Biomedical subjects

Alexei N Plotnikov

Publications and source records attributed to Alexei N Plotnikov.

11 recordsLinked to original sources

PubMed2006

Wild-type and mutant HCN channels in a tandem biological-electronic cardiac pacemaker.

BACKGROUND: Biological pacemakers (BPM) implanted in canine left bundle branch function competitively with electronic pacemakers (EPM). We hypothesized that BPM engineered with the use of mE324A mutant murine HCN2 (mHCN2) genes would improve function over mHCN2 and that BPM/EPM tandems confer advantage over either approach alone. METHODS AND RESULTS: In cultured neonatal rat myocytes, activation midpoint was -46.9 mV in mE324A versus -66.1 mV in mHCN2 (P < 0.05). mE324A manifested a positive shift of voltage dependence of gating kinetics of activation and deactivation compared with mHCN2 (P < 0.05) in myocytes as well as Xenopus oocytes. In intact dogs in complete atrioventricular block, saline (control), mHCN2, or mE324A virus was injected into left bundle branch, and EPM were implanted (VVI 45 bpm). Twenty-four-hour ECGs were monitored for 14 days. With EPM discontinued, there was no difference in duration of overdrive suppression among groups. However, basal heart rates in controls were less than those in mHCN2, which did not differ from those in E324A (45 versus 57 versus 53 bpm; P < 0.05). When spontaneous rate fell below 45 bpm, EPM intervened at that rate, triggering 83% of beats in control, contrasting (P < 0.05) with 26% (mHCN2) and 36% (mE324A). On day 14, epinephrine (1 microg/kg per minute IV) induced a 50% heart rate increase in all mE324A, one third of mHCN2, and one fifth of control (P < 0.05 mE324A versus control or mHCN2). CONCLUSIONS: mE324A induces faster, more positive pacemaker current activation than mHCN2 and stable, catecholamine-sensitive rhythms in situ that compete with EPM comparably but more catecholamine responsively than mHCN2. BPM/EPM tandems function reliably, reduce the number of EPM beats, and confer sympathetic responsiveness to the tandem.

Animals↗

PubMed2006

I(Kr) contributes to the altered ventricular repolarization that determines long-term cardiac memory.

OBJECTIVE: Cardiac memory (CM) is characterized by an altered T-wave morphology, which reflects altered repolarization gradients. We hypothesized that the delayed rectifier currents, I(Kr) and I(Ks), might contribute to these repolarization changes. METHODS: We studied conscious, chronically instrumented dogs paced from the postero-lateral left ventricular (LV) wall at rates 5-10% faster than sinus rate for 3 weeks. ECGs during sinus rhythm were recorded on days 0, 7, 14 and 21 of pacing. Within 3 weeks, CM achieved steady state, hearts were excised, and epicardial and endocardial tissues and myocytes were studied. RESULTS: In unpaced controls, action potential duration to 50% and 90% repolarization (APD) in epicardium was shorter than in endocardium (P < 0.05); in CM epicardial APD increased at CL > or = 500 ms, while endocardial APD was either unchanged or decreased such that the transmural gradient seen in controls diminished (P < 0.05). A transmural I(Kr) gradient occurred in controls (epicardium>endocardium, P < 0.05) and was reversed in CM. No I(Ks) transmural gradient was found in controls, while in CM endocardial I(Ks) was greater than epicardial at greater than +50 mV. Canine ERG (cERG) mRNA and protein in epicardium > endocardium in controls (P < 0.05), and this difference was lost in CM. Expression levels of KCNQ1 and KCNE1 protein were similar in all groups. CONCLUSIONS: A transcriptionally induced change in epicardial I(Kr) contributes to the altered ventricular repolarization that characterizes CM.

Action Potentials↗

PubMed2005

Repolarization gradients in the canine left ventricle before and after induction of short-term cardiac memory.

BACKGROUND: Questions remain about the contributions of transmural versus apicobasal repolarization gradients to the configuration of the T wave in control settings and after the induction of short-term cardiac memory. METHODS AND RESULTS: Short-term cardiac memory is seen as T-wave changes induced by altered ventricular activation that persists after restoration of sinus rhythm. We studied cardiac memory in anesthetized, open-chest dogs paced from the ventricle for 2 hours. Unipolar electrograms were recorded from as many as 98 epicardial and 144 intramural sites, and activation times and activation-recovery intervals (ARIs) were measured. In separate experiments, epicardial monophasic action potentials were recorded. We found no appreciable left ventricular intramural gradients in repolarization times (activation time+ARI) in either control conditions or after the induction of memory. In controls, there was a left ventricular apicobasal gradient, with the shortest repolarization times in anterobasal regions and longest repolarization times posteroapically. After induction of memory, repolarization times shortened uniformly throughout the ventricular wall. Monophasic action potential duration at 90% repolarization decreased by approximately 10 ms after induction of memory. CONCLUSIONS: In the intact canine left ventricle at physiological rates, there is no transmural gradient in repolarization. Apicobasal gradients in repolarization time, with shortest repolarization times in anterobasal areas and longest repolarization times in posteroapical regions, are important in the genesis of the T wave. Repolarization times and monophasic action potentials at the 90% repolarization level shorten after the induction of memory. The deeper T wave in the ECG after induction of memory may be explained by the more rapid phase 3 of the action potential.

Action Potentials↗

PubMed2005

The cAMP response element binding protein modulates expression of the transient outward current: implications for cardiac memory.

OBJECTIVE: Long-term cardiac memory (LTCM), expressed as a specific pattern of T-wave change on ECG, is associated with 1) reduced transient outward potassium current (I(to)), 2) reduced mRNA for the pore-forming protein of I(to), Kv4.3, 3) reduced cAMP response element binding protein (CREB), and 4) diminished binding to its docking site on the DNA, the cAMP response element (CRE). We hypothesized a causal link between the decrease of the transcription factor CREB and down-regulation of I(to) and one of its channel subunits, KChIP2, in LTCM. METHODS: After three weeks of left ventricular pacing to induce LTCM (8 paced, 7 sham control dogs), epicardial KChIP2 mRNA and protein levels were assessed by real-time PCR and Western blotting. Mimicking the CREB down-regulation in LTCM, CREB was knocked down in situ in other dogs using adenoviral anti-sense. Effects on the action potential notch, reflecting I(to), were investigated in situ using monophasic action potential (MAP) recordings and at the cellular level by the whole-cell patch clamp technique. CREB binding in the KChIP2 promoter region was ascertained by electrophoretic mobility-shift assays. RESULTS: In LTCM, epicardial KChIP2 mRNA and protein were reduced by 62% and 76%, respectively, compared to shams (p < 0.05). CREB binding by the canine KChIP2 promoter region was demonstrated. CREB knockdown led to disappearance of the phase1 notch in MAP and ablation of I(to). CONCLUSIONS: These results strengthen the hypothesis that down-regulation of CREB-mediated transcription underlies the attenuation of epicardial I(to) in LTCM. They also emphasize that ventricular pacing exerts effects at a subcellular level contributing to memory and conceivably to other forms of cardiac remodeling.

Action Potentials↗

PubMed2005

Atrial gradient as a potential predictor of atrial fibrillation.

OBJECTIVES: We tested the utility and comparability of the atrial gradient and atrial ERP as early markers of electrical remodeling and a propensity to atrial fibrillation (AF). BACKGROUND: Pacing at physiologic rates from the left atrium alters the atrial gradient and is associated with atrial tachyarrhythmias. At these physiologic rates, there is no change in the atrial effective refractory period (ERP). METHODS: Sixty-one chronically instrumented mongrel dogs in complete heart block were paced from the left or right atrium at 400 to 900 bpm for 46 +/- 3 days. Dogs were monitored weekly and electrophysiologic studies conducted to determine changes in the atrial gradient, ERP, and rhythm. RESULTS: Rapid atrial pacing was associated with concordant decreases in atrial gradient, ERP, and occurrence of AF. Incidence of AF increased with increasing pacing rate. Although there ultimately was an equal incidence of AF with left atrial and right atrial pacing, the onset of AF occurred earlier with left atrial pacing. As expected, ERP decreased in both atria. Animals with long control ERP did not fibrillate. CONCLUSIONS: Rapid pacing induces changes in atrial gradient, which can be used as a noninvasive marker of electrical remodeling. AF is accompanied by decreases in atrial gradient and ERP, and the incidence is highest in dogs with short control ERP.

Animals↗

PubMed2005

Cardiac memory: mechanisms and clinical implications.

Cardiac memory (CM) is identified as an altered T wave on electrocardiogram and vectorcardiogram that is seen when sinus rhythm resumes after a period of abnormal myocardial activation. Specifically, the sinus rhythm T wave tracks the QRS vector of the abnormal impulse. CM frequently is induced by ventricular pacing or arrhythmias and historically has been considered of minor relevance to medical practice. Although it has long been known that CM can mimic the T-wave inversions of myocardial ischemia, we learned more recently that CM can alter the actions of antiarrhythmic drugs. Furthermore, it provides a template for investigating the mechanisms whereby ventricular pacing affects myocardial physiology. In this article we review the mechanisms believed responsible for induction of CM and some of its more recently recognized clinical manifestations. We also discuss the controversies regarding atrial memory and its potential clinical implications.

Animals↗

PubMed2004

Cardiac memory evolves with age in association with development of the transient outward current.

BACKGROUND: Calcium-insensitive transient outward current (I(to)) is important to the development of cardiac memory (CM), which itself reflects the capacity of the heart to remodel electrophysiologically. We used cardiac pacing to test the hypothesis that CM evolution can be explained by developmental maturation of I(to). METHODS AND RESULTS: Acutely anesthetized dogs from 1 day old to adult were paced from the left ventricle (VP, n=29) or left atrial appendage (AP, n=12) to induce CM. T-wave vector displacement (TVD) obtained during VP was greater than with AP (adults, 0.39+/-0.06 mV; neonates, 0.04+/-0.01 mV; P<0.05). TVD began to increase at approximately 40 days of age, reaching adult levels by approximately 200 days. Microelectrode studies performed in 18 dogs (ages 3 to 94 days) after completing the CM protocol and 20 additional dogs (1 day old to adult) revealed that the epicardial action potential notch was absent in neonates, became apparent in the young, and was deepest in adults. The relationship between TVD and epicardial notch was such that as notch magnitude increased, TVD increased (r=-0.65, P<0.05). KChIP2 and Kv4.3 mRNA (measured via reverse transcription-polymerase chain reaction) also increased with age. CONCLUSIONS: The inducibility of CM gradually increases with age in association with evolution of the epicardial action potential notch and mRNA expression for KChIP2 and Kv4.3. This suggests that the capacity of the heart to remodel electrophysiologically and to manifest memory during development depends in part on evolution of the determinants of I(to).

Action Potentials↗

PubMed2004

Biological pacemaker implanted in canine left bundle branch provides ventricular escape rhythms that have physiologically acceptable rates.

BACKGROUND: We hypothesized that administration of the HCN2 gene to the left bundle-branch (LBB) system of intact dogs would provide pacemaker function in the physiological range of heart rates. METHODS AND RESULTS: An adenoviral construct incorporating HCN2 and green fluorescent protein (GFP) as a marker was injected via catheter under fluoroscopic control into the posterior division of the LBB. Controls were injected with an adenoviral construct of GFP alone or saline. Animals were monitored electrocardiographically for up to 7 days after surgery, at which time they were anesthetized and subjected to vagal stimulation to permit emergence of escape pacemakers. Hearts were then removed and injection sites visually identified and removed for microelectrode study of action potentials, patch clamp studies of pacemaker current, and/or immunohistochemical studies of HCN2. For 48 hours postoperatively, 7 of 7 animals subjected to 24-hour ECG monitoring showed multiple ventricular premature depolarizations and/or ventricular tachycardia attributable to injection-induced injury. Thereafter, sinus rhythm prevailed. During vagal stimulation, HCN2-injected dogs showed rhythms originating from the left ventricle, the rate of which was significantly more rapid than in the controls. Excised posterior divisions of the LBB from HCN2-injected animals manifested automatic rates significantly greater than the controls. Isolated tissues showed immunohistochemical and biophysical evidence of overexpressed HCN2. CONCLUSIONS: A gene-therapy approach for induction of biological pacemaker activity within the LBB system provides ventricular escape rhythms that have physiologically acceptable rates. Long-term stability and feasibility of the approach remain to be tested.

Action Potentials↗

PubMed2003

Cardiac memory is associated with decreased levels of the transcriptional factor CREB modulated by angiotensin II and calcium.

Cardiac memory (CM) has short- (STCM) and long-term (LTCM) components modulated by calcium and angiotensin II. LTCM is associated with reduced Ito and Kv4.3 mRNA levels. Because the cAMP response element binding protein, CREB, contributes to CNS memory transcription, we hypothesized that it might be a transcriptional factor in CM, influenced by calcium and angiotensin II. We studied STCM in dogs that were AV sequentially paced (AVP) for 2 hours or sham-operated. STCM was evaluated with ECG and vectorcardiogram (VCG), and subepicardial biopsies were taken at 5 to 120 minutes and investigated for CREB. LTCM was studied in dogs paced for 3 weeks and in sham controls. At 3 weeks the heart was excised, biopsies obtained, and CRE binding tested. STCM induction occurred in AVP dogs but not in sham or AVP dogs treated with saralasin or nifedipine. Nuclear CREB was significantly decreased at 2 hours in the AVP no-drug group only. LTCM dogs manifested reduced binding of nuclear proteins to CRE, and CRE binding activity in the promoter region of Kv4.3. In conclusion, there is an association between STCM induction and decreased nuclear CREB that is angiotensin-modulated and calcium-dependent. Moreover, the decreased CRE binding after 3 weeks of AVP combined with CRE binding activity in the Kv4.3 promoter can explain the Kv4.3 mRNA and Ito downregulation that characterize LTCM.

Angiotensin II↗

PubMed2003

Role of L-type calcium channels in pacing-induced short-term and long-term cardiac memory in canine heart.

BACKGROUND: We tested the hypothesis that ICa,L is important to the development of cardiac memory. METHODS AND RESULTS: The effects of L-type Ca2+ channel blockade and beta-blockade were tested on acutely anesthetized and on chronically instrumented, conscious dogs. Short-term memory (STM) was induced by 2 hours of ventricular pacing and long-term memory (LTM) by ventricular pacing for 21 days. STM dogs received placebo, nifedipine, or propranolol, and LTM dogs received placebo, atenolol, or amlodipine. AT1 receptor blockade (candesartan) and ACE inhibition (trandolapril) were also tested in LTM. Microelectrodes were used to record transmembrane potentials from isolated epicardial and endocardial slabs using a protocol simulating STM in intact animals. Left ventricular epicardial myocytes from LTM or sham control dogs were dissociated, and ICa,L was recorded (whole-cell patch-clamp technique). Evolution of STM and LTM was attenuated by ICa,L blockers but not beta-blockers. Neither AT1 receptor blockade nor ACE inhibition suppressed LTM. In microelectrode experiments, pacing induced an epicardial-endocardial gradient change mimicking STM that was suppressed by nifedipine. In patch-clamp experiments, peak ICa,L density in LTM and control were equivalent, but activation was more positive and time constants of inactivation longer in LTM (P<0.05). CONCLUSIONS: ICa,L blockade but not beta-adrenergic blockade suppresses cardiac memory. LTM evolution is unaffected by angiotensin II blockade and is associated with altered ICa,L kinetics.

Action Potentials↗

PubMed2003

Expression and function of a biological pacemaker in canine heart.

BACKGROUND: We hypothesized that localized overexpression of the hyperpolarization-activated, cyclic nucleotide-gated (HCN2) pacemaker current isoform in canine left atrium (LA) would constitute a novel biological pacemaker. METHODS AND RESULTS: Adenoviral constructs of mouse HCN2 and green fluorescent protein (GFP) or GFP alone were injected into LA, terminal studies performed 3 to 4 days later, hearts removed, and myocytes examined for native and expressed pacemaker current (I(f)). Spontaneous LA rhythms occurred after vagal stimulation-induced sinus arrest in 4 of 4 HCN2+GFP dogs and 0 of 3 GFP dogs (P<0.05). Native I(f) in nonexpressed atrial myocytes was 7+/-4 pA at -130 mV (n=5), whereas HCN2+GFP LA had expressed pacemaker current (I(HCN2)) of 3823+/-713 pA at -125 mV (n=10) and 768+/-365 pA at -85 mV. CONCLUSIONS: HCN2 overexpression provides an I(f)-based pacemaker sufficient to drive the heart when injected into a localized region of atrium, offering a promising gene therapy for pacemaker disease.

Adenoviridae↗