Subthreshold dynamics in periodically stimulated squid giant axons.
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Journal Article↗
Biomedical subjects
A Shrier
Publications and source records attributed to A Shrier.
Contribution of ATP-sensitive potassium channels to the electrophysiological effects of adenosine in guinea-pig atrial cells.
1. Adenosine caused dose-dependent action potential abbreviation in multicellular guinea-pig atrial preparations, an action antagonized by glyburide (IC50, 31 microM) in both physiological and low-chloride superfusate. 2. When 5 mM ATP was included in pipettes for whole-cell voltage clamp of isolated guinea-pig atrial myocytes, adenosine (10 microM) increased the holding current at -40 mV from 41 +/- 8 to 246 +/- 31 pA (mean +/- S.E.M., P < 0.01), and glyburide (20 microM) returned the holding current to 69 +/- 11 pA (P < 0.01 vs. adenosine alone). Acetylcholine (10 microM) also increased the holding current, but its effects were not altered by glyburide. 3. Both adenosine and acetylcholine induced an additional current component in response to 500 ms voltage steps. Glyburide partially inhibited the adenosine-induced current, but did not alter the effect of acetylcholine. In the presence of maximally effective acetylcholine concentrations, adenosine increased membrane conductance (P < 0.01), although to a lesser extent than in the absence of acetylcholine. 4. Single K+ channel activity was seen in only one of eight cell-attached patches in the absence of adenosine or acetylcholine (0.5 mM Ba2+ in bath and pipette solutions). With acetylcholine (10 microM) in the pipette, inwardly rectifying channels (conductance, 41 +/- 5 pS) were seen in five of six patches. With adenosine (10 microM) in the pipette, single-channel activity was seen in twelve of fourteen patches with two populations of channels, one similar to that induced by acetylcholine and another higher-conductance channel (72 +/- 5 pS) that showed less inward rectification. Glyburide (20 microM) suppressed the high-conductance channel (68 +/- 2 pS) leaving a single channel type with a conductance of 36 +/- 5 pS and strong inward rectification. 5. We conclude that K+ATP channels contribute to the electrophysiological actions of adenosine on guinea-pig atrium in the presence of physiological intracellular ATP levels, and may therefore play a role in the cardiac electrophysiological effects of adenosine in the absence of myocardial ischaemia.
Acetylcholine↗
A quantitative description of the E-4031-sensitive repolarization current in rabbit ventricular myocytes.
We have measured the E-4031-sensitive repolarization current (IKr) in single ventricular myocytes isolated from rabbit hearts. The primary goal of this analysis was a description of the IKr kinetic and ion transfer properties. Surprisingly, the maximum time constant of this component was 0.8 s at 33-34 degrees C, which is significantly greater than the value of 0.18 s previously reported under similar conditions in the original measurements of IKr from guinea pig ventricular myocytes. The primary, novel feature of our analysis concerns the relationship of the bell-shaped curve that describes the voltage dependence of the kinetics and the sigmoidal curve that describes the activation of IKr. The midpoint of the latter occurred at approximately +10 mV on the voltage axis, as compared to -30 mV for the point on the voltage axis at which the maximum time constant occurred. Moreover, the voltage dependence of the kinetics was much broader than the steepness of the activation curve would predict. Taken together, these results comprise a gating current paradox that is not resolved by the incorporation of a fast inactivated state in the analysis. The fully activated current-voltage relation for IKr exhibited strong inward-going rectification, so much so that the current was essentially nil at +30 mV, even though the channel opens rapidly in this voltage range. This result is consistent with the lack of effect of E-4031 on the early part of the plateau phase of the action potential. Surprisingly, the reversal potential Of /Kr was ~15 mV positive to the potassium ion equilibrium potential,which indicates that this channel carries inward current during the latter part of the repolarization phase of the action potential.
Animals↗
Differential responsiveness of atrial and ventricular myocytes to potassium channel openers.
We examined the effects of the potassium channel openers (PCOs) pinacidil and lemakalim (BRL 38227) on action potential (AP) configurations and outward currents in atrial and ventricular myocytes isolated from rabbit and guinea pig hearts, using the whole-cell configuration of the patch clamp technique at 33 degrees-35 degrees C. The PCOs known to activate ATP-sensitive K+ current (IKATP) in various tissues, induced this current and decreased AP duration (APD) in rabbit ventricular myocytes. In contrast, in rabbit atrial myocytes, PCOs either had no effect or increased duration and plateau amplitude of the AP. The predominant outward current in rabbit atrial myocytes is a 4-aminopyridine (4-AP)-sensitive transient outward current (Ito). The PCOs caused a decrease in Ito without inducing IKATP in rabbit atria. In identical experimental conditions, PCOs activated IKATP in both guinea pig atrial and ventricular myocytes. Our results suggest that (a) a species as well as cardiac tissue difference exists in responsiveness to PCOs, and (b) the decrease in Ito without concomitant induction of IKATP can lead to changes in AP configuration opposite to that expected from IKATP activation. Different effects of PCOs on distinct parts of the heart could lead to disparity in APD and refractoriness that may contribute to arrhythmogenesis.
Action Potentials↗
Overdrive suppression of spontaneously beating chick heart cell aggregates: experiment and theory.
In spontaneously beating chick heart cell aggregates, sustained periodic stimulation at a rate faster than the intrinsic frequency is generally followed by a transient slowing of the automatic rhythm called "overdrive suppression." We characterize the qualitative aspects of overdrive suppression using three sets of experimental protocols: 1) stimulation at a fixed frequency with various numbers of stimuli, 2) stimulation at different frequencies, 3) stimulation with different intensities. We develop a mathematical model based on a system of nonlinear ordinary differential equations to account for the experimental observations. The main idea of the model is that overdrive suppression arises as a result of a hyperpolarizing current that is induced by action potentials. This work shows that the frequency of action potentials is the major determinant of overdrive suppression. Consequently, during periodic pacing of spontaneous oscillators at different rates, the fastest frequency where 1:1 entrainment can be maintained is associated with maximal overdrive suppression. This type of model is complementary to the development of a rigorous ionic model and can help provide insight into the physiological mechanisms of overdrive suppression.
Animals↗
Mechanisms of conduction time hysteresis in rabbit atrioventricular node.
The functional origin of atrioventricular nodal hysteresis was studied in isolated rabbit heart preparations. This hysteresis is characterized by asymmetric changes in nodal conduction time (NCT) occurring for symmetric changes in cycle length. The respective contribution of the nodal properties of recovery, facilitation, and fatigue to the beat-to-beat changes in NCT observed during paired symmetric ramps of decreasing and increasing cycle length was determined with specifically design stimulation protocols. Nodal hysteresis was found to be entirely accounted for by variations in the contribution of nodal recovery and fatigue properties observed at corresponding cycle lengths. The study establishes how this contribution varies on a beat-to-beat basis as a result of cycle length history. This holds true for the numerous changes in hysteresis observed in response to changes in the sequence and slope of the ramps. Facilitation clearly affected NCT during these responses but did not contribute to the hysteresis. Moreover, the study demonstrates that there is no inherent change in the characteristics of nodal function with the direction of the ramp that could account for the hysteresis. Thus nodal hysteresis arises from nodal functional properties of recovery and fatigue but does not constitute a distinct independent intrinsic property of the node.
Animals↗
Ionic mechanisms and nonlinear dynamics of embryonic chick heart cell aggregates.
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Animals↗
A review of the effects of three cardioactive agents on the electrical activity from embryonic chick heart cell aggregates: TTX, ACh, and E-4031.
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Acetylcholine↗
Relationship between different recovery curves representing rate-dependent AV nodal function in rabbit heart.
INTRODUCTION: The rate-dependent changes in atrioventricular (AV) nodal conduction time show different characteristics depending upon whether the conduction times are plotted against the atrial interval (AA-recovery curve) or His-atrial interval (HA-recovery curve). This study characterizes these differences in the context of controlled changes of nodal functional properties, determines their functional significance, and tests the hypothesis that they are related solely to the nodal conduction time of the last beat (last conduction time) before the premature beat. METHODS AND RESULTS: Premature nodal conduction times obtained in isolated rabbit heart preparations under various steady-state and transient conditions were plotted as a function of the corresponding HA and AA intervals, as well as the AA interval corrected for the last conduction time. Under all conditions, the corrected AA-recovery curve was indistinguishable in shape from the HA-recovery curve, and as such reflected similar underlying nodal functional properties. Moreover, a selective increase in the last conduction time, induced in the absence of time-dependent effects associated with the functional property of fatigue, shifted the AA-recovery but not the HA-recovery curve upward with respect to the control curve. CONCLUSION: The last conduction time accounts entirely for differences between AA-recovery and HA-recovery curves that otherwise reflect the same underlying nodal functional state. Thus, a consistent assessment of rate-dependent changes in nodal function can be achieved with either measure of recovery time.
Animals↗
Inhibition of metabolism abolishes transient outward current in rabbit atrial myocytes.
Outward currents were measured in single rabbit atrial myocytes using the whole cell configuration of the patch-clamp technique in the presence of tetrodotoxin (5-10 microM) and MnCl2 (2 mM) to block inward currents. Depolarizing voltage-clamp steps from a holding potential of -80 mV elicited a predominant 4-aminopyridine (4-AP)-sensitive transient outward current (Ito). Inhibitors of oxidative metabolism, 2,4-dinitrophenol (DNP; 100 microM) and cyanide (3 mM) abolished Ito and caused a large increase in the steady-state outward current. This steady-state outward current was inhibited by glibenclamide (5 microM), a blocker of the ATP-regulated potassium current (IKATP). In the presence of DNP, glibenclamide (5 microM) not only inhibited IKATP but also partially restored Ito. Absence of ATP from the pipette produced effects on outward currents similar to those induced by DNP or cyanide. We conclude that metabolic inhibition abolishes Ito in rabbit atrial myocytes and suggest that ATP may be required for the activation of the channel.
2,4-Dinitrophenol↗
Pacemaker current in single cells and in aggregates of cells dissociated from the embryonic chick heart.
1. We have measured the time-dependent pacemaker current, I(f), in single cells, or small clusters of two or three cells dissociated from embryonic chick hearts with the whole-cell patch clamp technique, and in multicellular reaggregates of dissociated cells with the two-microelectrode voltage clamp technique. 2. We observed time-dependent current (I(f)) in the -90 to -60 mV range from aggregates of ventricular cells, as in our earlier results from this preparation, which we previously attributed to the potassium ion current mechanism, IK2. We also observed I(f) in single atrial cells and aggregates of atrial cells. 3. The range of activation of I(f) was -120 to -90 mV in atrial preparations (either single cells or aggregates). The activation range of I(f) in ventricular cells was also -120 to -90 mV which is approximately 30 mV negative to the I(f) activation range in ventricular cell aggregates. The reasons for this shift of I(f) in ventricular preparations are unknown. 4. The I(f) component clearly underlies the spontaneous pacemaker depolarization which is observed in ventricular heart cell aggregates during the first week of embryonic development. However, I(f) is not a significant factor underlying spontaneous activity in atrial preparations. The pacemaker current in these cells is a net inward background component, which is significantly reduced in amplitude with development, as is the I(f) component in the ventricle.
Animals↗
The topology of phase response curves induced by single and paired stimuli in spontaneously oscillating chick heart cell aggregates.
The topological properties of the phase resetting of biological oscillators by an isolated stimulus delivered at various phases of the cycle depend on whether the stimulus is "weak" or "strong." When multiple stimuli are delivered to the oscillator, the response to stimulation also depends on the time between the stimuli, and the rate at which the oscillator returns to an underlying limit cycle attractor. If the time between two consecutive "weak" stimuli is sufficiently short, the effects produced by the pair of stimuli may be characteristic of a single "strong" stimulus. These results are demonstrated in a model experimental system, spontaneously beating aggregates of cells derived from embryonic chick heart, and are illustrated by consideration of a simple theoretical model of nonlinear oscillators, the Poincaré oscillator.
Animals↗
Voltage-dependent inactivation of the potassium current of embryonic chick hepatocytes.
The whole-cell patch electrode voltage clamp technique was used to study the inactivation properties of the delayed rectifying potassium current of single cultured embryonic chick hepatocytes at 20 degrees C. The potassium current activates maximally within 250-500 ms of membrane depolarization, after which it decays with a monoexponential time course. Both steady-state activation and inactivation are voltage dependent. Steady-state inactivation declines from 100% at -5 mV to 0 near -70 mV. with half inactivation at -41 mV. At the resting potential (EM) of these cells (-21.5 +/- 6.0 mV, n = 36) 6-18% of the IK channels are not inactivated and less than 5% are open. Development and removal of inactivation follow single exponential time courses. The inactivation time constant attains a maximum of around 30 s at -35 mV and is sharply voltage dependent at the EM of these cells. Measurement of EM under current clamp shows random oscillations of 5-10 mV amplitude. We suggest that the voltage- and time-dependent properties of IK, in tandem with a time- and voltage-independent, non-selective current also seen here, would provide the mechanism for a fluctuating EM.
Animals↗
Evolution of rhythms during periodic stimulation of embryonic chick heart cell aggregates.
During periodic stimulation of spontaneously beating chick heart cell aggregates, there is often an evolution of coupling patterns between the stimulator and the aggregate action potential. For example, at rapid stimulation frequencies, a rhythm that is initially 1:1 (stimulus frequency:aggregate frequency) can evolve to other rhythms such as 5:4 and 4:3. Time-dependent effects generated during periodic stimulation are characterized by three types of experiments to determine 1) the effect of periodic stimulation on the intrinsic cardiac beat rate (overdrive suppression), 2) the effect of periodic stimulation on the phase resetting properties of the aggregate, and 3) the time-dependent changes in the coupling patterns between the stimulator and the aggregate during periodic stimulation. The protocols involved variations in the duration and rate of periodic stimulation. A mathematical model is developed in the form of a two-dimensional finite difference equation based on the data from experiments 1 and 2. The model is used to predict the data generated by experiment 3. There is good correspondence with the experiments in that the theory reproduces complex transitions between various rhythms and displays irregular rhythms similar to those observed experimentally. These results have implications for the evolution of cardiac arrhythmias such as atrioventricular heart block and modulated parasystole.
Animals↗
Theoretical computation of phase locking in embryonic atrial heart cell aggregates.
The effects of periodic stimulation of spontaneously beating aggregates of chick atrial heart cells are considered. Provided the effects of a single stimulus do not change the properties of the oscillation, and that the oscillation is re-established rapidly following a stimulus, this system can be modeled by one-dimensional finite difference equations. These equations employ experimentally generated phase resetting data that describe the effects of a single isolated stimulus at different phases of the oscillation. A complete analysis of the predicted dynamics is given over a broad range of stimulation frequencies and amplitudes. Prominent features of the dynamics include phase locking, bistability, chaos, and disappearance of Arnold tongues at large stimulation amplitudes. The fine details of the bifurcations are sensitive to properties of the phase resetting curves, and consequently, the observed bifurcations are not expected to be "universal" for larger stimulation amplitudes. Experimental traces show many correspondences with theoretical computations.
Animals↗
Phase resetting of embryonic chick atrial heart cell aggregates. Experiment and theory.
The influence of brief duration current pulses on the spontaneous electrical activity of embryonic chick atrial heart cell aggregates was investigated experimentally and theoretically. A pulse could either delay or advance the time of the action potential subsequent to the pulse depending upon the time in the control cycle at which it was applied. The perturbed cycle length throughout the transition from delay to advance was a continuous function of the time of the pulse for small pulse amplitudes, but was discontinuous for larger pulse amplitudes. Similar results were obtained using a model of the ionic currents which underlie spontaneous activity in these preparations. The primary ion current components which contribute to phase resetting are the fast inward sodium ion current, INa, and the primary, potassium ion repolarization current, IX1. The origin of the discontinuity in phase resetting of the model can be elucidated by a detailed examination of the current-voltage trajectories in the region of the phase response curve where the discontinuity occurs.
Action Potentials↗
Effects of tetrodotoxin on heart cell aggregates. Phase resetting and annihilation of activity.
The influence of relatively low concentrations of tetrodotoxin (TTX) on phase resetting of spontaneous activity of embryonic chick atrial heart cell aggregates by brief duration current pulses was investigated experimentally and theoretically. The maximal upstroke velocity, Vmax, of the spontaneous action potential was reduced by TTX in a concentration-dependent manner for [TTX] less than 10(-7) M. However, the beat rate was unaffected in this concentration range. Application of a depolarizing current pulse of brief duration during a critical region of the spontaneous cycle annihilated activity in some preparations exposed to [TTX] approximately 10(-7) M. These results were analyzed with the model of electrical activity described in the previous paper (Clay, J.R., R.M. Brochu, and A. Shrier. 1990. Biophys. J. 58:609-621) based on a tonic block of the INa channel by TTX with a dissociation constant, KD, of 50 nM.
Action Potentials↗
NASPE young investigator awardee-1990. Complex rhythms resulting from overdrive suppression in electrically stimulated heart cell aggregates.
Spontaneously beating embryonic chick heart cell aggregates were stimulated with current pulses delivered either as periodic trains, or at a fixed delay after each action potential. Following stimulation at fixed rates faster than the intrinsic rate, there was a transient slowing of the spontaneous rhythm. This response, called overdrive suppression, can lead to a complex evolution of rhythms. During periodic stimulation there is a continuum of dropped beat patterns, and during fixed delay stimulation bursting activity appears. This study provides a conceptual basis for understanding analogous rhythms in the intact heart.
Action Potentials↗