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Biomedical subjects

Steven M Pogwizd

Publications and source records attributed to Steven M Pogwizd.

At least 19 recordsLinked to original sources
PubMed2006

Creating a cardiac pacemaker by gene therapy.

While electronic cardiac pacing in its various modalities represents standard of care for treatment of symptomatic bradyarrhythmias and heart failure, it has limitations ranging from absent or rudimentary autonomic modulation to severe complications. This has prompted experimental studies to design and validate a biological pacemaker that could supplement or replace electronic pacemakers. Advances in cardiac gene therapy have resulted in a number of strategies focused on beta-adrenergic receptors as well as specific ion currents that contribute to pacemaker function. This article reviews basic pacemaker physiology, as well as studies in which gene transfer approaches to develop a biological pacemaker have been designed and validated in vivo. Additional requirements and refinements necessary for successful biopacemaker function by gene transfer are discussed.

Arrhythmias, Cardiac↗
PubMed2006

Three-dimensional myocardial activation imaging in a rabbit model.

This study evaluated a recently developed three-dimensional (3-D) electrocardiographic imaging (3DECI) approach in a closed-chest rabbit model. First, the performance and sensitivity of parameters of 3DECI were evaluated using a geometrically realistic rabbit heart-torso model. Second, a 3-D intracardiac mapping procedure was evaluated based on the heart-torso rabbit model. Third, comparisons were made among the forward-simulated ventricular activation sequence, the activation sequence derived by the intracardiac mapping, and the 3DECI inverse solution. Finally, the present procedures were tested in vivo in a rabbit, in which the relative error between the measured and imaged activation sequence was 0.20 and the localization error was 5.1 mm. The present simulation and experimental results suggest the merits of the 3DECI imaging approach, and the validity of intracardiac mapping as a tool to evaluate the 3DECI.

Animals↗
PubMed2006

Micropatterns of propagation.

Alterations in microscopic conduction could contribute to microreentry and arrhythmogenesis in pathological settings. This chapter reviews microconduction in the ventricular myocardium. Gap junctions play a significant role in longitudinal and transverse propagation of the action potential wavefront in the ventricle. Studies of microscopic conduction in patterned cultures of neonatal rodent myocytes have provided novel insights into the role of gap junctions, the effects of uncoupling versus altered excitability, and the contribution of discontinuities and branching. Decreased gap junctional coupling can contribute to slowing of conduction and development of unidirectional block. However, in the setting of structural inhomogeneities and unbalanced current source and load, decreased coupling can, at times, improve conduction and be 'anti-arrhythmic,' attesting to the complexity of intercellular coupling as a therapeutic target. Genetically engineered mouse models of Cx43 depletion demonstrate slow conduction and arrhythmogenesis that appears to be reentrant in nature. Studies in these models provide novel insights into the contribution of gap junctions to impulse propagation and arrhythmogenesis in the intact heart. Overall, gap junction expression, distribution and heterogeneity are important contributors to microscopic conduction, and alterations in any of these can contribute to the development of an arrhythmogenic substrate in pathological states.

Action Potentials↗
PubMed2005

Ca2+/calmodulin-dependent protein kinase modulates cardiac ryanodine receptor phosphorylation and sarcoplasmic reticulum Ca2+ leak in heart failure.

Abnormal release of Ca from sarcoplasmic reticulum (SR) via the cardiac ryanodine receptor (RyR2) may contribute to contractile dysfunction and arrhythmogenesis in heart failure (HF). We previously demonstrated decreased Ca transient amplitude and SR Ca load associated with increased Na/Ca exchanger expression and enhanced diastolic SR Ca leak in an arrhythmogenic rabbit model of nonischemic HF. Here we assessed expression and phosphorylation status of key Ca handling proteins and measured SR Ca leak in control and HF rabbit myocytes. With HF, expression of RyR2 and FK-506 binding protein 12.6 (FKBP12.6) were reduced, whereas inositol trisphosphate receptor (type 2) and Ca/calmodulin-dependent protein kinase II (CaMKII) expression were increased 50% to 100%. The RyR2 complex included more CaMKII (which was more activated) but less calmodulin, FKBP12.6, and phosphatases 1 and 2A. The RyR2 was more highly phosphorylated by both protein kinase A (PKA) and CaMKII. Total phospholamban phosphorylation was unaltered, although it was reduced at the PKA site and increased at the CaMKII site. SR Ca leak in intact HF myocytes (which is higher than in control) was reduced by inhibition of CaMKII but was unaltered by PKA inhibition. CaMKII inhibition also increased SR Ca content in HF myocytes. Our results suggest that CaMKII-dependent phosphorylation of RyR2 is involved in enhanced SR diastolic Ca leak and reduced SR Ca load in HF, and may thus contribute to arrhythmias and contractile dysfunction in HF.

Animals↗
PubMed2005

Expression and phosphorylation of the na-pump regulatory subunit phospholemman in heart failure.

Intracellular [Na] is approximately 3 mmol/L higher in heart failure (HF; in our arrhythmogenic rabbit model), and this can profoundly affect cardiac Ca and contractile function via Na/Ca exchange and Na/H exchange. Na/K-ATPase is the primary mechanism of Na extrusion. We examine here in HF rabbits (and human hearts) expression of Na/K-ATPase isoforms and phospholemman (PLM), a putative Na/K-ATPase regulatory subunit that inhibits pump function and is a major cardiac phosphorylation target. Na/K-ATPase alpha1- and alpha2-isoforms were reduced in HF in rabbit ventricular homogenates (by 24%) and isolated myocytes (by 30% and 17%), whereas alpha3 was increased (50%) in homogenates and decreased (52%) in myocytes (P<0.05). Homogenate Na/K-ATPase activity in left ventricle was also decreased in HF. However, we showed previously that Na/K-ATPase characteristics in intact ventricular myocytes were unaltered in HF. To reconcile these findings, we assessed PLM expression, phosphorylation, and association with Na/K-ATPase. PLM coimmunoprecipitated with Na/K-ATPase alpha1 and alpha2 in control and HF rabbit myocytes. PLM expression was reduced in HF by 42% in isolated rabbit left ventricular (LV) myocytes, by 48% in rabbit LV homogenates, and by 24% in human LV homogenate. The fraction of PLM phosphorylated at Ser-68 was increased dramatically in HF. Our results are consistent with a role for PLM analogous to that of phospholamban for SR Ca-ATPase (SERCA): inhibition of Na/K-ATPase function that is relieved on PLM phosphorylation. So reduced Na/K-ATPase expression in HF may be functionally offset by lower inhibition by PLM (because of reduced PLM expression and higher PLM phosphorylation).

Animals↗
PubMed2005

Noninvasive three-dimensional electrocardiographic imaging of ventricular activation sequence.

Imaging the myocardial activation sequence is critical for improved diagnosis and treatment of life-threatening cardiac arrhythmias. It is desirable to reveal the underlying cardiac electrical activity throughout the three-dimensional (3-D) myocardium (rather than just the endocardial or epicardial surface) from noninvasive body surface potential measurements. A new 3-D electrocardiographic imaging technique (3-DEIT) based on the boundary element method (BEM) and multiobjective nonlinear optimization has been applied to reconstruct the cardiac activation sequences from body surface potential maps. Ultrafast computerized tomography scanning was performed for subsequent construction of the torso and heart models. Experimental studies were then conducted, during left and right ventricular pacing, in which noninvasive assessment of ventricular activation sequence by means of 3-DEIT was performed simultaneously with 3-D intracardiac mapping (up to 200 intramural sites) using specially designed plunge-needle electrodes in closed-chest rabbits. Estimated activation sequences from 3-DEIT were in good agreement with those constructed from simultaneously recorded intracardiac electrograms in the same animals. Averaged over 100 paced beats (from a total of 10 pacing sites), total activation times were comparable (53.3 +/- 8.1 vs. 49.8 +/- 5.2 ms), the localization error of site of initiation of activation was 5.73 +/- 1.77 mm, and the relative error between the estimated and measured activation sequences was 0.32 +/- 0.06. The present experimental results demonstrate that the 3-D paced ventricular activation sequence can be reconstructed by using noninvasive multisite body surface electrocardiographic measurements and imaging of heart-torso geometry. This new 3-D electrocardiographic imaging modality has the potential to guide catheter-based ablative interventions for the treatment of life-threatening cardiac arrhythmias.

Algorithms↗
PubMed2005

Adenoviral gene transfer of mutant phospholamban rescues contractile dysfunction in failing rabbit myocytes with relatively preserved SERCA function.

In heart failure (HF) a main factor in reduced contractility is reduced SR Ca2+ content and reversed force-frequency response (FFR), ie, from positive to negative. Our arrhythmogenic rabbit HF model exhibits decreased contractility mainly due to an increase in Na/Ca exchange (NCX) activity (with only modest decrease in SR Ca2+-ATPase (SERCA) function), similar to many end-stage HF patients. Here we test whether phospholamban (PLB) inhibition using a dominant-negative mutant PLB adenovirus (K3E/R14E, AdPLB-dn, with beta-galactosidase adenovirus as control) could enhance SERCA function and restore Ca2+ transients and positive FFR in ventricular myocytes from these HF rabbits. HF myocytes infected with AdPLB-dn (versus control) had enhanced Ca2+ transient amplitude (2.0+/-0.1 versus 1.6+/-0.05 F/Fo at 0.5 Hz, P<0.05) and had a positive FFR, whereas acutely isolated HF myocytes or those infected with Adbetagal had negative FFR. Ca2+ transients declined faster in AdPLB-dn versus Adbetagal myocytes (RT50%: 317+/-29 versus 551+/-90 ms at 0.5 Hz, P<0.05) and had an increased SR Ca2+ load (3.5+/-0.3 versus 2.6+/-0.2 F/Fo at 0.5 Hz, P<0.05), indicative of increased SERCA function. Furthermore, this restoration of function was not due to changes in NCX or SERCA expression. Thus, increasing SERCA activity in failing myocytes by AdPLB-dn gene transfer reversed the contractile dysfunction (and restored positive FFR) by increasing SR Ca2+ load. This approach could enhance contractile function in failing hearts of various etiologies, even here where reduced SERCA activity is not the main dysfunction.

Adenoviridae↗
PubMed2004

Connexin 43 downregulation and dephosphorylation in nonischemic heart failure is associated with enhanced colocalized protein phosphatase type 2A.

In nonischemic heart failure (HF), ventricular tachycardia initiates by a nonreentrant mechanism, but there is altered conduction (that could lead to re-entry) that could arise from changes in gap junctional proteins, especially connexin43 (Cx43). We studied Cx43 expression and phosphorylation state in the left ventricle (LV) from an arrhythmogenic rabbit model of nonischemic HF and from patients with HF attributable to idiopathic dilated cardiomyopathy. We also investigated the role of protein phosphatases that dephosphorylate Cx43--PP1 and PP2A. In HF rabbit LV, Cx43 mRNA and total protein were decreased by 29% and 34%, respectively (P<0.05 and P<0.001). In controls, Cx43 was primarily in the phosphorylated state, but with HF there was a 64% increase in nonphosphorylated Cx43 (Cx43-NP, normalized to total Cx43; P<0.05). Similar results were noted in HF rabbit myocytes (P<0.05) and in human idiopathic dilated cardiomyopathy LV (P<0.05). We found that PP1 and PP2A colocalized with Cx43 in rabbit LV. With HF, the level of colocalized PP2A increased >2.5-fold (P<0.002), whereas colocalized PP1 was unchanged. We also found intercellular coupling (assessed by Lucifer Yellow dye transfer) was markedly reduced in HF. However, okadaic acid (10 nmol/L) reduced the amount of Cx43-NP and significantly improved cell coupling in HF. Thus, in nonischemic HF in rabbits and humans, there is a decrease in both Cx43 expression and phosphorylation that contributes to uncoupling. Increased levels of PP2A that colocalize with Cx43 can underlie enhanced levels of Cx43-NP in HF. Modulation of Cx43 phosphorylation may be a potential therapeutic target to improve conduction in HF.

Animals↗
PubMed2004

Cellular basis of triggered arrhythmias in heart failure.

Ventricular tachycardia in nonischemic heart failure (HF) initiates by a nonreentrant mechanism that appears to be due to triggered activity primarily from delayed afterdepolarizations that arise from altered cellular Ca handling and ionic currents. In HF, factors that conspire to enhance triggered arrhythmias include upregulated Na/Ca exchange, preserved beta-adrenergic responsiveness, and decreased I(K1). Overall, the further delineation of key factors that underlie triggered arrhythmias in HF will provide the basis for new therapeutic strategies directed toward novel targets that can reduce the high incidence of sudden death in patients with HF.

Animals↗
PubMed2003

Elevated sarcoplasmic reticulum Ca2+ leak in intact ventricular myocytes from rabbits in heart failure.

Altered sarcoplasmic reticulum (SR) Ca2+-ATPase and Na+-Ca2+ exchange (NCX) function have been implicated in depressing SR Ca2+ content and contractile function in heart failure (HF). Enhanced diastolic ryanodine receptor (RyR) leak could also lower SR Ca2+ load in HF, but direct cellular measurements are lacking. In this study, we measure SR Ca2+ leak directly in intact isolated rabbit ventricular myocytes from a well-developed nonischemic HF model. Abrupt block of SR Ca2+ leak by tetracaine shifts Ca2+ from the cytosol to SR. The tetracaine-induced decline in [Ca2+]i and increase total SR Ca2+ load ([Ca2+]SRT) directly indicate the SR Ca2+ leak (before tetracaine). Diastolic SR Ca2+ leak increases with [Ca2+]SRT, and for any [Ca2+]SRT is greater in HF versus control. Mathematical modeling was used to compare the relative impact of alterations in SR Ca2+ leak, SR Ca2+-ATPase, and Na+-Ca2+ exchange on SR Ca2+ load in HF. We conclude that increased diastolic SR Ca2+ leak in HF may contribute to reductions in SR Ca2+ content, but changes in NCX in this HF model have more impact on [Ca2+]SRT.

Animals↗
PubMed2003

Clinical potential of sodium-calcium exchanger inhibitors as antiarrhythmic agents.

The Na(+)/Ca(2+) exchanger (NaCaX) plays an important role in calcium handling in myocytes, but in the setting of calcium overload NaCaX can also contribute to the activation of an arrhythmogenic transient inward current (I(ti)). Therefore, approaches to inhibit NaCaX could have potential antiarrhythmic effects in pathophysiological states such as heart failure (HF) or myocardial ischaemia and reperfusion. NaCaX typically functions in a forward (Ca(2+) extrusion) mode but can also function in a reverse (Ca(2+) influx) mode. The determining factors for the directionality of NaCaX ion movement are the electrochemical gradients of calcium and sodium, and membrane potential (E(m)). In HF, upregulated NaCaX plays a dual role: it decreases sarcoplasmic reticulum (SR) calcium load, which leads to contractile dysfunction, and it underlies the I(ti) responsible for delayed after-depolarisations (DADs) and ventricular arrhythmias. In myocardial ischaemia and reperfusion, increases in [Na(+)](i) (as a result of acidosis and activation of the Na(+)/H(+) exchanger [NHE]) lead to calcium overload via the NaCaX and arrhythmogenesis is probably mediated by I(ti) activation due to NaCaX. As such, inhibition of NaCaX could provide a novel therapeutic approach to the prevention and treatment of arrhythmias. Unfortunately, it is difficult to assess the efficacy of such an approach since there are no specific NaCaX inhibitors. Currently available agents are hampered by their nonspecific effects on other ion channels and carriers. The potential utility of specific inhibition of forward or reverse mode NaCaX as an antiarrhythmic approach in the settings of HF and ischaemia/ reperfusion is discussed within the context of current knowledge of myocyte calcium and sodium handling. NaCaX is a challenging and complex therapeutic target because of the delicate balance of SR calcium load (too little contributes to contractile dysfunction and too much leads to calcium overload and arrhythmogenesis). Further understanding of NaCaX function, [Na(+)](i) and [Ca(2+)](i) in HF and ischaemia/reperfusion, combined with the development and assessment of specific NaCaX inhibitors, will ultimately define the potential role of NaCaX inhibition in the prevention and treatment of ventricular arrhythmias.

Animals↗
PubMed2002

Intracellular Na(+) concentration is elevated in heart failure but Na/K pump function is unchanged.

BACKGROUND: Intracellular sodium concentration ([Na(+)](i)) modulates cardiac contractile and electrical activity through Na/Ca exchange (NCX). Upregulation of NCX in heart failure (HF) may magnify the functional impact of altered [Na(+)](i). METHODS AND RESULTS: We measured [Na(+)](i) by using sodium binding benzofuran isophthalate in control and HF rabbit ventricular myocytes (HF induced by aortic insufficiency and constriction). Resting [Na(+)](i) was 9.7+/-0.7 versus 6.6+/-0.5 mmol/L in HF versus control. In both cases, [Na(+)](i) increased by approximately 2 mmol/L when myocytes were stimulated (0.5 to 3 Hz). To identify the mechanisms responsible for [Na(+)](i) elevation in HF, we measured the [Na(+)](i) dependence of Na/K pump-mediated Na(+) extrusion. There was no difference in V(max) (8.3+/-0.7 versus 8.0+/-0.8 mmol/L/min) or K(m) (9.2+/-1.0 versus 9.9+/-0.8 mmol/L in HF and control, respectively). Therefore, at measured [Na(+)](i) levels, the Na/K pump rate is actually higher in HF. However, resting Na(+) influx was twice as high in HF versus control (2.3+/-0.3 versus 1.1+/-0.2 mmol/L/min), primarily the result of a tetrodotoxin-sensitive pathway. CONCLUSIONS: Myocyte [Na(+)](i) is elevated in HF as a result of higher diastolic Na(+) influx (with unaltered Na/K-ATPase characteristics). In HF, the combined increased [Na(+)](i), decreased Ca(2+) transient, and prolonged action potential all profoundly affect cellular Ca(2+) regulation, promoting greater Ca(2+) influx through NCX during action potentials. Notably, the elevated [Na(+)](i) may be critical in limiting the contractile dysfunction observed in HF.

Action Potentials↗
PubMed2002

Intracellular [Na+] and Na+ pump rate in rat and rabbit ventricular myocytes.

Intracellular [Na+] ([Na+]i) is centrally involved in regulation of cardiac Ca2+ and contractility via Na+-Ca2+ exchange (NCX) and Na+-H+ exchange (NHX). Previous work has indicated that [Na+]i is higher in rat than rabbit ventricular myocytes. This has major functional consequences, but the reason for the higher [Na+]i in rat is unknown. Here, resting [Na+]i was measured using the fluorescent indicator SBFI, with both traditional calibration and a novel null-point method (which circumvents many limitations of prior methods). In rabbit, resting [Na+]i was 4.5 +/- 0.4 mM (traditional calibration) and 4.4 mM (null-point). Resting [Na+]i in rat was significantly higher using both the traditional calibration (11.1 +/- 0.7 mM) and the null-point approach (11.2 mM). The rate of Na+ transport by the Na+ pump was measured as a function of [Na+]i in intact cells. Rat cells exhibited a higher V(max) than rabbit (7.7 +/- 1.1 vs. 4.0 +/- 0.5 mM x min(-1)) and a higher K(m) (10.2 +/- 1.2 vs. 7.5 +/- 1.1 mM). This results in little difference in pump activity for a given [Na+]i below 10 mM, but at measured resting [Na+]i levels the pump-mediated Na+ efflux is much higher in rat. Thus, Na+ pump rate cannot explain the higher [Na+]i in rat. Resting Na+ influx rate was two to four times higher in rat, and this accounts for the higher resting [Na+]i. Using tetrodotoxin, HOE-642 and Ni2+ to block Na+ channels, NHX and NCX, respectively, we found that all three pathways may contribute to the higher resting Na+ influx in rat (albeit differentially). We conclude that resting [Na+]i is higher in rat than in rabbit, that this is caused by higher resting Na+ influx in rat and that a higher Na+,K+-ATPase pumping rate in rat is a consequence of the higher [Na+]i.

Animals↗
PubMed2002

Upregulated Na/Ca exchange is involved in both contractile dysfunction and arrhythmogenesis in heart failure.

Systolic heart failure (HF) is characterized by reduced systolic function and often by arrhythmias. We studied a rabbit model of HF (induced by combined aortic insufficiency and stenosis) which shows both contractile dysfunction and arrhythmogenesis. In this model we find an approximately 100% increase in Na/Ca exchange (NaCaX) expression at the level of mRNA, protein and function, but only a modest decrease in SR Ca-ATPase (approximately 24%, only detectable in cellular function). This combination results in a 40% reduction in SR Ca content in HF, which is sufficient to explain the 40% reduction in twitch Ca transients and 30-38% decrease in contractile function in this HF model. When stimulated by isoproterenol the SR Ca load readily reaches the threshold for spontaneous SR Ca release (this threshold Ca load is unchanged in HF). This SR Ca release activates a transient inward current (I(ti)) carried exclusively by NaCaX. For a given SR Ca release there is greater I(ti) in HF (due to higher NaCaX). We also find a 49% decrease in the inward rectifier potassium current (I(K1)), which allows greater depolarization for a given I(ti). Thus, higher NaCaX and lower I(K1) greatly increase the likelihood that an SR Ca release-induced delayed afterdepolarization (DAD) will trigger an arrhythmogenic action potential. We conclude that NaCaX contributes in major ways to both contractile dysfunction (by reducing SR Ca) and increased propensity for triggered arrhythmias (by increasing I(ti) and DADs).

Animals↗
PubMed2002

Calcium cycling in heart failure: the arrhythmia connection.

Ventricular tachycardia in nonischemic heart failure (HF) arises from a nonreentrant mechanism most likely due to delayed afterdepolarizations from activation of a transient inward current (I(ti)). We present data and a paradigm in which an up-regulated Na/Ca exchanger, residual beta-adrenergic responsiveness, and decreased inward rectifying K current (I(K1)) in HF all conspire to markedly increase the propensity for triggered arrhythmias. The up-regulated Na/Ca exchanger plays an additional critical role in unloading the sarcoplasmic reticulum of Ca, thereby causing the mechanical dysfunction. It is imperative that therapeutic approaches for ventricular tachycardia in HF take into consideration cellular Ca handling and excitation-contractile coupling, and their alteration in the failing heart.

Animals↗
PubMed2002

Na/Ca exchange in heart failure: contractile dysfunction and arrhythmogenesis.

Congestive heart failure (HF) is characterized by contractile dysfunction and a high incidence of sudden death from nonreentrant ventricular arrhythmias, both of which involve altered intracellular calcium handling. The focus of this article is on the critical role of the Na/Ca exchanger. We demonstrate that upregulation of Na/Ca exchanger unloads the sarcoplasmic reticulum (SR), leading to contractile dysfunction. At the same time, Na/Ca exchanger underlies the arrhythmogenic transient inward current (I(ti)) in HF. Preserved beta-adrenergic responsiveness in HF plays a crucial role in increasing SR Ca load, leading to SR Ca release and activation of I(ti). In addition, decreased I(K1) (inward rectifier) current in HF destabilizes resting membrane potential (E(m)) and further enhances arrhythmogenesis mediated by the upregulated Na/Ca exchanger. We thus propose a new paradigm in which upregulated Na/Ca exchanger in HF plays a dual role in underlying both the contractile dysfunction and arrhythmogenesis in the failing heart. Therapeutic approaches to the treatment of HF will need to balance increasing SR Ca load with the arrhythmogenic effects of SR Ca overload that involve activation of I(ti) carried by Na/Ca exchanger.

Animals↗