of protein expression in the SF showed that 5'-nucleotidase was more expressed in hypothyroid than in the control group and that the phosphorylated state of PKC observed in this condition may contribute to a possible mechanism of 5'-nucleotidase modulation by thyroid status.

SIGNIFICANCE

Taken together, these data reveal that TH can influence adenosine production by modulating 5'-nucleotidase activity and expression, which may contribute to the cardioprotective effect and the maintenance of cardiac function under TH privation.

Oxidative stress and heart failure in altered thyroid States

Increased or reduced action of thyroid hormone on certain molecular pathways in the heart and vasculature causes relevant cardiovascular derangements. It is well established that hyperthyroidism induces a hyperdynamic cardiovascular state, which is associated with a faster heart rate, enhanced left ventricular systolic and diastolic function whereas hypothyroidism is characterized by the opposite changes. Hyperthyroidism and hypothyroidism represent opposite clinical conditions, albeit not mirror images. Recent experimental and clinical studies have suggested the involvement of ROS tissue damage under altered thyroid status. Altered-thyroid state-linked changes in heart modify their susceptibility to oxidants and the extent of the oxidative damage they suffer following oxidative challenge. Chronic increase in the cellular levels of ROS can lead to a catastrophic cycle of DNA damage, mitochondrial dysfunction, further ROS generation and cellular injury. Thus, these cellular events might play an important role in the development and progression of myocardial remodeling and heart failure in altered thyroid states (hypo- and hyper-thyroidism). The present review aims at elucidating the various signaling pathways mediated via ROS and their modulation under altered thyroid state and the possibility of antioxidant therapy.

Excessive thyroxine enhances susceptibility to apoptosis and decreases contractility of cardiomyocytes

Excessive thyroid hormone induces cardiac hypertrophy and promotes heart failure in patients with hyperthyroidism, but the mechanism remains elusive. Rats were orally administered with levothyroxine (100 microg/kg, T(4)) for 4 weeks to induce hyperthyroidism. The calculated stroke volume decreased and the shortening amplitude-frequency relationship in unloaded contraction of isolated cardiomyocytes was negative in T(4)-treated rats. Apoptotic rates increased and DNA laddering was also detectable in T(4)-treated rat hearts. By contrast, in primary cultured cardiomyocytes, T(3) induced dose-dependent hypertrophy but did not affect the apoptotic rate. Angiotensin II further increased the apoptotic rate of T(3)-induced hypertrophied cardiomyocytes. The apoptotic rate was dependent on the extent of cardiomyocyte hypertrophy. These results suggest that cardiac contractility is enhanced during the early stage of hyperthyroidism, but decreased during the late stage of hyperthyroidism. The hypertrophied cardiomyocytes were also susceptible to apoptotic stimulation by angiotensin II. Depressed cardiac contractility and enhanced apoptosis may lead to heart failure in hypertrophied myocardium.

Thyroid hormones and cardiac arrhythmias

Thyroid hormone plays an important role in cardiac electrophysiology and Ca2+ handling through both genomic and nongenomic mechanisms of action, while both actions can interfere. Chronic changes in the amount of circulating thyroid hormone due to thyroid dysfunction or systemic disease result in structural, electrophysiological and Ca2+ handling remodeling, while acute changes may affect basal activity of cardiac cells membrane systems. Consequently, long-term or rapid modulation of sarcolemmal ion channels, Ca2+ cycling proteins and intercellular communicating channels by thyroid hormone may affect heart function as well as susceptibility of the heart to arrhythmias. This aspect including pro- and anti-arrhythmic potential of thyroid hormone is highlighted in this review.

Protein kinase D links Gq-coupled receptors to cAMP response element-binding protein (CREB)-Ser133 phosphorylation in the heart

Many growth regulatory stimuli promote cAMP response element-binding protein (CREB) Ser(133) phosphorylation, but the physiologically relevant CREB-Ser(133) kinase(s) in the heart remains uncertain. This study identifies a novel role for protein kinase D (PKD) as an in vivo cardiac CREB-Ser(133) kinase. We show that thrombin activates a PKCdelta-PKD pathway leading to CREB-Ser(133) phosphorylation in cardiomyocytes and cardiac fibroblasts. alpha(1)-Adrenergic receptors also activate a PKCdelta-PKD-CREB-Ser(133) phosphorylation pathway in cardiomyocytes. Of note, while the epidermal growth factor (EGF) promotes CREB-Ser(133) phosphorylation via an ERK-RSK pathway in cardiac fibroblasts, the thrombin-dependent EGFR transactivation pathway leading to ERK-RSK activation does not lead to CREB-Ser(133) phosphorylation in this cell type. Adenoviral-mediated overexpression of PKCdelta (but not PKCepsilon or PKCalpha) activates PKD; PKCdelta and PKD1-S744E/S748E overexpression both promote CREB-Ser(133) phosphorylation. Pasteuralla multocida toxin (PMT), a direct Galpha(q) agonist that induces robust cardiomyocyte hypertrophy, also activates the PKD-CREB-Ser(133) phosphorylation pathway, leading to the accumulation of active PKD and Ser(133)-phosphorylated CREB in the nucleus, activation of a CRE-responsive promoter, and increased Bcl-2 (CREB target gene) expression in cardiomyocyte cultures. Cardiac-specific Galpha(q) overexpression also leads to an increase in PKD-Ser(744)/Ser(748) and CREB-Ser(133) phosphorylation as well as increased Bcl-2 protein expression in the hearts of transgenic mice. Collectively, these studies identify a novel Galpha(q)-PKCdelta-PKD-CREB-Ser(133) phosphorylation pathway that is predicted to contribute to cardiac remodeling and could be targeted for therapeutic advantage in the setting of heart failure phenotypes.

Thyroid hormone attenuates cardiac remodeling and improves hemodynamics early after acute myocardial infarction in rats

OBJECTIVE

Cardiac remodeling of viable myocardium occurs after acute myocardial infarction (AMI) and further contributes to cardiac dysfunction. The present study explored whether thyroid hormone (TH) administered shortly after AMI in rats can attenuate cardiac remodeling and improve cardiac function. TH regulates important structural and regulatory proteins in the myocardium including myosin isoform expression and calcium cycling proteins.

METHODS

AMI was induced in Wistar male rats by ligating left coronary artery (AMI, n=10), while sham-operated rats were used as controls (SHAM, n=10). Animals with acute myocardial infarction were also treated with 0.05% thyroid powder in food (AMI-THYR, n=10). Within 2 weeks, cardiac function was impaired as assessed by echocardiography and under isometric conditions in Langendorff preparations.

RESULTS

Ejection fraction (EF%) was 71.5 (SEM, 2.7) in SHAM versus 30.0 (2.0) in AMI, P<0.05. +dp/dt was 3886 (566) in SHAM versus 2266 (206) in AMI hearts, P<0.05 and -dp/dt was 1860 (46) in SHAM versus 1633 (120) in AMI hearts, P=ns. Such changes were associated with alterations in myosin isoform expression in the non-infarcted area; AMI hearts expressed 34% alpha-MHC and 66% beta-MHC versus 52% alpha-MHC and 48% beta-MHC in SHAM, P<0.05, while the expression of SERCA and phospholamban (PLB) remained unchanged. Furthermore, a mismatch of left ventricular size and cardiac mass (2*Posterior Wall thickness/LVIDd was decreased) was observed. After TH treatment, AMI-THYR hearts expressed 71% alpha-MHC and 29% beta-MHC, P<0.05 versus SHAM and AMI and the ratio of SERCA/PLB was increased by 2.0-fold, P<0.05 versus SHAM and AMI. These changes corresponded to a marked improvement in cardiac function; EF% was raised to 45.8 (1.7), P<0.05 versus AMI while +dp/dt and -dp/dt were 3800 (435) and 2600 (200), respectively, in AMI-THYR hearts, P<0.05 versus AMI. The ratio of 2*Posterior Wall thickness/LVIDd was normalized.

CONCLUSIONS

Thyroid hormone administration early after infarction attenuates cardiac remodeling and significantly improves myocardial performance.

Effects of triiodo-thyronine on angiotensin-induced cardiomyocyte hypertrophy: reversal of increased beta-myosin heavy chain gene expression

Thyroid hormone-induced cardiac hypertrophy is similar to that observed in physiological hypertrophy, which is associated with high cardiac contractility and increased alpha-myosin heavy chain (alpha-MHC, the high ATPase activity isoform) expression. In contrast, angiotensin II (Ang II) induces an increase in myocardial mass with a compromised contractility accompanied by a shift from alpha-MHC to the fetal isoform beta-MHC (the low ATPase activity isoform), which is considered as a pathological hypertrophy and inevitably leads to the development of heart failure. The present study is designed to assess the effect of thyroid hormone on angiotensin II-induced hypertrophic growth of cardiomyocytes in vitro. Cardiomyocytes were prepared from hearts of neonatal Wistar rats. The effects of Ang II and 3,3',5-triiodo-thyronine (T3) on incorporations of [3H]-thymine and [3H]-leucine, MHC isoform mRNA expression, PKC activity, and PKC isoform protein expression were studied. Ang II enhanced [3H]-leucine incorporation, beta-MHC mRNA expression, PKC activity, and PKCepsilon expression and inhibited alpha-MHC mRNA expression in cardiomyocytes. T3 treatment prevented Ang II-induced increases in PKC activity, PKCepsilon, and beta-MHC mRNA overexpression and favored alpha-MHC mRNA expression. Thyroid hormone appears to be able to reprogram gene expression in Ang II-induced cardiac hypertrophy, and a PKC signal pathway may be involved in such remodeling process.

Thyroid hormones increase inducible nitric oxide synthase gene expression downstream from PKC-ζ in murine tumor T lymphocytes

Regulation of cell proliferation by thyroid hormone (TH) has been demonstrated, but the effect of THs and the mechanisms involved in lymphocyte activity have not been elucidated. Differential expression of PKC isoenzymes and high nitric oxide synthase (NOS) activity have been described in tumor T lymphocytes. We have analyzed the direct actions of TH on normal T lymphocytes and BW5147 T lymphoma cells in relation to PKC and NOS activities. THs increased tumor and mitogen-induced normal T lymphocyte proliferation. PKC isoenzyme-selective blockers impaired these effects in both cell types, indicating the participation of Ca2+-dependent and -independent isoenzymes in normal and tumor cells, respectively. TH actions were blunted by extra- and intracellular Ca2+ blockers only in normal T lymphocytes, whereas NOS blockers impaired TH-induced proliferation in T lymphoma cells. Incubation for 24 h with TH induced a rise in total and membrane-associated PKC activities in both cell types and led to a rapid and transient effect only in tumor cells. THs increased atypical PKC-ζ expression in BW5147 cells and classical PKC isoenzymes in mitogen-stimulated normal T cells. TH augmented NOS activity and inducible NOS protein and gene expression only in tumor cells. Blockade of PKC and the atypical PKC-ζ isoform inhibited TH-mediated stimulation of inducible NOS and cell proliferation. These results show, for the first time, that differential intracellular signals are involved in TH modulation of lymphocyte physiology and pathophysiology.

Nuclear localization of protein kinase C-α induces thyroid hormone receptor-α1 expression in the cardiomyocyte

Maladaptive cardiac hypertrophy results in phenotypic changes in several genes that are thyroid hormone responsive, suggesting that thyroid hormone receptor (TR) function may be altered by cellular kinases, including protein kinase C (PKC) isozymes that are activated in pathological hypertrophy. To investigate the role of PKC signaling in regulating TR function, cultured neonatal rat ventricular myocytes were transduced with adenovirus (Ad) expressing wild-type (wt) or kinase-inactive (dn) PKCα or constitutively active (ca) PKCδ and PKCε. Overexpression of wtPKCα, but not caPKCδ or caPKCε, induced a 28-fold increase ( P < 0.001) in TRα1 protein in the nuclear compartment and a smaller increase in the cytosol. Furthermore, TRα1 mRNA was increased 55-fold ( P < 0.001). This effect of PKCα was dependent on its kinase activity because dnPKCα was without effect. Phorbol 12-myristate 13-acetate (PMA) induced nuclear translocation of endogenous PKCα and Ad-wtPKCα concomitantly with an increase in nuclear TRα1 protein. In contrast, PMA-induced nuclear translocation of dnPKCα resulted in a decrease of TRα1. The increase in TRα1 protein in Ad-wtPKCα-transduced cardiomyocytes was not the result of a reduced rate of protein degradation, nor was the half-life of TRα1 mRNA prolonged, suggesting a PKCα-mediated effect on TRα transcription. Although phosphorylation of ERK1/2 was increased in Ad-wtPKCα-transduced cells, inhibition of phospho-ERK did not change TRα1 expression. PKCα overexpression in cardiomyocytes caused marked repression of triiodothyronine (T3)-responsive genes, α-myosin heavy chain, and the sarcoplasmic reticulum calcium-activated adenosinetriphosphatase SERCA2. Treatment with T3for 4 h resulted in significant reductions of PKCα in nuclear and cytosolic compartments, and decreased TRα1 mRNA and protein, with normalization of phenotype. These results implicate PKCα as a regulator of TR function and suggest that nuclear localization of PKCα may control transcription of the TRα gene, and consequently, affect cardiac phenotype.

Thyroid hormone receptors alpha1 and beta1 are downregulated in the post-infarcted rat heart: consequences on the response to ischaemia-reperfusion

There is accumulating evidence that thyroid hormone metabolism is altered after myocardial infarction (AMI) but its physiological relevance remains largely unknown. The present study investigated the possible role of thyroid hormone signaling in the response of the post-infarcted heart to ischaemia-reperfusion. Wistar rats were subjected to left coronary artery ligation (AMI), or sham operation (SHAM). After 8 weeks, hearts from AMI and SHAM rats were perfused in Langendorff mode and subjected to 20 min of zero-flow global ischaemia (I) and 45 min of reperfusion (R); AMI(I/R), n = 7 and SHAM(I/R), n = 7. Basal left ventricular pressure (LVDP), +dp/dt, and -dp/dt were significantly reduced. Left ventricular weight of the viable myocardium was increased by 14% in the AMI as compared to SHAM hearts, P < 0.05. T(3) and T(4) plasma levels in nM were 1.83 (0.08) and 53.3 (2.9) for SHAM and 1.76 (0.06) and 59.4 (5.2) for AMI rats, respectively, P > 0.05. TRalpha1 and TRbeta1 expression levels were 1.3- and 1.8-fold less in AMI than in SHAM hearts, P < 0.05. Furthermore, SERCA and NHE1 expression levels were 2.1- and 1.8-fold less in AMI than in SHAM, P < 0.05. PKCepsilon was 1.35-fold more in AMI compared to SHAM, P < 0.05. Myocardial glycogen content (in micromol/g) was 7.8 (1.2) in AMI as compared to 4.4 (0.5) for SHAM hearts, P < 0.05. After I/R, left ventricular end-diastolic pressure at 45 min of R (LVEDP45 in mmHg) was 20.3 (3.2) for AMI(I/R) vs 50.6 (4.8) mmHg for SHAM(I/R), P < 0.05. LDH release per gram of tissue was 251 (103) for AMI(I/R) and 762 (74) for SHAM(I/R), P < 0.05. In conclusion, TRalpha1 and TRbeta1 are downregulated after myocardial infarction and this was associated with altered expression of thyroid hormone responsive genes and increased tolerance of the post-infarcted heart to ischaemia-reperfusion injury.

Stimulus-specific Differences in Protein Kinase Cδ Localization and Activation Mechanisms in Cardiomyocytes

Protein kinase C (PKC) isoforms play key roles in the regulation of cardiac contraction, ischemic preconditioning, and hypertrophy/failure. Models of PKC activation generally focus on lipid cofactor-induced PKC translocation to membranes. This study identifies tyrosine phosphorylation as an additional mechanism that regulates PKC delta actions in cardiomyocytes. Using immunoblot analysis with antibodies to total PKC delta and PKC delta-pY(311), we demonstrate that PKC delta partitions between soluble and particulate fractions (with little Tyr(311) phosphorylation) in resting cardiomyocytes. Phorbol 12-myristate 13-acetate (PMA) promotes PKC delta translocation to membranes and phosphorylation at Tyr(311). H(2)O(2) also increases PKC delta-pY(311) in association with its release from membranes. Both PMA- and H(2)O(2)-dependent increases in PKC delta-pY(311) are mediated by Src family kinases, but they occur via different mechanisms. The H(2)O(2)-dependent increase in PKC delta-pY(311) results from Src activation and increased Src-PKC delta complex formation. The PMA-dependent increase in PKC delta-pY(311) results from a lipid cofactor-induced conformational change that renders PKC delta a better substrate for phosphorylation by precomplexed Src kinases (without Src activation). PKC delta-Y(311) phosphorylation does not grossly alter the kinetics of PMA-dependent PKC delta down-regulation. Rather, tyrosine phosphorylation regulates PKC delta kinase activity. PKC delta is recovered from the soluble fraction of H(2)O(2)-treated cardiomyocytes as a tyrosine-phosphorylated, lipid-independent enzyme with altered substrate specificity. In vitro PKC delta phosphorylation by Src also increases lipid-independent kinase activity. The magnitude of this effect varies, depending upon the substrate, suggesting that tyrosine phosphorylation fine-tunes PKC delta substrate specificity. The stimulus-specific modes for PKC delta signaling identified in this study allow for distinct PKC delta-mediated phosphorylation events and responses during growth factor stimulation and oxidant stress in cardiomyocytes.

Caveolae-associated proteins in cardiomyocytes: caveolin-2 expression and interactions with caveolin-3

Caveolin-3 the muscle-specific caveolin isoform, acts like the more ubiquitously expressed caveolin-1 to sculpt caveolae, specialized membrane microdomains that serve as platforms to organize signal transduction pathways. Caveolin-2 is a structurally related isoform that alone does not drive caveolae biogenesis; rather, caveolin-2 cooperates with caveolin-1 to form caveolae in nonmuscle cells. Although caveolin-2 might be expected to interact in an fashion analogous to that of caveolin-3, it generally has not been detected in cardiomyocytes. This study shows that caveolin-2 and caveolin-3 are detected at low levels in ventricular myocardium and increase dramatically with age or when neonatal cardiomyocytes are placed in culture. In contrast, flotillins (caveolin functional homologs) are expressed at relatively constant levels in these preparations. In neonatal cardiac cultures, caveolin-2 and -3 expression is not influenced by thyroid hormone (a postnatal regulator of other cardiac gene products). The further evidence that caveolin-2 coimmunoprecipitates with caveolin-3 and floats with caveolin-3 by isopycnic centrifugation in cardiomyocyte cultures suggests that caveolin-2 may play a role in caveolae biogenesis and influence cardiac muscle physiology.

Cross-regulation of novel protein kinase C (PKC) isoform function in cardiomyocytes. Role of PKC epsilon in activation loop phosphorylations and PKC delta in hydrophobic motif phosphorylations

Recent studies identify conventional protein kinase C (PKC) isoform phosphorylations at conserved residues in the activation loop and C terminus as maturational events that influence enzyme activity and targeting but are not dynamically regulated by second messengers. In contrast, this study identifies phorbol 12-myristoyl 13-acetate (PMA)- and norepinephrine-induced phosphorylations of PKC epsilon (at the C-terminal hydrophobic motif) and PKC delta (at the activation loop) as events that accompany endogenous novel PKC (nPKC) isoform activation in neonatal rat cardiomyocytes. Agonist-induced nPKC phosphorylations are prevented (and the kinetics of PMA-dependent PKC down-regulation are slowed) by pharmacologic inhibitors of nPKC kinase activity. PKC delta is recovered from PMA-treated cultures with increased in vitro lipid-independent kinase activity (and altered substrate specificity); the PMA-dependent increase in PKC delta kinase activity is attenuated when PKC delta activation loop phosphorylation is prevented. To distinguish roles of individual nPKC isoforms in nPKC phosphorylations, wild-type (WT) and dominant negative (DN) PKC delta and PKC epsilon mutants were introduced into cardiomyocyte cultures using adenovirus-mediated gene transfer. WT-PKC delta and WT-PKC epsilon are highly phosphorylated at activation loop and hydrophobic motif sites, even in the absence of allosteric activators. DN-PKC delta is phosphorylated at the activation loop but not the hydrophobic motif; DN-PKC epsilon is phosphorylated at the hydrophobic motif but not the activation loop. Collectively, these results identify a role for PKC epsilon in nPKC activation loop phosphorylations and PKC delta in nPKC hydrophobic motif phosphorylations. Agonist-induced nPKC isoform phosphorylations that accompany activation/translocation of the enzyme contribute to the regulation of PKC delta kinase activity, may influence nPKC isoform trafficking/down-regulation, and introduce functionally important cross-talk for nPKC signaling pathways in cardiomyocytes.

Role of PKC in autocrine regulation of rat ventricular K+ currents by angiotensin and endothelin

Transient and sustained K(+) currents were measured in isolated rat ventricular myocytes obtained from control, steptozotocin-induced (Type 1) diabetic, and hypothyroid rats. Both currents, attenuated by the endocrine abnormalities, were significantly augmented by in vitro incubation (>6 h) with the angiotensin-converting enzyme inhibitor quinapril or the angiotensin II (ANG II) receptor blocker saralasin. Western blots indicated a parallel increase in Kv4.2 and Kv1.2, channel proteins that underlie the transient and (part of the) sustained currents. Under diabetic and hypothyroid conditions, both currents were also augmented by an endothelin receptor blocker (PD142893) or by an endothelin-converting enzyme inhibitor. Kv4.2 density was also enhanced by PD142893. Incubation (>5 h) with the PKC inhibitor bis-indolylmaleimide augmented both currents, whereas the PKC activator dioctanoyl-rac-glycerol (DiC8) prevented the augmentation of currents by quinapril. DiC8 also prevented the augmentation of Kv4.2 density by quinapril. Specific peptides that activate PKC translocation indicated that PKC-epsilon and not PKC-delta is involved in ANG II action on these currents. In control myocytes, quinapril and PD142893 augmented the sustained late current but had no effect on peak current. It is concluded that an autocrine release of angiotensin and endothelin in diabetic and hypothyroid conditions attenuates K(+) currents by suppressing the synthesis of some K(+) channel proteins, with the effects mediated at least partially by PKC-epsilon.

Alterations in brain protein kinase C isoforms following developmental exposure to a polychlorinated biphenyl mixture

PCBs have been shown to alter several neurochemical end-points and are implicated in the etiology of some neurological diseases. Recent in vivo studies from our laboratory indicated that developmental exposure to a commercial PCB mixture, Aroclor 1254, caused perturbations in calcium homeostasis and changes in protein kinase C (PKC) activities in rat brain. However, it is not known which molecular substances are targets for PCB-induced developmental neurotoxicity. Since the PKC signaling pathway has been implicated in the modulation of motor behavior as well as learning and memory, and the roles of PKC are subspecies specific, the present study attempted to analyze the effects on selected PKC isozymes in the cerebellum and the hippocampus following developmental exposure (gestational day 6 through postnatal day 21) to a PCB mixture, Aroclor 1254. The results indicated that the developmental exposure to PCBs caused significant hypothyroxinemia and age-dependent alterations in the translocation of PKC isozymes; the effects were greatly significant at postnatal day (PND) 14. Immunoblot analysis of PKC-alpha (alpha) from both cerebellum and hippocampus revealed that developmental exposure to Aroclor 1254 caused a significant decrease in cytosolic fraction and an increase in particulate fraction. There was no significant difference between these two brain regions on the level of fractional changes. However, the ratio between the fractions (particulate/cytosol) from cerebellum only was increased in a dose-dependent manner. Analysis of PKC-gamma (gamma) in cerebellum on PND14 showed a decrease in cytosolic fraction in both dose groups and an increase in particulate fraction at high dose (6 mg/kg) only. The ratio between the two fractions was increased in a dose-dependent manner. In the hippocampus, there was a significant decrease in PKC-gamma in cytosolic fraction of the high-dose group and a significant increase in particulate fraction of the low-dose group. But, the ratio between the fractions showed a significant increase (2.6-fold increase in high dose on PND14). Analysis of PKC-epsilon (epsilon) in cerebellum showed a significant decrease in cytosolic fraction at PND14, while particulate PKand an increase in ratio between fractions at 6 mg/kg on PND14. The results from this study indicate that the patterns of subcellular distributions of PKC isoforms following a developmental PCB exposure were PKC isozyme- and developmental stage-specific. Considering the significant role of PKC signaling in motor behavior, learning and memory, it is suggested that altered subcellular distribution of PKC isoforms at critical periods of brain development may be a possible mechanism of PCB-induced neurotoxic effects and that PKC-alpha, gamma, and epsilon may be among the target molecules implicated with PCB-induced neurological impairments during developmental exposure. It is believed that this is the first report successfully identifying PKC isoforms responding to PCBs during developmental exposure.

Streptozotocin diabetes protects against arrhythmias in rat isolated hearts: role of hypothyroidism

We examined the contribution of hypothyroidism to streptozotocin diabetes-induced alterations in the arrhythmia susceptibility of ex vivo hearts to regional zero-flow ischaemia. Diabetic rats received either protamine zinc insulin (10 IU/kg/day, s.c.) or triiodothyronine (10 microg/kg/day, s.c.) for 8 weeks commencing 72 h after injection of streptozotocin (60 mg/kg, i.p.). Arrhythmias were determined in ex vivo Langendorff-perfused hearts, subjected to a 30-min main left coronary artery occlusion, followed by 30-min reperfusion. Serum free thyroxine concentrations, rectal temperature and ex vivo heart rate were significantly decreased in the 8-week diabetic group (P<0.001). These changes were prevented by administration of triiodothyronine or insulin. Ventricular fibrillation during reperfusion was abolished in hearts from diabetic rats. This protection was prevented by treatment with either triiodothyronine or insulin. Hearts from methimazole-hypothyroid rats also showed no ventricular fibrillation during reperfusion. The protection against ischaemia-reperfusion-arrhythmias observed in hearts from streptozotocin-diabetic rats may be due to diabetes-induced hypothyroidism.

Role of protein kinase C in the signal pathways that link Na+/K+-ATPase to ERK1/2

We have shown before that Na(+)/K(+)-ATPase acts as a signal transducer, through protein-protein interactions, in addition to being an ion pump. Interaction of ouabain with the enzyme of the intact cells causes activation of Src, transactivation of EGFR, and activation of the Ras/ERK1/2 cascade. To determine the role of protein kinase C (PKC) in this pathway, neonatal rat cardiac myocytes were exposed to ouabain and assayed for translocation/activation of PKC from cytosolic to particulate fractions. Ouabain caused rapid and sustained stimulation of this translocation, evidenced by the assay of Ca(2+)-dependent and Ca(2+)-independent PKC activities and by the immunoblot analysis of the alpha, delta, and epsilon isoforms of PKC. Dose-dependent stimulation of PKC translocation by ouabain (1-100 microm) was accompanied by no more than 50% inhibition of Na(+)/K(+)-ATPase and doubling of [Ca(2+)](i), changes that do not affect myocyte viability and are known to be associated with positive inotropic, but not toxic, effects of ouabain in rat cardiac ventricles. Ouabain-induced activation of ERK1/2 was blocked by PKC inhibitors calphostin C and chelerythrine. An inhibitor of phosphoinositide turnover in myocytes also antagonized ouabain-induced PKC translocation and ERK1/2 activation. These and previous findings indicate that ouabain-induced activation of PKC and Ras, each linked to Na(+)/K(+)-ATPase through Src/EGFR, are both required for the activation of ERK1/2. Ouabain-induced PKC translocation and ERK1/2 activation were dependent on the presence of Ca(2+) in the medium, suggesting that the signal-transducing and ion-pumping functions of Na(+)/K(+)-ATPase cooperate in activation of these protein kinases and the resulting regulation of contractility and growth of the cardiac myocyte.

Transmural differences in rat ventricular protein kinase C epsilon correlate with its functional regulation of a transient cardiac K+ current

The effects of PKC activation on a transient (It) and a sustained (Iss) cardiac K+ current and the subcellular distribution of the epsilon isoform of PKC (PKC(epsilon)) were compared in epicardial and endocardial regions of the rat ventricle. Activation of PKC(epsilon) with a diacylglycerol analogue (di-octanoyl-glycerol (DiC8), 20 (mu)M) leads to differential effects in epicardial and endocardial cells. In epicardial cells (n = 20) It and Iss are attenuated by 17.7 +/- 2.1 % and 11.9 +/- 3.1 %, respectively (means +/- S.E.M.). In endocardial cells It attenuation was significantly smaller (4.6 +/- 1.6 %, n = 14, P < 0.0005). Iss attenuation was similar to that in epicardial cells (10.5 +/- 3.8 %). PKC[epsilon] expression was measured by Western blotting. Calculated endocardial/epicardial ratios showed no regional differences in total protein extracts (1.04 +/- 0.11, mean +/- S.E.M, n = 4), but PKC[epsilon] distribution in the cytosolic fraction showed a marked difference, with significantly (P < 0.05) higher levels in endocardial extracts. The cytosolic endocardial/epicardial PKC[epsilon] ratio was 2.64 +/- 0.24 (n = 4), indicating a reduced amount of PKC[epsilon] in the membrane fraction of the endocardium. This could account for the reduced effect of DiC8 on It in endocardial myocytes. Under both hypothyroid and streptozotocin-induced diabetic conditions the difference in endocardial and epicardial cytosolic PKC[epsilon] levels was absent (ratios of 0.86 +/- 0.21 (n = 4) and 1.09 +/- 0.16 (n = 3), respectively; means +/- S.E.M.). Ratios in the total protein extracts were not significantly different from those in control conditions. The results show transmural differences in the functional effects of PKC(epsilon) activation on a cardiac K+ current, and in the subcellular distribution of PKC(epsilon). These differences are absent in diabetic and hypothyroid conditions.

Sympathetic innervation alters activation of pacemaker current (If) in rat ventricle

Pacemaker current (If) exists in both neonatal and adult ventricles, but activates at more negative voltages in the adult. This study uses whole-cell patch clamp to investigate the factors that may contribute to the maturational shift of If, comparing neonatal rat ventricular myocytes that were cultured for 4-6 days either alone, in co-culture with sympathetic nerves, or with neurotransmitters chronically present in culture. If recorded from nerve-muscle co-cultures had a significantly more negative and shallower activation-voltage relation than that from control muscle cultures, which was reflected in the midpoint potential (V50) and slope factor (K) of activation. This effect of innervation was prevented by the sustained presence in the culture of the alpha1-adrenergic antagonist prazosin (Pz) at 10(-7) M. In parallel experiments, myocytes treated with noradrenaline (NA) at 10(-7) M or neuropeptide Y (NPY) at 10(-7) M during culture had the same If activation as control cells, but cells treated with NA and NPY together had a significantly more negative and shallower activation curve. Maximum conductance and reversal potential were unchanged. The effect of chronic exposure to NA + NPY was prevented by the sustained presence of either Pz or the NPY Y2 selective antagonist T4-[NPY(33-36)]4 (3.5 x 10(-7) M) in the culture, indicating a requirement for both alpha1-adrenergic and NPY Y2 activation. Substituting NA with the alpha1A-adrenergic selective agonist A61603 (5(-10) x 10(-9) M), in the presence of NPY, did not alter If, suggesting the involvement of alpha1B- rather than alpha1A-adrenoceptors. Further, sequential exposure to NPY followed by NA was effective in reproducing the action of chronic simultaneous exposure to these agonists, but sequential exposure to NA followed by NPY was ineffective. The results are consistent with past studies indicating that NPY affects the functional expression of the alpha1B-adrenergic cascade and suggest that sympathetic innervation induces a negative shift of If in ventricle via a combined action at alpha1B-adrenergic and NPY Y2 receptors. This effect of innervation probably contributes to the developmental maturation of If activation.

The alpha(1)-adrenoceptor subtype- and protein kinase C isoform-dependence of Norepinephrine's actions in cardiomyocytes

Catecholamines modulate cardiac function at least in part through alpha(1)-adrenergic receptors linked to the activation of protein kinase C (PKC). This study examines the molecular forms of the alpha(1)-receptor and PKC that mediate norepinephrine's actions in cardiomyocytes; distinct approaches (activation-dependent down-regulation of PKC isoforms) and novel reagents (A61603, an alpha(1A/c)-receptor agonist) are used to resolve this issue which has been the focus of dispute in previous studies. Norepinephrine (NE) induces a rise in diacylglycerol levels which is sustained for 24 h and is associated with the translocation (at 5 min) and down-regulation (at 24 h) of PKC delta and PKC xi (but not PKC alpha). The selective targeting of the alpha(1)-adrenergic receptor to activate novel PKC isoforms is remarkable, given an 8-fold greater abundance of PKC alpha relative to PKC xi in this preparation. NE activates the extracellular signal-regulated protein kinase (ERK) subfamily of mitogen-activated protein kinases through a PKC delta/PKC xi -dependent pathway. WB-4101 (alpha(1A/c)- and alpha(1D)-receptor antagonist) and 5-methylurapidil (alpha(1A/c)-receptor antagonist) inhibit norepinephrine-dependent accumulation of inositol phosphate and diacylglycerol, down-regulation of PKC delta and PKC xi, and activation of ERK. Each of these responses is stimulated by A61603, but not attenuated by high concentrations of chloroethylclonidine (which irreversibly inactivates the alpha(1B)-, and to a lesser extent, the alpha(1D)-receptor) or BMY 7378 (selective alpha(1D)-receptor antagonist). A61603 also activates p38-MAPK and induces hypertrophy. These studies establish that NE's actions in cardiomyocytes can be attributed to the alpha(1A/c)-adrenergic receptor subtype and nPKC isoforms, thereby identifying specific targets for the development of pharmaceuticals to influence cardiac contractile function and/or growth responses.

Protein kinase C regulation of K+ currents in rat ventricular myocytes and its modification by hormonal status

1. The effects of protein kinase C (PKC) activation on cardiac K+ currents were studied in rat ventricular myocytes, using whole-cell voltage clamp methods. Control rats were compared to hypothyroid or diabetic rats, in which PKC expression and activity were enhanced. 2. In control myocytes, two calcium-independent outward K+ currents, the transient It and the sustained Iss, were attenuated by 18.9 +/- 2.0 and 16.8 +/- 3.5 %, respectively (mean +/- s.e.m.), following addition of a synthetic analogue of diacylglycerol, DiC8 (20 microM). In myocytes from hypothyroid or diabetic rats, It and Iss were not affected by DiC8. 3. The effects of DiC8 were restored in myocytes from thyroidectomized rats by injection of physiological doses of tri-iodothyronine (T3; 10 microg kg-1 for 6-8 days). Incubating cells from diabetic rats with 100 nM insulin for 5-9 h also restored the ability of DiC8 to attenuate It and Iss. 4. The attenuation of K+ currents by DiC8 in control cells was absent in the presence of a peptide known to inhibit the translocation of the isoform PKCepsilon (EAVSKPLT, 24 microM introduced through the recording pipette). A scrambled peptide (LSETKPAV) was without effect. 5. Under hypothyroid conditions the inhibitory peptide restored the effects of DiC8 on It and Iss. These currents were attenuated by 11.9 +/- 1. 5 and 9.8 +/- 1.5 %, respectively, which was significantly (P < 0. 001) more than without the peptide or with the scrambled peptide. 6. These results show that the PKC-mediated suppression of cardiac K+ currents is normally mediated by PKCepsilon translocation. This effect is absent under hypothyroid and diabetic conditions, presumably due to prior PKC activation and translocation. A PKCepsilon translocation inhibitor restores the ability of DiC8 to attenuate K+ currents under hypothyroid conditions. This presumably reflects a (partial) reversal of a chronic translocation and a shift in the balance between PKC and its anchoring proteins.

Neuropeptide Y contributes to innervation-dependent increase in I(Ca, L) via ventricular Y2 receptors

The developmental increase in L-type Ca current (I(Ca,L)) density in the rat ventricle is reproduced in vitro by culturing neonatal myocytes with sympathetic neurons. We tested whether this effect of sympathetic innervation results from a chronic or sustained action of neurally released neuropeptide Y (NPY). Ventricular myocytes from newborn rats were cultured in serum-free medium with or without sympathetic neurons, NPY, or NPY analogs. Ca currents were measured in single myocytes at room temperature using the perforated patch clamp. In all cell groups (control, innervated, or NPY treated), the current-voltage relation for I(Ca,L) was represented by a bell-shaped curve with maximal value near 0 mV. The current density at 0 mV normalized to that of corresponding mean control values was 1.63 +/- 0.12 and 1.52 +/- 0.16 for innervated and NPY-treated myocytes, respectively. Both groups differed significantly from control (P < 0.05). NPY analogs exhibited the following rank order of effectiveness: NPY >/= NPY-(13-36) >/= PYY >> [Leu31Pro34]NPY, suggesting that the NPY effect occurs via a Y2-receptor subtype. In confirmation, chronic treatment of innervated cultures with a Y2-selective NPY antagonist prevented the innervation-dependent increase in I(Ca,L). These results indicate that sympathetic innervation contributes to the developmental increase in I(Ca,L) via neurally released NPY acting at Y2 receptors on the ventricular myocytes.

Direct effects of thyroid hormones on rat coronary artery: nongenomic effects of triiodothyronine and thyroxine

To determine whether thyroid hormones, triiodothyronine (T3) and thyroxine (T4), have any direct, nongenomic effects on vascular smooth muscle cells, we evaluated the effects of these hormones on rat coronary arteries. Bolus injection of T3 or T4 elicited a transient, dose-dependent decrease in coronary perfusion pressure (CPP), as well as an increase in arterial vasodilation. Vasodilation occurred immediately after injection, peaked at 15 seconds, and lasted 80 seconds. Reverse T3 had no effect on CPP or vasodilation. The rapidity of these effects suggests that they are not mediated by the T3-nuclear receptor, but are direct, nongenomic effects of thyroid hormones. Our results also suggest that thyroid hormones may play a role in preventing myocardial ischemia by inducing coronary artery vasodilation.

Lead exposure promotes translocation of protein kinase C activities in rat choroid plexus in vitro, but not in vivo

Lead (Pb) exposure reportedly modulates PKC activity in brain endothelial preparations, which may underlie Pb-induced damage at the blood-brain barrier. Our previous work indicates that Pb accumulates in the choroid plexus and causes dysfunction of this blood-cerebrospinal fluid (CSF) barrier. The present studies were undertaken to test the hypothesis that Pb in the choroid plexus may alter PKC activity and thus affect the functions of the blood-CSF barrier. When choroidal epithelial cells in a primary culture were exposed to Pb (10 microM in culture medium), the membrane-bound PKC activity increased by 5.2-fold, while the cytosolic PKC activities decreased, an indication of the induction of PKC translocation by Pb. The effect of Pb on cellular PKC was concentration dependent in the range of 0.1-10 microM. We further evaluated PKC activity of the choroid plexus in rats chronically exposed to Pb in the drinking water (control, 50 or 250 micrograms Pb/ml) for 30, 60, or 90 days. Two-way analysis of variance revealed a significant age-related decline of PKC activities in both cytosol and membrane of the choroid plexus. However, Pb treatment did not alter plexus PKC activities. In addition, we found that short-term, acute Pb exposure in rats did not significantly change PKC activities nor did it affect the expression of PKC isoenzymes in the choroid plexus. Our results suggest that Pb exposure may promote the translocation of PKC from cytosol to membrane in rat blood-CSF barrier in vitro, but not in vivo.

Aldosterone rapidly represses protein kinase C activity in neonatal rat cardiomyocytes in vitro

Aldosterone lowers protein kinase C (PKC) activity in myocyte-enriched cultures from neonatal Sprague-Dawley rat hearts, with activity measured by the transfer of phosphate to myristolated alanine-rich C-kinase substrate, in the presence of Ca2+, phosphatidylserine, and diolein. The effect is rapid, with a significant effect after 1 min exposure, half maximal at < or = 1 nM aldosterone, with steroids showing a hierarchy of potency aldosterone = 9alpha fluorocortisol > deoxycorticosterone > corticosterone > spironolactone. Both Ca2+-dependent and -independent PKC activity appear equally inhibited by aldosterone, and PMA-stimulated increases in PKC activity appear similarly aldosterone-sensitive. No displaceable binding of [3H]aldosterone to purified PKC can be shown, evidence against a direct effect of aldosterone on PKC; aldosterone does not alter basal or PMA-stimulated PKC activity in cardiac fibroblasts, evidence for a cell-specific mediator of the myocyte effect. Taken with the previous demonstration of the potentiation of aldosterone-specific MR-mediated effects by PKC activation, the present data argue for the existence of a complex cross-talk mechanism between aldosterone and factors affecting PKC activity in the heart.