Pacemaker Depolarization Research Articles

Effects of forskolin on intracellular sodium activity in resting and stimulated cardiac Purkinje fibers from sheep

In the present investigation, the effects of forskolin on intracellular sodium activity were studied in quiescent and electrically stimulated cardiac Purkinje fibers from sheep using Na+-sensitive microelectrodes. Also assessed, were the effects of this promoter of cytosolic cAMP production on resting membrane potential, action potential and twitch tension. In the quiescent fibers, forskolin (12 microM) caused intracellular sodium activity to decrease in the face of cellular depolarization. This cellular depolarization was occasionally accompanied by spontaneous firing of action potentials. In the stimulated fibers, forskolin (10 microM) also caused intracellular sodium activity to decrease. Moreover, it caused a marked acceleration of phase 4 pacemaker depolarization, an elevation of the plateau of the action potential and an increase in twitch tension. When the Na+ pump was inhibited by either strophanthidin (1 microM) or by 0 mM extracellular K+, forskolin had no effect on intracellular sodium activity. In summary, the results of the present study indicate that forskolin, presumably by increasing intracellular cAMP, causes the following to occur in cardiac Purkinje fibers from sheep: (a) a decrease in intracellular sodium activity when the Na+ pump is functioning normally; (b) a promotion of membrane depolarization in quiescent fibers; (c) an increase in the steepness of the pacemaker potential in electrically stimulated fibers, and (d) an increase in the force of contraction. Therefore, forskolin will be a useful tool for investigating the role of cAMP in physiological function of cardiac cells.

Abnormal pacemaking is modulated by sarcoplasmic reticulum in partially-depolarized myocardium from dilated right atria in humans

Fifty human atrial specimens removed at time of cardiac surgery were studied in vitro. Thirty-four samples were selected as presenting partial cell depolarization and exhibiting slow response action potentials. Twenty of these preparations were automatic whereas 14 were not. Neither mean maximum diastolic potential (-52.8 +/- 1.3 mV and -49.3 +/- 2.2 mV respectively) nor maximum rate of depolarization (Vmax) (1.1 +/- 0.1 V/s and 1.3 +/- 0.8 V/s) significantly differed between these two groups. Abnormal automaticity due to phase 4 depolarization occurred in 12/13 preparations dissected from markedly dilated atria whereas it occurred in only 2/10 preparations sampled from non-dilated atria. A statistically significant relationship between in vitro abnormal pacemaking and atrial dilatations was found. We investigated the effects on abnormal pacemaker depolarization and automaticity of a reduction in the extracellular Na and Ca and of sarcoplasmic reticulum (SR) inhibitors. Abnormal pacemaker depolarization appeared to be much more sensitive to a reduction in the extracellular Na than in the extracellular Ca. Both Sr and Mg slowed the automatic rate. Ryanodine 3 X 10(-6) M, a specific SR inhibitor, irreversibly lengthened the spontaneous basic cycle duration to about 300% of control. Epinephrine up to 10(-4) M was ineffective in accelerating the residual spontaneous rhythm that persists after ryanodine action, although epinephrine markedly enhanced the overshoot and Vmax of the slow responses. It is concluded that, in the human atrial myocardium, abnormal pacemaking that develops at low level of membrane potential: is promoted by chamber dilatation; is strongly modulated by SR-dependent processes.

Membrane currents linked to barium-induced pacemaker activity in the bullfrog atrium.

Ionic mechanisms underlying Ba2+-induced automaticity in the bullfrog atrial muscle were studied by performing current and voltage clamp experiments using the double sucrose-gap technique. The application of Ba2+ of more than 0.01 mM induced membrane depolarization, and its amplitude became larger with increasing [Ba2+]o. Pacemaker activity was initiated above 0.3 mM Ba2+. Voltage clamp experiments revealed a marked decrease in the membrane conductance during hyperpolarizing steps, suggesting a depression of anomalous rectifier (Ikl). The time course of the outward current tail was fitted with a sum of fast (tau not equal to 0.8 s) and slow (tau not equal to 4-5 s) exponential components. The slow component (Ia) was markedly depressed by Ba2+, but the fast component (Ixs) remained. There was no significant change in the slow inward current (Isi). The hyperpolarization-activated current had an activation threshold of about -90 mV and was depressed by Ba2+ at concentrations higher than 1 mM. The pacemaker activity was suppressed by depleting either Na+ or Ca2+ in the bath solutions. The activation of the Isi might be responsible for both the later phase of the pacemaker depolarization and the rapid rising of the spontaneous action potential during low-voltage oscillation; the sodium current might take a similar role in the high-voltage oscillation. The Ixs may play the major role in generating the diastolic depolarization. The suppression of the Ikl and Ia by Ba2+ may provide a low background K conductance, which is essential for the initiation of the pacemaker activity.

Action potentials and net membrane currents of isolated smooth muscle cells (urinary bladder of the guinea-pig)

Cells were isolated by incubating chunks of tissue from the urinary bladder of the guinea-pig in a high potassium, low chloride medium containing 0.2 mM calcium plus the enzymes collagenase and pronase. After isolation, the cells were superfused with a physiological salt solution (PSS) containing 150 mM NaCl, 3.6 mM CaCl2 and 5.4 mM KCl (35 degrees C). Patch electrodes filled with an isotonic KCl-solution were used for whole cell recordings. With a single electrode voltage clamp we measured a capacitance of 50 +/- 5 pF per cell, an input resistance of 200 +/- 25 kOhm X cm2 and a series resistance of 44 +/- 4 Ohm X cm2. The cells had resting potentials of -52 +/- 2 mV. They did not beat spontaneously but responded to stimuli with single action potentials (APs) which rose from the threshold (-38 mV) with a maximal rate of 6.5 +/- 1.8 V/s to an overshoot of 22 +/- 3 mV. The AP lasted for 36 +/- 4 ms (measured between threshold and -40 mV). Continuous cathodal current produced repetitive activity, a pacemaker depolarization followed the AP and preceded the next upstroke. Net membrane currents evoked by clamp steps to positive potentials were composed of an inward and an outward component. The inward component generating the upstroke of the AP was carried by Ca ions (iCa, Klöckner and Isenberg 1985). The repolarization resulted from a potassium outward current iK. Ca-channel blockers (5 mM NiCl2) reduced iK suggesting that (part of) iK was Ca-activated. iK rose within about 100 ms to a peak of 40-200 muA/cm2 from which it inactivated slowly and incompletely. The inactivating iK followed a bell-shaped voltage-dependence, the noninactivating iK an outwardly rectifying one. Both parts had similar steady state inactivation curves with a half maximal inactivation potential at -36 mV and a slope of 9 mV. Repolarization to -50 mV induced outward tail currents which reversed polarity at -85 mV (the calculated potassium equilibrium potential). The amplitude and the time course of the envelope of the tail currents varied in proportion to iK during the prestep. Thus, the tail current is suggested to reflect the turning off of a potassium conductance which had been activated during the prepulse. iK was largely reduced but not blocked by 20 or 150 mM tetraethylammonium (TEA). TEA did not significantly change the resting potential, but it prolonged the AP and facilitated upstroke and overshoot.(ABSTRACT TRUNCATED AT 400 WORDS)

A model of cardiac electrical activity incorporating ionic pumps and concentration changes

A model of cardiac electrical activity incorporating ionic pumps and concentration changes

Electrophysiological investigation of the antennal heart of Periplaneta americana and its reactions to proctolin

Some physiological parameters of the antennal heart, an accessory circulatory organ in the head of Periplaneta americana and the effect of the neuropeptide proctolin on it were investigated. The beat frequency of the antennal heart in vivo or semiisolated is about 2–3 times slower than that of the dorsal vessel and not coordinated with the latter. The extracellular ECG of the antennal heart has a simple biphasic shape with a total duration of 588.7 ± 38.2 ms. Intracellularly recorded parameters showed characteristics typical of myogenic rhythmicity: a slow depolarization with a rate of rise of 7.5 ± 0.7 mV/s, followed by an action potential of 54.9 ± 1.2 mV with a relatively long duration of 201.6 ± 10.8 ms, absence of overshoots and resistance to TTX. Proctolin produced a marked enhancement of the frequency of beat of the antennal heart up to about 400% with a high sensitivity (threshold concentrations: 5·10 −9M). The dose-response curve shows a linear relationship between the logarithm of the concentration and the percentage increase in beat frequency. The electrical event most influenced by proctolin was the slow pacemaker depolarization, whose rate of rise was enhanced up to 240%. The action potential remained unchanged; the depolarization of the resting potential was very small and the input resistance did not change. The antennal heart responds to neurohormone D, another neuropeptide in insects, in a similar way as it does to proctolin. The mode of action of proctolin on the antennal heart is discussed in comparison to that found in other systems.

A mechanism for production of phase shifts in a pattern generator.

During motor activity of the pyloric system of the lobster stomatogastric ganglion, there are rhythmic alternations between activity in the pyloric dilator (PD) and pyloric (PY) motor neurons. We studied the phase relations between PD motor neuron activity and PY motor neuron activity in preparations cycling at a wide range of frequencies and after altering the activity of the PD neurons. The PY neurons fall into two classes, early (PE) and late (PL) (21), distinguished by the different phases in the pyloric cycle at which they fire. The phase at which PE neurons fired and the phase at which PL neurons fired was independent of pyloric cycle frequency over a range of frequencies from 0.5 to 2.25 Hz. The anterior burster (AB) interneuron is electrically coupled to the PD motor neurons. Together the AB and PD neurons form the pacemaker for the pyloric system. Synchronous depolarization of the AB and PD neurons evokes a complex inhibitory post-synaptic potential (IPSP) in PY neurons. This IPSP has two components: an early, AB neuron-derived component and a late, PD neuron-derived component. Deletion of the PD neurons from the pyloric circuit by photoinactivation removed the PD-evoked component of the pacemaker-evoked IPSP. This resulted in a decrease in the duration of the IPSP evoked by pacemaker depolarization and a significant advance in the firing phase of PY neurons. Bath application of dopamine was used to hyperpolarize and inhibit the PD neurons (30), causing them to release less neurotransmitter. As a consequence, the duration of the IPSP evoked by pacemaker depolarization was decreased and the firing phase of the PY neurons was significantly advanced. Stimulation of the inferior ventricular nerve (IVN) produces a slow excitation of the PD neurons (30), causing them to release more neurotransmitter. Consequently, the duration of the IPSP evoked by pacemaker depolarization was increased and the firing phase of the PY neurons was significantly retarded for several cycles of pyloric activity following IVN stimulation. Thus, modulation of the strength of PD-evoked inhibition in PY neurons is responsible for altering the firing phase of the PY neurons with respect to the pyloric pacemaker. We suggest that frequency of the pyloric output and the phase relations of the elements within the pyloric cycle can be regulated independently. The potential implications of these data for modulation of synaptic efficacy in other preparations are discussed.

Does the "pacemaker current" generate the diastolic depolarization in the rabbit SA node cells?

Small preparations of spontaneously beating rabbit sino-atrial node (SA node) were voltage clamped with the two-microelectrode technique. The effects of 0.25-5 mM Cs+ on the spontaneous pacing rate and the time-dependent inward "pacemaker" current, ih, were studied. In the presence of 2 mM Cs+, the spontaneous pacing rate decreased only slightly even though ih was strongly depressed at potentials negative to -60 mV Cs+ had little or no effect on other time-dependent currents observed with clamp pulses less negative than -50 mV. Since no voltage-dependence to the Cs+ effect on ih could be measured (between -90 mV and -20 mV), it was considered unlikely that the lack of Cs+ effect on the rate of diastolic depolarization results from a voltage-dependent effect of Cs+ on the ih channel. Adrenaline produced a marked positive chronotropic effect in Cs+-treated SA node cells. This effect was accompanied by marked enhancement of the slow inward current (isi) with no change in the Cs+-blocked ih current. These results are consistent with the idea that ih plays a minor role in generation of pacemaker depolarization, and suggest a more prominent role of isi in the generation of diastolic depolarization in SA nodal cells.

Effects of nystatin-mediated intracellular ion substitution on membrane currents in calf purkinje fibres.

1. Calf cardiac Purkinje fibres were exposed briefly to the ionophore nystatin to promote exchange of caesium for intracellular potassium. The effects of Cs loading were stable for at least 30 min, but they could be reversed by nystatin-mediated K loading.2. After Cs loading, the resting potential shifted to about -20 mV and the current-voltage relationship showed a strong inhibition of inwardly rectifying K channels.3. Anodal break stimulation evoked normal action potential upstrokes and twitch contractions. The early repolarization (phase 1) was markedly slowed.4. Cs loading simplified the pattern of current changes evoked by step depolarizations over the plateau range. Membrane current reached an inward peak and then declined monotonically.5. The current signal showed no hint of the transient outward current found in untreated or K-loaded preparations. Furthermore, Cs loading abolished the outward tails associated with deactivation of transient outward current, and occluded the blocking effect of the K-channel inhibitor 4-aminopyridine.6. Inhibition of transient outward current revealed a maximal inward current of about 5 muA/muF in 5.4 mM-Ca(o), which is considerably larger than the net inward current without Cs loading.7. The inward current was attributed to Ca channels on the basis of its sensitivity to membrane potential, extracellular Ca, D600, Mn and Cd.8. Cs loading also reduced slow current changes associated with delayed rectification and pace-maker depolarization.9. The results support the hypothesis that the transient outward current is carried by K(+) ions, while providing a method for unmasking inward Ca current.

Analysis of subthreshold pace-maker currents in chick embryonic heart cells.

1. Small re-aggregates of cells dissociated from the ventricles of 7-day-old chick embryonic hearts beat spontaneously in low external potassium concentration (Ko = 1.3 mM) tissue culture medium. This activity was blocked by the addition of tetrodotoxin (TTX) or potassium ions to the external medium. 2. A two-micro-electrode voltage-clamp technique was used to analyse the subthreshold currents responsible for the pace-maker depolarization. 3. Voltage-clamp steps 6-10 sec in duration revealed a time-dependent current having first order kinetics. Its membrane potential range of steady-state activation was -90 to -60 mV. 4. The current kinetics were qualitatively similar to those of Hodgkin & Huxley (1952b) with a peak time constant of approximately 1 sec at V = -75 mV. The kinetics were independent of Ko. 5. The time-dependent current was attributed to gated membrane channels. The fully activated current-voltage (I-V) relation of the channels was determined from the ratio of the amplitudes of the time-dependent currents during and after voltage-clamp steps following the procedure of Noble & Tsien (1968). 6. The fully activated I-V relation displayed inward rectification with negative slope conductance at potentials more than 15 mV positive to its reversal potential. Changes of Ko shifted the I-V curve along the voltage axis like a potassium electrode. 7. The time-independent (background) current was obtained by subtracting the gated channel current from the steady-state I-V curve. This current also rectified in the inward direction. 8. The inwardly rectifying I-V relations were theoretically described by a channel having a row of ion-selective sites along which ions move in a single file (Hodgkin & Keynes, 1955), and a membrane-bound particle which blocked the channel in a voltage-dependent manner. 9. The relationship of the voltage-clamp results to spontaneous activity is discussed and comparisons are made with measurements from whole embryonic heart and other cardiac tissues.

Potassium current during the pacemaker depolarization in rabbit sinoatrial node cell.

Voltage clamp experiments were carried out on the rabbit sinoatrial (S-A) node. The delayed outward current in the voltage range between --60 mV and --22 mV almost disappeared in the presence of 5 mM Ba2+. The slow inward current and the hyperpolarization-activated current remained unaffected. In the absence of the time-dependent potassium current the S-A node cell generated spontaneous action potentials, provided that the membrane was hyperpolarized by constant outward current. Therefore it seems unlikely that the potassium current plays an essential role in generating the pacemaker potential in the S-A node. The time course of the potassium current (iK) during the cardiac cycle was calculated using equations simulating the kinetics of iK. According to this computation, the change of iK in the S-A node is small during pacemaker depolarization. It is proposed that the gradual decay of potassium conductance is less important for the development of the pacemaker potential than the contribution of the slow inward current.

Reconstruction of sino-atrial node pacemaker potential based on the voltage clamp experiments.

The pacemaker activity of the S-A node cell was explained by reconstructing the pacemaker potential using a Hodgkin-Huxley type mathematical model which was based on the reported voltage clamp data. In this model four dynamic currents, the sodium current iNa, the slow inward current, is, the potassium current, iK, and the hyperpolarization-activated current, ih, in addition to a time-dependent leak current, i1 were included. The model simulated the spontaneous action potential the current voltage relationship, and the voltage clamp experiment in normal Tyrode solution of the rabbit S-A node. Furthermore, the changes of activity induced by the potassium current blocker Ba2+, by applying constant current, acetylcholine, and epinephrine were also reconstructed. It was strongly suggested that the pacemaker depolarization in the S-A node cell is mainly due to a gradual increase of iS during diastole, and that the contribution of iK is much less compared to the potassium current iK2 in the Purkinje fiber pacemaker depolarization. The rising phase of the action potential was due to iS and the plateau phase is determined by both the inactivation of iS and activation of iK.

Fluctuations in interbeat interval in rhythmic heart-cell clusters. Role of membrane voltage noise

Small clusters of ventricular cells prepared from 7-d chick heart maintain spontaneous, stationary, rhythmic beating in culture for many hours. For clusters containing I-125 cells, mean interbeat interval (IBI) is 0.45 +/- 0.08 s and is independent of cell number (N), whereas, the coefficient of variation of IBI (C) is proportional to N-1/2. Because membrane voltage noise in such clusters would also be expected to vary as N-1/2, we propose a model relating fluctuation in IBI (sigma IBI) to voltage noise (sigma v). A simplified model consisting of random voltage fluctuations superimposed upon a linear pacemaker depolarization of slope a is used to analyze the N-dependent shape of the IBI histogram. Values of sigma v derived from the relation sigma IBI = sigma v/a, or calculated from the skewness of the measured IBI histograms, both agree well with those extrapolated from steady-state noise recorded from resting heart-cell aggregates.

Cellular and subcellular mechanisms of cardiac pacemaker oscillations.

Rhythmic oscillations in the membrane potential of heart cells are important in normal cardiac pacemaker activity as well as cardiac arrhythmias. Two fundamentally different mechanisms of oscillatory activity can be distinguished at the cellular and subcellular level. The first mechanism, referred to as a surface membrane oscillator, can be represented by a control loop in which membrane potential changes evoke delayed conductance changes and vice versa. Since the surface membrane potential is a key variable within the control loop, the oscillation can be interrupted at any time by holding the membrane potential constant with a voltage clamp. This mode of oscillation seems to describe spontaneous pacemaker activity in the primary cardiac pacemaker (sinoatrial node) as well as other regions (Purkinje fibre, atrial or ventricular muscle). In all tissues studied so far, the pacemaker depolarization is dominated by the slow shutting-off of an outward current, largely carried by potassium ions. The second mechanism can be called an internal oscillator since it depends upon a subcellular rhythm generator which is largely independent from the surface membrane. Under voltage clamp, the existence of the internal oscillation is revealed by the presence of oscillations in membrane conductance or contractile force which occur even though the membrane potential is held fixed. The two oscillatory mechanisms are not mutually exclusive; the subcellular mechanism can be preferentially enhanced in any given cardiac cell by conditions which elevate intracellular calcium. Such conditions include digitalis intoxication, high Cao, low Nao, low or high Ko, cooling, or rapid stimulation. Several lines of evidence suggest that the subcellular mechanism involves oscillatory variations in myoplasmic calcium, probably due to cycles of Ca uptake and release by the sarcoplasmic reticulum. The detailed nature of the Cai oscillator and its interaction with the surface membrane await further investigation.

Spontaneous electrical activity and interaction of large and small cells in cardiac ganglion of the crab, Portunus sanguinolentus

1. Semi-isolated preparations of the nine-celled cardiac ganglion of the crab, Portunus sanguinolentus, were studied electrophysiologically, using simultaneous recording from extracellular and two or three intracellular electrodes. Nine penetrations of small cells were achieved. 2. Three large (80 x 120 micron) cells lie near the anterior end of the 5-mm main trunk; two large and four small (less than 50 micron) cells at the posterior end. Large-cell axons pass along the main trunk and then exit to innervate cardiac muscle; small-cell axons do not leave the ganglion. 3. The semi-isolated ganglion produces spontaneous electrical activity organized into regularly patterned, rhythmic bursts of large- and small-cell impulses recurring at rates of 0.3-0.6/s and lasting 500-800 ms. Small impulse activity commences and ends each burst. Small cells fire trains during the burst, but impulses are not synchronized among them. Large-cell trains are synchronous, are at about one-half the frequency, and have fewer impulses than small-cell trains. 4. Intracellular recordings from small cells show a slow, pacemaker depolarization from a maximum membrane potential of -54 mV leading with only a slight inflection at ca. -50 mV to a depolarized plateau at ca. -40 mV; nonovershooting impulses are superimposed on this but cease before it repolarizes. Impulses, therefore, arise at a site distant from the soma and do not invade it. Deflections suggesting synaptic potentials are not seen. 5. Intracellular recordings from large cells show complex depolarizations corresponding to extracellularly recorded bursts. These represent excitatory postsynaptic potentials (EPSPs) corresponding with individual small-cell impulses, attenuated, non-overshooting spikes, and an underlying slow depolarization; usually no pacemaker depolarization is apparent between bursts. Chemically mediated transmission is probable for the EPSPs because they show delay, increase in amplitude with hyperpolarization, sometimes show facilitation, and are reduced in saline having one-third Ca, 3 x Mg. 6. EPSPs, impulses, and the slow depolarization occur synchronously among the large cells. Potentials recorded from posterior cells are attenuated and slower than those of the anterior cells. This is interpreted to reflect sites of occurrence more distant from the soma in the posterior than in the anterior cells. Impulses do not invade the somata. 7. Intracellular recordings from large-cell axons 4 mm from the soma show overshooting action potentials arising sharply from a base line. EPSPs are absent or highly attenuated and there is little underlying depolarization (less than 2 mV). 8. Current passing with electrodes intracellular to two cells has established directly that all large cells are electrotonically coupled and that an anterior cell and a small cell are coupled. Changes of burst rate during current passing into any large cell indicate that all large cells and small cells are electrotonically coupled. 9...

How does adrenaline accelerate the heart?

THE way in which adrenaline acts on the sinoatrial (SA) node to accelerate the heart rate has hitherto been obscure. However, in various other parts of the heart adrenaline increases the slow inward (Ca2+/Na+) current1–4, and voltage-recording experiments have indicated that adrenaline also has this action in the sinus region5–7. In the voltage-clamp experiments reported here, we find that adrenaline does indeed increase the slow inward current in the SA node of the rabbit, but that it also augments the outward current which would tend to decelerate pacemaker depolarisation. We find that an additional current, if, is activated within the range of voltage where the pacemaker depolarisation occurs: this could be important both in normal pacemaking and in adrenaline-induced acceleration.

Isolation and characterization of slow, depolarizing responses of cardiac ganglion neurons in the crab, Portunus sanguinolentus.

1. Tetrodotoxin-resistant, active responses to depolarization of the large cardiac ganglion cells were studied in semi-isolated preparations from the crab, Portunus sanguinolentus. Impulse activity was monitored with extracellular electrodes, simultaneous recordings from two or three large cells were made with intracellular electrodes, and current was passed via a bridge or second intracellular electrode. Preparations were continuously perfused with saline containing 3 x 10(-7) M tetrodotoxin (TTX). 2. About 20 min after introduction of TTX, small-cell impulses and resultant EPSPs in large cells cease, while rhythmic, spontaneous bursting of large cells continues. A pacemaker depolarization between bursts and slow depolarizations underlying the impulse bursts are prominent at this time. Shortly after, spontaneous burst rate slows, and at ca. 25 min, the ganglion becomes electrically quiescent. 3. In the quiescent, TTX-perfused ganglion, injection of depolarizing current into any one of the large cells results in active responses. At current strengths of sufficient intensity and duration (e.g., 20 nA, 20 ms; 5 nA, 500 ms) to depolarize a large cell by ca. 10 mV from resting potential (-53 mV, avg), the graded responses become regenerative and of constant form, provided the stimulation rate is less thna 0.15/s. Such responses have been termed "driver potentials." At more rapid rates, thresholds are increased and responses reduced. 4. Driver potentials of anterior large cells reach peak amplitudes of ca. 20 mV (to -32 mV), have maximum rates of rise of 0.45 V/s and of fall of 0.2 V/s, and a duration of ca. 250 ms. They are followed by hyperpolarizing afterpotentials, a rapidly decaying one (1 s) to -58 mV, followed by a slowly decaying one (7.5 s), -55 mV. Responses of posterior large cells are smaller (16 mV) and slower; the site of active response may be at a distance from the soma. 5. The ability of elicit near-synchronous responses and the identity of amplitude and form of responses among anterior cells and of posterior cells, regardless of which cell receives depolarizing current, indicates that all cells undergo active responses and are stimulated by electrotonic spread of depolarization. 6. The responses involve a conductance increase since memses during a driver potential are much reduced. 7. Depolarization by steady current increases the absolute threshold, decreases the maximum depolarization of the peak, and slows rates of rise and fall. Hyperpolarization increases rates of rise and fall; the absolute value reached by the peak depolarization is unchanged. Hyperpolarization reduces the amplitude of the rapid after-potential relative to the displaced resting potential. 8. Hyperpolarizing current pulses imposed during the rise and peak of driver-potential responses are followed by redevelopment of a complete response. Sufficiently strong hyperpolarization can terminate a response. The current strength needed to terminate a response decreases the later during the response the pulse is given...

Electrical properties associated with wide intercellular clefts in rabbit Purkinje fibres.

1. Rabbit Purkinje fibres were studied using micro-electrode recordings of electrical activity or a two-micro-electrode voltage clamp. Previous morphological work had suggested that these preparations offer structural advantages for the analysis of ionic permeability mechanisms. 2. Viable preparations could be obtained consistently by exposure to a K glutamate Tyrode solution during excision and recovery. In NaCl Tyrode solution, the action potential showed a large overshoot and fully developed plateau, but no pacemaker depolarization at negative potentials. 3. The passive electrical properties were consistent with morphological evidence for the accessibility of cleft membranes within the cell bundle. Electrotonic responses to intracellular current steps showed the behaviour expected for a simple leaky capacitative cable. Capacitative current transients under voltage clamp were changed very little by an eightfold reduction in the external solution conductivity. 4. Slow current changes attributable to K depletion were small compared to those found in other cardiac preparations. The amount of depletion was close to that predicted by a cleft model which assumed free K diffusion in 1 micron clefts. 5. Step depolarizations over the plateau range of potentials evoked a slow inward current which was resistant to tetrodotoxin but blocked by D600. 6. Strong depolarizations to potentials near 0 mV elicited a transient outward current and a slowly activating late outward current. Both components resembled currents found in sheep or calf Purkinje fibres. 7. These experiments support previous interpretations of slow plateau currents in terms of genuine permeability changes. The rabbit Purkinje fibre may allow various ionic channels to be studied with relatively little interference from radial non-uniformities in membrane potential or ion concentration.

Ionic basis of transient inward current induced by strophanthidin in cardiac Purkinje fibres.

1. Voltage clamp experiements studied the ionic basis of the strophathidin-induced transient inward current (TI) in cardiac Purkinje fibres. 2. The reversal potential of TI (Erev) was determined in the presence of various bathing solutions. Erev averaged --5 m V in the standard modified Tyrode solution (Kass, Lederer, Tsien & Weingart, 1978). Erev was displaced toward more negative potentials when the external Na concentration (NaO) was reduced by replacement of NaCl with Tris Cl, choline Cl or sucrose. 3. A sudden reduction of NaO evoked a temporary increase in TI, followed after a few minutes by a sustained diminution. The initial increase was closely paralleled by an enhanced aftercontraction and could be explained by an indirect effect of NaO on internal Ca. The subsequent fall in TI amplitude could be accounted for by the reduced driving force, E--Erev. 4. Erev was not significantly changed by replacing extracellular Cl with methyl-sulphate, or by limited variations in external Ca (2.7--16.2 mM) or external K (1--8 MM). 5. These results are consistent with an increase in membrane permeability to Na and perhaps K. 6. TI was not directly affected by TTX, which blocks excitatory Na channels, or by Cs, which inhibits inwardly rectifying K channels. TI may be distinguished from the slow inward current by its kinetic, pharmacological and ionic properties. 7. TI might be carried by a pre-existing ionic pathway such as the 'leak' channel which provides inward current underlying normal pace-maker depolarization. Another possibility is that TI reflects Ca extrusion by an electrogenic Ca--Na exchange.

Membrane currents underlying activity in frog sinus venosus

1. The spontaneous electrical activity of small strips of muscle from the sinus venosus region of the heart of Rana catesbeiana was investigated using the double sucrose gap technique. The voltage clamp was used to record the ionic currents underlying the pace-maker depolarization and the action potential.2. The records of spontaneous electrical activity are very similar to those obtained from the sinus venosus using micro-electrodes. Moreover, the pace-maker activity is almost completely insensitive to tetrodotoxin (TTX) at 2.0 x 10(-6) g/ml., which suggests that the pace-maker responses can be classified as primary, as opposed to follower pacing.3. In response to short rectangular depolarizing voltage clamp pulses, only one inward current is activated. This current is almost completely insensitive to TTX but can be blocked by manganese ions. It appears, therefore, to be equivalent to the slow inward (Ca(2+)/Na(+)) current, I(si), of other cardiac tissues. The threshold for I(si) is near to the maximum diastolic potential, indicating that it must be activated during the pace-maker depolarization.4. Interruption of the normal pace-maker depolarization by rapid activation of the voltage clamp circuit reveals the time-dependent decay of outward current. This current reverses between -75 and -90 mV and, therefore, is probably carried mainly by potassium ions.5. Outward current decay is not a simple exponential, and Hodgkin-Huxley analysis suggests that two distinct components of outward current may be present. One of these is activated in the potential range of the pace-maker depolarization and the other at more positive potentials. Both outward currents reach full, steady-state activation at about zero mV, i.e. within the ;plateau' range of the sinus action potential.6. These results are compared with other recently published voltage clamp data from the rabbit sino-atrial node.7. A hypothesis for the generation of pace-maker activity is presented which involves (i) decay of outward current and (ii) activation of the slow inward current, I(si).