Rapid Rates Of Stimulation Research Articles

Mechanisms underlying atrial-selective block of sodium channels by Wenxin Keli: Experimental and theoretical analysis

IntroductionAtrial-selective inhibition of cardiac sodium channel current (INa) and INa-dependent parameters has been shown to contribute to the safe and effective management of atrial fibrillation. The present study was designed to examine the basis for the atrial-selective actions of Wenxin Keli. MethodsWhole cell INa was recorded at room temperature in canine atrial and ventricular myocytes. Trains of 40 pulses were elicited over a range of pulse durations and interpulse intervals to determine tonic and use-dependent block. A Markovian model for INa that incorporates interaction of Wenxin Keli with different states of the channel was developed to examine the basis for atrial selectivity of the drug. ResultsOur data indicate that Wenxin Keli does not bind significantly to either closed or open states of the sodium channel, but binds very rapidly to the inactivated state of the channel and dissociates rapidly from the closed state. Action potentials recorded from atrial and ventricular preparations in the presence of 5g/L Wenxin Keli were introduced into the computer model in current clamp mode to simulate the effects on maximum upstroke velocity (Vmax). The model predicted much greater inhibition of Vmax in atrial vs. ventricular cells at rapid stimulation rates. ConclusionOur findings suggest that atrial selectivity of Wenxin Keli to block INa is due to more negative steady-state inactivation, less negative resting membrane potential, and shorter diastolic intervals in atrial vs. ventricular cells at rapid activation rates. These actions of Wenxin Keli account for its relatively safe and effective suppression of atrial fibrillation.

Morphological Changes in the Middle Latency Response Using Maximum Length Sequence Stimuli

The middle latency response (MLR) can be a powerful tool for assessing integrity of cortical and subcortical auditory structures. Most research on the MLR, however, is constrained to relatively slow repetition rates by the time window necessary for response acquisition. Maximum length sequence (MLS) paradigms enable the recording of the MLR at high repetition rates, which could reduce test time and provide information about the behavior of auditory structures at rapid rates of stimulation. To examine potential timing advantages and differences in waveforms between an MLS-MLR paradigm compared to a conventional MLR paradigm (Experiment 1), and to examine effects of rate on MLR morphology within the MLS paradigm (Experiment 2). A repeated measures study. All subjects within each experiment underwent every condition for that experiment. Ten young adult female subjects participated in each experiment of this study. All subjects had normal hearing and negative neurological history. Latency and amplitude values as well as the presence/absence of Na, Pa, Nb, and Pb components of the MLR were measured. Comparisons were made between the MLS-MLR and conventional MLR paradigms, as well as between repetition rates within the MLS-MLR paradigm. Significant latency and amplitude differences were found between MLS and conventional MLR paradigms. The late components of the MLR (Nb, Pb) were present significantly more often when the MLS paradigm was used. No timing advantage was found with the MLS-MLR paradigm. Within the MLS paradigm, as repetition rate increases, latency becomes significantly shorter and amplitude significantly lower. Middle latency responses evoked by the MLS-MLR paradigm show significant differences from those evoked by a conventional MLR paradigm. The MLS-MLR paradigm can reliably evoke the late components (Nb, Pb) of the MLR and may be an important clinical tool for future investigation of these elements.

Effects of EGIS-7229 (S 21407), a novel class III antiarrhythmic drug, on myocardial refractoriness to electrical stimulation in vivo and in vitro.

The I(Kr) blocker EGIS-7229 (S-21407), displays class Ib and class IV effects that may alter its pharmacologic profile compared with those of pure I(Kr) blockers. Therefore, the concentration- and frequency-dependent effects of EGIS-7229, and of the I(Kr) blockers d,l-sotalol and dofetilide, on the effective refractory period (ERP) were measured in isolated right ventricular papillary muscle of the rabbit in vitro. The effects of these drugs on right ventricular fibrillation threshold (RVFT) at increasing intravenous doses were also determined in anesthetized cats. Dofetilide and d,l-sotalol increased ERP in a concentration-dependent manner (dofetilide: 3-100 nM; d,l-sotalol: 3-100 microM) with strong reverse frequency dependence at high concentrations. EGIS-7229 concentration dependently lengthened ERP at 1-30 microM. Its effect on ERP was clearly reverse frequency dependent at 3 microM, but this feature of the drug diminished at 10 microM and was not apparent at 30 microM. The effect of EGIS-7229 (30 microM) on ERP was devoid of reverse frequency dependence as it was more effective (31%) than dofetilide (16 %) at high-pacing rate (3 Hz), whereas it was less effective (50%) than dofetilide (70%) at slow-pacing rate (1 Hz). Reverse frequency-dependent ERP effect of dofetilide (100 nM) was similarly abolished by the addition of lidocaine (30 microM). EGIS-7229 (1-8 mg/kg iv), d,l-sotalol (1-8 mg/kg iv), and dofetilide (10-80 microg/kg iv) caused a dose-dependent increase in RVFT. The minimum effective dose of d,l-sotalol and EGIS-7229 was 1 and 2 mg/kg, respectively, whereas that of dofetilide was 10 microg/kg. EGIS-7229 induced a smaller peak effect in RVFT than sotalol or dofetilide. In conclusion, EGIS-7229 markedly increased refractoriness to electrical stimulation in vitro and in vivo. Compared with pure I(Kr) blockers, the benefits of EGIS-7229 seem to be a greater lengthening of effective refractory period at rapid stimulation rates, suggesting a strong antiarrhythmic action, and a smaller effect at slow stimulation rates, suggesting low potential to induce early afterdepolarizations.

Inhibitory effects of class I and IV antiarrhythmic drugs on the Na+-activated K+ channel current in guinea pig ventricular cells.

Recently we have reported that class III antiarrhythmic drugs including amiodarone inhibit the Na+-activated K+ (KNa) channels in isolated cardiac cells. In this study effects of antiarrhythmic drugs having class I and/or IV properties on the single KNa channel current were examined in inside-out membrane patches of guinea pig ventricular cells by using patch clamp techniques. The KNa channel current, which was activated by increasing [Na+]i from 0 mM to 100 mM in the presence of 150 mM [K+]o, showed a large slope conductance (212 pS) and inward-going rectification. Quinidine (100 microM), mexiletine (100 microM) and flecainide (10 microM) were selected as representative of class Ia, Ib and Ic drugs, respectively. These drugs at relatively high concentrations incompletely inhibited the KNa channel by decreasing the open time (flickering block). The class IV drug verapamil inhibited the KNa channel current mainly by decreasing the open probability although the IC50 value of verapamil (3.36 microM) was higher than the therapeutic concentrations. Bepridil and SD-3212, antiarrhythmic drugs having both class I and IV properties, potently inhibited the KNa channel current by decreasing the open probability. The IC50 values of bepridil and SD-3212 for inhibiting the KNa channel current was 0.51 microM and 0.53 microM, respectively, both of which are within the therapeutic range. Most antiarrhythmic drugs inhibit cardiac KNa channels by different modes and at different concentrations. The KNa channel blocking action of bepridil and SD-3212 may partly contribute to the prolongation of the action potential duration by these drugs at rapid stimulation rates.

Postrepolarization refractoriness as a potential anti-atrial fibrillation mechanism of pilsicainide, a pure sodium channel blocker with slow recovery kinetics.

The antifibrillatory effect of pilsicainide, a sodium channel blocker with slow recovery kinetics, was investigated in a canine model of atrial fibrillation. Prolonging the atrial effective refractory period is an important mechanism for pharmacological termination of atrial fibrillation. However, the effectiveness of potassium channel blockers has been questioned because of their reverse-use-dependent property. In eight open-chest dogs, the duration of the atrial endocardial monophasic action potential and the atrial effective refractory period were determined using a Franz catheter. Conduction velocity was obtained from a 96-channel mapping electrode at multiple cycle lengths. Inducibility of sustained atrial fibrillation (> 30 minutes) was confirmed by atrial burst pacing during bilateral vagal stimulation, and local fibrillation cycle lengths were measured. Five minutes after restarting fibrillation, pilsicainide (0.6 mg/kg + 0.04 mg/kg/min) was administered. After fibrillation was terminated, measurements were repeated. Pilsicainide successfully terminated atrial fibrillation in 7 of 8 dogs after the median time of 5.1 minutes. The conduction velocity decreased significantly. Although pilsicainide did not affect monophasic action potential duration, it caused use-dependent prolongation of the atrial effective refractory period (P < 0.05), creating postrepolarization refractoriness. Accordingly, pilsicainide prolonged the atrial fibrillation cycle length from 80.6 to 113.8 ms (P < 0.05) before termination of fibrillation. Sodium channel blockers with slow recovery kinetics can prolong the atrial effective refractory period without affecting monophasic action potential duration. Unlike potassium channel blockers, these sodium channel blockers maintain postrepolarization refractoriness, even at rapid stimulation rates, thus exerting a salutary antifibrillatory effect.

The Use of Maximum Length Sequences to Obtain Brainstem Auditory Evoked Responses at Rapid Rates of Stimulation.

The Use of Maximum Length Sequences to Obtain Brainstem Auditory Evoked Responses at Rapid Rates of Stimulation.

Effect of rate on changes in conduction velocity and extracellular potassium concentration during acute ischemia in the in situ pig heart.

The purpose of our study was to determine if the slowing of longitudinal intraventricular conduction in the in situ porcine heart during acute regional no-flow ischemia was rate dependent. Further, we investigated whether any rate dependence could be correlated to a rate-dependent component of the ischemia-induced rise in extracellular potassium concentration, [K+]e. We studied in situ hearts in nine anesthetized open chest pigs in which acute no-flow ischemia was induced by occlusion of the left anterior descending coronary artery. To determine the effects of steady-state rate on the slowing of conduction and rise in [K+]e during ischemia, we varied the rate of stimulation during sequential occlusions from 90 to 150 beats/min. Longitudinal conduction velocity was determined by unipolar electrodes embedded in a plaque that was sutured to the epicardial surface in the center of the ischemic zone. Myocardial [K+]e was determined simultaneously by potassium-sensitive electrodes placed at or within 1 to 2 mm of the epicardium in close proximity to the activation recording electrodes. Conduction velocity decreased more rapidly at the more rapid rates of stimulation although the reduction in conduction velocity occurring prior to the onset of conduction block was similar at both rates. The potassium change was not rate dependent and rose at the same rate regardless of the rate of stimulation. Our study demonstrates that the steady-state rate-dependent component of the slowing of intraventricular conduction induced by acute ischemia in the in situ porcine heart occurs in the absence of a rate-dependent component in the rise of [K+]e. Between rates of 90 and 150 beats/min, the rate dependence of the conduction slowing may be attributed to one or more potassium-independent factors such as the rate-dependent changes in resting membrane potential, in Vmax of the action potential upstroke, and in cell-to-cell uncoupling, which have been observed in other models of acute ischemia.

Bepridil prolongs the action potential duration of guinea pig ventricular muscle only at rapid rates of stimulation

1. We examined the electromechanical effects of the calcium antagonist, bepridil (1–20 μM), on isolated guinea pig ventricular muscles, driven at various stimulus frequencies (0.1, 0.5, 1, 2 and 5 Hz) in Tyrode's solution containing various K + concentrations (1.4–43.2 mM). 2. Conventional microelectrode and tension-recording techniques were used. 3. We found that bepridil decreased the maximum upstroke velocity ( V ̇ max ) of the action potential with no change in the resting membrane potential (RMP). 4. The former effect depended on both stimulus frequency and the drug concentration used. 5. Bepridil lengthened the duration of the action potential at the level of 25% repolarization (APD 25) at the highest frequency (5 Hz), but shortened it at lower frequencies (⩽2 Hz). 6. The drug also lengthened the APD 90 at the highest frequency (5 Hz) but without significant effect at lower frequencies (⩽2 Hz). 7. Bepridil depolarized the RMP at relatively low extracellular K + concentrations (⩽2.7 mM), accompanied by a prolongation of APD 90. 8. There were no such effects at much higher K + concentrations (⩾5.4 mM), and the drug markedly depressed the V ̇ max and the action potential amplitude. 9. The drug eliminated the positive staircase phenomenon of twitch contraction, in a concentration-dependent manner. 10. All these findings taken together suggest that bepridil prolongs the action potential duration by inhibiting outward potassium currents ( I K and I KI), at rapid rates of stimulation (∼300/min), which is comparable to the physiological heart rate of a guinea pig. 11. The prolongation of APD seemed to be secondary to the bepridil-induced reduction of intracellular Ca 2+ concentration, [Ca 2+] i.

A comparison of maximum length and Legendre sequences for the derivation of brain-stem auditory-evoked responses at rapid rates of stimulation

Experiments were performed in which brain-stem auditory-evoked responses (BAERs) were elicited by two types of pseudorandom pulse trains: maximum length sequences (MLS) and Legendre sequences (LGS). In experiment 1, each pulse sequence was presented at 50 dB nHL with minimum pulse intervals varying from 1 to 10 ms. Wave V latency increased and wave V amplitude decreased with decreasing minimum pulse intervals, with no significant effect of the type of sequence (MLS vs LGS), and no significant interaction between sequence and minimum pulse interval in terms of wave V amplitude or latency. In a second experiment, the minimum pulse interval was held constant at 4 ms, while MLS and LGS levels were varied from 20 to 60 dB nHL. With increasing click intensity, there is a decrease in wave V latency and an increase in wave V amplitude. There was no significant effect of type of sequence (LGS vs MLS) or interaction between type of sequence and stimulus intensity for wave V amplitude or latency. Despite the obvious violation of the assumptions (linearity and stationarity) underlying the application of maximum length sequence analysis and Legendre sequence analysis, both techniques produced reliable responses remarkably similar in morphology to evoked responses obtained by conventional averaging. The results of these experiments support the possibility that analysis methods based on pseudorandom pulse sequences may prove more efficient in data collection and provide a more thorough description of the electrophysiologic behavior of the auditory system compared to conventional averaging.

Effects of halothane dose and stimulus rate on canine spinal, far-field and near-field somatosensory evoked potentials

Evidence that canine spinal, far-field and near-field somatosensory evoked potentials resemble those recorded in humans and other species has been presented, and the vulnerability of each component to varying depths of halothane anesthesia is reported. Lumbar spinal peak latencies are not affected by halothane dose, but the negative peak is significantly prolonged by rapid rates of stimulation. Elevated stimulus rates and halothane doses reduce lumbar spinal cord potential amplitudes. Early far-field cephalic components are refractory to halothane. Late far-field components and near-field cortical potentials are substantially altered by increments in halothane dose. Both near-field and far-field responses are more readily identified in vertex-neck than vertex-brow derivations. Early far-field somatosensory evoked potentials recorded from vertex to neck, together with lumbar spinal cord potentials, may be the preferred monitoring technique when the use of halothane anesthesia is desired. Rapid rates of stimulation may facilitate earlier recognition of cord dysfunction, but supplement rather than replace baseline recordings at slo stimulus rates.

Effects of hypoxia, hyperkalemia, and metabolic acidosis on canine subendocardial action potential conduction.

We have studied the individual and combined effects of elevated external potassium concentration (8 mM [K+], metabolic acidosis (pH = 6.8), and hypoxia at different stimulation 400 milliseconds) on Purkinje (P) and ventricular (V) conduction velocities and on Purkinje-ventricular junctional conduction delay (PVJ delay) in in vitro preparations from canine ventricles. Elevated [K+] had opposite effects on P and V velocities, increasing V velocity by 8% while reducing P velocity by 7%. Acidosis reduced P velocity by 9% while reducing V velocity by only 4%. Hypoxia and rapid stimulation rates had no significant effect on either P or V velocities. All test solutions (except hypoxia alone) significantly increased the PVJ delay. The magnitude of the increase in PVJ delay was much greater than the effects on either P or V velocity. In addition, hypoxia and rapid stimulation augmented the increase in PVJ delay in the presence of elevated [K+] and/or acidosis. The special features of conduction at the PV junctional sites may produce altered pathways of excitation of the ventricles during myocardial ischemia.

Statistical analysis of single sodium channels. Effects of N-bromoacetamide

Currents were obtained from single sodium channels in outside-out excised patches of membrane from the cell line GH3. The currents were examined in control patches and in patches treated with N- bromoacetamide ( NBA ) to remove inactivation. The single-channel current-voltage relationship was linear over the range -60 to + 10 mV, and was unaffected by NBA . The slope conductance at 9.3 degrees C was 12 pS, and the Q10 for single channel currents was about 1.35. The currents in both control and NBA -treated patches showed evidence of a slow process similar to desensitization in acetylcholine-receptor channels. This process was especially apparent at rapid rates of stimulation (5 Hz), where openings occurred in clusters of records. The clustering of records with and without openings was analyzed by runs analysis, which showed a statistically significant trend toward nonrandom ordering in the responses of channels to voltage pulses. NBA made this nonrandom pattern more apparent. The probability that an individual channel was "hibernating" during an activating depolarization was estimated by a maximum likelihood method. The lifetime of the open state was also estimated by a maximum likelihood method, and was examined as a function of voltage. In control patches the open time was mildly voltage-dependent, showing a maximum at about -50 mV. In NBA -treated patches the open time was greater than in the control case and increased monotonically with depolarization; it asymptotically approached that of the control patches at hyperpolarized potentials. By comparing channel open times in control and NBA -treated patches, we determined beta A and beta I, the rate constants for closing activation gates and fast inactivation gates. Beta I was an exponential function of voltage, increasing e-fold for 34 mV. beta A had the opposite voltage dependence. The probability of an open channel closing its fast inactivation gate, rather than its activation gate, increased linearly with depolarization from -60 to -10 mV. These results indicate that inactivation is inherently voltage dependent.

Evoked potentials recorded from scalp and spinous processes during spinal column surgery

Peroneal nerve evoked potentials were simultaneously recorded from scalp and from wire electrodes inserted into lumbar and thoracic spinous processes at multiple levels during surgery for correction of spinal column curvature in 43 patients. Spinal potentials progressively increased in latency rostrally. Over cauda equina and rostral spinal cord initially positive triphasic potentials were recorded. Over caudal spinal cord the response consisted of initial positive-negative diphasic potentials that merged with broad large negative and positive potentials. At rapid rates of stimulation, the initial diphasic component was stable but the subsequent potentials significantly diminished in amplitude. This suggests that the diphasic component reflects presynaptic activity arising in the intramedullary continuations of dorsal root fibers and that the subsequent components reflect largely postsynaptic activity. Scalp recordings at restricted bandpass (30–3000 c/sec) revealed well defined positive and negative potentials with mean peak latencies of 25.9 and 29.9 msec (P V-N 1). The amplitudes and latencies of P V-N 1 remained relatively stable throughout general anesthesia with halogenated agents which suggests that this component may be a reliable monitor of conduction within spinal cord afferent pathways during spinal surgery. Data are presented which suggest that selective filtering may help to distinguish faster frequency, synchronous axonal events from slower frequency, asynchronous axonal or synaptic events.

Field potentials, inhibition and the effect of pentobarbitone in the rat olfactory cortex slice.

1. Field potentials were evoked in slices of rat olfactory cortex by stimulating the lateral olfactory tract. In addition to previously described components of the wave-form, a further distinct surface-negative potential of low amplitude and long duration (I-wave) has been described. 2. Pentobarbitone, at concentrations of 10(-5) M and above, markedly enhanced enhanced the amplitude and duration of the I-wave with only minimal effect on other components of the field potential. 3. The I-wave was reversibly reduced by the GABA antagonists bicuculline and picrotoxin and was also attenuated at rapid rates of stimulation. Low chloride medium usually caused a transient increase in amplitude of the I-wave followed by a gradual reduction, suggesting that a chloride-mediated depolarization was involved. 4. Evoked inhibition, which was most probably post-synaptic, occurred in parallel with the I-wave. This was monitored as a suppression of, or increase in latency of the population spike evoked by a second stimulus at appropriate intervals after the first. Pentobarbitone substantially increased the duration of the post-synpatic inhibition, without obvious changes in the presynaptic inhibitory phenomenon associated with antidromic firing in the lateral olfactory tract. 5. It is proposed that the I-wave is the field potential representation of a population depolarizing i.p.s.p. and that the main action of pentobarbitone is to enhance this inhibition.

Neuromuscular Transmission in a Mammalian Preparation during Exposure to Enflurane

The effects of enflurane on motor nerve terminals and muscle endplates were studied in the rat phrenic nerve-diaphragm preparation using standard microelectrode recording techniques. Muscle endplate interaction with enflurane was suggested by depression of the amplitude of miniature endplate potentials (MEPP's) without change in frequency, increased duration of MEPP's and endplate potentials (EPP's) increased threshold for generation of muscle action potentials, and inhibition of the endplate depolarization induced by succinylcholine. Evidence of nerve terminal effects of enflurane was limited to a greater relative decline of EPP amplitude during tetanus, indicative of failure to maintain transmitter output at rapid rates of stimulation. It is concluded that the depressant effect of enflurane on indirectly elicited muscle twitch is primarily due to an impairment of the action of muscle endplate on adjacent membrane; however, the relative importance of prejunctional depression, seen only during rapid nerve stimulation, cannot be assessed from the present data.

Visual evoked responses elicited by rapid stimulation

An assessment of the technique of eliciting VERs with rapid rates of stimulation has revealed that differences in evoked response due to stimulus parameters may be minimized under these conditions. The probable reason is that addition of ongoing activity from one flash to that elicited by another occurs; this addition can obscure or enhance wave form differences present in the complete evoked response. A comparison of visual-evoked responses (VERs) to rapid flash rates which were empirically determined with VERs synthesized from a linear addition of the first two segments of the complete evoked response reveals a rather remarkable agreement in most cases. On the other hand, some failures to predict empirical data in this manner to occur. These failures are not due to temporal variations and their etiology is unknown at this time. Until the electrogenesis of the VER is understood, we conclude the use of both a fast and a slow rate to obtain VERs is a good methodological technique for studying brain functioning.

Functional characteristics of the gating mechanism in the canine A-V conducting system.

The areas of maximum action potential duration, or gates, of the isolated canine distal A-V conducting system were studied by microelectrode techniques to determine the characteristics of their responses to premature impulses and to rapid rates of stimulation. The results indicate that premature responses will be conducted at the same velocity as driven responses until the coupling interval between the driven and premature impulses approaches the refractory period of the gate. When delayed conduction of the premature impulse develops, it occurs over a segment of the conducting system incorporating the gate. The magnitude of the delay which can develop at coupling intervals just in excess of the functional refractory period tends to be small in healthy tissue and larger in depressed tissue. Suitable delays allowed premature impulses having coupling intervals shorter than the refractory period at the gate to be conducted distally. The refractory period in response to rapid rates of stimulation was discordant in the several false tendons in six of nine preparations studied. Both delayed conduction and 2:1 conduction were shown to occur near the limit of response to rapid stimulation. The data presented suggest certain functional analogies between the A-V node and the gating mechanism.

Electrophysiologic effects of quinidine. Studies using chronically implanted electrodes in awake dogs with and without cardiac denervation.

Experiments were performed on awake dogs with chronic recording electrodes implanted on the SA node, His bundle, and right bundle branch. Quinidine at serum concentrations of 5 to 10 mg/liter had little effect on AV conduction, but slowed conduction in Purkinje tissue, prolonged ventricular activation, decreased the excitability of atrium and ventricle, and prolonged the atrial refractory period without a consistent effect on the ventricular refractory period. In awake dogs that had previously undergone cardiac denervation, quinidine decreased the excitability and prolonged the refractory period of atrium and ventricle. The effects of quinidine on excitability were dependent on the duration of the stimulus used to test excitability. The actions of quinidine on intraventricular conduction and excitability were enhanced at rapid rates of stimulation. These data indicate that slowing of intraventricular conduction is observed consistently at serum quinidine concentrations that are generally considered to be within the therapeutic range, but changes of the refractory period of the ventricle are not observed consistently. Although the vagolytic properties of quinidine may contribute to changes of heart rate, they do not appear to influence significantly the effects of quinidine on atrial refractoriness and AV conduction.

The T deflection of isolated mammalian heart muscle electrogram.

Both bipolar and unipolar electrograms recorded from well-oxygenated isolated papillary muscle preparations consist of R and T deflections in the same direction. The T is inverted by anoxia, rapid late, and injury. When cut linear strips of ventricular muscle are observed for many hours, during which time they presumably recover from the initial injury, the T deflection becomes upright in most cases, and then can be depressed and inverted by anoxia or rapid rates of stimulation. Since, in the absence of modifying factors, the T is in the same direction as the R deflection in a simple linear strip, it probably cannot be adequately explained on the basis of membrane repolarization. Since the nerve after-potentials are believed to represent processes involving oxidative metabolism, the T deflection may also be produced by metabolic processes, although the possibility cannot be excluded that it represents a combination of membrane repolarization and other processes.