Dog Heart Research Articles

Abstract 635: The Proximal AV Bundle in the Human Heart and Related Morbidity

INTRODUCTION: The proximal AV bundle (PAVB) has been shown to be the only input to the AV node (AVN) in the canine heart in anatomoelectrical reports over the past 20 years. The anatomic studies utilized photographic correlations of epi- and endocardial aspects of whole hearts through blocking, and serial histologic parallel, perpendicular and transverse plane Goldner Trichrome stained sections of the flattened heart; Karnovsky’s fixative at pH 7.2 and sucrose buffer rinses; direct 3D and stereotaxic analysis. Electrical studies, under direct observation of in-vitro superfused hearts, delineated unique wire, catheter, and micropipet electrode potentials via high K, transections, Lucifer Yellow iontophoresis and photoablations during spontaneous and paced SA node rhythms and with simultaneous SA node, atrionodal bundles, PAVB, AVN and distal AV bundle recordings. HYPOTHESIS: The PAVB exists in the human heart. METHODS AND RESULTS: Explanted normal human hearts, deemed unsuitable for transplantation, processed as above with transverse sections, revealed that the AVN (Figs. A–C ) is joined by the PAVB at a 90-degree angle (Figs. B, C ). These “normal” hearts from older patients (57– 80 yr) had atrophic or absent atrial myocardium. In Figure C , most of the right medial atrial wall myocardium, but not the left atrium (LA), had been replaced by fat. CONCLUSIONS: PAVB is the only AVN input in the human heart. As in the canine heart, PAVB also runs away from the annulus and is apposed to LA. Knowledge of the PAVB should be helpful in decreasing morbidity associated with clinical procedures. Care must be taken in ablating the fast superior atrionodal bundle pathway input to the PAVB. Figures A and B are from the same 60 yr and C is from a 71 yr old heart. AVN (A) is apposed to the left ventricular outflow tract (LVOFT and dotted line) along with the PAVB ( B and C ). But as seen in C , PAVB assumes a position apposed to LA, And, as in the dog heart, thereafter (not shown here) PAVB is completely apposed to LA.

Abstract 355: Snf1-related kinase is Upregulated in the Ischemic Heart and Regulates Cellular Metabolism and Death

Sucrose non-fermenting (Snf1)-related kinase (SNRK) is a novel kinase homologous to the Snf1 kinase of yeast and the AMP-activated protein kinases (AMPK) of mammals. SNRK is expressed in a variety of tissues; however, its role in the cardiovascular system is unclear. While AMPK and Snf1 regulate metabolism, the primary function of SNRK is not known. We hypothesized that like AMPK, SNRK regulates cellular metabolism and influences cell survival. Western blots of protein extracts from several cell types showed that SNRK is widely expressed and exists in different forms in the nucleus and cytoplasm. We next assessed the effects of ischemia, hypoglycemia and increased AMP on SNRK levels. Dog hearts subjected to ischemia displayed a 41% increase in SNRK levels ( P <0.01), while explanted hearts from patients with ischemic cardiomyopathy showed a 220% increase compared to controls ( P <0.05). SNRK mRNA levels and subcellular location were altered by hypoglycemia and increased AMP levels. The role of SNRK on cell death and metabolism was then assessed by transfecting HEK293 cells with GFP, a SNRKGFP fusion construct, or SNRKGFP containing a mutation in the nuclear localization sequence (SNRKnls). SNRKGFP overexpression increased cell death by 10% ( P =0.007) compared to controls both in the absence and presence of oxidant stress. Nuclear respiratory factors (NRF)2α and -2β (transcription factors that induce mitochondrial biogenesis) were upregulated with SNRK overexpression by 30% and 90%, respectively ( P <0.05). SNRKnls did not induce a significant change. SNRK overexpression, but not SNRKnls, resulted in a 15% reduction in cellular ATP content compared to controls ( P =0.0001). A gene array performed on cells overexpressing either SNRKGFP or GFP showed that SNRK overexpression strongly correlates with numerous nuclear and metabolic functions. These data show that SNRK is upregulated in ischemic heart disease and suggest that it induces cell death. SNRK also appears to be a component of the cellular metabolic machinery, likely exerting its actions via entry into the nucleus. Thus, by altering cellular metabolism, SNRK may play an important role in the response of the heart to ischemia, and may provide a novel target for cardiovascular therapeutic interventions.

Cardiac changes induced by excess exogenous growth hormone in juvenile miniature poodles

The transient elevated plasma growth hormone (GH) levels that occur at a young age in giant breed dogs may have consequences in adult life. The aim of this study was to investigate whether excess juvenile GH has consequences for cardiac function and morphology. To simulate the naturally occurring juvenile hypersomatotropism in giant breed dogs, elevated plasma GH and insulin-like growth factor-I (IGF-I) concentrations were induced in six miniature poodles (GH dogs) by daily administration of supraphysiological doses of GH starting at 12 weeks of age. Eight miniature poodles of the same age that received vehicle only served as controls. Cardiac anatomy and function were evaluated by echocardiography. After euthanasia at 21 weeks of age, the hearts were examined for weight, myocyte dimensions and collagen fraction. The hearts of the GH dogs had larger atria (+22%), a thicker left ventricular wall (+21%), greater weight (+84%), and their cardiomyocytes were 15% longer, 25% thicker, and 92% greater in volume than those of control dogs. The mean collagen fraction was also higher in the GH dogs (5.6%) than in the controls (3.1%). In conclusion, excess GH in juvenile miniature poodles resulted in myocardial hypertrophy and increased collagen content. These findings are consistent with observations in acromegalic human patients and in rats treated with GH.

Tissue Structure and Ventricular Wall Mechanics

Considering that the left ventricle is a thick-walled body that maintains its volume while reducing intracavitary volume by two thirds, it should come as no surprise that the ventricular wall thickens during systole and that there is a transmural gradient in wall thickening. Wall thickening on the inner wall may exceed values of 40% measured by a variety of techniques. These values far exceed the 8% diameter increase estimated by simple conservation of volume in an individual myocyte shortening by 15%. Physiologists have long understood that changes in the macrostructure of the wall (cellular rearrangement) must account for the difference between wall thickening and estimates of cell thickening.1 Reorientation of laminae or sheets of muscle cells has been proposed to account for this change in macrostructure.2,3 The dichotomy between wall thickening and estimated change in myocyte diameter has been elegantly framed by Cheng et al4 in this issue of Circulation , which shows nearly identical transmural fiber shortening at 2 ventricular sites, with very different wall thickening in the ovine ventricle. Similar data have been shown for other species.5,6 However, the data presented by Cheng et al4 provide strong evidence that the cellular rearrangements are different at the 2 sites. Cheng et al4 used a method developed by Costa et al5 in which regional histological measurements of muscle fiber and sheet orientations were used to resolve 3-dimensional transmural strain distributions obtained from implanted arrays of radiopaque markers into components. These components were defined by local axes that are parallel to the fibers, transverse to the fibers in the sheet plane, and transverse to the sheets. Wall thickening then can be resolved into components of sheet motion. Costa et al5 and Takayama et al6 reported that the interlaminar shear-strain term (Esn) and …

Repolarization of ventricular myocardium in atrioventricular electrical stimulation of the heart in dogs

The duration of ventricular myocardium excitation increases during atrioventricular stimulation of dog heart and the sequence of depolarization of the right and left ventricles is desynchronized. Significant shortening of the activation-recovery interval in the intramural and subendocardial layers of the left ventricular base leads to modification of the repolarization sequence in this area and to an increase of total dispersion of activation-recovery intervals in the cardiac ventricles, as a result of which repolarization sequence starts partially repeating the depolarization sequence.

Myocardial perfusion characteristics during machine perfusion for heart transplantation

Optimal parameters for machine perfusion preservation of hearts prior to transplantation have not been determined. We sought to define regional myocardial perfusion characteristics of a machine perfusion device over a range of conditions in a large animal model. Dog hearts were connected to a perfusion device (LifeCradle, Organ Transport Systems, Inc, Frisco, TX) and cold perfused at differing flow rates (1) at initial device startup and (2) over the storage interval. Myocardial perfusion was determined by entrapment of colored microspheres. Myocardial oxygen consumption (MVO(2)) was estimated from inflow and outflow oxygen differences. Intra-myocardial lactate was determined by (1)H magnetic resonance spectroscopy. MVO(2) and tissue perfusion increased up to flows of 15 mL/100 g/min, and the ratio of epicardial:endocardial perfusion remained near 1:1. Perfusion at lower flow rates and when low rates were applied during startup resulted in decreased capillary flow and greater non-nutrient flow. Increased tissue perfusion correlated with lower myocardial lactate accumulation but greater edema. Myocardial perfusion is influenced by flow rates during device startup and during the preservation interval. Relative declines in nutrient flow at low flow rates may reflect greater aortic insufficiency. These factors may need to be considered in clinical transplant protocols using machine perfusion.

Chemical ablation of the Purkinje system causes early termination and activation rate slowing of long-duration ventricular fibrillation in dogs

Endocardial mapping has suggested that Purkinje fibers may play a role in the maintenance of long-duration ventricular fibrillation (LDVF). To determine the influence of Purkinje fibers on LDVF, we chemically ablated the Purkinje system with Lugol solution and recorded endocardial and transmural activation during LDVF. Dog hearts were isolated and perfused, and the ventricular endocardium was exposed and treated with Lugol solution (n = 6) or normal Tyrode solution as a control (n = 6). The left anterior papillary muscle endocardium was mapped with a 504-electrode (21 x 24) plaque with electrodes spaced 1 mm apart. Transmural activation was recorded with a six-electrode plunge needle on each side of the plaque. Ventricular fibrillation (VF) was induced, and perfusion was halted. LDVF spontaneously terminated sooner in Lugol-ablated hearts than in control hearts (4.9 +/- 1.5 vs. 9.2 +/- 3.2 min, P = 0.01). After termination of VF, both the control and Lugol hearts were typically excitable, but only short episodes of VF could be reinduced. Endocardial activation rates were similar during the first 2 min of LDVF for Lugol-ablated and control hearts but were significantly slower in Lugol hearts by 3 min. In control hearts, the endocardium activated more rapidly than the epicardium after 4 min of LDVF with wave fronts propagating most often from the endocardium to epicardium. No difference in transmural activation rate or wave front direction was observed in Lugol hearts. Ablation of the subendocardium hastens VF spontaneous termination and alters VF activation sequences, suggesting that Purkinje fibers are important in the maintenance of LDVF.

Molecular identity of the late sodium current in adult dog cardiomyocytes identified by Nav1.5 antisense inhibition

Late Na(+) current (I(NaL)) is a major component of the action potential plateau in human and canine myocardium. Since I(NaL) is increased in heart failure and ischemia, it represents a novel potential target for cardioprotection. However, the molecular identity of I(NaL) remains unclear. We tested the hypothesis that the cardiac Na(+) channel isoform (Na(v)1.5) is a major contributor to I(NaL) in adult dog ventricular cardiomyocytes (VCs). Cultured VCs were exposed to an antisense morpholino-based oligonucleotide (Na(v)1.5 asOligo) targeting the region around the start codon of Na(v)1.5 mRNA or a control nonsense oligonucleotide (nsOligo). Densities of both transient Na(+) current (I(NaT)) and I(NaL) (both in pA/pF) were monitored by whole cell patch clamp. In HEK293 cells expressing Na(v)1.5 or Na(v)1.2, Na(v)1.5 asOligo specifically silenced functional expression of Na(v)1.5 (up to 60% of the initial I(NaT)) but not Na(v)1.2. In both nsOligo-treated controls and untreated VCs, I(NaT) and I(NaL) remained unchanged for up to 5 days. However, both I(NaT) and I(NaL) decreased exponentially with similar time courses (tau = 46 and 56 h, respectively) after VCs were treated with Na(v)1.5 asOligo without changes in 1) decay kinetics, 2) steady-state activation and inactivation, and 3) the ratio of I(NaL) to I(NaT). Four days after exposure to Na(v)1.5 asOligo, I(NaT) and I(NaL) amounted to 68 +/- 6% (mean +/- SE; n = 20, P < 0.01) and 60 +/- 7% (n = 11, P < 0.018) of those in VCs treated by nsOligo, respectively. We conclude that in adult dog heart Na(v)1.5 sodium channels have a "functional half-life" of approximately 35 h (0.69tau) and make a major contribution to I(NaL).

Arrhythmogenic right ventricular cardiomyopathy in Boxer dogs is associated with calstabin2 deficiency

Objective To examine the presence and effect of calstabin2-deficiency in Boxer dogs with arrhythmogenic right ventricular cardiomyopathy (ARVC). Animals Thirteen Boxer dogs with ARVC. Materials and methods Tissue samples were collected for histopathology, oligonucleotide microarray, PCR, immunoelectrophoresis, ryanodine channel immunoprecipitation and single-channel recordings, and calstabin2 DNA sequencing. Results In cardiomyopathic Boxer dogs, myocardial calstabin2 mRNA and protein were significantly decreased as compared to healthy control dogs (calstabin2 protein normalized to tetrameric cardiac ryanodine receptor (RyR2) complex: affected, 0.51 ± 0.04; control, 3.81 ± 0.22; P < 0.0001). Calstabin2 deficiency in diseased dog hearts was associated with a significantly increased open probability of single RyR2 channels indicating intracellular Ca 2+ leak. PCR-based sequencing of the promoter, exonic and splice site regions of the canine calstabin2 gene did not identify any causative mutations. Conclusions Calstabin2 deficiency is a potential mechanism of Ca 2+ leak-induced ventricular arrhythmias and heart disease in Boxer dogs with ARVC.

Sympathetic activation causes focal adhesion signaling alteration in early compensated volume overload attributable to isolated mitral regurgitation in the dog.

We reported that left ventricular (LV) dilatation after 4 weeks of isolated mitral regurgitation (MR) in the dogs is marked by extracellular matrix loss and an increase in adrenergic drive. Given that extracellular matrix proteins and their receptor integrins influence beta-adrenergic receptor (beta-AR) responses in vitro, we tested whether beta1-AR activation modulates focal adhesion (FA) signaling and LV remodeling in these same dogs with isolated MR. Normal dogs were compared with dogs with MR of a 4-week duration and with MR dogs treated with beta(1)-AR blockade (beta(1)-RB) (extended-release metoprolol succinate, 100 mg QD) that was started 24 hours after MR induction. In MR LVs, a decrease in collagen accumulation compared with normal dogs was associated with a decrease in FA kinase tyrosine phosphorylation, along with FA kinase interaction with adapter and cytoskeletal proteins, p130(Cas) and paxillin, respectively, as determined by immunoprecipitation assays. There was increased phosphorylation of stress related molecules p38 mitogen-activated protein kinase (MAPK) and Hsp27 and survival signaling kinases extracellular signal-regulated kinase 1/2 and AKT, with no evidence of cardiomyocyte apoptosis. beta(1)-RB attenuated FA signaling loss and prevented p38 MAPK, Hsp27, and AKT phosphorylation induced by MR and significantly increased LV epicardial collagen content. However, beta(1)-RB did not improve LV endocardial collagen loss or LV dilatation induced by MR. Isolated myocytes from normal and MR dog hearts treated with beta(1)- or beta(2)-AR agonists demonstrated no difference in FA kinase, p38 MAPK, Hsp27, or AKT phosphorylation. These results showed that chronic stimulation of beta(1)-AR during early compensated MR impairs FA signaling that may affect myocyte/fibroblast-extracellular matrix scaffolding necessary for LV remodeling.

NOS dependent gene regulation in cardiac tissues of conscious type I diabetic dogs

We have shown that in conscious dogs with type I diabetes (DM) that baseline CBF was significantlydecreased, endothelium‐dependent (ACh) coronary vasodilation were impaired, and reflex cholinergic NO‐dependent coronary vasodilation was selectivelydepressed. The likely mechanism responsible for the depressedreflex cholinergic NO‐dependent coronary vasodilation was thedecreased bioactivity of NO from the vascular endothelium. Bradykinin reduced oxygen consumption in normal dog heart in vitro (38 ± 1.3%) and this was reduced in type I DM (18 ± 1.6%). The effect of bradykinin was restored by apocynin (30 ± 0.9%), tempol (30 ± 1.1%) or vit C (29 ± 0.7%) indicating a role for NADPH oxidase derived superoxide. Affymetrix Canine Arrays were used to study gene expression in dogs with alloxan‐induced diabetes. Cardiac RNA was extracted from the control and DM (n=4). The data revealed that 797 genes were differentially expressed (P&lt;0.01; fold change of at least ±2). There was no change in eNOS mRNA. There was up regulation of some components of the NADPH oxidase subunits (gp91 (2.2 fold), P&lt;0.03), glutathione peroxidase and down‐regulation of SOD1 (3 fold, P&lt;0.001) and decrease (4–40 fold) in a number of genes encoding mitochondrial enzymes such as cytochrome c oxidase, NADH‐ubiquinone oxidoreductase, ubiquinol‐cytochrome c reductase, ATP synthase, isocitrate dehydrogenase 3, succinate dehydrogenase and aconitase 2. There was down‐regulation of Ca2+ cycling genes (ryanodine receptor; SERCA2 Calcium ATPase), and structural proteins (actin alpha). In conclusion, these data show a direct effect of diabetes on the heart by impairing NO bioavailability through oxidative stress. Supported by HL‐PO1‐43023.

Epicardial and intramural excitation during ventricular pacing: effect of myocardial structure

Published studies show that ventricular pacing in canine hearts produces three distinct patterns of epicardial excitation: elliptical isochrones near an epicardial pacing site, with asymmetric bulges; areas with high propagation velocity, up to 2 or 3 m/s and numerous breakthrough sites; and lower velocity areas (<1 m/s), where excitation moves across the epicardial projection of the septum. With increasing pacing depth, the magnitude of epicardial potential maxima becomes asymmetric. The electrophysiological mechanisms that generate the distinct patterns have not been fully elucidated. In this study, we investigated those mechanisms experimentally. Under pentobarbital anesthesia, epicardial and intramural excitation isochrone and potential maps have been recorded from 22 exposed or isolated dog hearts, by means of epicardial electrode arrays and transmural plunge electrodes. In five experiments, a ventricular cavity was perfused with diluted Lugol solution. The epicardial bulges result from electrotonic attraction from the helically shaped subepicardial portions of the wave front. The high-velocity patterns and the associated multiple breakthroughs are due to involvement of the Purkinje network. The low velocity at the septum crossing is due to the missing Purkinje involvement in that area. The asymmetric magnitude of the epicardial potential maxima and the shift of the breakthrough sites provoked by deep stimulation are a consequence of the epi-endocardial obliqueness of the intramural fibers. These results improve our understanding of intramural and epicardial propagation during premature ventricular contractions and paced beats. This can be useful for interpreting epicardial maps recorded at surgery or inversely computed from body surface ECGs.

Edmund H. Sonnenblick

Edmund H. Sonnenblick

389: Development and Evaluation of an In-Vitro Reperfusion Circuit for Reconditioning of Predamaged Donor-Lungs

Purpose: Perfusion preservation may extend the storage interval and permit utilization of marginal donor hearts. A limitation of this technique is predicting reperfusion cardiac function prior to implantation. We hypothesized that H magic angle spinning magnetic resonance spectroscopy (MASMRS) could be used to rapidly determine the metabolic state of preserved myocardium. We applied MASMRS to a large animal model of cardiac transplantation to compare metabolism after standard static storage and perfusion storage. Methods and Materials: Dog hearts were harvested after cardioplegic arrest. Animals were randomized to either static storage in Celsior (n 5) or perfusion preservation (n 6) in a device (Lifecradle® ,Organ Transport Systems) that delivers Celsior to the coronaries at 10 mL/100g/min. After 4 hours storage at 5°C, small left ventricular biopsies (10 mg) were taken and H MASMRS was carried out with a gHx nanoprobe on a Varian 14.1T spectrometer. Spectral data were analyzed with a peak-picking, partial least squares algorithm to identify biomarkers that were indicative of the storage method. Results: Analysis time for each sample was less than ten minutes. The peak-picking algorithm identified 5 resonances at .89 (triglyceride methyl), 2.02 (alpha to a triglyceride double bond), 3.92 (creatine), 4.63 (unassigned), and 5.22ppm (glycerol) that were sufficient to categorize spectra as either static storage or perfusion preservation with 100% efficiency. The algorithm selects the minimum number of resonances needed to classify the spectrum. Other compounds that were significantly different between the two preparations included lactate and myo-inositol, for example. Conclusions: H MASMRS can identify metabolic differences between cardiac preservation methods. The ability to use minute tissue samples and obtain metabolic data make this technique an attractive option for cardiac transplantation, and may provide clinicians with important information on the status of a donor heart without deleterious extension of the ischemic interval.

Both Purkinje Cells and Left Ventricular Posteroseptal Reentry Contribute to the Maintenance of Ventricular Fibrillation in Open-Chest Dogs and Swine Effects of Catheter Ablation and the Ventricular Cut-and-Sew Operation

Radiofrequency catheter ablation (RFCA) targeting the left ventricular posteroseptum (LVPS) and posterior papillary muscle (PPM) terminates or prevents ventricular fibrillation (VF) in rabbit and dog hearts. However, whether the mechanism of VF maintenance is reentry or focal Purkinje firing is unclear. In the present study the effects of RFCA (endocardial ablation of PPM+LVPS in 7 dogs and 7 swine), left ventricular anterolateral wall ablation (LVAL in 7 dogs), and the cut-and-sew operation (CSO: along the left ventricular posterior wall (LVPW) beside PPM in 7 swine) on VF inducibility were compared. (1) VF inducibility was decreased from 100+/-0% to 21.9+/-31.2% (p<0.0001) by PPM+LVPS endocardial ablation, but not by LVAL ablation in dogs. (2) LVPW CSO reduced VF inducibility (100+/-0% to 43.6+/-9.5%, p<0.0001) in swine. (3) In contrast to the canine Purkinje network, which is mostly localized to the subendocardium, the swine Purkinje network extends to the subepicardial layer with a higher density (p<0.001). Both PPM+LVPS ablation (Purkinje destruction) in dogs and LVPW CSO (blocking reentry) in swine reduce VF inducibility, suggesting that in both species focal firing from the Purkinje network and reentry around the PPM contributes to the maintenance of VF.

Multiple antiapoptotic targets of the PI3K/Akt survival pathway are activated by epoxyeicosatrienoic acids to protect cardiomyocytes from hypoxia/anoxia

Epoxyeicosatrienoic acids (EETs) reduce infarction of the myocardium after ischemia-reperfusion injury to rodent and dog hearts mainly by opening sarcolemmal and mitochondrial potassium channels. Other mediators for the action of EET have been proposed, although no definitive pathway or mechanism has yet been reported. Using cultured cells from two rodent species, immortalized myocytes from a mouse atrial lineage (HL-1) and primary myocytes derived from neonatal rat hearts, we observed that pretreatment with EETs (1 microM of 14,15-, 11,12-, or 8,9-EET) attenuated apoptosis after exposure to hypoxia and reoxygenation (H/R). EETs also preserved the functional beating of neonatal myocytes in culture after exposure to H/R. We demonstrated that EETs increased the activity of the prosurvival enzyme phosphatidylinositol 3-kinase (PI3K). In fact, cardiomyocytes pretreated with EET and exposed to H/R exhibited antiapoptotic changes in at least five downstream effectors of PI3K, protein kinase B (Akt), Bcl-x(L)/Bcl-2-associated death promoter, caspases-9 and -3 activities, and the expression of the X-linked inhibitor of apoptosis, compared with vehicle-treated controls. The PI3K/Akt pathway is one of the strongest intracellular prosurvival signaling systems. Our studies show that EETs regulate multiple molecular effectors of this pathway. Understanding the targets of action of EET-mediated protection will promote the development of these fatty acids as therapeutic agents against cardiac ischemia-reperfusion.

Atrial Antifibrillatory Effects of Structurally Distinct IKurBlockers 3-[(Dimethylamino)methyl]-6-methoxy-2-methyl-4-phenylisoquinolin-1(2H)-one and 2-Phenyl-1,1-dipyridin-3-yl-2-pyrrolidin-1-yl-ethanol in Dogs with Underlying Heart Failure

Drug discovery efforts have focused recently on atrial-selective targets, including the Kv1.5 channel, which underlies the ultrarapid delayed rectifier current, I(Kur), to develop novel treatments for atrial fibrillation (AF). Two structurally distinct compounds, a triarylethanolamine TAEA and an isoquinolinone 3-[(dimethylamino)-methyl]-6-methoxy-2-methyl-4-phenylisoquinolin-1(2H)-one (ISQ-1), blocked I(Kur) in Chinese hamster ovary cells expressing human Kv1.5 with IC(50) values of 238 and 324 nM, respectively. In anesthetized dogs, i.v. infusions of TAEA and ISQ-1 elicited comparable 16% increases in atrial refractory period, with no effect on ventricular refractory period or QTc interval. Plasma concentrations at end infusion for TAEA and ISQ-1 were 58.5 +/- 23.6 and 330.3 +/- 43.5 nM, respectively. The abilities of TAEA and ISQ-1 to terminate AF, with comparison to the rapidly activating component of delayed rectifier potassium current blocker (+)-N-[1'-(6-cyano-1,2,3,4-tetrahydro-2(R)-naphthalenyl)-3,4-dihydro-4(R)-hydroxyspiro(2H-1-benzopyran-2,4'-piperidin)-6-yl]methanesulfonamide] monohydrochloride (MK-499) and the class IC 1-[2-[2-hydroxy-3-(propylamino)-propoxy]phenyl]-3-phenyl-1-propanone (propafenone), were assessed in conscious dogs with heart failure and inducible AF (entry criterion). All test agents administered in i.v. bolus regimens terminated AF in at least half of animals tested; conversely no agent was universally effective. MK-499, ISQ-1, TAEA, and propafenone terminated AF in five of six, four of seven, four of six, and five of six animals at plasma concentrations of 32.6 +/- 18.7, 817 +/- 274, 714 +/- 622, and 816 +/- 240 nM, respectively. Directed cardiac electrophysiologic studies in anesthetized dogs using i.v. bolus (consistent with AF studies) plus infusion regimens with TAEA and ISQ-1 demonstrated significant increases in atrial refractory period (12-15%), A-H and P-A intervals, but no effects on ventricular refractory period, H-V, and HEG intervals. The demonstration of AF termination with TAEA and ISQ-1 in the dog heart failure model extends the profile of antiarrhythmic efficacy of Kv1.5 blockade.

L-type calcium current recovery versus ventricular repolarization: preserved membrane-stabilizing mechanism for different QT intervals across species

Long QT syndrome is associated with early after-depolarization (EAD) that may result in torsade de pointes (TdP). Interestingly, the corrected QT interval seems to be proportional to body mass across species under physiologic conditions. The purpose of this study was to test whether recovery of L-type calcium current (I(Ca,L)), the primary charge carrier for EADs, from its inactivated state matches ventricular repolarization time and whether impairment of this relationship leads to development of EAD and TdP. Transmembrane action potentials from the epicardium, endocardium, or subendocardium were recorded simultaneously with a transmural ECG in arterially perfused left ventricular wedges isolated from cow, dog, rabbit, and guinea pig hearts. I(Ca,L) recovery was examined using action potential stimulation in isolated left ventricular myocytes. The ventricular repolarization time (action potential duration at 90% repolarization [APD(90)]), ranging from 194.7 +/- 1.8 ms in guinea pig to 370.2 +/- 9.9 ms in cows, was linearly related to the thickness of the left ventricular wall among the species studied. The time constants (tau) of I(Ca,L) recovery were proportional to APD(90), making the ratios of tau to APD(90) fall into a relatively narrow range among these species despite markedly different ventricular repolarization time. Drugs with risk for TdP in humans were shown to impair this intrinsic balance by either prolongation of the repolarization time and/or acceleration of I(Ca,L) recovery, leading to the appearance of EADs capable of initiating TdP. An adequate balance between I(Ca,L) recovery and ventricular repolarization serves as a "physiologic stabilizer" of ventricular action potentials in repolarization phases.

Drug development for treatment of cardiac arrhythmias: targeting the gap junctions

reentrant excitation ([12][1]) is likely the most important mechanism for serious life-threatening arrhythmias. Traditionally, a major focus of antiarrhythmic drug development has been to explore the actions of drugs designed to block conduction in reentrant pathways(s). During the latter part of

Oral Bioavailability of the Selective Cardiac Myosin Activator CK-1827452: Chronic Oral Inotropic Therapy for Heart Failure?

Introduction: CK-1827452 (CK-452) is a novel small molecule activator of cardiac myosin. In a dog heart failure model, it increased left ventricular (LV) systolic function and cardiac output, while filling pressures, heart rate, and total peripheral resistance decreased. Intravenous (iv) CK-452 also increased LV systolic function in healthy subjects, and is now under study in heart failure patients. An oral formulation of CK-452 could enable chronic oral inotropic therapy for heart failure. Purpose: The objective of the current study was to determine the absolute bioavailability of CK-452 in healthy volunteers.