Leonid V. Rosenshtraukh

Interactions between antiarrhythmic drugs and cardiac memory.

OBJECTIVE Ventricular pacing or arrhythmias can induce cardiac memory (CM). We hypothesized that clinically administered antiarrhythmic drugs alter the expression of CM, and that the repolarization changes characteristic of CM can modulate the effects of antiarrhythmic drugs. METHODS We studied conscious, chronically-instrumented dogs paced for two 1-h periods to study the effects of drugs on the evolution of memory (protocol 1) or for 21 days (protocol 2) to observe the effects of steady-state memory on drug actions. Dogs were treated in both settings with quinidine, lidocaine or E4031, in random order, and within therapeutic serum concentration ranges. RESULTS Pacing, alone, for 2 h significantly prolonged ERP only near the left ventricular pacing site, whereas pacing alone for 21 days prolonged ERP at all sites (P<0.05). Quinidine and E4031, but not lidocaine, prolonged repolarization and ERP and suppressed evolution of CM in protocol 1. However, quinidines effect in prolonging repolarization was diminished in both protocols, while its effect in prolonging ERP was diminished in the 21-day protocol only. In contrast, the effects of E4031 were additive to those of CM, prolonging repolarization and ERP in both protocols, while lidocaine showed no changes in effect at all. CONCLUSIONS Pacing to induce CM significantly affects ventricular repolarization and refractoriness, and there are interactions between CM, quinidine and E4031. Depending on the specific drug, these interactions have the potential to be anti- or proarrhythmic, and may impact importantly on the clinical efficacy of drugs as well as on electrophysiologic testing of drug actions.

Local cholinergic suppression of pacemaker activity in the rabbit sinoatrial node.

The effects of transmural vagal stimulation and acetylcholine (ACh) superfusion on primary and latent pacemaker cells of the rabbit sinoatrial node were studied by using microelectrodes. Both ACh and vagal stimulation lengthened atrial cycle length by 40-60% as compared with control. In the cells from the primary pacemaker area, both ACh superfusion and vagal stimulation suppressed action potential (AP) amplitude and then induced inexcitability. In contrast, cells from subsidiary pacemaker area as well as atrium remained excitable. These effects were completely reversible and also were abolished by atropine, 10(-7) M. Cholinergically induced suppression of AP amplitude is predictable based on the maximal rate of AP upstroke (dV/dt). The probability of amplitude suppression was the highest among pacemaker cells (dV/dt, <3 V/s), in which ACh suppressed amplitude in 27 (93%) of 29 cells, and vagal stimulation did so in 38 (81%) of 47 cells. With increasing upstroke velocity, the probability of amplitude suppression decreased. Inexcitability did not occur in cells whose dV/dt was >15 V/s. The suppression of AP amplitude by ACh occurred in a concentration-dependent manner: the concentration inducing suppression of amplitude in 50% of pacemaker cells was approximately 10 microM. These results indicate that cholinergic effects on typical pacemaker and subsidiary pacemaker cells are different: whereas subsidiary pacemaker cells remain excitable, typical pacemaker cells become quiescent. We hypothesize that quiescent cells create quiescent regions in the center of the sinoatrial node that might functionally be an obstacle for reentrant tachycardias.

Electrophysiological mechanisms of antiarrhythmic protection during hypothermia in winter hibernating versus nonhibernating mammals.

BACKGROUND Robust cell-to-cell coupling is critically important in the safety of cardiac conduction and protection against ventricular fibrillation (VF). Hibernating mammals have evolved naturally protective mechanisms against VF induced by hypothermia and reperfusion injury. OBJECTIVE We hypothesized that this protection strategy involves a dynamic maintenance of conduction and repolarization patterns through the improvement of gap junction functions. METHODS We optically mapped the hearts of summer-active (SA) and winter-hibernating (WH) ground squirrels Spermophilus undulatus from Siberia and nonhibernating rabbits during different temperatures (+3 degrees C to +37 degrees C). RESULTS Midhypothermia (+17 degrees C) resulted in nonuniform conduction slowing, increased dispersion of repolarization, shortened wavelength, and consequently enhanced VF induction in SA ground squirrels and rabbits. In contrast, wavelength was increased during hypothermia in WH hearts in which VF was not inducible at any temperature. In SA and rabbit hearts, but not in WH, conduction anisotropy was significantly increased by pacing acceleration, thus promoting VF induction during hypothermia. WH hearts maintained the same rate-independent anisotropic propagation pattern even at 3 degrees C. connexin 43 (Cx43) had more homogenous transmural distribution in WH ventricles as compared to SA. Moreover, Cx43 and N-cadherins (N-cad) densities as well as the percentage of their colocalization were significantly higher in WH compared to SA epicardium. CONCLUSION Rate-independent conduction anisotropy ratio, low dispersion of repolarization, and long wavelength-these are the main electrophysiological mechanisms of antiarrhythmic protection in hibernating mammalian species during hypothermia. This strategy includes the improved gap junction function, which is due to overexpression and enhanced colocalization of Cx43 and N-cad.

Vagally induced block and delayed conduction as a mechanism for circus movement tachycardia in frog atria.

Episodes of tachycardia induced by strong vagal stimulation in spontaneously beating isolated atria of frog (Rana temporaria) were studied with multielectrode mapping technique. These episodes were inducible in 19 of 39 preparations. The arrhythmia started several seconds after cessation of vagal stimulation strong enough to cause sinus arrest, without electrical stimulation of the myocardium. The arrhythmia consisted of two to 20 beats (6±4, mean±SD, n=42) with a cycle length of 100-500 msec. Recording from 32 sites with spatial resolution of 1-2 mm showed that the arrhythmia was due to intra-atrial circus movement. The estimated perimeter of the reentrant circuit ranged from 6 to 20 mm. In circuits of the minimal size, the average conduction velocity along the circuit was as low as 2-3 cm/sec. Paroxysms of the tachycardia were always preceded by vagally induced nonuniform depression of conduction, with some areas of atria being completely blocked. As the vagal influence decreased, the blocked areas recovered in an inhomogeneous manner, their unblocking being significantly (p<0.05) delayed after inhibition of tissue cholinesterase by proserine. The reentrant tachycardia was initiated when a sinus impulse arrived during certain phase of the unblocking. Unlike the well-known mechanism of reentrant excitation, which is based on inhomogeneous refractoriness and critically timed extrabeat(s), the circus movement in our model depended on vagally induced conduction block and could be launched by a single sinus impulse.

Modulation of rabbit sinoatrial node activation sequence by acetylcholine and isoproterenol investigated with optical mapping technique

Aims:  Changes in the rabbit sinoatrial node (SAN) activation sequence with the cholinergic and adrenergic factors were studied. The correlation between the sinus rhythm rate and the leading pacemaker site shift was determined. The hypothesis concerning the cholinergic suppression of nodal cell excitability as one of the mechanisms associated with pacemaker shift was tested.

Non‐quantal release of acetylcholine from parasympathetic nerve terminals in the right atrium of rats

Acetylcholinesterase (AChE) inhibitors provoke typical cholinergic effects in the isolated right atrium of the rat due to the accumulation of acetylcholine (ACh). Our study was designed to show that in the absence of vagal impulse activity, ACh is released from the parasympathetic nerve fibres by means of non‐quantal secretion. The conventional microelectrode technique was used to study changes in action potential (AP) configuration in the right atrium preparation of rats during application of AChE inhibitors. Staining with the lipophilic fluorescent dye FM1‐43 was used to demonstrate the presence of endocytosis in cholinergic endings. The AChE inhibitors armin (10−7–10−5 m) and neostigmine (10−7 to 5 × 10−6 m) caused a reduction of AP duration and prolonged the cycle length. These effects were abolished by atropine and were therefore mediated by ACh accumulated in the myocardium during AChE inhibition. Putative block of impulse activity of the postganglionic neurons by tetrodotoxin (5 × 10−7 m) and blockade of ganglionic transmission by hexomethonium (2 × 10−4 m), as well as blockade of all forms of quantal release with Clostridium botulinum type A toxin (50 U ml−1), did not alter the effects of armin. Experiments with FM1‐43 dye confirmed the effective block of exocytosis by botulinum toxin. Selective inhibition of the choline uptake system using hemicholinium III (10−5 m), which blocks non‐quantal release at the neuromuscular junction, suppressed the effects of AChE inhibitors. Thus, accumulation of ACh is likely to be caused by non‐quantal release from cholinergic terminals. We propose that non‐quantal release of ACh, shown previously at the neuromuscular junction, is present in cholinergic postganglionic fibres of the rat heart in addition to quantal release.

Electrophysiological responses of canine atrial endocardium and epicardium to acetylcholine and 4-aminopyridine

OBJECTIVES Prior studies demonstrated marked electrophysiological and pharmacological differences between canine ventricular epicardium and endocardium. For atrium, however, it has been assumed that, because of the thin wall, electrical properties of epicardium and endocardium are similar. The aim of the present study was to compare the action potential (AP) characteristics in epicardial and endocardial atrial cells before and following addition of acetylcholine (ACh) and 4-aminopyridine (4-AP). METHODS AND RESULTS Microelectrode techniques were used to study the effects of ACh (10(-7)-10(-5) M) and 4-AP (0.5 mM) on epicardial and endocardial AP of canine right atrial free wall at cycle lengths (CL) of 250 to 2000 ms. ACh hyperpolarized epicardial and endocardial cells (by 5-8 mV at 10(-5) M). In control, AP duration to 90% repolarization (APD90) was longer in endocardium at all CL. ACh shortened APD90 in either tissue with more prominent effect in endocardium (at 10(-5) M and CL = 2000 ms, from 179 +/- 10 to 90 +/- 11 ms in epicardium and from 209 +/- 10 to 65 +/- 6 ms in endocardium, P < 0.05). As a result, at 10(-5) M, APD90 in endocardium was shorter than in epicardium at all CL 4-AP effects on AP duration were similar in both tissue types. No effects of 4-AP was seen at CL = 250 ms and at long CL, the compound shortened APD90 and prolonged AP duration to 50% repolarization. CONCLUSIONS (1) ACh exerts direct effects on atrial epicardial and endocardial AP; (2) 4-AP-sensitive transient outward current (Itol) is expressed both in canine atrial epicardial and endocardial cells; (3) differential response of epicardial and endocardial APD to ACh may alter the gradient of repolarization across the atrial wall and contribute to vagally induced atrial flutter and fibrillation.

Effects of a new class III antiarrhythmic drug nibentan in a canine model of vagally mediated atrial fibrillation.

Nibentan, a new class III antiarrhythmic drug, is highly effective in patients with atrial flutter and fibrillation. However, its mechanism of action remains unclear. The aim of this study was to investigate the effects of nibentan using a canine model of vagally sustained atrial fibrillation (AF). Nibentan was intravenously infused to anesthetized open-chest dogs during vagally induced AF. Cumulative doses of nibentan (0.063, 0.125, and 0.250 mg/kg) successfully terminated AF in 78, 88, and 100% as well as prevented AF reinduction in 11, 63, and 90% of cases, respectively. All doses of nibentan significantly and rate-independently increased atrial effective refractory period (AERP) with and without vagal stimulation. Activation mapping (224 epicardial electrodes) during AF showed that nibentan reduced the number of simultaneously occurring reentrant wavelets. Herewith the atrial excitation slowed down until conduction failure of reentrant wavelets led to arrhythmia termination. These changes in activation patterns can be accounted for by nibentan-induced increase of AERP (55 +/- 9%, 82 +/- 12%, and 90 +/- 6%; p < 0.01) and wavelength for reentry (47 +/- 7%, 68 +/- 12%, and 72 +/- 4%; p < 0.01) at rapid atrial rates in the presence of vagal stimulation. In conclusion, the high efficacy of nibentan against AF was associated with significant rate-independent increase in AERP and in wavelength, and might be in part explained by block of both delayed rectifier (I(K)) and muscarinic I(K,ACh) currents.

Cholinergic modulation of activation sequence in the atrial myocardium of non-mammalian vertebrates

Cholinergic changes of electric activity were studied in isolated atrium preparations from fishes (cod and carp), amphibians (frog) and reptilians (lizard) using the microelectrode technique and high-resolution optical mapping. Perfusion of isolated atrium with acetylcholine (10(-6)-5.10(-5) M) caused gradual suppression of action potential generation and, eventually, completely blocked the excitation in a part of the preparation. Other regions of atrium, situated close to the sinoatrial and atrioventricular junctions, remained excitable. Such cholinergic suppression of electric activity was observed in the atrial myocardium of frog and in both fish species, but not in reptilians. Ba(2+) (10(-4) M), which blocks the acetylcholine-dependent potassium current (I(KACh)), prevented cholinergic reduction of action potential amplitude. In several preparations of frog atrium, cholinergic suppression of excitation coincided with episodes of atrial fibrillation. We conclude that the phenomenon of cholinergic suppression of electric activity is typical for atria of fishes and amphibians. It is likely to be caused by I(KACh) activation and may be important for initiation of atrial arrhythmias.

Mechanisms of Cardiac Muscle Insensitivity to a Novel Acetylcholinesterase Inhibitor C-547

We compared the effects of the novel acetylcholinesterase (AChE) inhibitor C-547 on action potential configuration and sinus rhythm in the isolated right atrium preparation of rat with those of armin and neostigmine. Both armin (10−7, 10−6, and 10−5 M) and neostigmine (10−7, 10−6, and 5 × 10−6 M) produced a marked decrease in action potential duration and slowing of sinus rate. These effects were abolished by atropine and are attributable to the accumulation of acetylcholine in the myocardium. The novel selective AChE inhibitor C-547 (10−9 to 10−7 M), an alkylammonium derivative of 6-methyluracil, had no such effects. The inhibition constant of C-547 on cardiac AChE is 40-fold higher than that on extensor digitorum longus muscle AChE. These results suggest that C-547 might be employed to treat diseases such as myasthenia gravis or Alzheimer disease, without having unwanted effects on the heart.