Cellular distribution of Ca2+ pumps and Ca2+ release channels in rat cardiac hypertrophy induced by aortic stenosis

Increased SERCA2a sub-cellular heterogeneity in right-ventricular heart failure inhibits excitation-contraction coupling and modulates arrhythmogenic dynamics

The intracellular calcium handling system of cardiomyocytes is responsible for controlling excitation-contraction coupling (ECC) and has been linked to pro-arrhythmogenic cellular phenomena in conditions such as heart failure (HF). SERCA2a, responsible for intracellular uptake, is a primary regulator of calcium homeostasis, and remodelling of its function has been proposed as a causal factor underlying cellular and tissue dysfunction in disease. Whereas adaptations to the global (i.e. whole-cell) expression of SERCA2a have been previously investigated in the context of multiple diseases, the role of its spatial profile in the sub-cellular volume has yet to be elucidated. We present an approach to characterize the sub-cellular heterogeneity of SERCA2a and apply this approach to quantify adaptations to the length-scale of heterogeneity (the distance over which expression is correlated) associated with right-ventricular (RV)-HF. These characterizations informed simulations to predict the functional implications of this heterogeneity, and its remodelling in disease, on ECC, the dynamics of calcium-transient alternans and the emergence of spontaneous triggered activity. Image analysis reveals that RV-HF is associated with an increase in length-scale and its inter-cellular variability; simulations predict that this increase in length-scale can reduce ECC and critically modulate the vulnerability to both alternans and triggered activity. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

ECG conduction disturbances and ryanodine receptor expression levels in occupational lead exposure workers

Objectives

A significant number of researches have evidenced that occupational lead (Pb) exposure increased risks of cardiovascular disease. However, evidences about the potential effects of Pb on the cardiac conduction system are sparse and inconclusive. Besides, ryanodine receptors (RyRs) induced dysfunction of cardiac excitation contraction coupling which is considered to be one of the mechanisms in cardiovascular diseases. Therefore, we examined the association between occupational Pb exposure and ECG conduction abnormalities, as well as RyRs in Pb-induced ECG abnormalities.

Methods

We investigated 529 Pb smelter workers, and measured blood lead (BPb), zinc protoporphyrin (ZPP), ECG outcomes and RyR expression levels. Based on BPb levels, the workers were divided into three groups: the BPb not elevated group, the BPb elevated group and the Pb poisoning group. Descriptive and multivariable analyses were performed.

Results

Compared with the BPb not elevated group, the Pb poisoning group had a higher incidence of high QRS voltage, and a lower level of RyR1 gene expression (p<0.05). Further unconditional multivariable logistic regression analyses showed that high QRS voltage was positively related to BPb (OR=1.045, 95% CI 1.014 to 1.078) and inversely associated with RyR1 expression (OR=0.042, 95% CI 0.002 to 0.980) after adjusting for potential confounders. In addition, multiple linear regression analyses showed that the QTc interval was positively associated with ZPP (β=0.299, 95% CI 0.130 to 0.468) after adjusting for potential confounders.

Conclusions

Our study provided evidences that occupational exposure to Pb may be associated with worse ECG outcomes (high QRS voltage), which might be related to decreased levels of RyR1.

TSH inhibits SERCA2a and the PKA/PLN pathway in rat cardiomyocytes

Elevated thyroid-stimulating hormone (TSH) levels often accompany impaired LV diastolic function and subtle systolic dysfunction in subclinical hypothyroidism (sHT). These cardiac dysfunctions are characterized by increases in mean aortic acceleration and pre-ejection/ejection time ratios. To explore the mechanism underlying these pathologies, we investigated the effects of TSH on sarcoplasmic reticulum calcium ATPase (SERCA2a) activity and expression in neonatal rat cardiomyocytes. TSH inhibited SERCA2a activity and expression by binding to TSH receptors in cardiomyocyte membranes and inhibiting the protein kinase A/phoshpolamban (PKA/PLN) signaling pathway. These results suggest that increases in serum TSH levels contribute to the development of cardiac diastolic and systolic dysfunction.

Functionally redundant control of cardiac hypertrophic signaling by inositol 1,4,5-trisphosphate receptors

Calcium plays an integral role to many cellular processes including contraction, energy metabolism, gene expression, and cell death. The inositol 1, 4, 5-trisphosphate receptor (IP₃R) is a calcium channel expressed in cardiac tissue. There are three IP₃R isoforms encoded by separate genes. In the heart, the IP₃R-2 isoform is reported to being most predominant with regards to expression levels and functional significance. The functional roles of IP₃R-1 and IP₃R-3 in the heart are essentially unexplored despite measureable expression levels. Here we show that all three IP₃Rs isoforms are expressed in both neonatal and adult rat ventricular cardiomyocytes, and in human heart tissue. The three IP₃R proteins are expressed throughout the cardiomyocyte sarcoplasmic reticulum. Using isoform specific siRNA, we found that expression of all three IP₃R isoforms are required for hypertrophic signaling downstream of endothelin-1 stimulation. Mechanistically, IP₃Rs specifically contribute to activation of the hypertrophic program by mediating the positive inotropic effects of endothelin-1 and leading to downstream activation of nuclear factor of activated T-cells. Our findings highlight previously unidentified functions for IP₃R isoforms in the heart with specific implications for hypertrophic signaling in animal models and in human disease.

BAPTA-AM decreases cellular pH, inhibits acidocalcisome acidification and autophagy in amino acid-starved T. brucei

To investigate the role of Ca²⁺ signaling in starvation-induced autophagy in Trypanosoma brucei, the causative agent of human African trypanosomiasis, we used cell-permeant Ca²⁺ chelator BAPTA-AM and cell impermeant chelator EGTA, and examined the potential involvement of several intracellular Ca²⁺ signaling pathways in T. brucei autophagy. The results showed an unexpected effect of BAPTA-AM in decreasing cellular pH and inhibiting acidocalcisome acidification in starved cells. The implication of these results in the role of Ca²⁺ signaling and cellular/organellar pH in T. brucei autophagy is discussed.

Cardiac inositol 1,4,5-trisphosphate receptors

Calcium is a second messenger that regulates almost all cellular functions. In cardiomyocytes, calcium plays an integral role in many functions including muscle contraction, gene expression, and cell death. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) are a family of calcium channels that are ubiquitously expressed in all tissues. In the heart, IP₃Rs have been associated with regulation of cardiomyocyte function in response to a variety of neurohormonal agonists, including those implicated in cardiac disease. Notably, IP₃R activity is thought to be essential for mediating the hypertrophic response to multiple stimuli including endothelin-1 and angiotensin II. In this review, we will explore the functional implications of IP₃R activity in the heart in health and disease.

Transmural gradients of myocardial structure and mechanics: Implications for fiber stress and strain in pressure overload

Although a truly complete understanding of whole heart activation, contraction, and deformation is well beyond our current reach, a significant amount of effort has been devoted to discovering and understanding the mechanisms by which myocardial structure determines cardiac function to better treat patients with cardiac disease. Several experimental studies have shown that transmural fiber strain is relatively uniform in both diastole and systole, in contrast to predictions from traditional mechanical theory. Similarly, mathematical models have largely predicted uniform fiber stress across the wall. The development of this uniform pattern of fiber stress and strain during filling and ejection is due to heterogeneous transmural distributions of several myocardial structures. This review summarizes these transmural gradients, their contributions to fiber mechanics, and the potential functional effects of their remodeling during pressure overload hypertrophy.

The SR/ER-mitochondria calcium crosstalk is regulated by GSK3β during reperfusion injury

Glycogen synthase kinase-3β (GSK3β) is a multifunctional kinase whose inhibition is known to limit myocardial ischemia-reperfusion injury. However, the mechanism mediating this beneficial effect still remains unclear. Mitochondria and sarco/endoplasmic reticulum (SR/ER) are key players in cell death signaling. Their involvement in myocardial ischemia-reperfusion injury has gained recognition recently, but the underlying mechanisms are not yet well understood. We questioned here whether GSK3β might have a role in the Ca(2+) transfer from SR/ER to mitochondria at reperfusion. We showed that a fraction of GSK3β protein is localized to the SR/ER and mitochondria-associated ER membranes (MAMs) in the heart, and that GSK3β specifically interacted with the inositol 1,4,5-trisphosphate receptors (IP3Rs) Ca(2+) channeling complex in MAMs. We demonstrated that both pharmacological and genetic inhibition of GSK3β decreased protein interaction of IP3R with the Ca(2+) channeling complex, impaired SR/ER Ca(2+) release and reduced the histamine-stimulated Ca(2+) exchange between SR/ER and mitochondria in cardiomyocytes. During hypoxia reoxygenation, cell death is associated with an increase of GSK3β activity and IP3R phosphorylation, which leads to enhanced transfer of Ca(2+) from SR/ER to mitochondria. Inhibition of GSK3β at reperfusion reduced both IP3R phosphorylation and SR/ER Ca(2+) release, which consequently diminished both cytosolic and mitochondrial Ca(2+) concentrations, as well as sensitivity to apoptosis. We conclude that inhibition of GSK3β at reperfusion diminishes Ca(2+) leak from IP3R at MAMs in the heart, which limits both cytosolic and mitochondrial Ca(2+) overload and subsequent cell death.

Alteration in mitochondrial Ca(2+) uptake disrupts insulin signaling in hypertrophic cardiomyocytes

BACKGROUND

Cardiac hypertrophy is characterized by alterations in both cardiac bioenergetics and insulin sensitivity. Insulin promotes glucose uptake by cardiomyocytes and its use as a substrate for glycolysis and mitochondrial oxidation in order to maintain the high cardiac energy demands. Insulin stimulates Ca(2+) release from the endoplasmic reticulum, however, how this translates to changes in mitochondrial metabolism in either healthy or hypertrophic cardiomyocytes is not fully understood.

RESULTS

In the present study we investigated insulin-dependent mitochondrial Ca(2+) signaling in normal and norepinephrine or insulin like growth factor-1-induced hypertrophic cardiomyocytes. Using mitochondrion-selective Ca(2+)-fluorescent probes we showed that insulin increases mitochondrial Ca(2+) levels. This signal was inhibited by the pharmacological blockade of either the inositol 1,4,5-triphosphate receptor or the mitochondrial Ca(2+) uniporter, as well as by siRNA-dependent mitochondrial Ca(2+) uniporter knockdown. Norepinephrine-stimulated cardiomyocytes showed a significant decrease in endoplasmic reticulum-mitochondrial contacts compared to either control or insulin like growth factor-1-stimulated cells. This resulted in a reduction in mitochondrial Ca(2+) uptake, Akt activation, glucose uptake and oxygen consumption in response to insulin. Blocking mitochondrial Ca(2+) uptake was sufficient to mimic the effect of norepinephrine-induced cardiomyocyte hypertrophy on insulin signaling.

CONCLUSIONS

Mitochondrial Ca(2+) uptake is a key event in insulin signaling and metabolism in cardiomyocytes.

Regulation of autophagy in cardiomyocytes by Ins(1,4,5)P(3) and IP(3)-receptors

Autophagy is a process that removes damaged proteins and organelles and is of particular importance in terminally differentiated cells such as cardiomyocytes, where it has primarily a protective role. We investigated the involvement of inositol(1,4,5)trisphosphate (Ins(1,4,5)P(3)) and its receptors in autophagic responses in neonatal rat ventricular myocytes (NRVM). Treatment with the IP(3)-receptor (IP(3)-R) antagonist 2-aminoethoxydiphenyl borate (2-APB) at 5 or 20 μmol/L resulted in an increase in autophagosome content, defined as puncta labeled by antibody to microtubule associated light chain 3 (LC3). 2-APB also increased autophagic flux, indicated by heightened LC3II accumulation, which was further enhanced by bafilomycin (10nmol/L). Expression of Ins(1,4,5)P(3) 5-phosphatase (IP(3)-5-Pase) to deplete Ins(1,4,5)P(3) also increased LC3-labeled puncta and LC3II content, suggesting that Ins(1,4,5)P(3) inhibits autophagy. The IP(3)-R can act as an inhibitory scaffold sequestering the autophagic effector, beclin-1 to its ligand binding domain (LBD). Expression of GFP-IP(3)-R-LBD inhibited autophagic signaling and furthermore, beclin-1 co-immunoprecipitated with the IP(3)-R-LBD. A mutant GFP-IP(3)-R-LBD with reduced ability to bind Ins(1,4,5)P(3) bound beclin-1 and inhibited autophagy similarly to the wild type sequence. These data provide evidence that Ins(1,4,5)P(3) and IP(3)-R act as inhibitors of autophagic responses in cardiomyocytes. By suppressing autophagy, IP(3)-R may contribute to cardiac pathology.

Differential calcium handling in two canine models of right ventricular pressure overload

BACKGROUND

The purpose of this investigation was to characterize differential right atrial (RA) and ventricular (RV) molecular changes in Ca(2+)-handling proteins consequent to RV pressure overload and hypertrophy in two common, yet distinct models of pulmonary hypertension: dehydromonocrotaline (DMCT) toxicity and pulmonary artery (PA) banding.

METHODS

A total of 18 dogs underwent sternotomy in four groups: (1) DMCT toxicity (n = 5), (2) mild PA banding over 10 wk to match the RV pressure rise with DMCT (n = 5); (3) progressive PA banding to generate severe RV overload (n = 4); and (4) sternotomy only (n = 4).

RESULTS

In the right ventricle, with DMCT, there was no change in sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) or phospholamban (PLB), but we saw a trend toward down-regulation of phosphorylated PLB at serine-16 (p[Ser-16]PLB) (P = 0.07). Similarly, with mild PA banding, there was no change in SERCA or PLB, but p(Ser-16)PLB was down-regulated by 74% (P < 0.001). With severe PA banding, there was no change in PLB, but SERCA fell by 57% and p(Ser-16)PLB fell by 67% (P < 0.001). In the right atrium, with DMCT, there were no significant changes. With both mild and severe PA banding, p(Ser-16)PLB fell (P < 0.001), but SERCA and PLB did not change.

CONCLUSIONS

Perturbations in Ca(2+)-handling proteins depend on the degree of RV pressure overload and the model used to mimic the RV effects of pulmonary hypertension. They are similar, but blunted, in the atrium compared with the ventricle.

Decreased cardiac SERCA2 expression, SR Ca uptake, and contractile function in hypothyroidism are attenuated in SERCA2 overexpressing transgenic rats

The sarco/endoplasmic reticulum (SR) Ca(2+)-ATPase SERCA2a has a key role in controlling cardiac contraction and relaxation. In hypothyroidism, decreased expression of the thyroid hormone (TH)-responsive SERCA2 gene contributes to slowed SR Ca(2+) reuptake and relaxation. We investigated whether cardiac expression of a TH-insensitive SERCA2a cDNA minigene can rescue SR Ca(2+) handling and contractile function in female SERCA2a-transgenic rats (TG) with experimental hypothyroidism. Wild-type rats (WT) and TG were rendered hypothyroid by 6-N-propyl-2-thiouracil treatment for 6 wk; control rats received no treatment. In vivo measured left ventricular (LV) hemodynamic parameters were compared with SERCA2a expression and function in LV tissue. Hypothyroidism decreased LV peak systolic pressure, dP/dt(max), and dP/dt(min) in both WT and TG. However, loss of function was less in TG. Thus slowed relaxation in hypothyroidism was found to be 1.5-fold faster in TG compared with WT (P < 0.05). In parallel, a 1.4-fold higher V(max) value of homogenate SR Ca(2+) uptake was observed in hypothyroid TG (P < 0.05 vs. hypothyroid WT), and the hypothyroidism-caused decline of LV SERCA2a mRNA expression in TG by -24% was markedly less than the decrease of -49% in WT (P < 0.05). A linear relationship was observed between the SERCA2a/PLB mRNA ratio values and the V(max) values of SR Ca(2+) uptake when the respective data of all experimental groups were plotted together (r = 0.90). The data show that expression of the TH-insensitive SERCA2a minigene compensates for loss of expressional activity of the TH-responsive native SERCA2a gene in the female hypothyroid rat heart. However, SR Ca(2+) uptake and in vivo heart function were only partially rescued.

Effects of sex differences on constitutive nitric oxide synthase expression and activity in response to pressure overload in rats

Clinical studies have documented sex differences in left ventricular (LV) hypertrophy patterns, but the mechanisms are so far poorly defined. This study aimed to determine whether 1) severe pressure overload altered expression and/or activity of cardiac constitutive nitric oxide synthase (NOS1 and NOS3) and 2) these changes were modulated according to sex. Analyses were performed 0.4–20 wk after thoracic aortic constriction (TAC) in male and female Wistar rats. Male rats with TAC exhibited early signs of cardiac dysfunction, as shown by echocardiographic and LV end-diastolic pressure measurements, whereas females with TAC exhibited higher LV hypertrophy (+96% vs. males at 20 wk; P < 0.05). After TAC, cardiac NOS1 expression was rapidly induced (0.4 wk) and stable afterward in males ( P < 0.05 vs. sham groups), whereas it was delayed in females. Accordingly, specific NOS1 activity was increased by 2 wk in male rats with TAC (+122%; P < 0.001 vs. sham groups) and only by 20 wk in females (+220%; P < 0.001 vs. sham groups). NOS1 activity was correlated with NOS1 level. Regarding cardiac NOS3, expression was unaffected by TAC, and the decrease in activity observed at early and late times in male and female rats with TAC, respectively, is shown to be related to NOS3 allosteric regulator caveolin-1 level. The data demonstrated a unique sex-dependent regulation of the constitutive NOSs in response to TAC in rats; such a difference might play a role in the sex-dependent adaptability of the heart in response to pressure overload.

17beta-estradiol regulates constitutive nitric oxide synthase expression differentially in the myocardium in response to pressure overload

Estrogens [E(2)] exert direct and indirect effects that can modulate the development of cardiac disease. However, the precise mechanisms that are involved remain undefined. Our objective was to investigate whether E(2) affected the activity and expression of constitutive nitric oxide synthase (NOS) isoforms (NOS3 and NOS1) in cardiac hypertrophy induced by thoracic aortic constriction (TAC). Ovariectomized (Ovx) and nonovariectomized Wistar rats were subjected to TAC. Ovx animals received E(2) or placebo 3 wk after surgery for 11 wk. Afterward cardiac function and degree of left ventricular hypertrophy were assessed by echocardiography. NOS activity and expression were studied by biochemical techniques. TAC led to significant left ventricular hypertrophy (>90%) irrespective of hormonal status. Cardiac performance declined more in TAC+Ovx (-20%, P < 0.015) than in the two other TAC groups [TAC and TAC+Ovx+E(2)]. Total NOS activity decreased significantly in the Ovx groups. In response to TAC, total NOS activity increased whatever the E(2) status. Specific NOS3 activity dramatically decreased in the Ovx groups (-55%, P < 0.009) and was unaltered by TAC. By using coimmunoprecipitation assays, we showed that NOS3/caveolin-1 complexes negatively regulated NOS3 activity as a function of E(2) status. On the other hand, NOS1 expression and activity were markedly increased in hypertrophied myocardium (P < 0.003), irrespective of E(2) status. This study demonstrates a differential regulation of NOS expression and activity in response to pressure overload and E(2) status, the former being mainly involved in the induction of NOS1, whereas the latter regulated NOS3 activity and in turn cardiac function.

Oxidative stress of myosin contributes to skeletal muscle dysfunction in rats with chronic heart failure

Intrinsic muscle abnormalities affecting skeletal muscle are often reported during chronic heart failure (CHF). Because myosin is the molecular motor of force generation, we sought to determine whether its dysfunction contributes to skeletal muscle weakness in CHF and, if so, to identify the underlying causative factors. Severe CHF was induced in rats by aortic stenosis. In diaphragm and soleus muscles, we investigated in vitro mechanical performance, myosin-based actin filament motility, myosin heavy (MHC) and light (MLC) chain isoform compositions, MLC integrity, caspase-3 activation, and oxidative damage. Diaphragm and soleus muscles from CHF exhibited depressed mechanical performance. Myosin sliding velocities were 16 and 20% slower in CHF than in sham in diaphragm (1.9 ± 0.1 vs. 1.6 ± 0.1 μm/s) and soleus (0.6 ± 0.1 vs. 0.5 ± 0.1 μm/s), respectively (each P < 0.05). The ratio of slow-to-fast myosin isoform did not differ between sham and CHF. Immunoblots with anti-MLC antibodies did not detect the presence of protein fragments, and no activation of caspase-3 was evidenced. Immunolabeling revealed oxidative damage in CHF muscles, and MHC was the main oxidized protein. Lipid peroxidation and expression of oxidized MHC were significantly higher in CHF than in shams. In vitro myosin exposure to increasing ONOO−concentrations was associated with an increasing amount of oxidized MHC and a reduced myosin velocity. These data provide experimental evidence that intrinsic myosin dysfunction occurs in CHF and may be related to oxidative damage to myosin.

Enhanced calcium mobilization in rat ventricular myocytes during the onset of pressure overload-induced hypertrophy

Early cardiovascular changes evoked by pressure overload (PO) may reveal adaptive strategies that allow immediate survival to the increased hemodynamic load. In this study, systolic and diastolic Ca2+ cycling was analyzed in left ventricular rat myocytes before ( day 2, PO-2d group) and after ( day 7, PO-7d group) development of hypertrophy subsequent to aortic constriction, as well as in myocytes from time-matched sham-operated rats (sham group). Ca2+ transient amplitude was significantly augmented in the PO-2d group. In the PO-7d group, intracellular Ca2+ concentration ([Ca2+]i) was reduced during diastole, and mechanical twitch relaxation (but not [Ca2+]i decline) was slowed. In PO groups, fractional sarcoplasmic reticulum (SR) Ca2+ release at a twitch, SR Ca2+ content, SR Ca2+ loss during diastole, and SR-dependent integrated Ca2+ flux during twitch relaxation were significantly greater than in sham-operated groups, whereas the relaxation-associated Ca2+ flux carried by the Na+/Ca2+ exchanger was not significantly changed. In the PO-7d group, mRNA levels of cardiac isoforms of SR Ca2+-ATPase (SERCA2a), phospholamban, calsequestrin, ryanodine receptor, and NCX were not significantly altered, but the SERCA2a-to-phospholamban ratio was increased 2.5-fold. Moreover, greater sensitivity to the inotropic effects of the β-adrenoceptor agonist isoproterenol was observed in the PO-7d group. The results indicate enhanced Ca2+ cycling between SR and cytosol early after PO imposition, even before hypertrophy development. Increase in SR Ca2+ uptake may contribute to enhancement of excitation-contraction coupling (augmented SR Ca2+ content and release) and protection against arrhythmogenesis due to buildup of [Ca2+]i during diastole.

Increased Ca2+ sensitivity and protein expression of SERCA 2a in situations of chronic beta3-adrenoceptor deficiency

This study investigated the influence of chronic beta(3)-adrenoceptor deficiency on myocardial function. Therefore, we investigated Ca(2+)-regulatory proteins, SERCA 2a activity, and myofibrillar and mitochondrial function in hearts of wild-type (WT, n=7) and beta(3)-adrenoceptor knockout mice (beta(3)-KNO, n=7). Morphometric heart analysis showed no difference between WT and beta(3)-KNO. No alterations were observed for the protein expression of the ryanodine receptor or phospholamban. However, in beta(3)-KNO mice, protein expression of SERCA 2a and phospholamban phosphorylation were significantly increased. These changes were accompanied by an increased SERCA 2a activity in beta(3)-KNO. Alterations in phospholamban phosphorylation were independent of alterations in beta(1)/beta(2)-adrenoceptor distribution and protein expression of G proteins in beta(3)-KNO. Measurement of myofibrillar Ca(2+) sensitivity showed no difference in the Ca(2+)/force relation for WT and beta(3)-KNO. The same seems to hold true for mitochondrial function since the protein expressions of cytochrome c, uncoupling protein 3 and cytochrome c oxidase subunit IV were similar in WT and beta(3)-KNO. The conclusion is that depression of beta(3)-adrenergic stimulation may modulate the protein expression of SERCA 2a and phospholamban phosphorylation, thereby improving sarcoplasmic reticulum Ca(2+) uptake. Thus, beta(3)-adrenergic depression may be a therapeutic aim in situations of impaired SERCA 2a activity, e.g. for the treatment of heart failure.

Decreased mAKAP, ryanodine receptor, and SERCA2a gene expression in mdx hearts

Duchenne muscular dystrophy (DMD) is a common genetic disease resulting from mutations in the dystrophin gene. The lack of dystrophin function as a cytoskeletal protein leads to abnormal intracellular Ca(2+) homeostasis, the actual source and functional consequences of which remain obscure. The mdx mouse, a mouse model of DMD, revealed alterations in contractile properties that are likely due to defective Ca(2+) handling. However, the exact mechanisms of the Ca(2+) handling defect are unclear. We performed suppressive subtractive hybridization to isolate differentially expressed genes between 5-month-old mdx and control mice. We observed a decrease in muscle A-kinase anchoring protein (mAKAP) in the mdx hearts. We noticed not only down-regulation of mAKAP mRNA but also decreased mRNA level of the molecules involved in Ca(2+) handling and excitation-contraction (E-C) coupling in the sarcoplasmic reticulum (SR), the cardiac ryanodine receptor, and the sarcoplasmic reticulum Ca(2+) ATPase. Therefore, dystrophin deficiency may cause an impairment of SR Ca(2+) homeostasis and E-C coupling in mdx hearts, in part, by decreased gene expression of molecules involved in SR Ca(2+) handling.

Hallmarks of ion channel gene expression in end-stage heart failure

Electrical conductance is greatly altered in end-stage heart failure, but little is known about the underlying events. We therefore investigated the expression of genes coding for major inward and outward ion channels, calcium binding proteins, ion receptors, ion exchangers, calcium ATPases, and calcium/calmodulin-dependent protein kinases in explanted hearts (n=13) of patients diagnosed with end-stage heart failure. With the exception of Kv11.1 and Kir3.1 and when compared with healthy controls, major sodium, potassium, and calcium ion channels, ion transporters, and exchangers were significantly repressed, but expression of Kv7.1, HCN4, troponin C and I, SERCA1, and phospholamban was elevated. Hierarchical gene cluster analysis provided novel insight into regulated gene networks. Significant induction of the transcriptional repressor m-Bop and the translational repressor NAT1 coincided with repressed cardiac gene expression. The statistically significant negative correlation between repressors and ion channels points to a mechanism of disease. We observed coregulation of ion channels and the androgen receptor and propose a role for this receptor in ion channel regulation. Overall, the reversal of repressed ion channel gene expression in patients with implanted assist devices exemplifies the complex interactions between pressure load/stretch force and heart-specific gene expression.

Dysregulated ryanodine receptors mediate cellular toxicity: restoration of normal phenotype by FKBP12.6

Ca2+ homeostasis is a vital cellular control mechanism in which Ca2+ release from intracellular stores plays a central role. Ryanodine receptor (RyR)-mediated Ca2+ release is a key modulator of Ca2+ homeostasis, and the defective regulation of RyR is pathogenic. However, the molecular events underlying RyR-mediated pathology remain undefined. Cells stably expressing recombinant human RyR2 (Chinese hamster ovary cells, CHOhRyR2) had similar resting cytoplasmic Ca2+ levels ([Ca2+]c) to wild-type CHO cells (CHOWT) but exhibited increased cytoplasmic Ca2+ flux associated with decreased cell viability and proliferation. Intracellular Ca2+ flux increased with human RyR2 (hRyR2) expression levels and determined the extent of phenotypic modulation. Co-expression of FKBP12.6, but not FKBP12, or incubation of cells with ryanodine suppressed intracellular Ca2+ flux and restored normal cell viability and proliferation. Restoration of normal phenotype was independent of the status of resting [Ca2+]c or ER Ca2+ load. Heparin inhibition of endogenous inositol trisphosphate receptors (IP3R) had little effect on intracellular Ca2+ handling or viability. However, purinergic stimulation of endogenous IP3R resulted in apoptotic cell death mediated by hRyR2 suggesting functional interaction occurred between IP3R and hRyR2 Ca2+ release channels. These data demonstrate that defective regulation of RyR causes altered cellular phenotype via profound perturbations in intracellular Ca2+ signaling and highlight a key modulatory role of FKBP12.6 in hRyR2 Ca2+ channel function.

Ca2+ channel remodeling in perfused heart: Effects of mechanical work and interventions affecting Ca2+ cycling on sarcolemmal and sarcoplasmic reticulum Ca2+ channels

We investigated whether changes in cardiac work or in Ca2+ fluxes may affect the expression of sarcolemmal or sarcoplasmic reticulum Ca2+ channels (DHPRs and RyRs, respectively). Isolated rat hearts were perfused at low Ca2+ concentration (0.8 mM instead of 1.5 mM), at low preload (5 cm instead of 20 cm), in the presence of 100 nM nifedipine or with a cardioplegic solution. After 60 min, hypocalcemic perfusion produced significant reduction in [3H]-PN 200-110 and [3H]-ryanodine binding, due to approximately 30% reduction in Bmax (P<0.01), with unchanged Kd. Such modifications were reversible. Similar results were obtained in the nifedipine and cardioplegia groups. Low preload perfusion produced similar contractile effects as hypocalcemic perfusion, but it had no effect on radioligand binding. After hypocalcemic perfusion, DHPR and RyR gene expression, evaluated by RT-PCR, were not modified. Chelerythrine (protein kinase C inhibitor) and lavendustin C (Ca2+/calmodulin-dependent protein kinase II inhibitor), but not H-89 (protein kinase A inhibitor), abolished the effects of hypocalcemic perfusion on [3H]-PN 200-110 and [3H]-ryanodine binding. We conclude that reduced Ca2+ entry and/or intracellular Ca2+ cycling determines DHPR and RyR remodeling through posttranslational protein modifications. Both protein kinase C and Ca2+/calmodulin-dependent protein kinase II appear to play a role in this phenomenon.

Effect of chronic hypoxia on agonist-induced tone and calcium signaling in rat pulmonary artery

The effect of chronic hypoxia (CH) for 14 days on Ca2+ signaling and contraction induced by agonists in the rat main pulmonary artery (MPA) was investigated. In MPA myocytes obtained from control (normoxic) rats, endothelin (ET)-1, angiotensin II (ANG II), and ATP induced oscillations in intracellular Ca2+ concentration ([Ca2+]i) in 85-90% of cells, whereas they disappeared in myocytes from chronically hypoxic rats together with a decrease in the percentage of responding cells. However, both the amount of mobilized Ca2+ and the sources of Ca2+ implicated in the agonist-induced response were not changed. Analysis of the transient caffeine-induced [Ca2+]i response revealed that recovery of the resting [Ca2+]i value was delayed in myocytes from chronically hypoxic rats. The maximal contraction induced by ET-1 or ANG II in MPA rings from chronically hypoxic rats was decreased by 30% compared with control values. Moreover, the D-600- and thapsigargin-resistant component of contraction was decreased by 40% in chronically hypoxic rats. These data indicate that CH alters pulmonary arterial reactivity as a consequence of an effect on both Ca2+ signaling and Ca2+ sensitivity of the contractile apparatus. A Ca2+ reuptake mechanism appears as a CH-sensitive phenomenon that may account for the main effect of CH on Ca2+ signaling.

Characterization of pressure-induced calcium response in neuronal cell lines

BACKGROUND

Cation channels that respond to mechanical stress have been described in neuronal and nonneuronal cells. These nonselective cation ([C+(SA)]) channels are believed to regulate volume and osmolarity of cells in the central nervous system and are therefore believed to be involved in brain injury, resulting in intracellular calcium accumulation and cell death.

METHODS

Activation of pressure-sensitive channels was monitored as an increase in [Ca2+](i) by flow cytometry using indo-1. Several neuronal cell lines including NH15-CA2 neuroblastoma x glioma cells were stimulated by rectangular pressure increase.

RESULTS

Neuronal cell lines showed a pressure-sensitive increase in [Ca2+](i) but no pressure sensitivity was found in fibroblasts and embryonic P19 cells. [C+(SA)] channels in NH15-CA2 cells were not blocked by inhibitors of voltage-dependent calcium channels and G-proteins. Depletion of extracellular calcium and of internal Ca2+ stores inhibited pressure-induced [Ca2+](i) increase. Elevated [C+(SA)] channel activity was also observed in confluent NH15-CA2 thus accumulated in the G(0)/G(1)-phase of the cell cycle. P19 cells showed occurrence of [C+(SA)] channel activity only after neuronal differentiation.

CONCLUSION

Pressure-sensitive channel activity is present in cells of neuronal origin. This activity depends on neuronal differentiation and might have a pivotal role in neuronal development and differentiation.

Human isolated bronchial smooth muscle contains functional ryanodine/caffeine-sensitive Ca-release channels

Human bronchial smooth muscle (HBSM) contraction is implicated in a variety of respiratory diseases, including asthma. Yet, the presence of an operative calcium-induced calcium release (CICR) mechanism, identified in various smooth muscles, has not been established in HBSM. We therefore studied Ca-releasing mechanisms in HBSM obtained at thoracotomy with special attention to ryanodine-sensitive receptor channels (RyRs). In freshly isolated bronchial myocytes, ryanodine (0.5 to 50 microM) and caffeine (1 to 25 mM) induced transient increases in the cytoplasmic calcium concentration ([Ca(2+)](i)). Higher ryanodine concentrations (> 100 microM) inhibited the caffeine-induced [Ca(2+)](i) response, which was also blocked in the presence of tetracaine (300 microM) or ruthenium red (200 microM), two potent CICR inhibitors. In HBSM strips, caffeine induced a transient contraction which, likewise, was inhibited by ryanodine and tetracaine. However, ryanodine (200 microM) modified neither the [Ca(2+)](i) response nor the contraction induced by K(+)-rich (110 mM) solution. Reverse transcriptase/polymerase chain reaction (RT-PCR) and RNase protection assay performed in HBSM have revealed the existence of mRNAs encoding only the type 3 RyR. We also characterized acetylcholine-induced [Ca(2+)](i) and contractile responses. None of these responses was altered by ryanodine or by tetracaine. These results demonstrate, for the first time, the existence of functional RyRs in HBSM cells which, owing to the type of isoform or the amount of protein expressed, are not involved, under physiologic conditions, in depolarization- or agonist-induced contraction.

Biology of hypertensive cardiopathy

Available data suggest that hypertensive cardiopathy is principally determined by the phenoconversion that allows the myocyte to adapt to the new working conditions by re-expressing a fetal program. Nevertheless, in clinical conditions, the scheme is different. The above phenotype is modified by trophic factors, which originate from ischemia, senescence, diabetes, genetics, or neurohormonal reactions. This review only focuses on some of the most recent advances concerning the permanent changes in the myocyte. Changes in extracellular matrix have been excluded. Recently, emphasis has been on the kinetic basis of the myocardial dysfunction at the myosin level, the potential therapeutic utilization of transferring the adrenergic receptor gene, the participation of NO synthases in the adaptational process, the existence of an abnormal excitation-contraction coupling due to a redistribution of Ca2+ sparks, the role of the microtubule as a determinant of sarcomere motion, and the multifactorial origin of cell death by apoptosis.