Biomedical subjects

Gregory E Morley

Publications and source records attributed to Gregory E Morley.

13 recordsLinked to original sources

PubMed2006

Consequences of cardiac myocyte-specific ablation of KATP channels in transgenic mice expressing dominant negative Kir6 subunits.

Cardiac ATP-sensitive K+ (K(ATP)) channels are formed by Kir6.2 and SUR2A subunits. We produced transgenic mice that express dominant negative Kir6.x pore-forming subunits (Kir6.1-AAA or Kir6.2-AAA) in cardiac myocytes by driving their expression with the alpha-myosin heavy chain promoter. Weight gain and development after birth of these mice were similar to nontransgenic mice, but an increased mortality was noted after the age of 4-5 mo. Transgenic mice lacked cardiac K(ATP) channel activity as assessed with patch clamp techniques. Consistent with a decreased current density observed at positive voltages, the action potential duration was increased in these mice. Some myocytes developed EADs after isoproterenol treatment. Hemodynamic measurements revealed no significant effects on ventricular function (apart from a slightly elevated heart rate), whereas in vivo electrophysiological recordings revealed a prolonged ventricular effective refractory period in transgenic mice. The transgenic mice tolerated stress less well as evident from treadmill stress tests. The proarrhythmogenic features and lack of adaptation to a stress response in transgenic mice suggest that these features are intrinsic to the myocardium and that K(ATP) channels in the myocardium have an important role in protecting the heart from lethal arrhythmias and adaptation to stress situations.

ATP-Binding Cassette Transporters↗

PubMed2006

Gap junctions and propagation of the cardiac action potential.

Pacemaker cells in the heart generate periodic electrical signals that are conducted to the working myocardium via the specialized conduction system. Effective cell-to-cell communication is critical for rapid, uniform conduction of cardiac action potentials-- a prerequisite for effective, synchronized cardiac contraction. Local circuit currents form the basis of the depolarization wave front in the working myocardium. These currents flow from cell to cell via gap junction channels. In this chapter, we trace the path of the action potential from its generation in the sinus node to propagation through the working myocardium, with a detailed discussion of the role of gap junctions. First, we review the transmembrane ionic currents and the basic principles of conduction of the action potential to the working myocardium via the specialized tissues of the heart. Next, we consider the relative contribution of cell geometry, size, and gap junction conductance. These factors are examined in terms of their source-to-sink relationships. Lastly, we will discuss new insights into the importance of gap junctions in cardiac conduction in health and disease which have been gained from high resolution optical mapping in connexin-deficient mice.

Action Potentials↗

PubMed2005

Somatic events modify hypertrophic cardiomyopathy pathology and link hypertrophy to arrhythmia.

Sarcomere protein gene mutations cause hypertrophic cardiomyopathy (HCM), a disease with distinctive histopathology and increased susceptibility to cardiac arrhythmias and risk for sudden death. Myocyte disarray (disorganized cell-cell contact) and cardiac fibrosis, the prototypic but protean features of HCM histopathology, are presumed triggers for ventricular arrhythmias that precipitate sudden death events. To assess relationships between arrhythmias and HCM pathology without confounding human variables, such as genetic heterogeneity of disease-causing mutations, background genotypes, and lifestyles, we studied cardiac electrophysiology, hypertrophy, and histopathology in mice engineered to carry an HCM mutation. Both genetically outbred and inbred HCM mice had variable susceptibility to arrhythmias, differences in ventricular hypertrophy, and variable amounts and distribution of histopathology. Among inbred HCM mice, neither the extent nor location of myocyte disarray or cardiac fibrosis correlated with ex vivo signal conduction properties or in vivo electrophysiologically stimulated arrhythmias. In contrast, the amount of ventricular hypertrophy was significantly associated with increased arrhythmia susceptibility. These data demonstrate that distinct somatic events contribute to variable HCM pathology and that cardiac hypertrophy, more than fibrosis or disarray, correlates with arrhythmic risk. We suggest that a shared pathway triggered by sarcomere gene mutations links cardiac hypertrophy and arrhythmias in HCM.

Animals↗

PubMed2005

Altered right atrial excitation and propagation in connexin40 knockout mice.

BACKGROUND: Intercellular coupling via connexin40 (Cx40) gap junction channels is an important determinant of impulse propagation in the atria. METHODS AND RESULTS: We studied the role of Cx40 in intra-atrial excitation and propagation in wild-type (Cx40(+/+)) and knockout (Cx40(-/-)) mice using high-resolution, dual-wavelength optical mapping. On ECG, the P wave was significantly prolonged in Cx40(-/-) mice (13.4+/-0.5 versus 11.4+/-0.3 ms in Cx40(+/+)). In Cx40(+/+) hearts, spontaneous right atrial (RA) activation showed a focal breakthrough at the junction of the right superior vena cava, sulcus terminalis, and RA free wall, corresponding to the location of the sinoatrial node. In contrast, Cx40(-/-) hearts displayed ectopic breakthrough sites at the base of the sulcus terminalis, RA free wall, and right superior vena cava. Progressive ablation of such sites in 4 Cx40(-/-) mice resulted in ectopic focus migration and cycle length prolongation. In all Cx40(-/-) hearts the focus ultimately shifted to the sinoatrial node at a very prolonged cycle length (initial ectopic cycle length, 182+/-20 ms; postablation sinus cycle length, 387+/-44 ms). In a second group of experiments, epicardial pacing at 10 Hz revealed slower conduction in the RA free wall of 5 Cx40(-/-) hearts than in 5 Cx40(+/+) hearts (0.61+/-0.07 versus 0.94+/-0.07 m/s; P<0.05). Dominant frequency analysis in Cx40(-/-) RA demonstrated significant reduction in the area of 1:1 conduction at 16 Hz (40+/-10% versus 69+/-5% in Cx40(+/+)) and 25 Hz (36+/-11% versus 65+/-9% in Cx40(+/+)). CONCLUSIONS: This is the first demonstration of intra-atrial block, ectopic rhythms, and altered atrial propagation in the RA of Cx40(-/-) mice.

Algorithms↗

PubMed2005

Cardiac-specific loss of N-cadherin leads to alteration in connexins with conduction slowing and arrhythmogenesis.

The remodeling of ventricular gap junctions, as defined by changes in size, distribution, or function, is a prominent feature of diseased myocardium. However, the regulation of assembly and maintenance of gap junctions remains poorly understood. To investigate N-cadherin function in the adult myocardium, we used a floxed N-cadherin gene in conjunction with a cardiac-specific tamoxifen-inducible Cre transgene. The mutant animals appeared active and healthy until their sudden death approximately 2 months after deleting N-cadherin from the heart. Electrophysiologic analysis revealed abnormal conduction in the ventricles of mutant animals, including diminished QRS complex amplitude consistent with loss of electrical coupling in the myocardium. A significant decrease in the gap junction proteins, connexin-43 and connexin-40, was observed in N-cadherin-depleted myocytes. Perturbation of connexin function resulted in decreased ventricular conduction velocity, as determined by optical mapping. Our data suggest that perturbation of the N-cadherin/catenin complex in heart disease may be an underlying cause, leading to the establishment of the arrythmogenic substrate by destabilizing gap junctions at the cell surface.

Animals↗

PubMed2005

Alk3/Bmpr1a receptor is required for development of the atrioventricular canal into valves and annulus fibrosus.

Endocardial cushions are precursors of mature atrioventricular (AV) valves. Their formation is induced by signaling molecules originating from the AV myocardium, including bone morphogenetic proteins (BMPs). Here, we hypothesized that BMP signaling plays an important role in the AV myocardium during the maturation of AV valves from the cushions. To test our hypothesis, we used a unique Cre/lox system to target the deletion of a floxed Alk3 allele, the type IA receptor for BMPs, to cardiac myocytes of the AV canal (AVC). Lineage analysis indicated that cardiac myocytes of the AVC contributed to the tricuspid mural and posterior leaflets, the mitral septal leaflet, and the atrial border of the annulus fibrosus. When Alk3 was deleted in these cells, defects were seen in the same leaflets, ie, the tricuspid mural leaflet and mitral septal leaflet were longer, the tricuspid posterior leaflet was displaced and adherent to the ventricular wall, and the annulus fibrosus was disrupted resulting in ventricular preexcitation. The defects seen in mice with AVC-targeted deletion of Alk3 provide strong support for a role of Alk3 in human congenital heart diseases, such as Ebstein's anomaly. In conclusion, our mouse model demonstrated critical roles for Alk3 signaling in the AV myocardium during the development of AV valves and the annulus fibrosus.

Animals↗

PubMed2005

Focal gap junction uncoupling and spontaneous ventricular ectopy.

Genetic studies in the mouse have demonstrated that conditional cardiac-restricted loss of connexin43 (Cx43), the major ventricular gap junction protein, is highly arrhythmogenic. However, whether more focal gap junction remodeling, as is commonly seen in acquired cardiomyopathies, influences the propensity for arrhythmogenesis is not known. We examined electrophysiological properties and the frequency of spontaneous and inducible arrhythmias in genetically engineered chimeric mice derived from injection of Cx43-deficient embryonic stem cells into wild-type recipient blastocysts. Chimeric mice had numerous well-circumscribed microscopic Cx43-negative foci in their hearts, comprising approximately 15% of the total surface area as determined by immunohistochemical analysis. Systolic function in the chimeric mice was significantly depressed as measured echocardiographically (19.0% decline in fractional shortening compared with controls, P < 0.05) and by invasive hemodynamics (17.6% reduction in change of pressure over time, P < 0.01). Chimeras had significantly more spontaneous arrhythmic events than controls (P < 0.01), including frequent runs of nonsustained ventricular tachycardia in some of the chimeric mice. However, in contrast to mice with conditional cardiac-resricted loss of Cx43 in the heart, no sustained ventricular tachyarrhythmias were observed. We conclude that focal areas of uncoupling in the myocardium increase the likelihood of arrhythmic triggers, but more widespread uncoupling is required to support sustained arrhythmias.

Animals↗

PubMed2005

Reduced intercellular coupling leads to paradoxical propagation across the Purkinje-ventricular junction and aberrant myocardial activation.

Ventricular tachycardia is a common heart rhythm disorder and a frequent cause of sudden cardiac death. Aberrant cell-cell coupling through gap junction channels, a process termed gap junction remodeling, is observed in many of the major forms of human heart disease and is associated with increased arrhythmic risk in both humans and in animal models. Genetically engineered mice with cardiac-restricted knockout of Connexin43, the major cardiac gap junctional protein, uniformly develop sudden cardiac death, although a detailed electrophysiological understanding of their profound arrhythmic propensity is unclear. Using voltage-sensitive dyes and high resolution optical mapping techniques, we found that uncoupling of the ventricular myocardium results in ectopic sites of ventricular activation. Our data indicate that this behavior reflects alterations in source-sink relationships and paradoxical conduction across normally quiescent Purkinje-ventricular muscle junctions. The aberrant activation profiles are associated with wavefront collisions, which in the setting of slow conduction may account for the highly arrhythmogenic behavior of Connexin43-deficient hearts. Thus, the extent of gap junction remodeling in diseased myocardium is a critical determinant of cardiac excitation patterns and arrhythmia susceptibility.

Animals↗

PubMed2004

Modulation of cardiac gap junction expression and arrhythmic susceptibility.

Connexin43 (Cx43), the predominant ventricular gap junction protein, is critical for maintaining normal cardiac electrical conduction, and its absence in the mouse heart results in sudden arrhythmic death. The mechanisms linking reduced Cx43 abundance in the heart and inducibility of malignant ventricular arrhythmias have yet to be established. In this report, we investigate arrhythmic susceptibility in a murine model genetically engineered to express progressively decreasing levels of Cx43. Progressively older cardiac-restricted Cx43 conditional knockout (CKO) mice were selectively bred to produce a heart-specific Cx43-deficient subline ("O-CKO" mice) in which the loss of Cx43 in the heart occurs more gradually. O-CKO mice lived significantly longer than the initial series of CKO mice but still died suddenly and prematurely. At 25 days of age, cardiac Cx43 protein levels decreased to 59% of control values (P<0.01), but conduction velocity was not significantly decreased and no O-CKO mice were inducible into sustained ventricular tachyarrhythmias. By 45 days of age, cardiac Cx43 abundance had decreased in a heterogeneous fashion to 18% of control levels, conduction velocity had slowed to half of that observed in control hearts, and 80% of O-CKO mice were inducible into lethal tachyarrhythmias. Enhanced susceptibility to induced arrhythmias was not associated with altered invasive hemodynamic measurements or changes in ventricular effective refractory period. Thus, moderately severe reductions in Cx43 abundance are associated with slowing of impulse propagation and a dramatic increase in the susceptibility to inducible ventricular arrhythmias.

Age Factors↗

PubMed2004

Transitions in ventricular activation revealed by two-dimensional optical mapping.

While cardiac function in the mature heart is dependent on a properly functioning His-Purkinje system, the early embryonic tubular heart efficiently pumps blood without a distinct specialized conduction system. Although His-Purkinje system precursors have been identified using immunohistological techniques in the looped heart, little is known whether these precursors function electrically. To address this question, we used high-resolution optical mapping and fluorescent dyes with two CCD cameras to describe the motion-corrected activation patterns of 76 embryonic chick hearts from tubular stages (stage 10) to mature septated hearts (stage 35). Ventricular activation in the tubular looped heart (stages 10-17) using both calcium-sensitive fluo-4 and voltage-sensitive di-4-ANEPPS shows sequentially uniform propagation. In late looped hearts (stages 18-22), domains of the dorsal and lateral ventricle are preferentially activated before spreading to the remaining myocardium and show alternating regions of fast and slow propagation. During stages 22-26, action potentials arise from the dorsal ventricle. By stages 27-29, action potential breakthrough is also observed at the right ventricle apex. By stage 31, activation of the heart proceeds from foci at the apex and dorsal surface of the heart. The breakthrough foci correspond to regions where putative conduction system precursors have been identified immunohistologically. To date, our study represents the most detailed electrophysiological characterization of the embryonic heart between the looped and preseptated stages and suggests that ventricular activation undergoes a gradual transformation from sequential to a mature pattern with right and left epicardial breakthroughs. Our investigation suggests that cardiac conduction system precursors may be electrophysiologically distinct and mature gradually throughout cardiac morphogenesis in the chick.

Action Potentials↗

PubMed2003

Subdiaphragmatic murine electrophysiological studies: sequential determination of ventricular refractoriness and arrhythmia induction.

Programmed electrical stimulation (PES) is a crucial aspect of the evaluation of the risk of arrhythmias in cardiac patients and provides a powerful tool for understanding the mechanisms of arrhythmia in experimental models. Whereas PES in the mouse is well characterized, the procedures allowing for follow-up studies in the same animal have not been developed. In this report, we describe a novel subdiaphragmatic approach that allows for repeat electrophysiological studies in the mouse. Under inhaled anesthesia, PES was performed in 36 wild-type mice via a stimulating electrode introduced through an epigastric incision and placed directly into the diaphragmatic surface of the heart. The procedure was repeated 7 days later. Ventricular effective refractory periods (VERP) did not change significantly between the initial and follow-up trials. Chronic treatment with amiodarone, however, was associated with a 70% prolongation in VERP from initial to follow-up studies (P < or = 0.001). In addition, PES of a genetically modified strain with sudden cardiac death, the connexin43 conditional knockout mouse consistently induced lethal polymorphic ventricular tachycardia. Thus sequential PES in mice is feasible with the use of a subdiaphragmatic approach, yields reproducible VERP values, and can be used to follow pharmacologically induced changes in VERP and identify mice at risk of lethal ventricular arrhythmias.

Action Potentials↗

PubMed2003

Patterning of the mouse conduction system.

The cardiac conduction system (CCS) is a network of cells responsible for the rhythmic and coordinated excitation of the heart. Components of the murine conduction system, including the peripheral Purkinje fibres, are morphologically indistinguishable from surrounding cardiomyocytes and there exists a paucity of molecular markers to specifically identify these cells. Recently, we identified a line of transgenic mice in which the lacZ reporter gene is expressed within the embryonic CCS beginning at 8.25 days post-conception (dpc); its expression appears to delineate the full extent of the CCS, including the distal Purkinje fibre network, throughout all subsequent stages of development. Moreover, using the highly sensitive technique of optical mapping of electrical activity in embryonic murine hearts, we provided evidence for functional specialization of components of the CCS as early as 10.5 dpc. Here, we summarize these findings and describe our initial efforts utilizing the CCS-lacZ mice to identify novel factors that promote CCS specialization.

Animals↗

PubMed2002

Neuregulin-1 promotes formation of the murine cardiac conduction system.

The cardiac conduction system is a network of cells responsible for the rhythmic and coordinated excitation of the heart. Components of the murine conduction system, including the peripheral Purkinje fibers, are morphologically indistinguishable from surrounding cardiomyocytes, and a paucity of molecular markers exists to identify these cells. The murine conduction system develops in close association with the endocardium. Using the recently identified CCS-lacZ line of reporter mice, in which lacZ expression delineates the embryonic and fully mature conduction system, we tested the ability of several endocardial-derived paracrine factors to convert contractile cardiomyocytes into conduction-system cells as measured by ectopic reporter gene expression in the heart. In this report we show that neuregulin-1, a growth and differentiation factor essential for ventricular trabeculation, is sufficient to induce ectopic expression of the lacZ conduction marker. This inductive effect of neuregulin-1 was restricted to a window of sensitivity between 8.5 and 10.5 days postcoitum. Using the whole mouse embryo culture system, neuregulin-1 was shown to regulate lacZ expression within the embryonic heart, whereas its expression in other tissues remained unaffected. We describe the electrical activation pattern of the 9.5-days postcoitum embryonic mouse heart and show that treatment with neuregulin-1 results in electrophysiological changes in the activation pattern consistent with a recruitment of cells to the conduction system. This study supports the hypothesis that endocardial-derived neuregulins may be the major endogenous ligands responsible for inducing murine embryonic cardiomyocytes to differentiate into cells of the conduction system.

Animals↗