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PubMed2003

Cellular mechanism of pituitary adenylate cyclase-activating polypeptide-induced atrial tachyarrhythmia in canine isolated arterially perfused right atria.

1. Pituitary adenylate cyclase-activating polypeptide (PACAP) induces atrial tachyarrhythmia (AT). However, the cellular mechanism responsible for this remains unclear. 2. In six canine isolated arterially perfused right atria, high-resolution optical mapping techniques were used to measure action potentials during control conditions and after PACAP injection (1 nmol). 3. During steady state pacing at a cycle length of 300 msec, the action potential duration was shorter during PACAP than during control (P < 0.001). In addition, maximum repolarization gradients during PACAP (4 +/- 1 msec/mm) were similar to those during control (5 +/- 1 msec/mm; n = 6). Transmural repolarization gradients were also similar between the two groups. 4. After PACAP, AT was easily initiated with a single premature extrastimulus and was associated with a focal pattern of activation. However, AT was not initiated by a single premature stimulus during control. 5. In conclusion, the PACAP-induced AT is associated with a focal pattern of activation that is independent of local repolarization gradients. These data suggest that increased dispersion of repolarization is not necessarily required for the induction of AT.

Action Potentials↗
PubMed1993

Spiral wave activity: a possible common mechanism for polymorphic and monomorphic ventricular tachycardias.

Several mechanisms have been proposed to explain the electrocardiographic patterns observed during various forms of polymorphic ventricular tachycardias, including torsades de pointes. Such mechanisms include the coexistence of either multiple foci or multiple exit pathways from single foci giving rise to various forms of aberrant ventricular activation sequences. For example, the simultaneous firing of two widely spaced foci at slightly different frequencies has been used to explain the undulating electrocardiogram that is characteristic of torsades de pointes. However, in spite of some supporting experimental evidence, such an idea remains conjectural from the clinical point of view. Here I discuss a mechanism that has been proposed recently to explain both monomorphic and polymorphic patterns (including undulating patterns) of ventricular tachycardia. The hypothesis is derived from the theory of spiral wave activity in excitable media, and from recent experiments using high resolution optical mapping in isolated two-dimensional ventricular muscle preparations that demonstrate that spiral wave activity may account for self-sustaining reentrant activation. Such studies have led to the observation that the behavior of the spiral center, the core, plays a key role in determining the electrocardiographic manifestation of the arrhythmia. Indeed, a stationary position of the core results in a monomorphic pattern of activation. On the other hand, beat-to-beat changes in the core position (i.e., drifting) leads to irregular patterns of activation. In fact, when drifting occurs in one direction, it gives rise to a Doppler shift in the excitation period in such a way that two coexisting frequencies are manifest, one ahead of and one behind the drifting core. The activation frequency in the region ahead of the core is always higher than that behind the core. Under such conditions, electrocardiographic recordings of the activity demonstrate an undulating pattern, which resembles that of torsades de pointes. When the core drifts in various directions, a polymorphic pattern is manifest. Thus, depending on spiral core dynamics, monomorphic, undulating, or completely irregular patterns may be observed. Moreover, transitions between such patterns can also occur. For example, drifting spirals giving rise to polymorphic activation can become stationary and result in monomorphic activation as a result of anchoring of the core to a small discontinuity (e.g., an artery or small scar) in the tissue. Direct extrapolation of such results to clinical cases is not appropriate. However, the observations discussed in this article offer a new testable hypothesis in which a common mechanism is postulated for the electrocardiographic patterns associated with monomorphic and polymorphic tachycardias.

Animals↗
PubMed1998

Hypoxia and hypothermia enhance spatial heterogeneities of repolarization in guinea pig hearts: analysis of spatial autocorrelation of optically recorded action potential durations.

INTRODUCTION: Regional dispersions of repolarization (DOR) are arrhythmogenic perturbations that are closely associated with reentry. However, the characteristics of DOR have not been well defined or adequately analyzed because previous algorithms did not take into account spatial heterogeneities of action potential durations (APDs). Earlier simulations proposed that pathologic conditions enhance DOR by decreasing electrical coupling between cells, thereby unmasking differences in cellular repolarization between neighboring cells. Optical mapping indicated that gradients of APD and DOR are associated with fiber structure and are largely independent of activation. We developed an approach to quantitatively characterize APD gradients and DOR to determine how they are influenced by tissue anisotropy and cell coupling during diverse arrhythmogenic insults such as hypoxia and hypothermia. METHODS AND RESULTS: Voltage-sensitive dyes were used to map APs from 124 sites on the epicardium of Langendorff-perfused guinea pig hearts during (1) cycles of hypoxia and reoxygenation and (2) after 30 minutes of hypothermia (32 degrees to 25 degrees C). We introduce an approach to quantitate DOR by analyzing two-dimensional spatial autocorrelation of APDs along directions perpendicular and parallel to the longitudinal axis of epicardial fibers. A spatial correlation length L was derived as a statistical measure of DOR. It corresponds to the distance over which APDs had comparable values, where L is inversely related to DOR. Hypoxia (30 min) caused a negligible decrease in longitudinal thetaL (from 0.530 +/- 0.138 to 0.478 +/- 0.052 m/sec) and transverse thetaT (from 0.225 +/- 0.034 to 0.204 +/- 0.021 m/sec) conduction velocities and did not alter thetaL/thetaT or activation patterns. In paced hearts (cycle length [CL] = 300 msec), hypoxia decreased APDs (123 +/- 18.2 to 46 +/- 0.6 msec; P < 0.001) within 10 to 15 minutes and enhanced DOR, as indicated by reductions of L from 1.8 +/- 0.9 to 1.1 +/- 0.5 mm (P < 0.005). Hypothermia caused marked reductions of thetaL (0.53 +/- 0.138 to 0.298 +/- 0.104 m/sec) and thetaT (0.225 +/- 0.034 to 0.138 +/- 0.027 m/sec), increased APDs (128 +/- 4.4 to 148 +/- 14.5 msec), and reduced L from 2.0 +/- 0.3 to 1.3 +/- 0.6 mm (P < 0.05). L decreased with increased time of hypoxia and recovered upon reoxygenation. Hypoxia and hypothermia reduced L measured along the longitudinal (L(L)) and transverse (L(T)) axes of cardiac fibers while the ratio of L(L)/L(T) remained constant. CONCLUSION: Conventional indexes of DOR (i.e., APD "range" or "standard deviation," evaluated with extracellular electrodes) did not convey the spatial inhomogeneities of repolarization revealed by L. Spatial autocorrelation analysis provides a statistically significant measurement of DOR, which can take into account intrinsic heterogeneities of APDs and fiber orientation. The data show that hypoxia and hypothermia produce reductions of L, even though they have different effects on mean APD and conduction velocity. The preservation of a constant L(L)/L(T) ratio during hypoxia and hypothermia, despite large reductions in L, is consistent with a mechanism in which reduced cell-to-cell coupling unmasks intrinsic dispersions of APD and reduces L(L) and L(T) by the same factor. Thus, the spatial autocorrelation of APDs provides a sensitive index of DOR under normal and arrhythmogenic conditions. It incorporates the anisotropic nature of the myocardium and therefore is preferable to conventional indexes of DOR.

Action Potentials↗
PubMed1999

Connexins and impulse propagation in the mouse heart.

Gap junction channels are essential for normal cardiac impulse propagation. Three gap junction proteins, known as connexins, are expressed in the heart: Cx40, Cx43, and Cx45. Each of these proteins forms channels with unique biophysical and electrophysiologic properties, as well as spatial distribution of expression throughout the heart. However, the specific functional role of the individual connexins in normal and abnormal propagation is unknown. The availability of genetically engineered mouse models, together with new developments in optical mapping technology, makes it possible to integrate knowledge about molecular mechanisms of intercellular communication and its regulation with our growing understanding of the microscopic and global dynamics of electrical impulse propagation during normal and abnormal cardiac rhythms. This article reviews knowledge on the mechanisms of cardiac impulse propagation, with particular focus on the role of cardiac connexins in electrical communication between cells. It summarizes results of recent studies on the electrophysiologic consequences of defects in the functional expression of specific gap junction channels in mice lacking either the Cx43 or Cx40 gene. It also reviews data obtained in a transgenic mouse model in which cell loss and remodeling of gap junction distribution leads to increased susceptibility to arrhythmias and sudden cardiac death. Overall, the results demonstrate that these are potentially powerful strategies for studying fundamental mechanisms of cardiac electrical activity and for testing the hypothesis that certain cardiac arrhythmias involve gap junction or other membrane channel dysfunction. These new approaches, which permit one to manipulate electrical wave propagation at the molecular level, should provide new insight into the detailed mechanisms of initiation, maintenance, and termination of cardiac arrhythmias, and may lead to more effective means to treat arrhythmias and prevent sudden cardiac death.

Animals↗
PubMed2000

Contact fluorescence imaging of reentry in monolayers of cultured neonatal rat ventricular myocytes.

INTRODUCTION: We present a novel contact fluorescence imaging (CFI) approach to monitor transmembrane potentials in monolayers of cultured neonatal rat ventricular cells. We apply CFI to demonstrate, for the first time, long-term recordings as well as electrical induction and termination of reentrant activity in this in vitro model. METHODS AND RESULTS: CFI was performed in confluent cell monolayers stained with di-8-ANEPPS. An anatomic obstacle (6 x 0.5 mm) was created in the center of the monolayers. Reentry was induced with a premature stimulus after pacing at 2 Hz (both via field stimulation). Seven sustained (>3 min) reentrant episodes, anchored to the anatomic obstacle, were observed in three monolayers. Field stimulation (30 V/cm) was applied to successfully terminate 6 of the 7 reentries. Analysis of reentrant activity showed similarities with anatomic reentry in tissue preparations, such as reduced conduction velocity around the core, variable conduction velocity along the reentrant pathway due to wavefront curvature effects, and field-induced activation at the obstacle borders leading to reentry termination (cardioversion). CONCLUSION: This study demonstrates the feasibility of CFI for macroscopic optical mapping of transmembrane potentials in a single layer of cultured cells. Our results suggest that the monolayer cell culture model is an attractive complement to tissue models of reentry and cardioversion.

Animals↗
PubMed2000

Reversal of repolarization gradient does not reverse the chirality of shock-induced reentry in the rabbit heart.

INTRODUCTION: Two hypotheses have been proposed to explain the mechanisms of vulnerability and related failure of defibrillation therapy: the cross-field-induced critical point hypothesis and the virtual electrode-induced phase singularity hypothesis. These two hypotheses predict the opposite effect of preshock repolarization on the chirality (direction of rotation) of shock-induced reentry. The former suggests its reversal upon reversal of repolarization, whereas the latter suggests its preservation. The aim of this study was to determine, by reversing the repolarization sequence, which of the mechanisms is responsible for internal shock-induced arrhythmia in the Langendorff-perfused rabbit heart. METHODS AND RESULTS: We used high-resolution optical mapping to assess the chirality of postshock reentry in 11 hearts. Hearts were paced at a coupling interval of 300 msec at various sites around the field of view (13.5 x 13.5 to 16.5 x 16.5 mm). Cathodal monophasic implantable cardioverter defibrillator shocks (-100 V, 8 msec) were applied during the T wave from a 10-mm coil electrode placed into the right ventricular cavity. We used 3.5 +/- 0.8 different pacing sites per heart. Change in direction of repolarization did not result in change of chirality. Chirality was constant in all 11 hearts despite the complete reversal of activation and repolarization patterns. However, the position of resulting vortices depended on transmembrane polarization gradient inverted delta Vm and amplitude of negative polarization Vm (deexcitation). Stronger gradients and deexcitation produced earlier epicardial break excitation (P = 0.04 and P < 0.0001, respectively). CONCLUSION: Virtual electrode-induced phase singularity mechanism underlies internal shock-induced arrhythmia in this model.

Animals↗
PubMed2005

The mechanism of pause-induced torsade de pointes in long QT syndrome.

INTRODUCTION: Torsade de pointes (TdP), is often preceded by a short-long cycle length sequence. However, the causal relationship between the pause associated with a short-long cycle length sequence and TdP is not completely understood. This study tests the hypothesis that a pause enhances both dispersion of repolarization and EAD formation; however, EADs that form where APD is longest will be less likely to initiate TdP. METHODS AND RESULTS: We used optical mapping to measure transmural action potentials from the canine left ventricular wedge preparation. D-sotalol and ATX-II were used to mimic LQT2 and LQT3, respectively. The pause significantly enhanced mean APD (from 356 +/- 20 to 381 +/- 25 msec in LQT2, P < 0.05; from 609 +/- 92 to 675 +/- 98 msec in LQT3, P < 0.05) and transmural dispersion (from 35 +/- 9 to 46 +/- 11 msec in LQT2, P < 0.05; from 121 +/- 85 to 171 +/- 98 msec in LQT3, P < 0.05) compared to steady state pacing. Under LQT3 condition EADs, EAD-induced triggered activity, and TdP were more likely to occur following a pause. Interestingly, the triggered beat following a pause always broke through at the region of maximum local repolarization gradient. CONCLUSION: These data suggest that a pause accentuates transmural repolarization gradients and facilitates the formation of EADs and EAD-induced triggered activity. In contrast to our hypothesis, the findings of this study support the concept that M-cells (where APD is longest) can play an important role in both the origination of EAD-induced triggered activity and unidirectional block associated with TdP.

Action Potentials↗
PubMed2005

Action potential duration and QT interval during pinacidil infusion in isolated rabbit hearts.

INTRODUCTION: Acute myocardial ischemia, which opens K(ATP) channel, is associated with shortened action potential duration (APD) but prolonged QT interval. This discrepancy has not been adequately explained. We hypothesize that the duration of intracellular calcium (Ca(i)) transient (DCaT) may play a role in determining QT interval. METHODS AND RESULTS: We performed simultaneous optical mapping of voltage and Ca(i) in 15 isolated rabbit hearts during a K(ATP) channel opener (pinacidil) infusion. Anterior epicardial mapping (n = 7) showed no difference of APD(90), QT interval, and the DCaT(90) at baseline. When perfused with 80 microM pinacidil, the APD(90), the QT interval, and the DCaT(90) were 105 +/- 10 msec, 199 +/- 14 msec, and 189 +/- 13 msec, respectively, during right ventricular (RV) pacing (P < 0.05). Posterior epicardial mapping (n = 4) showed that the APD(90) was significantly (P < 0.05) shorter than QT interval and DCaT(90) during pinacidil infusion. The results of the transmural mapping studies (n = 4) showed that the QT interval during RV pacing was not different than the DCaT(90) in the epicardium, midmyocardium, and endocardium, but was significantly (P < 0.01) longer than the APD(90) in epicardium, midmyocardium, and endocardium, respectively. There was a good correlation between the DCaT(90) and QT interval at baseline (r = 0.92, P < 0.0001) and during pinacidil infusion (r = 0.74, P < 0.0001). CONCLUSION: We conclude that K(ATP) channel opening shortened APD but not the QT interval. Because Ca(i) did not return to diastolic level at the end of action potential, it may have created a heterogeneous membrane potential distribution that determined the QT interval.

Action Potentials↗
PubMed2006

Rotigaptide (ZP123) improves atrial conduction slowing in chronic volume overload-induced dilated atria.

Chronic atrial dilation is associated with atrial conduction velocity slowing and an increased risk of developing atrial tachyarrhythmias. Rotigaptide (ZP123) is a selective gap junction modifier that increases cardiac gap junctional intercellular communication. We hypothesised that rotigaptide treatment would increase atrial conduction velocity and reduce the inducibility to atrial tachyarrhythmias in a model of chronic volume overload induced chronic atrial dilatation characterized by atrial conduction velocity slowing. Chronic volume overload was created in Japanese white rabbits by arterio-venous shunt formation. Atrial conduction velocity and atrial tachyarrhythmias inducibility were examined in Langendorff-perfused chronic volume overload hearts (n=12) using high-resolution optical mapping before and after treatment with rotigaptide. Moreover, expression levels of atrial gap junction proteins (connexin40 and connexin43) were examined in chronic volume overload hearts (n=6) and compared to sham-operated controls (n=6). Rotigaptide treatment significantly increased atrial conduction velocity in chronic volume overload hearts, however, rotigaptide did not decrease susceptibility to the induction of atrial tachyarrhythmias. Protein expressions of Cx40 and Cx43 were decreased by 32% and 72% (P<0.01), respectively, in chromic volume overload atria compared to control. To conclude, rotigaptide increased atrial conduction velocity in a rabbit model of chromic volume overload induced atrial conduction velocity slowing. The demonstrated effect of rotigaptide on atrial conduction velocity did not prevent atrial tachyarrhythmias inducibility. Whether rotigaptide may possess antiarrhythmic efficacy in other models of atrial fibrillation remains to be determined.

Animals↗
PubMed2002

Spatial heterogeneity of transmembrane potential responses of single guinea-pig cardiac cells during electric field stimulation.

Changes in transmembrane voltage (V(m)) of cardiac cells during electric field stimulation have a complex spatial- and time-dependent behaviour that differs significantly from electrical stimulation of space-clamped membranes by current pulses. A multisite optical mapping system was used to obtain 17 or 25 microm resolution maps of V(m) along the long axis of guinea-pig ventricular cells (n = 57) stained with voltage-sensitive dye (di-8-ANEPPS) and stimulated longitudinally with uniform electric field (2, 5 or 10 ms, 3-62 V cm(-1)) pulses (n = 201). The initial polarizations of V(m) responses (V(mr)) varied linearly along the cell length and reversed symmetrically upon field reversal. The remainder of the V(m) responses had parallel time courses among the recording sites, revealing a common time-varying signal component (V(ms)). V(ms) was depolarizing for pulses during rest and hyperpolarizing for pulses during the early plateau phase. V(ms) varied in amplitude and time course with increasing pulse amplitude. Four types of plateau response were observed, with transition points between the different responses occurring when the maximum polarization at the ends of the cell reached values estimated as 60, 110 and 220 mV. Among the cells that had a polarization change of > 200 mV at their ends (for fields > 45 V cm(-1)), some (n = 17/25) had non-parallel time courses among V(m) recordings of the various sites. This implied development of an intracellular field (E(i)) that was found to increase exponentially with time (tau = 7.2 +/- 3.2 ms). Theoretical considerations suggest that V(ms) represents the intracellular potential (phi(i)) as well as the average polarization of the cell, and that V(mr) is the manifestation of the extracellular potential gradient resulting from the field stimulus. For cells undergoing field stimulation, phi(i) acts as the cellular physiological state variable and substitutes for V(m), which is the customary variable for space-clamped membranes.

Animals↗
PubMed2006

Up-regulation of the inward rectifier K+ current (I K1) in the mouse heart accelerates and stabilizes rotors.

Previous studies have suggested an important role for the inward rectifier K+ current (I K1) in stabilizing rotors responsible for ventricular tachycardia (VT) and fibrillation (VF). To test this hypothesis, we used a line of transgenic mice (TG) overexpressing Kir 2.1-green fluorescent protein (GFP) fusion protein in a cardiac-specific manner. Optical mapping of the epicardial surface in ventricles showed that the Langendorff-perfused TG hearts were able to sustain stable VT/VF for 350 +/- 1181 s at a very high dominant frequency (DF) of 44.6 +/- 4.3 Hz. In contrast, tachyarrhythmias in wild-type hearts (WT) were short-lived (3 +/- 9 s), and the DF was 26.3 +/- 5.2 Hz. The stable, high frequency, reentrant activity in TG hearts slowed down, and eventually terminated in the presence of 10 mum Ba2+, suggesting an important role for I K1. Moreover, by increasing I K1 density in a two-dimensional computer model having realistic mouse ionic and action potential properties, a highly stable, fast rotor (approximately 45 Hz) could be induced. Simulations suggested that the TG hearts allowed such a fast and stable rotor because of both greater outward conductance at the core and shortened action potential duration in the core vicinity, as well as increased excitability, in part due to faster recovery of Na+ current. The latter resulted in a larger rate of increase in the local conduction velocity as a function of the distance from the core in TG compared to WT hearts, in both simulations and experiments. Finally, simulations showed that rotor frequencies were more sensitive to changes (doubling) in I K1, compared to other K+ currents. In combination, these results provide the first direct evidence that I K1 up-regulation in the mouse heart is a substrate for stable and very fast rotors.

Animals↗
PubMed2006

Dispersion of repolarization and refractoriness are determinants of arrhythmia phenotype in transgenic mice with long QT.

Enhanced dispersion of repolarization (DR) and refractoriness may be a unifying mechanism central to arrhythmia genesis in the long QT (LQT) syndrome. The role of DR in promoting arrhythmias was investigated in several strains of molecularly engineered mice: (a) Kv4.2 dominant negative transgenic (Kv4.2DN) that lacks the fast component of the transient outward current, I(to,f), have action potential (AP) and QT prolongation, but no spontaneous arrhythmias, (b) Kv1.4 targeted mice (Kv1.4-/-) that lack the slow component of I(to) (I(to,s)), have no QT prolongation and no spontaneous arrhythmias, and (c) double transgenic (Kv4.2DN x Kv1.4-/-) mice that lack both I(to,f) and I(to,s), have AP and QT prolongation, and spontaneous ventricular tachyarrhythmias. Hearts were perfused, stained with di-4-ANEPPS and optically mapped. Activation patterns and conduction velocities were similar between the strains but AP duration at 75% recovery (APD75) was longer in Kv4.2DN (28.0 +/- 2.5 ms, P < 0.01, n = 6), Kv1.4-/- (28.4 +/- 0.4 ms, P < 0.01, n = 5) and Kv4.2DN x Kv1.4-/- (34.3 +/- 2.6 ms, P < 0.01, n = 6) mice than controls (20.3 +/- 1.0 ms, n = 5). Dispersion of refractoriness between apex and base was markedly reduced in Kv4.2DN (0.3 +/- 0.5 ms, n = 6, P < 0.05) but enhanced in Kv1.4-/- (14.2 +/- 2.0 ms, n = 5, P < 0.05) and Kv4.2DN x Kv1.4(-/-) (15.0 +/- 3 ms, n = 5, P < 0.5) mice compared with controls (10 +/- 2 ms, n = 5). A premature pulse elicited ventricular tachycardia (VT) in Kv1.4-/- (n = 4/5) and Kv4.2DN x Kv1.4-/- hearts (n = 5/5) but not Kv4.2DN hearts (n = 0/6). Voltage-clamp recordings showed that I(to,f) was 30% greater in myocytes from the apex than base which may account for the absence of DR in Kv4.2DN mice. Thus, dispersion of repolarization (DR) appears to be an important determinant of arrhythmia vulnerability.

Action Potentials↗
PubMed2006

Propagation of the cardiac impulse in the diabetic rat heart: reduced conduction reserve.

Diabetes mellitus is a growing epidemic with severe cardiovascular complications. Although much is known about mechanical and electrical cardiac dysfunction in diabetes, few studies have investigated propagation of the electrical signal in the diabetic heart and the associated changes in intercellular gap junctions. This study was designed to investigate these issues, using hearts from control and diabetic rats. Diabetic conditions were induced by streptozotocin (STZ), given i.v. 7-14 days before experiments. Optical mapping with the voltage-sensitive dye di-4-ANEPPS, using hearts perfused on a Langendorff apparatus, showed little change in baseline conduction velocity in diabetic hearts, reflecting the large reserve of function. However, both the gap junction uncoupler heptanol (0.5-1 mM) and elevated potassium (9 mM, to reduce cell excitability) produced a significantly greater slowing of impulse propagation in diabetic hearts than in controls. The maximal action potential upstroke velocity (an index of the sodium current) and resting potential was similar in single ventricular myocytes from control and diabetic rats, suggesting similar electrical excitability. Immunoblotting of connexin 43 (Cx43), a major gap junction component, showed no change in total expression. However, immunofluorescence labelling of Cx43 showed a significant redistribution, apparent as enhanced Cx43 lateralization. This was quantified and found to be significantly larger than in control myocytes. Labelling of two other gap junction proteins, N-cadherin and beta-catenin, showed a (partial) loss of co-localization with Cx43, indicating that enhancement of lateralized Cx43 is associated with non-functional gap junctions. In conclusion, conduction reserve is smaller in the diabetic heart, priming it for impaired conduction upon further challenges. This can desynchronize contraction and contribute to arrhythmogenesis.

Animals↗
PubMedDate not supplied

Use of light absorbers to alter optical interrogation with epi-illumination and transillumination in three-dimensional cardiac models.

Cardiac optical mapping currently provides 2-D maps of transmembrane voltage-sensitive fluorescence localized near the tissue surface. Methods for interrogation at different depths are required for studies of arrhythmias and the effects of defibrillation shocks in 3-D cardiac tissue. We model the effects of coloading with a dye that absorbs excitation or fluorescence light on the radius and depth of the interrogated region with specific illumination and collection techniques. Results indicate radii and depths of interrogation are larger for transillumination versus epi-illumination, an effect that is more pronounced for broad-field excitation versus laser scanner. Coloading with a fluorescence absorber lessens interrogated depth for epi-illumination and increases it for transillumination, which is confirmed with measurements using transillumination of heart tissue slices. Coloading with an absorber of excitation light consistently decreases the interrogated depths. Transillumination and coloading also decrease the intensities of collected fluorescence. Thus, localization can be modified with wavelength-specific absorbers at the expense of a reduction in fluorescence intensity.

Animals↗
PubMed1985

Contribution of the rib cage to breathing in tetraplegia.

In tetraplegia there is often paradoxical inward motion of the rib cage during inspiration. The volume of this negative contribution is difficult to estimate but can be obtained by optical mapping. The partitioning of ventilation between the rib cage and abdomen in six normal subjects, 10 stable tetraplegic patients, and one tetraplegic patient at intervals during rehabilitation has been studied by this technique. In normal subjects the tidal volume and the vital capacity were the sum of positive contributions from the rib cage and abdomen. In stable tetraplegic subjects with similar neurological levels, the rib cage contribution varied widely but the total chest wall displacement could not be predicted from the vital capacity. In the patient studied sequentially rib cage paradox reversed with time after injury, and this was associated with an absolute increase in vital capacity and an improvement in the action of the diaphragm.

Adolescent↗
PubMed2005

Electrical remodeling of cardiac myocytes from mice with heart failure due to the overexpression of tumor necrosis factor-alpha.

Mice that overexpress the inflammatory cytokine tumor necrosis factor-alpha in the heart (TNF mice) develop heart failure characterized by atrial and ventricular dilatation, decreased ejection fraction, atrial and ventricular arrhythmias, and increased mortality (males > females). Abnormalities in Ca2+ handling, prolonged action potential duration (APD), calcium alternans, and reentrant atrial and ventricular arrhythmias were previously observed with the use of optical mapping of perfused hearts from TNF mice. We therefore tested whether altered voltage-gated outward K+ and/or inward Ca2+ currents contribute to the altered action potential characteristics and the increased vulnerability to arrhythmias. Whole cell voltage-clamp recordings of K+ currents from left ventricular myocytes of TNF mice revealed an approximately 50% decrease in the rapidly activating, rapidly inactivating transient outward K+ current Ito and in the rapidly activating, slowly inactivating delayed rectifier current IK,slow1, an approximately 25% decrease in the rapidly activating, slowly inactivating delayed rectifier current IK,slow2, and no significant change in the steady-state current Iss compared with controls. Peak amplitudes and inactivation kinetics of the L-type Ca2+ current ICa,L were not altered. Western blot analyses revealed a reduction in the proteins underlying Kv4.2, Kv4.3, and Kv1.5. Thus decreased K+ channel expression is largely responsible for the prolonged APD in the TNF mice and may, along with abnormalities in Ca2+ handling, contribute to arrhythmias.

Action Potentials↗
PubMed2004

Intracellular Ca dynamics in ventricular fibrillation.

In the heart, membrane voltage (Vm) and intracellular Ca (Cai) are bidirectionally coupled, so that ionic membrane currents regulate Cai cycling and Cai affects ionic currents regulating action potential duration (APD). Although Cai reliably and consistently tracks Vm at normal heart rates, it is possible that at very rapid rates, sarcoplasmic reticulum Cai cycling may exhibit intrinsic dynamics. Non-voltage-gated Cai release might cause local alternations in APD and refractoriness that influence wavebreak during ventricular fibrillation (VF). In this study, we tested this hypothesis by examining the extent to which Cai is associated with Vm during VF. Cai transients were mapped optically in isolated arterially perfused swine right ventricles using the fluorescent dye rhod 2 AM while intracellular membrane potential was simultaneously recorded either locally with a microelectrode (5 preparations) or globally with the voltage-sensitive dye RH-237 (5 preparations). Mutual information (MI) is a quantitative statistical measure of the extent to which knowledge of one variable (Vm) predicts the value of a second variable (Cai). MI was high during pacing and ventricular tachycardia (VT; 1.13 +/- 0.21 and 1.69 +/- 0.18, respectively) but fell dramatically during VF (0.28 +/- 0.06, P < 0.001). Cai at sites 4-6 mm apart also showed decreased MI during VF (0.63 +/- 0.13) compared with pacing (1.59 +/- 0.34, P < 0.001) or VT (2.05 +/- 0.67, P < 0.001). Spatially, Cai waves usually bore no relationship to membrane depolarization waves during nonreentrant fractionated waves typical of VF, whereas they tracked each other closely during pacing and VT. The dominant frequencies of Vm and Cai signals analyzed by fast Fourier transform were similar during VT but differed significantly during VF. Cai is closely associated with Vm closely during pacing and VT but not during VF. These findings suggest that during VF, non-voltage-gated Cai release events occur and may influence wavebreak by altering Vm and APD locally.

Animals↗
PubMed2004

Attenuation of I(K,slow1) and I(K,slow2) in Kv1/Kv2DN mice prolongs APD and QT intervals but does not suppress spontaneous or inducible arrhythmias.

Overexpression of a truncated Kv1.1 or Kv2.1 channel polypeptide in the heart (Kv1DN or Kv2DN) resulted in mice with a prolonged action potential duration (APD) due to marked attenuation of rapidly activating, slowly inactivating K+ current (I(K,slow1)) or slowly inactivating outward K(+) current (I(K,slow2)) in ventricular myocytes. ECG monitoring, optical mapping, and programmed electrical stimulation of Kv1DN mice revealed spontaneous and inducible reentrant ventricular tachycardia due to spatial dispersion of repolarization and refractoriness. Recently, we demonstrated upregulation of I(K,slow2) in apical cardiomyocytes derived from Kv1DN mice. We therefore hypothesized that the selective upregulation of Kv2.1-encoded currents underlies the apex-to-base dispersion of repolarization and the reentrant arrhythmias. To test this hypothesis, the Kv1DN line was crossbred with the Kv2DN line to produce Kv1/Kv2DN lines. Whole cell voltage-clamp recordings from left ventricular cells of Kv1/Kv2DN confirmed that the 4-aminopyridine- and tetraethylammonium-sensitive components of IK,slow were eliminated, resulting in marked APD prolongation compared with wild-type, Kv1DN, and Kv2DN cells. Telemetric ECG recordings revealed prolongation of the corrected QT in Kv1/Kv2DN compared with Kv1DN and Kv2DN mice. However, attenuation of Kv2.1-encoded currents in Kv1DN mice did not suppress the arrhythmias. Thus, the elimination of I(K,slow2) prolongs APD and the QT intervals, but does not have an antiarrhythmic effect.

Action Potentials↗