Faculty Bibliography

Myocardial injuries often lead to fibrotic deposition. This review presents evidence supporting the concept that fibroblasts in the heart electrically couple to myocytes.

SCN5A encodes the alpha subunit of the major cardiac sodium channel NaV1.5. Mutations in SCN5A are associated with conduction disease and ventricular fibrillation (VF); however, the mechanisms that link loss of sodium channel function to arrhythmic instability remain unresolved. Here, we generated a large-animal model of a human cardiac sodium channelopathy in pigs, which have cardiac structure and function similar to humans, to better define the arrhythmic substrate. We introduced a nonsense mutation originally identified in a child with Brugada syndrome into the orthologous position (E558X) in the pig SCN5A gene. SCN5AE558X/+ pigs exhibited conduction abnormalities in the absence of cardiac structural defects. Sudden cardiac death was not observed in young pigs; however, Langendorff-perfused SCN5AE558X/+ hearts had an increased propensity for pacing-induced or spontaneous VF initiated by short-coupled ventricular premature beats. Optical mapping during VF showed that activity often began as an organized focal source or broad wavefront on the right ventricular (RV) free wall. Together, the results from this study demonstrate that the SCN5AE558X/+ pig model accurately phenocopies many aspects of human cardiac sodium channelopathy, including conduction slowing and increased susceptibility to ventricular arrhythmias.

Cardiac metabolism remains altered for an extended period of time after myocardial infarction. Studies have shown fibroblasts from normal hearts express KATP channels in culture. It is unknown whether fibroblasts from infarcted hearts express KATP channels and whether these channels contribute to scar and border zone electrophysiology. KATP channel subunit expression levels were determined in fibroblasts isolated from normal hearts (Fb), and scar (sMI-Fb) and remote (rMI-Fb) regions of left anterior descending coronary artery (LAD) ligated rat hearts. Whole cell KATP current density was determined with patch clamp. Action potential duration (APD) was measured with optical mapping in myocyte-only cultures and heterocellular cultures with fibroblasts with and without 100 mumol/l pinacidil. Whole heart optical mapping was used to assess KATP channel activity following LAD ligation. Pinacidil activated a potassium current (35.4 +/- 7.5 pA/pF at 50 mV) in sMI-Fb that was inhibited with 10 mumol/l glibenclamide. Kir6.2 and SUR2 transcript levels were elevated in sMI-Fb. Treatment with Kir6.2 short interfering RNA decreased KATP currents (87%) in sMI-Fb. Treatment with pinacidil decreased APD (26%) in co-cultures with sMI-Fb. APD values were prolonged in LAD ligated hearts after perfusion with glibenclamide. KATP channels are present in fibroblasts from the scar and border zones of infarcted hearts. Activation of fibroblast KATP channels could modulate the electrophysiological substrate beyond the acute ischemic event. Targeting fibroblast KATP channels could represent a novel therapeutic approach to modify border zone electrophysiology after cardiac injury.

Dynamic changes of the actin cytoskeleton are instrumental in morphogenetic processes including changes in cell shape and adhesion. Formin proteins regulate actin microfilament assembly and can specifically influence adherens junction formation. Previous studies in our lab have demonstrated remarkable plasticity of formin isoforms during heart development and in vitro cardiomyocyte differentiation. As gap junction stability is dependent on the presences of mechanical junctions we were interested if modulation of cardiac formins influences expression of Cx43 protein and gap junction function. Objective: To investigate the effect of cardiac formin knockdown (KD) on cell-cell contact formation and functional coupling of cardiomyocytes. Methods: Cardiomyocytes were isolated from neonatal rat hearts (NRCM) and cultured as monolayers (d0); NRCM were treated with transfection agent only (TF), control siRNA (Ctr) or formin specific rat siRNAs (Daam1; Fhod1; Fhod3; Dharmacon) (d1); cultures were subjected to high resolution optical mapping or processed for immunofluorescence analysis (d4). Results: KD of Fhod1 or Fhod3 lead to disruption of sarcomers, cell rounding and ultimately resulted in complete dissociation of NRCM. In contrast, Daam1 KD resulted in significant cell elongation without loss of cell-cell contacts (mean cell areas in mum²: 681.8 + 99.1 (Daam1) vs. 594.9 + 67.6 (TF), 564 + 53.3 (Ctr), 455.9 + 47.4 (Fhod1), 339.3 + 14.3 (Fhod3); P: 0.01, ANOVA). As expected, optical mapping data for discontinuous Fhod1 and Fhod3 monolayers were very variable due to areas of complete block of conduction. Optical mapping analysis of Daam1 silenced NRCM demonstrated significant increase in conduction velocity (0.241 + 0.004 m/s; n=4) compared to NRCM treated with TF only (0.197 + 0.010 m/s, n=3) or Ctr (0.207 + 0.005 m/s, n=3; P: 0.003, ANOVA). Average gap junction diameter (0.24 + 0.03 mum (n=489; Daam1) vs. 0.41 + 0.03 mum (n=550; DF), 0.38 + 0.04 mum (n=574; Ctr) P: 0.009, ANOVA), and total !

Fibroblasts play a major role in normal cardiac physiology and in the response of the heart to injury and disease. Cardiac electrophysiological research has primarily focused on the mechanisms of remodeling that accompany cardiac disease with an emphasis on myocyte electrophysiology. Recently, there has been increasing interest in the potential role of fibroblasts in cardiac electrophysiology. This review focuses on the arrhythmia mechanisms involving interactions between myocytes and fibroblasts. We also discuss the available evidence supporting the contribution of intracardiac and extracardiac sources to the fibroblast and myofibroblast populations in diseased hearts.

BACKGROUND: Spinal cord stimulation (SCS) has been shown to modulate atrial electrophysiology and confer protection against ischemia and ventricular arrhythmias in animal models. OBJECTIVE: To determine whether SCS reduces the susceptibility to atrial fibrillation (AF) induced by tachypacing (TP). METHODS: In 21 canines, upper thoracic SCS systems and custom cardiac pacing systems were implanted. Right atrial and left atrial effective refractory periods were measured at baseline and after 15 minutes of SCS. Following recovery in a subset of canines, pacemakers were turned on to induce AF by alternately delivering TP and searching for AF. Canines were randomized to no SCS therapy (CTL) or intermittent SCS therapy on the initiation of TP (EARLY) or after 8 weeks of TP (LATE). AF burden (percent AF relative to total sense time) and AF inducibility (percentage of TP periods resulting in AF) were monitored weekly. After 15 weeks, echocardiography and histology were performed. RESULTS: Effective refractory periods increased by 21 +/- 14 ms (P = .001) in the left atrium and 29 +/- 12 ms (P = .002) in the right atrium after acute SCS. AF burden was reduced for 11 weeks in EARLY compared with CTL (P <.05) animals. AF inducibility remained lower by week 15 in EARLY compared with CTL animals (32% +/- 10% vs 91% +/- 6%; P <.05). AF burden and inducibility were not significantly different between LATE and CTL animals. There were no structural differences among any groups. CONCLUSIONS: SCS prolonged atrial effective refractory periods and reduced AF burden and inducibility in a canine AF model induced by TP. These data suggest that SCS may represent a treatment option for AF.

Introduction: Spinal cord stimulation (SCS) has been shown to modulate atrial electrophysiology and confer protection against ischemia and ventricular arrhythmias in animal models. We hypothesized that SCS would reduce the susceptibility to tachypacing (TP) induced atrial fibrillation (AF). Methods: In 21 canines, an upperthoracicSCS system (EonC Model 3688, Octrode Model 3186, St. Jude Medical, Piano TX) and custom cardiac pacing system (PM, Model 5386 or 2215-36, St. Jude Medical, Sylmar CA) were implanted. Atrial effective refractory periods (ERPs) in the high right atrium (RA) and distal coronary sinus (LA) were measured at baseline and after 15 min of SCS, after which AV nodal ablation was performed. Following recovery in a subset of canines, PM was turned on to create TP induced AF by alternately delivering TP and searching for AF. TP was interrupted by detection of AF and resumed after return to sinus rhythm. Upon initiation of TP, canines were randomized to no SCS therapy (CTL, n=6) or intermittent SCS therapy (SCS-ON, n=4) and followed for 15 weeks. AF burden, defined as the percent of time in AF relative to the total sense time, and AF inducibility, defined as the percent of TP periods resulting in AF induction, were monitored weekly. Data are presented as mean +/- standard error. Results: ERPs were significantly longer after SCS compared to baseline, byan average of21 +/-14ms (p=0.001) in LA and 29+/-12ms (p=0.002) in RA. The AF burden was significantly decreased by 34 percentage points at week 15in SCS-ON compared to CTL (56 +/- 21% vs 90 +/- 12%, p<0.05). AF inducibility was significantly reduced by 60 percentage points at week 15 in SCS-ON compared to CTL (32 +/- 10% vs 91 +/- 6%, p<0.05). Conclusions: SCS prolonged atrial ERPs and reduced AF burden and inducibility in a canine atrial TP induced AF model. These data suggest that SCS therapy may represent a treatment option for AF