Action potential and contraction of heart muscle

Anionic Conjugated Polyelectrolytes for FRET-based Imaging of Cellular Membrane Potential

We report a Förster resonance energy transfer (FRET)-based imaging ensemble for the visualization of membrane potential in living cells. A water-soluble poly(fluorene-cophenylene) conjugated polyelectrolyte (FsPFc10) serves as a FRET donor to a voltage-sensitive dye acceptor (FluoVolt^™ ). We observe FRET between FsPFc10 and FluoVolt^™ , where the enhancement in FRET-sensitized emission from FluoVolt^™ is measured at various donor/acceptor ratios. At a donor/acceptor ratio of 1, the excitation of FluoVolt^™ in a FRET configuration results in a three-fold enhancement in its fluorescence emission (compared to when it is excited directly). FsPFc10 efficiently labels the plasma membrane of HEK 293T/17 cells and remains resident with minimal cellular internalization for ~ 1.5 h. The successful plasma membrane-associated colabeling of the cells with the FsPFc10-FluoVolt^™ donor-acceptor pair is confirmed by dual-channel confocal imaging. Importantly, cells labeled with FsPFc10 show excellent cellular viability with no adverse effect on cell membrane depolarization. During depolarization of membrane potential, HEK 293T/17 cells labeled with the donor-acceptor FRET pair exhibit a greater fluorescence response in FluoVolt^™ emission relative to when FluoVolt^™ is used as the sole imaging probe. These results demonstrate the conjugated polyelectrolyte to be a new class of membrane labeling fluorophore for use in voltage sensing schemes.

Dynamic changes of myocardial oxygen consumption at pacing increased heart rate - the first observation by the continuous measurement of systemic oxygen consumption

OBJECTIVES

To assess dynamic changes in myocardial oxygen consumption (myoVO(2)) during atrial pacing increased heart rate by continuous measurement of systemic oxygen consumption (sysVO(2)).

METHODS

Six mechanically ventilated pigs were atrially paced to increase heart rate from baseline 98 ± 9 to 120-140-160-180 bpm for 10 minutes at each stage, with 10 minute intervals without pacing between stages. sysVO(2) was continuously measured with a respiratory mass spectrometer. Left anterior descending coronary arterial flow, aorta and coronary sinus blood gases were measured to calculate index of whole heart myoVO(2).

RESULTS

sysVO(2) peaked at the initiation of pacing in the first two to three minutes, followed by a decrease and subsequent stabilization. As heart rate increased, sysVO(2) increased by 0.08 ± 0.06 ml/kg/min, 0.14 ± 0.05 ml/kg/min and 0.17 ± 0.10 ml/kg/min, representing a 1.2 ± 0.9%, 2.1 ± 0.7% and 3.0 ± 1.8% increase of sysVO(2) respectively; myoVO(2) increased by 0.16 ± 0.12 to 0.31 ± 0.14 to 0.36 ± 0.24 ml/100 g/min, representing a 11 ± 9%, 21 ± 9% and 26 ± 12% increase of myoVO(2), respectively. The absolute and relative increases in sysVO(2) were significantly correlated with the increases in myoVO(2).

CONCLUSIONS

On-line continuous sysVO(2) monitoring by respiratory mass spectrometry allows non-invasive assessments of dynamic changes in myoVO(2) in vivo. The mechanism for the peaked increase in sysVO(2) at the initiation of pacing remains to be explored.

Induction of contracture and extracellular Ca2+ influx in cardiac muscle by sanguinarine: a study on cardiotoxicity of sanguinarine

In this study, the toxic effect of sanguinarine (SANG) on heart was studied with isolated cardiac muscle strip isolated from Wistar rat. SANG induced positive inotropic action followed by contracture on the left ventricle and both atria strips. In addition, SANG dose-dependently inhibited spontaneous beat of the right atrium. SANG-induced contracture was completely suppressed by pretreatment with La3+ or in a Ca2+ free Tyrode solution containing 2.5 mM EGTA. Incubating isolated cardiomyocytes with SANG enhanced the 45Ca2+ influx, which could be inhibited by pretreatment with La3+. However, the SANG-induced 45Ca2+ influx could not be inhibited by pretreatment with other Ca2+ channel blockers, such as nifedipine, verapamil, diltiazem, nickel and manganese, and amiloride. Although antioxidants can inhibit the SANG-induced lipid peroxidation, they could not prevent the SANG-induced contracture. N-acetylcysteine and dithiothreitol, the sulfhydryl reducing agents, were shown to be effective in preventing the SANG-induced contracture. These data suggested that the SANG-induced contracture is caused by the influx of extracellular Ca2+ through a La3+-sensitive Ca2+ channel.

Electrical activity and contraction in cells isolated from rat and guinea-pig ventricular muscle: a comparative study

1. Contraction in single ventricular muscle cells from rat and guinea-pig heart was measured using an optical technique, while at the same time either action potentials were recorded or transmembrane currents were measured under voltage-clamp conditions. 2. When the membrane was depolarized to 0 mV, there was a phasic and a tonic component of the contraction in guinea-pig cells, whereas in rat cells only the phasic component was obvious. In both species the depolarizations evoked the second inward current (Isi). 3. In rat cells, when the membrane potential during a depolarization was varied over the range -40 to +60 mV, the amplitude of contraction first increased to a peak at a potential close to 0 mV, and then declined as the membrane potential became more positive. In contrast, contraction in guinea-pig cells measured under similar conditions continued to increase as the depolarization was increased, and the tonic component of contraction became more obvious at more positive potentials. Contraction amplitude in guinea-pig cells could also be increased by increasing pulse duration under conditions where the tonic component of contraction was prominent. 4. Contraction during depolarization was suppressed by ryanodine in rat cells, whereas in guinea-pig cells contraction persisted, but with a modified time course. Ryanodine did inhibit spontaneous contractions of guinea-pig cells during exposure to low extracellular sodium. 5. Nifedipine suppressed Isi and phasic contraction in both rat and guinea-pig cells. In guinea-pig cells these effects developed contemporaneously, but in rat cells substantial reduction of Isi occurred before marked suppression of contraction. 6. In rat cells exposed to strontium in place of external calcium, inactivation of Isi was slowed and contraction was prolonged, with a slower time-to-peak and relaxation. The time course of the action potential was modified and ryanodine no longer inhibited contraction of rat cells in the presence of strontium. 7. It is concluded that the amplitude of contraction in rat and guinea-pig ventricular cells is determined by calcium both entering through the surface membrane and released from internal stores, and that under normal conditions the balance is towards release from stores in rat cells, and towards entry through the surface in guinea-pig cells.

Electrical and mechanical restitution of the human heart at different rates of stimulation

Action potential duration and contractility are reduced following premature excitations, and gradually increase as the stimulus interval is lengthened. To examine these phenomena of electrical and mechanical restitution in the human heart, we simultaneously measured action potential duration and the maximum rate of left ventricular pressure in five patients undergoing electrophysiological study. Test beats were introduced at varying intervals after the last of a series of steady state intervals. By plotting action potential duration and maximum rate of left ventricular pressure as a function of the test interval, we formed electrical and mechanical restitution curves. When the rate of steady state pacing was increased, there was a decrease in action potential duration and an increase in the maximum rate of left ventricular pressure for all test intervals; i.e., a change in pacing rate affected action potential duration and maximum rate of left ventricular pressure of test responses in a reciprocal fashion. In addition, a higher steady state pacing rate allowed action potentials and contractile responses to be elicited at shorter test intervals, thereby displacing the electrical and mechanical restitution curves to the left. The magnitude of the leftward shift of both curves corresponded closely to the shortening of the steady state action potential duration induced by the increase in pacing rate. These findings confirm for the human heart that both electrical and mechanical restitution occur after membrane repolarization, i.e., as a function of the electrical diastolic interval preceding a beat, and not the stimulus interval.

The use of a microprocessor in routine cardiac assessment

Increasing use is being made of microprocessors in medicine to perform routine clinical measurements. Physiological parameters whose measurement involves data processing on the part of hospital technicians can in many cases be better obtained using such instrumentation. The application of a microprocessor to the measurement of ST-segment elevations in the electrocardiogram is described to illustrate the use of programmable instrumentation in medicine and demonstrate some of the ensuing benefits.

Role of calcium ions in transient inward currents and aftercontractions induced by strophanthidin in cardiac Purkinje fibres

1. Under the influence of strophantidin, Purkinje fibres exhibit transient inward current (TI) which contributes to arrhythmogenic activity. Voltage-clamp experiments were carried out to study the role of Ca ions in this phenomenon. 2. The amplitude of TI varied directly with the extracellular Ca concentration, CaO. Magnesium ions had an antagonistic effect. 3. TI was closely associated with a phasic increase in force ("aftercontraction"). Like TI, the aftercontraction was evoked by a preceding action potential or by the break of a strong depolarizing pulse. 4. TI and the aftercontraction displayed similar wave forms although peak current preceded peak force by 50--100 msec. Both transients were enhanced by increasing the strength or duration of the preceding depolarization pulse. Both events were slowed as the potential level following the pulse was displaced in the negative direction. 5. TI and the aftercontraction could be evoked in the absence of cardiotonic steroids by strongly elevating CaO. 6. Additional experiments were carried out to test the hypothesis the TI reflects an influx of Ca2+ ions. Moninhibited TI but the developed and removal of the inbibition lagged far behind the effects on the slow inward current. 7. TI could be suppressed and eventually inverted by varying the membrane potential in the positive direction. The inversion potential averaged -5mV and was not consistent with a Ca-specific pathway. The aftercontraction was more closely related to the phasic conductance change underlying the current than to thecurrent flow itself. 8. The results are consistent with the idea that an oscillatory release of Ca from an intracellular store is the primary event underlying both the aftercontraction and the conductance change which generates TI. Digitalis intoxication or very high CaO may promote such events by elevating intracellular Ca levels.

The canine heart as an electrocardiographic generator. Dependence on cardiac cell orientation

Traditionally it is assumed that during cardiac depolarization the macroscopic current generators that produce electrocardiographic voltages can be represented as a uniform double-layer source, coincident with the macroscopic boundary between resting and depolarized cardiac fibers as measured with extracellular electrodes ("uniform" hypothesis). A segment of this boundary is thus considered as a current dipole oriented perpendicular to the boundary. We present evidence that, contrary to the above, the effective dipoles largely parallel the long axes of cardiac fibers ("axial" hypothesis). Calculated potentials in volume conductors differ markedly in the two cases. The magnitudes of rapid local "intrinsic" deflections also differ markedly. In our experiments, potential fields prodlced by stimulation at several cardiac sites and measured magnitudes of intrinsic deflections during normal depolarization and that caused by stimulation support the axial hypothesis and are incompatible with the uniform hypothesis. Our results suggest that axial orientation of sources is sufficiently strong so that predictions assuming the uniform hypothesis would be seriously in error, although the axial theory alone does not exactly describe all the measured potentials. Axial orientation of current generators must be considered in quantitative prediction of electrocardiographic potentials. tfurther study of the geometry of the intracellular depolarization boundary and its relation to fiber direction and to the frequency of lateral intercellular junctions is required to describe the generators exactly.

Cardiac muscle: excitability and passive electrical properties

The field of cellular cardiac electrophysiology has made excellent progress in the last decade in its effort to understand the electrical properties of the heart. It has profited from progress in membrane electrophysiology that has increased our understanding of the basic ionic mechanisms. It has developed quantitative methods for study of these mechanisms, in spite of the geometrical complexity of the heart. Indeed this complexity has added a richness and challenge to this area. We have shared with skeletal muscle the problem of electromechanical coupling. This review has discussed the present state of our knowledge of basic membrane mechanisms of importance in understanding normal and pathological function of heart muscle. It is intended to lay the groundwork for the following articles dealing with special aspects of cardiac electrophysiology. There is a continual interchange of ideas and concepts between applied and basic electrophysiology, and, as a result of this interchange, we can expect both areas to grow greatly in the coming years.

Properties of visual cells in the lateral eye of Limulus in situ: intracellular recordings

Two types of potential fluctuations, large and small, recorded intracellularly from photoreceptors in the dark-adapted Limulus eye in situ underlie the dual properties of the impulse discharge of the optic nerve fibers. The small potential fluctuations (SPFs)--the well-known quantum bumps--were normally less than 20 mV in amplitude. The large potential fluctuations (LPFs) were up to 80 mV in amplitude. LPFs appear to be regenerative events triggered by SPFs that enable single photon absorptions in retinular cells to fire off nerve impulses in the eccentric cell. In the dark, SPFs and LPFs occur spontaneously. At low light intensities, LPFs are the major components of the receptor potential. At high intensities, LPFs are suppressed and SPFs become the major components. SPFs and LPFs together enable single photoreceptor cells to encode approximately a 9-log unit range of light intensity. Excising the eye from the animal or cutting off its blood supply generally abolishes LPFs and thereby reduces the range of light intensity coded in the optic nerve discharge.

The effect of temperature on potassium chloride contracture in cat myocardium

1. Contracture was induced in cat myocardium by exposure to 140 mM-KC1 In isotonic Tyrode solution. Force of contracture expressed as mg/mm2 (muscle cross-sectional area) falls with increasing cross-sectional area. 2. The effect of temperature on isometric force developed during contracture was evaluated both in normal (untreated) atrial and ventricular muscle and following treatment with sympatholytic drugs. 3. The force of contracture was not significantly affected by sympatholytic drugs at 36 degrees C. 4. In normal atrial and ventricular muscle, force of contracture decreased when the muscle was cooled from 36 to either 29 or 20 degrees C. 5. In atrial muscle, the effect of temperature was not changed by sympatholytic drugs. In contrast, exposure to sympatholytic drugs increased contracture force developed by ventricular muscle at 20 degrees C. Also, contracture force was significantly greater at 20 than at 36 degrees C in ventricular muscle from reserpine-pretreated cats. 6. It is suggested that ventricular muscle becomes more sensitive to the relaxing effects of endogenous catecholamines at temperature is lowered. 7. The differences shown between atrial and ventricular muscle with respect to the effect of temperature and sympatholytic drugs on contracture force may result from the differing amounts of sarcoplasmic reticulum found in these types of cardiac muscle and also from different mechanisms of "excitation-contracture" coupling in atrial and ventricular muscle.

Electrophysiologic properties of antidysrhythmic drugs as a rational basis for therapy

The often emergent nature of life-threatening cardiac dysrhythmias, the frequent seeming "resistance" of the abnormal heart rhythm to therapy, and the commonly encountered toxicity of antidysrhythmic agents combine to make treatment of cardiac dysrhythmias one of the strictest challenges to the practicing physician. Although electrophysiologic studies have markedly increased out understanding of dysrhythmogenesis and the actions of anti-dysrhythmic drugs, these numerous investigations have provided but little assistance to the practicing physician either as an intellectual framework or as a guide to patient care. The electrophysiologic classification of the antidysrhythmic drugs presented in this paper should be acceptable both to the electrophysiologist and the clinician since it is based on alterations in basic membrane properties and correlates well with clinical realities. It serves as a guide to initial drug selection, anticipated bioelectric complications, the use of alternative drugs, and combination antidysrhythmic therapy.

Reconstruction of the electrical activity of cardiac Purkinje fibres

1. The electrical activity of Cardiac Purkinje fibres was reconstructed using a mathematical model of the membrane current. The individual components of ionic curent were described by equations which wee based as closely as possible on previous experiments using the voltage clamp technique. 2. Membrane action potentials and pace-maker activity were calculated and compared with time course of underlying changes in two functionally distinct outeard currents, iX1 and iK2. 3. The repolarization of the theoretical action potential is triggered by the onset of iX1, which becomes activated over the plateau range of potentials. iK2 also activates during the plateau but does not play a controlling role in the repolarization. Hwever, iK2 does govern the slow pace-maker depolarization through its subsequent deactivation at negative potentials. 4. The individual phases of the calculated action potential and their 'experimental' modifications were compared with published records. The upstroke is generated by a Hodgkin-Huxley type sodium conductance (gNa), and rises with a maximum rate of 478 V/sec, somewhat less than experimentally observed values ( up to 800 V/sec). The discrepancy is discussed in relation to experimental attempts at measuring gNa. 5. The ole of the transient outward chloride current (called igr) was studied in calculations of the rapid phase of repolarization and 'notch' configuration...

Relationships between voltage and tension in sheep cardiac Purkinje fibers

The two-microelectrode technique of voltage clamping sheep cardiac Purkinje fibers was used to examine the changes in contraction which occur during trains of voltage clamps. (A "train" is defined as a series of voltage clamps delivered at a particular rate, beginning after a rest long enough that the effects of previous stimulation have died away.) Contractions showed striking staircases, or progressive changes in peak isometric tension, during trains. Short clamps, clamps to voltages more negative than --20 or --30 mV, or holding potentials less negative than the resting potential favored negative staircases, while long clamps, clamps to positive voltages, and holding potentials near the resting potential each favored positive staircases. The staircase behavior appeared to be due to changes in the initial rate of recovery of the ability to contract. The changes in staircase behavior as a function of clamp voltage suggested that the relationship between peak tension and clamp voltage should depend on the experimental design. When the steady-state contraction was plotted as a function of clamp voltage, voltage-tension relations like those recently reported for working ventricle were obtained, with a threshold between --30 and --40 mV and a steep relation between tension and voltage. When the first contraction after a rest was plotted, the threshold voltage was more negative, the curve was flatter, and the peak tensions at inside positive voltages were reduced.

Cyclic Adenosine Monophosphate Modulation of Slow Calcium Influx Channels in Guinea Pig Hearts

The relationship between cellular levels of cyclic adenosine monophosphate (AMP) and slow inward calcium ion (Ca 2+ ) current was studied in isolated perfused guinea pig hearts in which fast sodium ion (Na + ) channels had been inactivated by depolarization with potassium ions (K + ) or blockade with tetrodotoxin. Perfusion with 22 mM K + depolarized cardiac tissue to approximately -40 mv and rendered hearts inexcitable. Tetrodotoxin (3 x 10 -5 M) blocked excitability without altering resting membrane potential. Excitability and contractions could be restored to these hearts with a variety of inotropic agents that also raised the measured tissue levels of cyclic AMP or with high concentrations of Ca 2+ . The magnitude of steady-state tension developed by restored hearts was directly related to the external Ca 2+ concentration as well as to the concentration of the restoring agent used. The tension of restored hearts was markedly reduced by Ca 2+ -channel antagonists. Elevation of cyclic AMP levels preceded restoration of excitability to inactivated hearts. A highly significant positive correlation was found between the magnitude of Ca 2+ -dependent tension developed by restored hearts and the level of cyclic AMP in those hearts. Glucagon and ouabain, inotropic drugs that do not elevate myocardial levels of cyclic AMP, failed to restore depolarized or tetrodotoxin-blocked hearts. Therefore, cyclic AMP appears to modulate slow Ca 2+ influx channels in myocardial cells.

Calcium ion release in mechanically disrupted heart cells

In cardiac muscle fibers which have had their sarcolemma disrupted intracellular stores of calcium ions can be released by the same chemical stimuli which cause their release from the sarcoplasmic reticulum of skinned skeletal muscle fibers. These stimuli are increases in calcium or caffeine concentrations and substitution of chloride for propionate or sodium for potassium in solutions bathing the fibers.

Effects of agents which inhibit the slow channel on sinus node automaticity and atrioventricular conduction in the dog

A slow ionic current carried by calcium, sodium, or both constitutes transmembrane ionic flow through the slow channel; such a current may be involved in normal action potentials of sinus and atrioventricular (AV) nodal cells. In this study, we investigated the effects of the slow-channel inhibiting agents verapamil, D600, manganous chloride, and lanthanum chloride on sinus node automaticity and AV nodal conduction in open-chest dogs treated with atropine (0.5 mg/kg) and propranolol (1.0 mg/kg). The arteries to the sinus node and the AV node were cannulated and perfused with agents that inhibit the slow current. These agents slowed sinus node discharge rate, depressed AV nodal conduction, and lengthened the effective and the functional AV nodal refractory period. Effects were dose related and reversed with time. His-Purkinje conduction remained normal. Isoproterenol and epinephrine reversed the effects of slow-channel inhibiting agents, but calcium, sodium, glucagon, and phenylephrine did not. Concentrations of propranolol which produced beta-receptor blockade prevented Isoproterenol-induced reversal of the effects of slow-channel inhibitors. We concluded that (1) agents which inhibit the slow channel directly depress sinus node discharge rate and AV nodal conduction, (2) effects of slow-channel inhibiting agents are not mediated through the activation of cholinergic discharge or inhibition of adrenergic discharge, and (3) beta-receptor stimulation reverses these effects.