Changes In Action Potential Characteristics Research Articles

The Use of Ratiometric Fluorescence Measurements of the Voltage Sensitive Dye Di-4-ANEPPS to Examine Action Potential Characteristics and Drug Effects on Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes.

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) and higher throughput platforms have emerged as potential tools to advance cardiac drug safety screening. This study evaluated the use of high bandwidth photometry applied to voltage-sensitive fluorescent dyes (VSDs) to assess drug-induced changes in action potential characteristics of spontaneously active hiPSC-CM. Human iPSC-CM from 2 commercial sources (Cor.4U and iCell Cardiomyocytes) were stained with the VSD di-4-ANEPPS and placed in a specialized photometry system that simultaneously monitors 2 wavebands of emitted fluorescence, allowing ratiometric measurement of membrane voltage. Signals were acquired at 10 kHz and analyzed using custom software. Action potential duration (APD) values were normally distributed in cardiomyocytes (CMC) from both sources though the mean and variance differed significantly (APD90: 229 ± 15 ms vs 427 ± 49 ms [mean ± SD, P < 0.01]; average spontaneous cycle length: 0.99 ± 0.02 s vs 1.47 ± 0.35 s [mean ± SD, P < 0.01], Cor.4U vs iCell CMC, respectively). The 10–90% rise time of the AP (Trise) was ∼6 ms and was normally distributed when expressed as 1/ in both cell preparations. Both cell types showed a rate dependence analogous to that of adult human cardiac cells. Furthermore, nifedipine, ranolazine, and E4031 had similar effects on cardiomyocyte electrophysiology in both cell types. However, ranolazine and E4031 induced early after depolarization-like events and high intrinsic firing rates at lower concentrations in iCell CMC. These data show that VSDs provide a minimally invasive, quantitative, and accurate method to assess hiPSC-CM electrophysiology and detect subtle drug-induced effects for drug safety screening while highlighting a need to standardize experimental protocols across preparations.

Allosteric Modulation of Kv11.1 (hERG) Channels Protects Against Drug-Induced Ventricular Arrhythmias.

Ventricular arrhythmias as a result of unintentional blockade of the Kv11.1 (hERG [human ether-à-go-go-related gene]) channel are a major safety concern in drug development. In past years, several highly prescribed drugs have been withdrawn for their ability to cause such proarrhythmia. Here, we investigated whether the proarrhythmic risk of existing drugs could be reduced by Kv11.1 allosteric modulators. Using [(3)H]dofetilide-binding assays with membranes of human Kv11.1-expressing human embryonic kidney 293 cells, 2 existing compounds (VU0405601 and ML-T531) and a newly synthesized compound (LUF7244) were found to be negative allosteric modulators of dofetilide binding to the Kv11.1 channel, with LUF7244 showing the strongest effect at 10 μmol/L. The Kv11.1 affinities of typical blockers (ie, dofetilide, astemizole, sertindole, and cisapride) were significantly decreased by LUF7244. Treatment of confluent neonatal rat ventricular myocyte (NRVM) monolayers with astemizole or sertindole caused heterogeneous prolongation of action potential duration and a high incidence of early afterdepolarizations on 1-Hz electric point stimulation, occasionally leading to unstable, self-terminating tachyarrhythmias. Pretreatment of NRVMs with LUF7244 prevented these proarrhythmic effects. NRVM monolayers treated with LUF7244 alone displayed electrophysiological properties indistinguishable from those of untreated NRVM cultures. Prolonged exposure of NRVMs to LUF7244 or LUF7244 plus astemizole did not affect their viability, excitability, and contractility as assessed by molecular, immunological, and electrophysiological assays. Allosteric modulation of the Kv11.1 channel efficiently suppresses drug-induced ventricular arrhythmias in vitro by preventing potentially arrhythmogenic changes in action potential characteristics, raising the possibility to resume the clinical use of unintended Kv11.1 blockers via pharmacological combination therapy.

Decrease in density of INa is in the common final pathway to heart block in murine hearts overexpressing calcineurin

Overexpression of calcineurin in transgenic mouse heart results in massive cardiac hypertrophy followed by sudden death. Sudden deaths are caused by abrupt transitions from sinus rhythm to heart block (asystole) in calcineurin-overexpressing (CN) mice. Preliminary studies showed decreased maximum change in potential over time (dV/dt(max)) of phase 0 of the action potential. Accordingly, the hypothesis was tested that decreased activity of the sodium channel contributes to heart block. Profound decreases in activity of sodium currents (I(Na)) paralleled the changes in action potential characteristics. Progressive age-dependent decreases were observed such that at 42-50 days of life little sodium channel function existed. However, this was not paralleled by decreased protein expression as assessed by immunocytochemistry or by Western blot. Since calcineurin can interact with the ryanodine receptor, we assessed whether chronic in vitro treatment with BAPTA-AM, thapsigargin, and ryanodine could rescue the decrease of I(Na). All of these treatments rescued I(Na) to levels indistinguishable from wild type. The nonspecific PKC inhibitor bisindolylmaleimide I also rescued the decrease of I(Na). To assess whether decreased sodium channel activity contributes to sudden death in vivo, the response to encainide (20 mg/kg) was assessed: 6 of 10 young CN mice died because of asystole, whereas 0 of 10 wild-type mice died (P < 0.01). Moreover, encainide produced exaggerated prolongation of the QRS width in sinus beats before the heart block. Catecholamine tone appears necessary to support life in older CN mice because propranolol (1 mg/kg) triggered asystolic death in five of six CN mice. We conclude that decrease in sodium channel activity is in the common final pathway to asystole in CN mice.

4-Aminopyridine: effects on electrical activity during ischemia and reperfusion in perfused rat hearts.

We investigated the effects of 2 and 4 mM 4-aminopyridine (4-AP,--blocker of the transient outward current I(to) on the electrophysiological response to regional ischemia and reperfusion. Spontaneously beating rat hearts were subjected to coronary occlusion (10 min) followed by reperfusion. The surface electrogram and the membrane potential from subepicardial left ventricular cells were recorded throughout. The basal effect of 4-AP was a dose dependent increase in the action potential duration (APD90) without changes in the resting potential or the heart rate. During early ischemia resting depolarization (from 87.4 +/- 1.9-70.1 +/- 2.5 mV in the controls) was enhanced by 4 mM, 4-AP (84.3 +/- 1.4 mV vs. 61.7 +/- 1.3 mV) whereas APD90 increased by 73.5%. These effects resulted in a marked reduction in the duration of diastolic intervals that led to conduction failure and aborted responses. A partial recovery was found by the end of ischemia concomitant with APD90 shortening in both, control and 4-AP treated hearts. On reperfusion, 4-AP did not influence the initial incidence of ventricular tachyarrhythmias but decreased their duration from 531.5 +/- 56.3-260.7 +/- 100 sec (2 mM) and to 75.6 +/- 10.5 sec (4 mM). These data confirm others obtained by Henry et al. in isolated cells indicating that ischemia induces sequential changes in several K+ conductances. In addition, they show that changes in action potential characteristics may exert beneficial effects on reperfusion arrhythmias by acting on the arrhythmic substrate without suppressing the trigger mechanism.

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.

Mechanoelectric feedback in a model of the passively inflated left ventricle.

Mechanoelectric feedback has been described in isolated cells and intact ventricular myocardium, but the mechanical stimulus that governs mechanosensitive channel activity in intact tissue is unknown. To study the interaction of myocardial mechanics and electrophysiology in multiple dimensions, we used a finite element model of the rabbit ventricles to simulate electrical propagation through passively loaded myocardium. Electrical propagation was simulated using the collocation-Galerkin finite element method. A stretch-dependent current was added in parallel to the ionic currents in the Beeler-Reuter ventricular action potential model. We investigated different mechanical coupling parameters to simulate stretch-dependent conductance modulated by either fiber strain, cross-fiber strain, or a combination of the two. In response to pressure loading, the conductance model governed by fiber strain alone reproduced the epicardial decrease in action potential amplitude as observed in experimental preparations of the passively loaded rabbit heart. The model governed by only cross-fiber strain reproduced the transmural gradient in action potential amplitude as observed in working canine heart experiments, but failed to predict a sufficient decrease in amplitude at the epicardium. Only the model governed by both fiber and cross-fiber strain reproduced the epicardial and transmural changes in action potential amplitude similar to experimental observations. In addition, dispersion of action potential duration nearly doubled with the same model. These results suggest that changes in action potential characteristics may be due not only to length changes along the long axis direction of the myofiber, but also due to deformation in the plane transverse to the fiber axis. The model provides a framework for investigating how cellular biophysics affect the function of the intact ventricles.

Localisation and functional significance of ryanodine receptors during beta-adrenoceptor stimulation in the guinea-pig sino-atrial node.

Recent evidence shows that calcium released from the sarcoplasmic reticulum (SR) plays an important role in the regulation of heart rate. The aim of this study was to investigate the subcellular distribution of ryanodine receptors in the guinea-pig sino-atrial (SA) node and to determine their functional role in the regulation of pacemaker frequency in response to beta-adrenoceptor stimulation. Monoclonal antibodies raised against the cardiac ryanodine receptor were used with confocal microscopy to investigate ryanodine receptor distribution in single guinea-pig SA node cells. The functional role of ryanodine receptors was investigated in both multicellular SA node/atrial preparations and in single SA node cells. Ryanodine receptor labelling was observed in all SA node cells studied and showed both subsarcolemmal and intracellular staining. In the latter, labelling appeared as transverse bands with a regular periodicity of approximately 2 microm. This interval resembled that of the expected sarcomere spacing but did not, however, depend on the presence of transverse tubules. The bands of ryanodine receptors appeared to be located in the region of the Z lines, based on co-distribution studies with antibodies to alpha-actinin, myomesin and binding sites for phalloidin. Functional studies on single SA node cells showed that application of ryanodine (2 micromol/l) reduced the rate of firing of spontaneous action potentials (measured using the perforated patch clamp technique) and this was associated with changes in action potential characteristics. Ryanodine also significantly decreased the positive chronotropic actions of isoprenaline in both multicellular and single cell preparations. In single cells exposed to 100 nmol/l isoprenaline, ryanodine caused a decrease in the rate of firing and this was associated with a decrease in the amplitude of the measured calcium transients. These findings are the first to show immunocytochemical evidence for the presence and organisation of ryanodine receptor calcium release channels in mammalian SA node cells. This study also provides evidence of a role for ryanodine sensitive sites in the beta-adrenergic modulation of heart rate in this species.

Selective Depolarization of Interneurons in the Early Posttraumatic Dentate Gyrus: Involvement of the Na+/K+-ATPase

Interneurons innervating dentate granule cells are potent regulators of the entorhino-hippocampal interplay. Traumatic brain injury, a leading cause of death and disability among young adults, is frequently associated with rapid neuropathological changes, seizures, and short-term memory deficits both in humans and experimental animals, indicating significant posttraumatic perturbations of hippocampal circuits. To determine the pathophysiological alterations that affect the posttraumatic functions of dentate neuronal networks within the important early (hours to days) posttraumatic period, whole cell patch-clamp recordings were performed from granule cells and interneurons situated in the granule cell layer of the dentate gyrus of head-injured and age-matched, sham-operated control rats. The data show that a single pressure wave-transient delivered to the neocortex of rats (mimicking moderate concussive head trauma) resulted in a characteristic ( approximately 10 mV), transient (<4 days), selective depolarizing shift in the resting membrane potential of dentate interneurons, but not in neighboring granule cells. The depolarization was not associated with significant changes in action potential characteristics or input resistance, and persisted in the presence of antagonists of ionotropic and metabotropic glutamate, and GABA(A) and muscarinic receptors, as well as blockers of voltage-dependent sodium channels and of the h-current. The differential action of the cardiac glycosides oubain and stophanthidin on interneurons from control versus head-injured rats indicated that the depolarization of interneurons was related to the trauma-induced decrease in the activity of the electrogenic Na(+)/K(+)-ATPase. In contrast, the Na(+)/K(+)-ATPase activity in granule cells did not change. Intracellular injection of Na(+), Ca(2+)-chelator and ATP, as well as ATP alone, abolished the difference between the resting membrane potentials of control and injured interneurons. The selective posttraumatic depolarization increased spontaneous firing in interneurons, enhanced the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) in granule cells, and augmented the efficacy of depolarizing inputs to discharge interneurons. These results demonstrate that mechanical neurotrauma delivered to a remote site has highly selective effects on different cell types even within the same cell layer, and that the electrogenic Na(+)-pump plays a role in setting the excitability of hippocampal interneuronal networks after injury.

Role of cardiac chloride currents in changes in action potential characteristics and arrhythmias.

Various types of Cl- currents have been recorded in cardiac myocytes from different regions of the heart and in different species. With few exceptions, most of these currents are not active under basal conditions, but are activated under the influence of various agonists and by physical stress. These channels are distributed nonuniformly, depending on the cell type, tissue and region of the heart. Therefore, Cl- current activation may influence membrane potential and impulse formation differently in different cells, and may play a role in arrhythmogenesis. Among these Cl- currents, the protein kinase A-activated Cl- current (I Cl.PKA), the stretch- or swelling-activated Cl- current (I Cl.SWELL) and the Ca(2+)-activated Cl current (I Cl.Ca) comprise the major anion currents that modify cardiac electrical activity. These currents exhibit outward-going rectification, or are predominantly activated at depolarized voltages and, thus, contribute significantly to shortening of the action potential duration but little to diastolic depolarization. The action potential shortening by Cl- current activation may not only perpetuate reentry by shortening the refractory period in a reentry pathway, but may also prevent the development of early afterdepolarization and triggered activity caused by the prolongation of action potentials. I Cl.Ca contributes to delayed afterdepolarization at diastolic potentials in Ca(2+)-overloaded cells. Another factor limiting the influence of Cl- currents on diastolic potentials is the presence of a predominantly opposing background K+ current, except at the nodal regions that lack these K+ channels, or under conditions of decreased K+ conductance. Therefore, the contribution of Cl- currents to the genesis of arrhythmias may depend on their association with the conductance of other ions, especially that of K+.

Phase resetting and dynamics in isolated atrioventricular nodal cell clusters.

In the heart, the AV node is the primary conduction pathway between the atria and ventricles and subserves an important function by virtue of its rate-dependent properties. Cell clusters isolated from the rabbit atrioventricular (AV) node beat with a stable rhythm (cycle length: 300-520 ms) and are characterized by slow action potential upstroke velocities (7 to 30 V/s). The goal of this study is to better characterize the phase resetting and the rhythms during periodic stimulation of this slow inward current system. Single or periodic depolarizing pulses (20 ms in duration) were injected into AV nodal cell clusters using glass microelectrodes. Phase resetting curves of both strong, weak as well as discontinuous types were obtained by applying single current pulses of different intensities and latencies following every ten action potentials. Graded responses were elicited in a wide range of stimulus phases and amplitudes. A single premature stimulus caused a transient prolongation of the cycle length. Sustained periodic stimulation, at rates faster than the intrinsic beat rate, resulted in various N:M (stimulus frequency: action potential frequency) entrainment rhythms as well as periodic or irregular changes in action potential morphology. The changes in action potential characteristics were evaluated by computing the area under the action potential trace and above a fixed threshold (-45 mV). We show that the variations in action potential morphology play a major role in the onset of complicated dynamics observed in this experimental preparation. In this context, the prediction of entrainment rhythms using techniques based on the iteration of phase resetting curves (PRCs) is inadequate since the PRC does not carry information directly related to the changes in action potential morphology. This study demonstrates the need to consider graded events which, though not propagated, have important implications in the understanding of dynamical diseases of the heart. (c) 1995 American Institute of Physics.

Effects of ryanodine and BAY K 8644 on membrane properties and conduction during simulated ischemia

We studied the effect of 1.0 microM ryanodine and 0.1 microM BAY K 8644 (putative modulators of intracellular calcium) on the changes in action potential characteristics, cellular coupling, and longitudinal conduction induced by simulated ischemia (9.0 mM K, 6.5 pH, 0 glucose, 20 mmHg PO2) in superfused guinea pig papillary muscles. Simulated ischemia (SI) depolarized the resting membrane by 5 mV and caused a 28% decrease in action potential upstroke (Vmax), a 65% decrease in action potential duration at 90% (APD90), a 40% increase in internal longitudinal resistance (ri), and a 17% decrease in conduction velocity as compared with the 9-K Tyrode control solution. These changes were reversible and reproducible. The decrease in Vmax induced by SI was greater than that associated with a K(+)-induced change in resting membrane potential (RMP). Ryanodine lessened the SI-induced APD90 shortening by 26%, the decrease in Vmax by 42%, the increase in ri by 33%, and the decrease in conduction velocity by 21%. BAY K 8644 did not alter SI-induced APD90 shortening but augmented the decrease in Vmax by 23%, the increase in ri by 67%, and the decrease in conduction velocity by 59%. Neither ryanodine nor BAY K 8644 altered the SI-induced changes in RMP. Our results suggest that changes in intracellular calcium during SI not only influence cellular coupling but also contribute to the apparent non-RMP-dependent component of the change in Vmax and to the change in APD90 induced by SI.

Electrophysiological effects of thyroid hormones in guinea-pig ventricular muscle: Time course and relationships to blood levels

Chronic alterations in the thyroid state have been shown to induce marked changes in action potential characteristics, the most pronounced being shortening of action potential duration (APD) by hyperthyroidism and an increase in duration by hypothyroidism. In the present study our major objectives were to investigate the time course of the effect of thyroid hormone on action potential characteristics, to examine the relationships between thyroid hormone levels and these changes, and to test whether the electrophysiological alterations are induced both by thyroxine ( T 4) and triiodothyronine ( T 3). The major findings were that a single dose of either hormone (100 μg/kg) caused a marked shortening of APD, while resting potential, action potential amplitude and V ̇ max were unchanged. APD shortening was observed promptly after ip T 4 or T 3 administration, with maximal effect occurring within 2 to 3 hrs. Having determined thyroid hormone levels in the guinea-pigs used for the electrophysiological experiments, we found a close temporal association and inverse linear relationships (r = −0.82) between total T 4 levels and APD. To determine whether APD shortening is induced due to a direct effect of thyroid hormone on the myocardium, euthyroid papillary muscles were superfused with 10 −6 m T 3; within 3 hrs of superfusion with T 3, APD was shortened from 148.8 ± 4.7 ms to 117.7 ± 6.4 ms ( P < 0.01), an effect blocked by the protein synthesis inhibitor cycloheximide. Our results demonstrate that thyroid hormones affect the duration of the ventricular action potential in a concentration dependent fashion. Furthermore, the study suggests that thyroid hormones affect the myocardium directly via mechanism(s) that are probably associated with thyroid hormone-related protein synthesis.

Ligature‐induced injury in peripheral nerve: Electrophysiological observations on changes in action potential characteristics following blockade of potassium conductance

The effects of the potassium channel blocking agent 4-aminopyridine (4-AP) on action potential properties were studied in chronically injured rat sciatic nerves. In normal, mature myelinated fibers, application of 4-AP does not lead to any significant change in action potential waveform or firing pattern in response to single stimuli. In contrast, application of 4-AP to nerves injured by the placement of loose ligatures results in the appearance of late rippled components in the compound action potential. This alteration in waveform is present at the injury site, but not at nerve segments proximal or distal to this region. Paired stimulation experiments demonstrate that this oscillation of the whole nerve response reflects repetitive firing in response to single stimuli following application of 4-AP. Intra-axonal recording following 4-AP application demonstrates bursts of action potentials, with several spikes of reduced amplitude arising from a depolarizing potential following the initial spike. Refractory period for the late spike is greater than that of the primary action potential. These results demonstrate that potassium channels are present and functional in chronically injured nerves, where blockage of these channels results in repetitive firing in response to single stimuli.

Aminopyridine-sensitivity of spinal cord white matter studied in vitro.

Sections of dorsal columns of the spinal cord were removed from rats of various ages and studied in a sucrose gap chamber. The potassium channel blocking agent, 4-aminopyridine (4-AP), led to a pronounced increase in the area of the compound action potential of immature dorsal column axons. During the course of maturation this effect was attenuated but not lost. Occlusion experiments indicate that the 4-AP-elicited increase in area of the response was likely the result of multiple action potential discharge, an effect also present in dorsal root but not ventral root fibers (Bowe et al. 1985). These results indicate that the 4-AP-elicited changes in action potential characteristics previously described for sensory fibers in the peripheral nervous system are also present in the central nervous system extensions of these axons.

Suppression of Lysophosphatidylcholine-lnduced Abnormal Automaticity by Verapamil in Canine Purkinje Fibers

Effects of verapamil on electrophysiological alterations induced by lysophosphatidylcholine (LPC), a toxic membrane-derived phospholipid potentially responsible for ischemia-induced arrhythmias, were evaluated in canine Purkinje fibers. LPC (10–160 μM) induced concentration-dependent decreases in resting membrane potential, action potential amplitude and maximum upstroke velocity of phase 0. The depolarization was not associated with the reduction of the potassium equilibrium potential, which was calculated from the intracellular potassium ion activities measured by ion-selective electrodes. A graded increase in LPC concentration invariably induced abnormal automaticity arising from depolarized membrane potentials. Although verapamil did not prevent LPC-induced changes in action potential characteristics, it suppressed the appearance of rapid spontaneous activity at reduced membrane potential. These results suggest that verapamil may modify the dysrhythmias induced by LPC in ischemic myocardium.

Suppression of lysophosphatidylcholine-induced abnormal automaticity by verapamil in canine Purkinje fibers.

Effects of verapamil on electrophysiological alterations induced by lysophosphatidylcholine (LPC), a toxic membrane-derived phospholipid potentially responsible for ischemia-induced arrhythmias, were evaluated in canine Purkinje fibers. LPC (10-160 microM) induced concentration-dependent decreases in resting membrane potential, action potential amplitude and maximum upstroke velocity of phase O. The depolarization was not associated with the reduction of the potassium equilibrium potential, which was calculated from the intracellular potassium ion activities measured by ion-selective electrodes. A graded increase in LPC concentration invariably induced abnormal automaticity arising from depolarized membrane potentials. Although verapamil did not prevent LPC-induced changes in action potential characteristics, it suppressed the appearance of rapid spontaneous activity at reduced membrane potential. These results suggest that verapamil may modify the dysrhythmias induced by LPC in ischemic myocardium.

Maturation of mammalian myelinated fibers: changes in action-potential characteristics following 4-aminopyridine application.

Maturation of mammalian myelinated fibers: changes in action-potential characteristics following 4-aminopyridine application.

Developmental changes in the cardiac effects of amrinone in the dog.

Amrinone, a bipyridine compound, has been shown to exert a positive inotropic effect on the heart, without producing cardiac arrhythmias. Because of preliminary observations suggesting that the actions of amrinone might change significantly with growth and development, we studied its effects on the contractility and electrophysiology of isolated cardiac muscle of 0- to 96-day-old beagles and Purkinje fibers of 5-year-old beagles. Amrinone's effects on ventricular muscle contractility were age-related. A significant decrease in contractility of right ventricular trabeculae and papillary muscles (not associated with changes in action potential characteristics) was observed in the 0- to 3-day newborn, whereas, by day 4-10, amrinone increased contractility. The magnitude of the increase became greater through day 96 of life. The negative inotropic effect of amrinone is unassociated with changes in the action potential plateau, suggesting that the slow inward current is not involved in this mechanism.

Effects of verapamil and lidocaine on changes in action potential characteristics and conduction time induced by combined hypoxia, hyperkalemia, and acidosis in canine ventricular myocardium.

We examined the effects of verapamil and lidocaine on changes in action potential characteristics and conduction time in the isolated right ventricular myocardium of canine hearts during superfusion with altered Tyrode's solution imitating some of metabolic alterations that occur in acute myocardial ischemia (Po2 less than 50 mm Hg, KCl 8 mM, pH 6.80). Altered Tyrode's solution produced loss of resting membrane potential (RMP), action potential amplitude (APA), action potential duration, and upstroke velocity of action potential (Vmax), and prolonged conduction time (CT). In the presence of lidocaine (5 mg/L), altered Tyrode's solution aggravated the reductions of APA and Vmax and the prolongation of CT in both endocardial and epicardial muscle cells. On the other hand, in the presence of verapamil (1 mg/L), the degree of the reductions of APA and Vmax and of the prolongation of CT induced by altered Tyrode's solution was reduced in epicardial muscle cells, but not in endocardial ones. Neither verapamil nor lidocaine affected change in RMP. These results suggest that the improving effect of verapamil on ischemia-induced conduction delay can be partly explained by its salutary effect on the depressed fast channel.

Chapter 4 Differentiation of Excitable Membranes

This chapter provides a historical introduction to the electrical events that define the action potential (AP) in adult excitable tissues and explores the properties of the specific ionic conductance mechanisms that underlie each part of the AP. The chapter serves as a background for a more extensive review of the literature concerning developmental changes in AP characteristics and the differentiating mechanisms of excitablity. It emphasizes on cardiac tissue, but not to the exclusion of investigations of nerve or skeletal muscle. The mechanism of impulse initiation in excitable cells has been a question of major interest for centuries. The study of electrical properties of tissues had its origins in the observations of Galvani on “animal electricity,” published in 1791, and the controversy with Volta that ensued. It was only after the development of the first galvonometers at the beginning of the nineteenth century that it became possible to measure electric currents in animal tissues. The concept that the electrical properties of excitable cells may reside primarily in the cell membrane was introduced by Bernstein and an electrical equivalent circuit for the cell membrane was developed by Fricke and Cole (1928). The electrical activity of the normal cardiac pacemaker was variously described as a slow depolarization or as an oscillatory potential.