Myocardial expression of hyperpolarization-activated, cyclic nucleotide-gated proteins in healthy cats and cats with hypertrophic cardiomyopathy.

In this article the myocardial expression of different hyperpolarization-activated, cyclic nucleotide-gated (HCN) isoforms in myocardial tissue from healthy control cats and cats with hypertrophic cardiomyopathy (HCM) was evaluated. Myocardial tissue samples of the left ventricle of control cats (n = 12) and cats with HCM (n = 4) were collected. Expression of feline HCN was determined by immunoblot analysis using antibodies against HCN2 and HCN4. Optical densities of HCN bands were compared among groups by use of the Mann-Whitney Rank Sum test. HCN4 was reliably detected in myocardial tissue whereas HCN2 was not. HCN4 expression was significantly increased in left ventricular (LV) myocardial samples of cats with HCM (P = 0.036) compared to control cats. Results indicate that myocardial HCN4 expression can be evaluated in cats by immunoblot analysis and that HCN4 expression is upregulated in LV myocardial tissue of cats with HCM. The pathophysiological importance of HCN overexpression with regard to myocyte function and altered automaticity deserves further study.

Pharmacokinetics of oral ivabradine in healthy cats.

A liquid chromatography-tandem mass spectrometry (LC-MS/MS) analytical method for the measurement of the novel heart rate-lowering drug ivabradine and its major metabolite, S-18982, was cross-validated in the plasma of eight healthy cats. Plasma concentrations were then determined after single and repeated oral administration of ivabradine. Individual plasma concentrations versus time from each cat were used in compartmental analysis using the commercially available software WinNonlin. Both ivabradine and S-18982 reached their maximum concentrations of 103.33 and 3.86 ng/mL within 1 h. Following repeated administration, areas under the plasma concentration-time curves for ivabradine and S-18982 did not significantly increase. Two-compartmental and one-compartmental models with first-order input and elimination provided the best fit to the data for ivabradine and S-18982, respectively. Both models were combined to produce a single 4-compartment model characterizing ivabradine and S-18982 pharmacokinetics. The results of this study indicate that repeated oral doses of ivabradine produced plasma drug concentrations suitable for 12-h dosing intervals in healthy cats. Furthermore, the analytical assay and combined ivabradine/S-18982 model provide tools for further evaluation of ivabradine pharmacokinetics and pharmacodynamics in future studies in cats.

The plateau outward current in canine ventricle, sensitive to 4-aminopyridine, is a constitutive contributor to ventricular repolarization.

BACKGROUND AND PURPOSE: I(Kur) (Ultra-rapid delayed rectifier current) has microM sensitivity to 4-aminopyridine (4-AP) and is an important modulator of the plateau amplitude and action potential duration in canine atria. Kv1.5 encodes I(Kur) and is present in both atria and ventricles in canines and humans. We hypothesized that a similar plateau outward current with microM sensitivity to 4-AP is present in canine ventricle. EXPERIMENTAL APPROACH: We used established voltage clamp protocols and used 4-AP (50 and 100 microM) to measure a plateau outward current in normal canine myocytes isolated from the left ventricular mid-myocardium. KEY RESULTS: Action potential recordings in the presence of 4-AP showed significant prolongation of action potential duration at 50 and 90% repolarization at 0.5 and 1 Hz (P<0.05), while no prolongation occurred at 2 Hz. Voltage clamp experiments revealed a rapidly activating current, similar to current characteristics of canine atrial I(Kur), in approximately 70% of left ventricular myocytes. The IC(50) of 4-AP for this current was 24.2 microM. The concentration of 4-AP used in our experiments resulted in selective blockade of an outward current that was not I(to) or I(Kr). Beta-adrenergic stimulation with isoprenaline significantly increased the 4-AP sensitive outward current density (P<0.05), suggesting a role for this current during increased sympathetic stimulation. In silico incorporation into a canine ventricular cell model revealed selective AP prolongation after current blockade. CONCLUSIONS AND IMPLICATIONS: Our results support the existence of a canine ventricular plateau outward current sensitive to micromolar 4-AP and its constitutive role in ventricular repolarization.

n-3 (omega-3) polyunsaturated fatty acids prevent acute atrial electrophysiological remodeling.

BACKGROUND AND PURPOSE: Recent reports suggest that n-3 (omega-3) polyunsaturated fatty acids (PUFAs) may reduce atrial fibrillation (AF). Reduction of the atrial effective refractory period (ERP) is believed to be an important early remodeling event that favors the development and perpetuation of AF. We hypothesized that n-3 PUFAs would attenuate early atrial electrophysiolgical remodeling in a canine model of acute atrial tachypacing. EXPERIMENTAL APPROACH: Adult dogs of either sex received n-3 PUFAs (n=6), n-6 PUFAs (n=6), or saline (n=6) infused over 1 h. After a stable ERP was established, treatment was initiated concurrently with 6 h of rapid atrial pacing (400 b.p.m.). Serial right atrial ERPs were measured during rapid atrial pacing, and induction of atrial tachyarrhythmias was attempted at the conclusion of each study. KEY RESULTS: There was no change in P wave duration or in the PQ, QRS, QT or QTc intervals in any of the treatment groups. N-3 PUFA treatment significantly reduced the shortening of atrial ERP, compared to both control groups (P<0.05). In separate experiments, the same n-3 PUFA infusion was given to dogs remaining in normal sinus rhythm. During sinus rhythm, n-3 PUFA infusion did not alter any electrocardiogram (ECG) parameter or the atrial ERP. CONCLUSIONS AND IMPLICATIONS: We conclude that acute n-3 PUFA treatment prevents acute atrial electrophysiological remodeling during high rate activity, which may minimize the self-perpetuation of AF.

C-reactive protein elevation in patients with atrial arrhythmias: inflammatory mechanisms and persistence of atrial fibrillation.

BACKGROUND: Atrial fibrillation (AF) may persist due to structural changes in the atria that are promoted by inflammation. C-reactive protein (CRP), a marker of systemic inflammation, predicts cardiovascular events and stroke, a common sequela of AF. We hypothesized that CRP is elevated in patients with atrial arrhythmias. METHODS AND RESULTS: Using a case-control study design, CRP in 131 patients with atrial arrhythmias was compared with CRP in 71 control patients. Among arrhythmia patients, 6 had frequent atrial ectopy or tachycardia, 86 had paroxysmal AF, 39 had persistent AF lasting >30 days, and 70 had lone arrhythmias. CRP was higher in arrhythmia than in control patients (median, 0.21 versus 0.096 mg/dL; P<0.001). Arrhythmia patients in AF within 24 hours before sampling had higher CRP than those in sinus rhythm (0.30 versus 0.15 mg/dL; P<0.001). CRP in controls was not different than in patients with atrial ectopy or tachycardia. Lone arrhythmia patients had a CRP of 0.21 mg/dL, which was not significantly lower than arrhythmia patients with structural heart disease (CRP, 0.23 mg/dL) but higher than controls (P=0.002). Persistent AF patients had a higher CRP (0.34 mg/dL) than paroxysmal AF patients (0.18 mg/dL; P=0.008); both groups had higher CRP levels than controls (P

Electrophysiologic and hemodynamic effects of apomorphine in dogs.

Apomorphine is a dopamine receptor agonist used as an emetic, for Parkinson's disease, and for treating erectile dysfunction. This study was conducted to monitor cardiovascular function in dogs given the standard emetic dose (0.05 mg/kg) or 10 times that. Measurements were made during baseline and at 1, 5, 15, 30, 45, and 60 min after iv administration. There were no changes produced by the 0.05 mg/kg dose of apomorphine except for a decrease in mean systemic arterial pressure (AoPm) at the 1 through 15 min recordings. For the 0.5 mg/kg dose, there were reductions in systemic vascular resistance at the 1 and 5 min recordings and in AoPm at the 1 through 60 min recordings. Although not significant, when AoPm fell, heart rate, stroke volume, and cardiac output tended to increase. Action potentials were recorded from superfused Purkinje and endocardial ventricular fibers while exposed to 10(-9) to 10(-5) M apomorphine (10(-10) M is considered therapeutic and 10(-7) M is considered lethal). There were no changes in action potential characteristics of Purkinje fibers, but action potential duration at 90% repolarization prolonged approximately 10-12% in endocardium at concentrations of 10(-6) M and greater. At the usual emetic dose (0.05 mg/kg) apomorphine resulted in no signs of cardiovascular toxicity and, at 0.5 mg/kg, cardiovascular changes were minimal. The emetic dose is higher than that for Parkinson's disease or erectile dysfunction; thus apomorphine appears to be a safe compound for clinical use in dogs and by extrapolation to man.

Ascorbate attenuates atrial pacing-induced peroxynitrite formation and electrical remodeling and decreases the incidence of postoperative atrial fibrillation.

Atrial fibrillation (AF), the most common chronic arrhythmia, increases the risk of stroke and is an independent predictor of mortality. Available pharmacological treatments have limited efficacy. Once initiated, AF tends to self-perpetuate, owing in part to electrophysiological remodeling in the atria; however, the fundamental mechanisms underlying this process are still unclear. We have recently demonstrated that chronic human AF is associated with increased atrial oxidative stress and peroxynitrite formation; we have now tested the hypothesis that these events participate in both pacing-induced atrial electrophysiological remodeling and in the occurrence of AF following cardiac surgery. In chronically instrumented dogs, we found that rapid (400 min(-1)) atrial pacing was associated with attenuation of the atrial effective refractory period (ERP). Treatment with ascorbate, an antioxidant and peroxynitrite decomposition catalyst, did not directly modify the ERP, but attenuated the pacing-induced atrial ERP shortening following 24 to 48 hours of pacing. Biochemical studies revealed that pacing was associated with decreased tissue ascorbate levels and increased protein nitration (a biomarker of peroxynitrite formation). Oral ascorbate supplementation attenuated both of these changes. To evaluate the clinical significance of these observations, supplemental ascorbate was given to 43 patients before, and for 5 days following, cardiac bypass graft surgery. Patients receiving ascorbate had a 16.3% incidence of postoperative AF, compared with 34.9% in control subjects. In combination, these studies suggest that oxidative stress underlies early atrial electrophysiological remodeling and offer novel insight into the etiology and potential treatment of an enigmatic and difficult to control arrhythmia. The full text of this article is available at http://www.circresaha.org.

Impaired myofibrillar energetics and oxidative injury during human atrial fibrillation.

BACKGROUND: Atrial fibrillation (AF) is associated with severe contractile dysfunction and structural and electrophysiological remodeling. Mechanisms responsible for impaired contractility are undefined, and current therapies do not address this dysfunction. We have found that myofibrillar creatine kinase (MM-CK), an important controller of myocyte contractility, is highly sensitive to oxidative injury, and we hypothesized that increased oxidative stress and energetic impairment during AF could contribute to contractile dysfunction. Methods and Results-- Right atrial appendages were obtained from AF patients undergoing the Maze procedure and from control patients who were in normal sinus rhythm and undergoing cardiac surgery. MM-CK activity was reduced in AF patients compared with controls (25.4+/-3.4 versus 18.2+/-3.8 micromol/mg of myofibrillar protein per minute; control versus AF; P<0.05). No reduction in total CK activity or myosin ATPase activity was detected. This selective reduction in MM-CK activity was associated with increased relative expression of the beta-myosin isoform (25+/-6 versus 63+/-5%beta, CTRL versus AF; P<0.05). Western blotting of AF myofibrillar isolates demonstrated no changes in protein composition but showed increased prevalence of protein oxidation as detected by Western blotting for 3-nitrotyrosine (peroxynitrite biomarker) and protein carbonyls (hydroxyl radical biomarker; P<0.05). Patterns of these oxidative markers were distinct, which suggests discrete chemical events and differential protein vulnerabilities in vivo. MM-CK inhibition was statistically correlated to extent of nitration (P<0.01) but not to carbonyl presence. CONCLUSIONS: The present results provide novel evidence of oxidative damage in human AF that altered myofibrillar energetics may contribute to atrial contractile dysfunction and that protein nitration may be an important participant in this condition.

Effects of azimilide, acidemia, and the combination on defibrillation energy requirements.

We investigated the effects of azimilide, acidemia, and the combination on the defibrillation energy requirement (DER). An anesthetized canine model of internal transvenous defibrillation with biphasic shocks was used. Dogs were assigned to receive a 0.25N HCl infusion (target pH, 7.15), azimilide, azimilide with HCl infusion, or placebo (n = 7 per treatment). DERs were determined in triplicate using an increment-decrement protocol at baseline and during each treatment. Monophasic action potentials and ECG intervals were measured at baseline and during each treatment. Analysis of variance (ANOVA) with post hoc Tukey's test was used for statistical analysis. The DER was reduced by azimilide and increased above control values by both acidemia and the combination of acidemia and azimilide. All treatment groups resulted in a significant change compared with placebo (p < 0.05). The correlation between DER and various repolarization measurements was determined. The treatment-related changes in both QT intervals and monophasic action potential (MAP) durations were inversely correlated with DER. Azimilide reduces the DER, whereas acidemia increases the DER in our model. The combination of azimilide and acidemia still resulted in an increase in the DER. This finding may have clinical implications for the use of azimilide in settings such as myocardial ischemia, in which myocardial pH is reduced.

The influence of specific and nonspecific potassium current blockade on the defibrillation energy requirement of biphasic shock.

UNLABELLED: Block of delayed rectifier potassium current (IK) is known to decrease defibrillation energy requirements (DERs). We tested the hypothesis that there would be no difference in DER reduction with a nonspecific IK (IKr + IKs) blocker, ambasilide, and a specific IKr blocker, dofetilide. METHODS: An anesthetized canine model (n = 30) of internal transvenous defibrillation with biphasic shocks was used. Ambasilide (n = 9; dose: 4.8 mg/kg, then 9.6 mg/kg/hour), dofetilide (n = 10; dose: 10 (micrograms/kg, then 3.6 (micrograms/kg/hour), or matched placebo (n = 11) were administered. DERs (J) were determined in triplicate using an increment-decrement protocol at baseline and during each treatment. ECG intervals were measured at baseline and during each treatment. ANOVA with post-hoc Bonferroni test was used for statistical analysis. RESULTS: Ambasilide resulted in a +23.5 +/- 4.06% prolongation of the QTc interval, while dofetilide resulted in a +20.5% +/- 3.76% prolongation of the QTc interval. Thus, the two drugs resulted in comparable prolongation of the QTc interval (P < 0.05 compared to placebo). Both drugs significantly reduced the DER (-17.7% +/- 5.33% reduction by ambasilide, and -21.9% +/- 5.21% reduction by dofetilide, P < 0.05 compared to placebo). There was no difference in the magnitude of DER reduction between the two treatments. CONCLUSIONS: Administration of equipotent doses (as indicated by QTc changes) of ambasilide or dofetilide had comparable effects on DERs. Selectivity of IK blockade has no significant effect on the magnitude of reduction in DERs.

Drug and defibrillator interactions.

We reviewed the interactions of drugs and defibrillators, with emphasis on implantable cardioverter defibrillators. Articles were identified by searching MEDLINE from 1966 to the present; additional sources were identified from reference lists in these articles. Drugs have the potential for both beneficial and harmful interactions with electrical therapy. Beneficial interactions include reductions in the energy required to defibrillate the heart and in the occurrence of arrhythmia resulting in decreased shock frequency, prolonged device longevity, and improved patient comfort. Potentially harmful interactions include altering the detection of ventricular tachycardia; altering the pacing threshold, resulting in interference with bradycardia or antitachycardia pacing; development of incessant ventricular tachycardia; and increasing the energy required to defibrillate the heart. As the use of implantable cardioverter defibrillators increases, pharmacists should be aware of the potential for drug-device interactions.

Effect of intravenous anesthetics on inward rectifier potassium current in rat and human ventricular myocytes.

BACKGROUND: Inhibition of the inward rectifying potassium current (I(K1)) may cause cardiac dysrhythmias by decreasing resting membrane potential or prolonging action potential. METHODS: The effects of thiopental, ketamine, and propofol on I(K1) conductance were evaluated in rat ventricular myocytes. The effect of thiopental on I(K1) conductance was also evaluated in human ventricular myocytes. Currents were recorded using the nystatin-perforated whole-cell patch-clamp technique (holding potential, -50 mV; test potentials, -140 to -40 mV). Pipette solution contained 130 mM KCl, 5 mM MgCl2, 5 mM HEPES, and 5 mM EGTA,pH 7.2. Bath solution (32 degrees C) contained 134 mM NaCI, 4 mM KCl, 1 mM MgCl2, 1 mM CaCl2, 0.3 mM CdCl2, 5 mM HEPES, and 5 mM d-glucose,pH 7.4. Drug concentrations examined encompassed the range of clinically relevant unbound plasma concentrations. Currents were normalized for cell capacitance. Conductance was calculated as current density/delta mV from -140 to -100 mV. Analysis of variance was used to test for changes in conductance as a function of drug concentration. RESULTS: Thiopental reduced I(K1) conductance in a concentration-dependent manner (P < 0.0001). Thiopental-induced changes in I(K1) conductance in rat ventricular myocytes were fit to an inhibitory E(max) model, with a median inhibitory concentration of 10.5 microM. The effect of thiopental on I(K1) conductance in human ventricular cells was comparable to that observed in rat ventricular myocytes. Neither ketamine nor propofol altered I(K1) conductance. CONCLUSIONS: Thiopental reduces I(K1) conductance in a concentration-dependent manner at clinically relevant concentrations in both rat and human ventricular myocytes.

Evaluation of an open-loop, computer-based infusion system designed to achieve a series of constant, targeted plasma procainamide concentrations in patients undergoing electrophysiologic testing.

STUDY OBJECTIVE: To evaluate the performance of a computer-based procainamide infusion system in patients undergoing electrophysiologic testing. DESIGN: Prospective case series. SETTING: Electrophysiology laboratory in a university hospital. PATIENTS: Thirty-four patients with inducible sustained ventricular tachycardia. INTERVENTIONS: Intravenous infusion of procainamide to achieve and maintain targeted plasma concentrations. MEASUREMENTS AND MAIN RESULTS: System performance was assessed by comparing targeted and observed plasma concentrations. The population median absolute performance error (size of typical miss) was 12.6% (95% CI 11.2-14.1%). The population median performance error (system bias) was not significantly different from zero. A small but statistically significant improvement in performance over time was observed (population absolute performance error divergence -0.125%/min). Population wobble (overall system stability) was 7.6% (95% CI 6.8-8.3%). Population-based estimates of central compartment volume and volume of distribution at steady state were significantly higher and lower, respectively, than estimates used by the infusion system. CONCLUSION: The computer-based infusion system is capable of achieving and maintaining a series of targeted procainamide concentrations in patients undergoing electrophysiologic testing.

Electrophysiologic interactions of procainamide and N-acetylprocainamide in isolated canine cardiac Purkinje fibers.

The study objective was to characterize the electrophysiologic interactions of procainamide (PA) and its metabolite, N-acetylprocainamide (NAPA), in canine Purkinje fibers. Cell (N = 43) action potentials were measured in Tyrode's solution (K+ = 4.0 mM, 36 degrees C) at a basic cycle length of 1,000 ms using standard microelectrode techniques. Six PA concentrations (0.020-0.32 mM) and six NAPA concentrations (0.010-0.24 mM) were studied alone and in combination. PA caused concentration-dependent decreases in Vmax and APD50 but did not alter APD90, ERP, or RMP. NAPA caused a small but not significant concentration-dependent decrease in Vmax, no change in RMP, and significant concentration-dependent increases in APD50, APD90, and ERP. Low NAPA concentrations increased, intermediate concentrations did not affect, and high NAPA concentrations again increased PA's effect on Vmax. PA-NAPA combinations resulted in concentration-dependent changes in APD50 that were intermediate between the effects of PA or NAPA alone. PA did not significantly alter NAPA's effects on APD90 at NAPA concentrations less than or equal to 0.040 mM, while it antagonized NAPA's effect at higher concentrations. The effects of PA-NAPA combinations on ERP were generally similar to their effects on APD90. The electrophysiologic effects of PA-NAPA combinations in normal canine Purkinje fibers are complex functions of the relative and absolute concentrations of the two compounds.

Evaluation of very rapid emit Qst methods for measuring serum procainamide and N-acetylprocainamide concentrations.

This study assessed the Emit Qst procainamide (PA) and N-acetylprocainamide (NAPA) assays. Accuracy and intraday precision were evaluated by repeatedly measuring PA and NAPA concentrations in spiked serum samples using Qst and high-performance liquid chromatography methods. Interday precision was evaluated by measuring concentrations in spiked samples over 4 weeks. Correlation between methods was assessed in patient samples, and proportional, constant, and random errors were estimated. Intraday coefficients of variation (CVs) were below 6.4% for PA and NAPA for both methods; interday CVs were below 7.8%. The proportional, constant, and random errors of the PA Qst assay in patient samples were 5.7%, -0.224 mg/L, and +/- 0.574 mg/L, respectively. The same errors in the NAPA Qst assay were 17.2%, 0.229 mg/L, and +/- 1.79 mg/L, respectively. The Qst assays are rapid, accurate, and precise methods for routine clinical measurement of PA and NAPA, although the proportional error in the NAPA assay should be recognized.

Moricizine: a novel antiarrhythmic agent.

Moricizine is a phenothiazine derivative with Vaughan Williams class 1 antiarrhythmic properties. It undergoes extensive first-pass metabolism, has a bioavailability of 34-38 percent, and is 95 percent bound to plasma proteins. Moricizine is extensively metabolized and may have pharmacologically active metabolites. A recent clinical study has shown that moricizine is slightly less effective than encainide or flecainide in suppressing ventricular premature depolarizations. Compared with disopyramide and quinidine, moricizine was equally or more effective in suppressing ventricular premature depolarizations, couplets, and nonsustained ventricular tachycardia. Further studies are needed comparing moricizine with other class 1 agents in the treatment of life-threatening arrhythmias; available data suggest that moricizine is comparable with these agents in the treatment of ventricular tachycardias and fibrillation. Moricizine appears to have a low incidence of serious adverse effects compared with other antiarrhythmics. This combination of apparently similar efficacy with a decreased incidence of adverse effects makes moricizine a worthwhile addition to currently available antiarrhythmic agents.