Measurement and significance of the reversal potential for the pace-maker current (iK2) in sheep Purkinje fibres

When Is a Potassium Channel Not a Potassium Channel?

Ever since they were first observed in Purkinje fibers of the heart, funny channels have had close connections to potassium channels. Indeed, funny channels were initially thought to produce a potassium current in the heart called I _K2. However, funny channels are completely unlike potassium channels in ways that make their contributions to the physiology of cells unique. An important difference is the greater ability for sodium to permeate funny channels. Although it does not flow through the funny channel as easily as does potassium, sodium does permeate well enough to allow for depolarization of cells following a strong hyperpolarization. This is critical for the function of funny channels in places like the heart and brain. Computational analyses using recent structures of the funny channels have provided a possible mechanism for their unusual permeation properties.

A Brief History of Pacemaking

Cardiac pacemaking is a most fundamental cardiac function, thoroughly investigated for decades with a multiscale approach at organ, tissue, cell and molecular levels, to clarify the basic mechanisms underlying generation and control of cardiac rhythm. Understanding the processes involved in pacemaker activity is of paramount importance for a basic physiological knowledge, but also as a way to reveal details of pathological dysfunctions useful in the perspective of a therapeutic approach. Among the mechanisms involved in pacemaking, the "funny" (I_f) current has properties most specifically fitting the requirements for generation and control of repetitive activity, and has consequently received the most attention in studies of the pacemaker function. Present knowledge of the basic mechanisms of pacemaking and the properties of funny channels has led to important developments of clinical relevance. These include: (1) the successful development of heart rate-reducing agents, such as ivabradine, able to control cardiac rhythm and useful in the treatment of diseases such as coronary artery disease, heart failure and tachyarrhythmias; (2) the understanding of the genetic basis of disorders of cardiac rhythm caused by HCN channelopathies; (3) the design of strategies to implement biological pacemakers based on transfer of HCN channels or of stem cell-derived pacemaker cells expressing I_f, with the ultimate goal to replace electronic devices. In this review, I will give a brief historical account of the discovery of the funny current and the development of the concept of I_f-based pacemaking, in the context of a wider, more complex model of cardiac rhythmic function.

A model model: a commentary on DiFrancesco and Noble (1985) 'A model of cardiac electrical activity incorporating ionic pumps and concentration changes'

This paper summarizes the advances made by the DiFrancesco and Noble (DFN) model of cardiac cellular electrophysiology, which was published in Philosophical Transactions B in 1985. This model was developed at a time when the introduction of new techniques and provision of experimental data had resulted in an explosion of knowledge about the cellular and biophysical properties of the heart. It advanced the cardiac modelling field from a period when computer models considered only the voltage-dependent channels in the surface membrane. In particular, it included a consideration of changes of both intra- and extracellular ionic concentrations. In this paper, we summarize the most important contributions of the DiFrancesco and Noble paper. We also describe how computer modelling has developed subsequently with the extension from the single cell to the whole heart as well as its use in understanding disease and predicting the effects of pharmaceutical interventions. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.

Na/K pump regulation of cardiac repolarization: insights from a systems biology approach

The sodium-potassium pump is widely recognized as the principal mechanism for active ion transport across the cellular membrane of cardiac tissue, being responsible for the creation and maintenance of the transarcolemmal sodium and potassium gradients, crucial for cardiac cell electrophysiology. Importantly, sodium-potassium pump activity is impaired in a number of major diseased conditions, including ischemia and heart failure. However, its subtle ways of action on cardiac electrophysiology, both directly through its electrogenic nature and indirectly via the regulation of cell homeostasis, make it hard to predict the electrophysiological consequences of reduced sodium-potassium pump activity in cardiac repolarization. In this review, we discuss how recent studies adopting the systems biology approach, through the integration of experimental and modeling methodologies, have identified the sodium-potassium pump as one of the most important ionic mechanisms in regulating key properties of cardiac repolarization and its rate dependence, from subcellular to whole organ levels. These include the role of the pump in the biphasic modulation of cellular repolarization and refractoriness, the rate control of intracellular sodium and calcium dynamics and therefore of the adaptation of repolarization to changes in heart rate, as well as its importance in regulating pro-arrhythmic substrates through modulation of dispersion of repolarization and restitution. Theoretical findings are consistent across a variety of cell types and species including human, and widely in agreement with experimental findings. The novel insights and hypotheses on the role of the pump in cardiac electrophysiology obtained through this integrative approach could eventually lead to novel therapeutic and diagnostic strategies.

The role of the funny current in pacemaker activity

Abstract: Pacemaking is a basic physiological process, and the cellular mechanisms involved in this function have always attracted the keen attention of investigators. The "funny" (I(f)) current, originally described in sinoatrial node myocytes as an inward current activated on hyperpolarization to the diastolic range of voltages, has properties suitable for generating repetitive activity and for modulating spontaneous rate. The degree of activation of the funny current determines, at the end of an action potential, the steepness of phase 4 depolarization; hence, the frequency of action potential firing. Because I(f) is controlled by intracellular cAMP and is thus activated and inhibited by beta-adrenergic and muscarinic M2 receptor stimulation, respectively, it represents a basic physiological mechanism mediating autonomic regulation of heart rate. Given the complexity of the cellular processes involved in rhythmic activity, an exact quantification of the extent to which I(f) and other mechanisms contribute to pacemaking is still a debated issue; nonetheless, a wealth of information collected since the current was first described more than 30 years ago clearly agrees to identify I(f) as a major player in both generation of spontaneous activity and rate control. I(f)- dependent pacemaking has recently advanced from a basic, physiologically relevant concept, as originally described, to a practical concept that has several potentially useful clinical applications and can be valuable in therapeutically relevant conditions. Typically, given their exclusive role in pacemaking, f-channels are ideal targets of drugs aiming to pharmacological control of cardiac rate. Molecules able to bind specifically to and block f-channels can thus be used as pharmacological tools for heart rate reduction with little or no adverse cardiovascular side effects. Indeed a selective f-channel inhibitor, ivabradine, is today commercially available as a tool in the treatment of stable chronic angina. Also, several loss-of-function mutations of HCN4 (hyperpolarization-activated, cyclic-nucleotide gated 4), the major constitutive subunit of f-channels in pacemaker cells, are known today to cause rhythm disturbances, such as for example inherited sinus bradycardia. Finally, gene- or cell-based methods for in situ delivery of f-channels to silent or defective cardiac muscle represent novel approaches for the development of biological pacemakers eventually able to replace electronic devices.

Review of the If selective channel inhibitor ivabradine in the treatment of chronic stable angina

Coronary heart disease is the major cause of morbidity and mortality in industrialized countries, and its prevalence is predicted to grow as the population ages. Current drugs for chronic stable angina (such as beta-blockers, calcium-channel blockers, long- and short-acting nitrates, and potassium-channel activators) are often effective, either as monotherapy or in combination, but side effects and contraindications may limit their use. The "I(f)" (for "funny") channel, discovered in 1979, is expressed mainly in the membrane of pacemaker cells present in the sinus node, the atrioventricular node, the ventricular conduction pathways, and ventricular myocytes. By determining the slope of diastolic depolarization, which in turn controls action potential frequency, it is a key determinant of heart rate and so provides a new therapeutic target for controlling angina symptoms. A new antiangina drug, ivabradine, has been developed and licensed for clinical use. It exclusively reduces the heart rate by selectively blocking the I(f) channel of the sino-atrial node. As clinical trials have shown it to be remarkably well-tolerated, ivabradine offers an alternative for patients who cannot take, or are intolerant of, beta blockade. This review provides an insight into this new agent, its historical background, mechanism of action, and pathophysiologic basis, and provides up-to-date evidence-based information on its optimum use in stable angina.

Acetylcholine reverses effects of beta-agonists on pacemaker current in canine cardiac Purkinje fibers but has no direct action. A difference between primary and secondary pacemakers

We have investigated the actions of acetylcholine in the absence and presence of the beta-agonist isoproterenol in cardiac Purkinje fibers. beta-Agonists, like isoproterenol, increase the magnitude of the pacemaker current (If) in cardiac myocytes by shifting its activation voltage more positive on the voltage axis. We find that acetylcholine has no effect on If in the absence of isoproterenol. However, if If is first increased by beta-agonist stimulation, acetylcholine can then return If to control levels. This effect on If is exerted through muscarinic receptors since atropine prevents this action of acetylcholine. Functionally, this action of acetylcholine can guarantee the maintenance of ventricular pacemakers when there is high parasympathetic tone but can also prevent extra ventricular beats when sympathetic and parasympathetic tone are both high.

Alinidine modifies the pacemaker current in sheep Purkinje fibers

(1) The "specific bradycardic agent" alinidine reduces the slope of the diastolic depolarization in sinoatrial tissue and Purkinje fibers. In short Purkinje fibers of sheep, alinidine (28 microM) decreased the pacemaker current by a dual action. The voltage dependence of if activation was shifted in the hyperpolarizing direction by 7.8 +/- 0.6 mV (n = 18, p less than 0.001) and the conductance of the fully activated if current was reduced to 73 +/- 2% (n = 18, p less than 0.001) of its control value. These effects were reversible and dose-dependent. (2) Ionophoretic injections of alinidine caused reversible reductions of the diastolic depolarization rate and simultaneous transient hyperpolarizing shifts of the if activation range. (3) Some prolongation of the action potential duration was observed at 28 microM and more pronounced at higher concentration. This was presumably the consequence of a reduction by alinidine of outward repolarizing current carried by the background inward rectifier and plateau current ix. (4) The action of alinidine on if resulted in a slower activation of a reduced fraction of the pacemaker current at the maximal diastolic potential level. This explains the decrease of the diastolic depolarization rate observed in Purkinje fibers.

A new oil-gap method for internal perfusion and voltage clamp of single cardiac cells

(1.) We designed a new technique to achieve fast voltage clamp, combined with internal perfusion. The single guinea-pig cardiac cell, dissociated by collagenase treatment, was stretched across an oil-gap (30-40 micron wide) from a pool of Tyrode solution to a pool of internal solution. Part of the cell membrane was disrupted in the internal solution by crushing on the cell, a tapered tip of a glass capillary. Through the open end, the intracellular medium was equilibrated with test solutions and electrical current was injected for the voltage clamp of the membrane in the Tyrode pool. (2.) The capacitive transient on stepping the membrane potential decayed with a time constant of 10-60 microseconds, depending on the capacitive area (20-80 pF). The time course was a single exponential in 46% of the atrial cells and in 66% of the ventricular cells. In these tissues the series resistance, approximated by a ratio of the time constant and Cm, was 686 +/- 180 k omega (n = 37) in the ventricular cells or 812 +/- 143 k omega (n = 18) in the atrial cells. The stable seal resistance (Rseal) established in the oil-gap was around 33 M omega in the ventricular cells and 100 M omega in the atrial cells. (3.) A rapid increase in the inward current followed by a slow decay was observed on repolarization over the range negative to the potassium equilibrium potential. From the inward rectification of both peak and late currents and suppressive effects of Cs+ on the current, the current changes were attributed to activation and inactivation of the inward rectifier K channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of thallium on membrane currents at diastolic potentials in canine cardiac Purkinje strands

A two-micro-electrode voltage-clamp technique was used to record membrane currents from canine cardiac Purkinje strands during hyperpolarizing steps to potentials between -70 and -150 mV in Tyrode solutions containing K+ and/or Tl+. Complete replacement of external K+ by equimolar Tl+ increases the instantaneous inwardly rectifying current. The inwardly rectifying region of the instantaneous I-V relation is shifted to more positive potentials and its slope is increased. The diastolic time-dependent current is reduced or reversed. Partial substitution of equimolar Tl+ for K+ reduces the diastolic time-dependent current. The instantaneous I-V relation is shifted inward for molar fractions of Tl+ (YTl) greater than 0.5, and is slightly more inward or unchanged for YTl less than or equal to 0.5. Addition of small amounts of Tl+ shifts the instantaneous I-V relation inward and reduces the diastolic time-dependent current. Addition of Tl+ in solutions containing Ba2+ to block the background inward rectifier has no effect on the instantaneous I-V relation; the diastolic time-dependent (pace-maker) current is reduced. Block of the pace-maker current by Tl+ is largely independent of potential in Ba2+ Tyrode solution. Since Tl+ has opposite effects on the pace-maker current and the inward rectifier, these findings support other evidence that the pace-maker current is not part of the background inward rectifier.

Pace-maker current changes during intracellular pH transients in sheep cardiac Purkinje fibres

Intracellular acidosis, at constant extracellular pH, hyperpolarizes the resting potential and reduces the diastolic depolarization rate of cardiac Purkinje fibres. With alkaline pHi, the fibre depolarizes and spontaneous firing is observed. Intracellular pH transients induced either by superfusion with Tyrode buffered with 5% CO2/23 mM HCO3- or 16% CO2/61 mM HCO3-, or with solutions containing weak undissociated acids, transiently shifted the half-maximum activation potential E0.5 of the pace-maker current. Similar transients were observed when NH4Cl was added and subsequently withdrawn from the solution. Simultaneous pHi measurements demonstrate a close relation between the time course of the pHi and E0.5 variations. Acid pHi shifts E0.5 to more negative and alkaline pHi's to less negative potentials. These pace-maker current activation voltage shifts are interpreted as the direct consequence of fixed charges titration at the inside of the sarcolemma. Other effects, like the slowing-down and reduction of the pace-maker current by acid pHi, presumably result from other interactions of protons with the pace-maker channel.

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

Equations have been developed to describe cardiac action potentials and pacemaker activity. The model takes account of extensive developments in experimental work since the formulation of the M.N.T. (R. E. McAllister, D. Noble and R. W. Tsien, J. Physiol., Lond. 251, 1-59 (1975)) and B.R. (G. W. Beeler and H. Reuter, J. Physiol., Lond . 268, 177-210 (1977)) equations. The current mechanism i K2 has been replaced by the hyperpolarizing-activated current, i f . Depletion and accumulation of potassium ions in the extracellular space are represented either by partial differential equations for diffusion in cylindrical or spherical preparations or, when such accuracy is not essential, by a three-compartment model in which the extracellular concentration in the intercellular space is uniform. The description of the delayed K current, i K , remains based on the work of D. Noble and R. W. Tsien ( J. Physiol., Lond . 200, 205-231 (1969 a )). The instantaneous inward-rectifier, i K1 , is based on S. Hagiwara and K. Takahashi’s equation ( J. Membrane Biol . 18, 61-80 (1974)) and on the patch clamp studies ofB. Sakmann and G. Trube ( J. Physiol., Lond . 347, 641-658 (1984)) and of Y. Momose, G. Szabo and W. R. Giles ( Biophys. J . 41, 311a (1983)). The equations successfully account for all the properties formerly attributed to i K2 , as well as giving more complete descriptions of i K1 and i K . The sodium current equations are based on experimental data of T. J. Colatsky ( J.Physiol., Lond. 305, 215-234 (1980)) and A. M. Brown, K. S. Lee and T. Powell ( J.Physiol., Lond. , Lond. 318, 479-500 (1981)). The equations correctly reproduce the range and magnitude of the sodium ‘window’ current. The second inward current is based in part on the data of H. Reuter and H. Scholz ( J. Physiol., Lond . 264, 17-47 (1977)) and K. S. Lee and R. W. Tsien ( Nature, Lond . 297,498-501 (1982)) so far as the ion selectivity is concerned. However, the activation and inactivation gating kinetics have been greatly speeded up to reproduce the very much faster currents recorded in recent work. A major consequence of this change is that Ca current inactivation mostly occurs very early in the action potential plateau. The sodium-potassium exchange pump equations are based on data reported by D. C. Gadsby ( Proc. natn. Acad. Sci. U. S. A. 77, 4035-4039 (1980)) and by D. A. Eisner and W. J. Lederer ( J. Physiol., Lond . 303, 441-474 (1980)). The sodium-calcium' exchange current is based on L. J. Mullins’ equations ( J. gen.. Physiol. 70, 681-695 (1977)). Intracellular calcium sequestration is represented by simple equations for uptake into a reticulum store which then reprimes a release store. The repriming equations use the data of W. R. Gibbons & H. A. Fozzard ( J. gen. Physiol . 65, 367-384 (1975 b )). Following Fabiato & Fabiato’s work ( J. Physiol., Lond. 249, 469-495 (I975)), Ca release is assumed to be triggered by intracellular free calcium. The equations reproduce the essential features of intracellular free calcium transients as measured with aequorin. The explanatory range of the model entirely includes and greatly extends that of the M.N.T. equations. Despite the major changes made, the overall time-course of the conductance changes to potassium ions strongly resembles that of the M.N.T. model. There are however important differences in the time courses of Na and Ca conductance changes. The Na conductance now includes a component due to the hyperpolarizing-activated current, i r , which slowly increases during the pacemaker depolarization. The Ca conductance changes are very much faster than in the M.N.T. model so that in action potentials longer than about 50 ms the primary contribution of the fast gated calcium channel to the plateau is due to a steady-state ‘window’ current or non-inactivated component. Slower calcium or Ca-activated currents, such as the Na-Ca exchange current, or Ca-gated currents, or a much slower Ca channel must then play the dynamic role previously attributed to the kinetics of a single type of calcium channel. This feature of the model in turn means that the repolarization process should be related to the inotropic state, as indicated by experimental work. The model successfully reproduces intracellular sodium concentration changes produced by variations in [Na]0, or Na-K pump block. The sodium dependence of the overshoot potential is well reproduced despite the fact that steady state intracellular Na is proportional to extracellular Na, as in the experimental results of D. Ellis J. Physiol., Lond . 274, 211-240 (1977)). The model reproduces the responses to current pulses applied during the plateau and pacemaker phases. In particular, a substantial net decrease in conductance is predicted during the pacemaker depolarization despite the fact that the controlling process is an increase in conductance for the hyperpolarizing-activated current. The immediate effects of changing extracellular [K] are reproduced, including: (i) the shortening of action potential duration and suppression of pacemaker activity at high [K ]; (ii) the increased automaticity at moderately low [K ]; and (iii) the depolarization to the plateau range with premature depolarizations and low voltage oscillations at very low [K]. The ionic currents attributed to changes in Na-K pump activity are well reproduced. It is shown that the apparent K m for K activation of the pump depends strongly on the size of the restricted extracellular space. With a 30% space (as in canine Purkinje fibres) the apparent K m is close to the assumed real value of 1 mM . When the extracellular space is reduced to below 5% , the apparent K m increases by up to an order of magnitude. A substantial part of the pump is then not available for inhibition by low [K] b . These results can explain the apparent discrepancies in the literature concerning the K m for pump activation.

The surprising heart: a review of recent progress in cardiac electrophysiology

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Single cardiac Purkinje cells: general electrophysiology and voltage-clamp analysis of the pace-maker current

Single Purkinje cells from dog, sheep and cow hearts were isolated by injecting a Ca-free collagenase containing Tyrode solution in the space between the connective tissue sheath and the Purkinje cells. A small proportion of these cells survived the isolation procedure and these cells were used for further investigation. The cells showed electrophysiological properties similar to intact Purkinje fibres as indicated by the following results. Maximum diastolic potentials between -70 and -85 mV and specific membrane resistances of 21-32 k omega cm2 indicated that the single cells were not leaky or hyperpermeable . The action potential showed a rapid upstroke, with a maximum rate of rise, Vmax' between 150 and 750 V/s, and two phases of fast repolarization separated by a plateau phase with a duration of about 200 ms. Each action potential was followed by a spontaneous depolarization with an amplitude between 1 and 10 mV. The upstroke of the action potential could be blocked by tetrodotoxin (TTX) in a dose-dependent manner. The rate of depolarization of the action potential was sensitive to changes in membrane potential; the resulting S-shaped curve showed a half-maximum potential of -65 mV and a steepness of 0.46 mV-1. The duration of the action potential was sensitive to external K concentrations, catecholamines and TTX in a way similar to intact Purkinje fibres. Both application of catecholamines and lowering the external K concentration induced spontaneous activity. The cells were used to study the ionic nature of the pace-maker current under voltage-clamp conditions using the two-micro-electrode technique. This pace-maker current was blocked in a voltage-dependent manner by 1 mM-Cs, and was not affected by 1 mM-Ba. The steady-state activation curve was shifted in the depolarizing direction by application of adrenaline. In contrast to voltage-clamp data obtained on the pace-maker current of intact Purkinje fibres, the pace-maker current in a single cell did not reverse near the presumed equilibrium potential for K ions; no reversal could be seen in the voltage range negative to -50 mV. These observations together with preliminary results on the Na and K dependence of the pace-maker current are strong arguments in favour of the hypothesis that the pace-maker current in cardiac Purkinje fibres is an inward current carried by Na and K ions and activates upon hyperpolarization.

Simulation of potassium accumulation in clefts of Purkinje fibers: effect on membrane electrical activity

Purkinje fiber action potentials and concomitant intercellular cleft [K] variations were reconstructed by using modified McAllister, Noble & Tsien (1975) equations including the pump current, ip, and the pacemaker current, if. Three different mean cleft widths were chosen: 40, 200 and 1000 nm. Assuming a cylindrical arrangement of the cells in the bundle, the cleft [K] gradient across the bundle was calculated by using the radial cylindrical diffusion equation. The effects of varying several parameters (cleft width, tortuosity, ip and if) were studied in conditions corresponding to two different values of [K] in the bulk solution, namely 2.7 and 5.4 mM. The shortening influence on the action potential of the systolic increase in cleft [K] was detectable only in the case of the smallest cleft width. Reduction in electrogenic pump activity led to alterations of the electrical activity which depended on the cleft width. The evolution of the intercellular [K] during each action potential and the following diastolic period was normally biphasic; a small reaccumulation during the late part of the diastole was induced by the K component of the if current. Experimentally determined intercellular [K] variations described in the literature exhibit a monophasic evolution. Such a monophasic evolution could be reproduced after reduction of both if and the transient outward K current and suppression of the negative slope of the ik1-Em relationship. In this case the amplitude of the cyclic change in intercellular [K] was approximately equal to 0.2 mM (for a 200 nm cleft width), a value much lower than that experimentally recorded. Possible reasons for this discrepancy are discussed. A simplified three compartment model for K diffusion was also used. Results obtained with the two models demonstrated that the simplified model can be used as a reasonable approximation of the more complex radial diffusion model, with a reduction in computation time reaching 80% or more.

Block by Cs of K current iK1 and of carbachol induced K current iCch in frog atrium

1. In frog atrium, Cs ions block both the inward rectifier iK1 and the carbachol induced K current iCch. 2. Both iK1 and iCch display a high affinity for Cs with a K0.5 of 4 X 10(-5) M for iK1 and of 8 X 10(-5) M for iCch at V = -50 mV. 3. Block of both iK1 and iCch is strongly voltage dependent. When fitted by the block model of Woodhull (1973), delta is greater than 1 for the two currents. 4. From these similarities, action of Cch on frog atrium K permeability could be interpreted as a modification of iK1.

Actions of barium and rubidium on membrane currents in canine Purkinje fibres

The actions of Ba2+ and Rb+, two blockers of background K+ conductance, were investigated. Recent studies performed on ungulate Purkinje fibres have suggested that the pace-maker current is an inward current activated by hyperpolarization. This hypothesis is based on the assumption that Ba2+ reduces the inwardly rectifying background K+ conductance without affecting the pace-maker current. Addition of 5 mM-BaCl2 to the bathing Tyrode solution decreases background K+ permeability and eliminates the reversal of the pace-maker current. The reversal reappears on return to Ba2+-free Tyrode solution. 5 mM-BaCl2 also reduces the time-dependent current at pace-maker potentials positive to about -95 mV in 4 mM-K+ Tyrode solution. The pace-maker current in Ba2+ Tyrode solution usually does not have an exponential time course, and often decays non-monotonically. It can take more than two minutes to reach a steady state. The fast initial component of membrane current, which is observed on hyperpolarizing in the pace-maker potential range in Purkinje fibres and which has been called the 'depletion current', is still present in Ba2+ Tyrode solution, but is reduced or eliminated if 10 mM-CsCl is added to the Ba2+ Tyrode solution. The addition of Cs+ is accompanied by an outward shift in membrane current in Ba2+ Tyrode solution. Ba2+ reduces the background K+ permeability in a dose-dependent manner. Addition of between 0.5 and 1 mM-BaCl2 achieves a maximum effect. Raising the amount of BaCl2 above this level reduces the time-dependent current even when no further effect on background permeability is observed. Rb+ substitution for K+ reduces the magnitude of the pace-maker current at potentials positive to -100 mV, eliminates the reversal of the pace-maker current, shifts the activation range to more negative potentials, and decreases the voltage dependence of pace-maker current kinetics. Rb+ addition to Tyrode solution has little effect on pace-maker current magnitude or time course positive to -90 mV, but does shift the reversal to more negative potentials. The available evidence suggests that the pace-maker current in Ba2+ Tyrode solution is an inward current activated by hyperpolarization. However, Ba2+ blocks an unknown fraction of the pace-maker current in a dose-dependent, and possibly voltage-dependent manner. Also, the presence of a slow component of pace-maker decay suggests that the standard Hodgkin-Huxley formalism for the analysis of pace-maker currents is inappropriate.

The kinetics and temperature dependence of the pace-maker current if in sheep Purkinje fibres

The kinetic properties of the if channel in shortened sheep Purkinje fibres were investigated using a two-micro-electrode voltage-clamp technique in the presence of Ba2+, Mn2+ and tetrodotoxin (TTX). The time course of the hyperpolarization-activated if currents (DiFrancesco, 1981 b) at potentials within the activation range was found to depend on whether the channel was switching 'on' or 'off'. At potentials positive to the half-activation point if decay was faster than if onset; at potentials negative to the half-activation point if onset was faster than if decay. The time courses of if onset and decay become similar only at potentials close to the centre of the activation range. If a single exponential was fitted to the time course of if switching, the time constant (tau y) was found to vary as a function of potential from approximately 50 msec to several seconds. The tau y - voltage relation is an extremely steep bell-shaped distribution. Reducing external Na+ (range 140-17.5 mM) did not alter the voltage dependence of the if time course. Increasing external K+ (range 5-60 mM) shifted the if time constants and activation curve by similar amounts in a depolarizing direction. The temperature dependence of if was investigated over the range 27.5-41 degrees C. Cooling reversibly slowed the time course of if activation with a Q10 of 3.13 (S.D. +/- 0.85, n = 62). A reversible reduction in the slope of the fully-activated current-voltage relation was observed on cooling, the Q10 being 1.35 (S.D. +/- 0.07), and was usually accompanied by a small depolarizing shift of the half-activation point and the reversal potential Ef. It is concluded that the if time course shows a marked potential dependence and does not obey Hodgkin-Huxley kinetics. Its temperature dependence resembles that of if in the sino-atrial node (DiFrancesco & Ojeda, 1980).

Fast inward-rectifying current accounts for anomalous rectification in olfactory cortex neurones

The somatic membrane of guinea-pig olfactory cortex neurones in vitro (23 degrees C) was voltage clamped by means of a single-micro-electrode sample-and-hold technique. In most cells the current-voltage (I-V) relationship showed inward (anomalous) rectification with increasing hyperpolarization beyond the resting potential (ca. -80 mV). Under current-clamp conditions a time-dependent 'sag' of the hyperpolarizing electrotonic potentials was observed following an initial overshoot. No depolarizing after-potential was seen on return to the resting potential. Inward rectification was activated between -100 and -110 mV (irrespective of pre-set resting potential) and increased the membrane input conductance by up to three-fold. The rectification was unaffected by tetrodotoxin or Cd2+. Under somatic voltage clamp, hyperpolarization beyond -110 mV activated a rapid inward relaxation fitted by a single exponential. The relaxation time constant (tau on) decreased e-fold for a 40 mV hyperpolarization. (Typical values: 28 ms at -110 mV declining to 13 ms at -140 mV; external K+ concentration 3 mM, 23 degrees C). More extreme hyperpolarizations evoked a slower 'inactivation' phase (tau = 40-60 ms). A transient outward-decaying 'tail' current reflecting deactivation of inward rectification was seen on stepping from -140 mV to more positive potentials. tau off became slower with hyperpolarization. The tail current disappeared at a potential close to the expected VK but was rarely inverted to an inward-decaying tail. It is proposed that the fast inward-rectifying current of olfactory neurones (If.i.r.) is a K+ current analogous to the anomalous K+ rectifier of marine egg and frog muscle membranes. The behaviour of the inward rectifier was dependent on external K+ concentration in accordance with the unique 'V--VK' dependence of classical anomalous rectification; however, of several agents tested (external Cs+, Ba2+, Rb+, Tl+ or tetraethylammonium), only Cs+ and Ba2+ blocked If.i.r. in a time- and voltage-dependent manner. The effect of tetraethylammonium resembled that of an increase in external K+. The possible contribution of the inward rectifier to the passive cell membrane properties is discussed.

Voltage-clamp analysis of muscarinic excitation in hippocampal neurons

Pyramidal cells in the CA1 field of guinea pig hippocampal slices were voltage-clamped using a single microelectrode, at 23-30 degrees C. Small inwardly relaxing currents triggered by step hyperpolarizations from holding potentials of -80 to -40 mV were investigated. Inward relaxations occurring for negative steps between -40 mV and -70 mV resembled M-currents of sympathetic ganglion cells: they were abolished by addition of carbachol, muscarine or bethanechol, as well as by 1 mM barium; the relaxations appeared to invert at around -80 mV; they became faster at more negative potentials; and the inversion potential was shifted positively by raising external K+ concentration. Inward relaxations triggered by steps negative to -80 mV, in contrast, appeared to reflect passage of another current species, which has been labelled IQ. Thus IQ did not invert negative to -80 mV, it was insensitive to muscarinic agonists or to barium, and it was blocked by 0.5-3 mM cesium (which does not block IM). Turn-on of IQ causes the well known droop in the hyperpolarizing electrotonic potential in these cells. The combined effects of IQ and IM make the steady-state current-voltage relation of CA1 cells slightly sigmoidal around rest potential. It is suggested that activation of cholinergic septal inputs to the hippocampus facilitates repetitive firing of pyramidal cells by turning off the M-conductance, without much change in the resting potential of the cell.

A voltage clamp study of the effects of AR-L 115 BS on the pacemaker current of cardiac Purkinje fibres

The new cardiotonic agent AR-L 115 BS was investigated by means of the double-micro-electrode voltage clamp technique on sheep cardiac Purkinje fibres. Clinical and pharmacological studies show that AR-L 115 BS increases heart rate as a side effect at medium to high therapeutic doses. The classical analysis of the pacemaker current was therefore performed to study the possible mechanism of this effect at a cellular level. The kinetic parameter s infinity and the reversal potential of the pacemaker current were shifted in the depolarizing direction after exposure to AR-L 115 BS. Peak values of the fully activated pacemaker current were either increased or diminished, while potassium leakage was slightly increased. These results are not related to the action of AR-L 115 BS on beta-adrenergic receptors but possibly due to enhanced intracellular calcium (see third paper in this series). Despite its tendency to increase heart rate, high concentrations of AR-L 115 BS should not be expected to promote arrhythmias in the Purkinje system since the electrophysiological effects tend to counteract each other.

Effects of Cs on acetylcholine induced current. Is ik1 increased by acetylcholine in frog atrium?

1.20 mM Cs+ ions reduce background current and decrease drastically the K+ depletion process, probably as a consequence of the reduction of iK1. The background current-voltage relationship becomes linear. 2. 20 mM Cs+ ions completely abolish the current induced by acetylcholine. 3. The possibility that the K+ current induced by acetylcholine is due to an increase of iK1 is discussed.

Active and passive electrical properties of single bullfrog atrial cells

Single cells from the bullfrog (Rana catesbeiana) atrium have been prepared by using a modification of the enzymatic dispersion procedure described by Bagby et al. (1971. Nature [Long.]. 234:351--352) and Fay and Delise (1973. Proc. Natl. Acad. Sci. U.S.A. 70:641--645). Visualization of relaxed cells via phase-contrast or Nomarski optics (magnification, 400--600) indicates that cells range between 150 and 350 micrometers in length and 4 and 7 micrometers in diameter. The mean sarcomere length in relaxed, quiescent atrial cells in 2.05 micrometer. Conventional electrophysiological measurements have been made. In normal Ringer's solution (2.5 mM K+, 2.5 mM Ca++) acceptable cells have stable resting potentials of about -88 mV, and large (125 mV) long-duration (approximately 720 ms) action potentials can be elicited. The Vm vs. log[K+]0 relation obtained from isolated cells is similar to that of the intact atrium. The depolarizing phase of the action potential of isolated atrial myocytes exhibits two pharmacologically separable components: tetrodotoxin (10(-6) g/ml) markedly suppresses the initial regenerative depolarization, whereas verapamil (3 x 10(-6) M) inhibits the secondary depolarization and reduce the plateau height. A bridge circuit was used to estimate the input resistance (220 +/- 7 M omega) and time constant 20 +/- 7 ms) of these cells. Two-microelectrode experiments have revealed small differences in the electrotonic potentials recorded simultaneously at two different sites within a single cell. The equations for a linear, short cable were used to calculate the electrical constants of relaxed, single atrial cells: lambda = 921.3 +/- 29.5 micrometers; Ri = 118.1 +/- 24.5 omega cm; Rm = 7.9 +/- 1.2 x 10(3) omega cm2; Cm = 2.2 +/- 0.3 mu Fcm-2. These results and the atrial cell morphology suggest that this preparation may be particularly suitable for voltage-clamp studies.

A study of the ionic nature of the pace-maker current in calf Purkinje fibres

1. Properties of the pace-maker current (if) in Purkinje fibres were studied in the presence of Ba, which by partially blocking the iK1 channel reduces K depletion during hyperpolarizing voltage-clamp pulses, and eliminates the main cause of distortion in the current time course. 2. On raising the external potassium concentration (Kb), the if fully activated current--voltage relation (if(E)) increases in the inward direction. In the range 3--36 mM--Kb and negative to -50 mV the current is inward, and no cross-over is observed. 3. In normal conditions, the reversal potential (Ef) for if lies in the voltage region positive to -50 mV, and can be observed on lowering the external sodium concentration (Nab). Ef shifts to the negative direction when Nab is decreased. Slopes ranging between 29 and 35 mV/decade are found for Nernst plots of Ef against Nab. Changing Nab in the range 140--4.4 mM causes the if(E) relation to undergo a simple shift along the voltage axis, without significant change in its slope. 4. Ef also depends on Kb, as can be observed in low Nab (35 mM), and shifts to the positive direction by about 26 mV for every 10-fold change in Kb. The fully activated slope conductance increases when Kb is increased. 5. It is concluded that Na and K both participate in carrying if. The slope of the fully activated if(E) relation increases with Kb, but is unchanged in different Nab, indicating that the channel conductance depends on Kb, but not appreciably on Nab.

A new interpretation of the pace-maker current in calf Purkinje fibres

1. The properties of the 'pace-maker' current iK2 of Purkinje fibres are investigated to verify whether its behaviour during voltage-clamp pulses is consistent with the view that iK2 is a K current deactivating on hyperpolarization in the range -50 to -100 mV. 2. During voltage-clamp pulses in the range positive to the apparent reversal potential (Erev), low concentrations of Cs depress the time-dependent current change without altering the time-independent current component. The total current becomes more outward in Cs, which on the assumption that Cs only affects the iK1 and iK2 channels implies that iK2 is inward in the voltage range analysed. 3. Ba strongly reduces the time-independent component due to iK1 and therefore limits the contribution of K depletion to the total current time course during hyperpolarizations. In the presence of barium a current reversal is not obtained even with large hyperpolarizations in normal Tyrode solution. If in Ba-containing solutions the external K concentration is raised from 3 up to 48 mM, iK2 greatly increases in the inward direction during hyperpolarizations in the range -51 to -101 mV, implying that it cannot be carried by K only. 4. In the presence of Ba, measurements of the membrane conductance due to iK2 indicate a channel-opening process during hyperpolarizations and a channel-closing process during depolarizations, in the K-concentration range analysed. 5. It is concluded that iK2 is not, as had been previously thought, a pure K current, but is rather an inward current activated during hyperpolarizations negative to about -50 mV. This provides further evidence for a possible identity between iK2 in Purkinje fibres and the current if in the SA node.

Separation of current induced by potassium accumulation from acetylcholine-induced relaxation current in the rabbit S-A node

In a previous analysis on the rabbit S-A node the ACh-induced current was separated from the membrane current by subtracting the control from the current recorded in presence of ACh. In view of a possible interference of K accumulation processes, in the present paper the validity of the subtraction method was tested by studying the direct and indirect effects of ACh on the outward potassium current (iK). The following results were obtained. (1) The ACh-dependent channel activation and the iK-channel activation are different processes. (2) The activation curve of iK and the time constant of decay of iK current on return from a depolarizing clamp pulse were not affected by ACh. (3) In the majority of the experiments the presence of an accumulation component in the extra-current elicited by ACh could not be resolved. In a few cases the amplitude of the tail current was decreased in the presence of ACh. (4) In the case where iK was reduced, the fully-activated current-voltage relationship (i/K) was altered in the same way as that observed when the external K concentration was increased. In this case the difference between the control and the current recorded in the presence of ACh yielded a current component having a time constant similar to that of iK. We concluded that the decrease in the amplitude was due to an increase in K concentration in the clefts between the cells (K accumulation), associated with ACh application. No direct effect of ACh on the iK channel is apparent. (5) Because of the difference in the time constants of the relaxation current and the current change induced by accumulation the two processes could be clearly separated from each other.

The contribution of potassium accumulation to outward currents in frog atrium

1. Voltage-clamp experiments on frog atrial muscle were designed to distinguish effects due to K accumulation in extracellular spaces from those due to activation of K conductance mechanisms in the membrane. 2. The set of instantaneous current-voltage relations obtained at various external K concentrations following depolarization to about -10 mV for several seconds was found to be quite different from that obtained before the depolarization. Hence the process of increasing the extracellular K concentration cannot account for all the time-dependent changes in outward current during depolarization. 3. Although the instantaneous current-voltage relations obtained at different values of external K concentration before prolonged depolarization show the cross-over phenomenon (Noble, 1965), those obtained at the end of the depolarization did not show this feature. It is concluded that the current-voltage relations for the channels conducting the time-dependent K current do not show cross-over. 4. These results were used to construct a model involving both K activation and K accumulation. This model successfully reproduces the appearance of a very slow component in outward current decay tails which, when subtracted by semi-exponential curve-stripping leaves a component with the real time constant of conductance change. The model does not however reproduce the appearance of a fast decaying component without adding a second conductance mechanism, or assuming non-exponential decay of a single conductance mechanism. 5. It is therefore suggested that i chi, fast is not a perturbation of i chi, slow or of iK1 by the process of K accumulation. This conclusion is reinforced by the results of experiments showing that the relative magnitude of i chi, fast is not greatly changed by substantially increasing the external K concentration in order to reduce the proportionate effect of K accumulation on the K concentration.

Voltage-clamp investigations of membrane currents underlying pace-maker activity in rabbit sino-atrial node

1. Small preparations of spontaneously beating rabbit sino-atrial node have been investigated using the two micro-electrode voltage-clamp technique. 2. Hyperpolarizing clamp pulses given from holding potentials of about -45 mV reveal a time-dependent change of a membrane current, if, which is shown to overlap the pace-maker range (-65 mV to -45 mV) for these preparations. 3. The changes in if are shown to be quite distinct from the de-activation of the time-dependent outward current, iK. 4. The time-dependent changes of the if system increase during adrenaline application and therefore contribute to the chronotropic action of adrenaline on the heart. 5. Evidence for a link between slow inward current (iSi) and time-dependent outward current (iK) in rabbit sino-atrial node is presented and assessed.

Properties of the current if in the sino-atrial node of the rabbit compared with those of the current iK, in Purkinje fibres

1. Properties of the 'pace-maker' current if in rabbit sino-atrial node have been investigated by voltage clamp of small preparations and compared with those of the iK2 current in the Purkinje fibre. Besides having a similar voltage range of activation and responding in a similar way to adrenaline, if resembles iK2 in other respects. 2. When external Na is reduced, if decreases proportionally. In 25% Na the time-dependent current change due to if disappears. 3. 20 mM-Cs completely abolishes it. 4. The time constant of if during a hyperpolarizing voltage-clamp pulse displays a relatively high temperature dependence. 5. In spite of the similarities between the two current systems, experiments in high K solutions (48 mM) rule out the possibility that the current change seen on a hyperpolarization reflects the decay of a pure K current. 6. From conductance measurements during onset of if it is deduced that if behaves as an inward current activated by hyperpolarizations.

Action of salicylate ions on the electrical properties of sheep cardiac Purkinje fibres

1. In sheep Purkinje fibre preparations, salicylate ions produce reversible changes in resting potential and in action potential duration. In most preparations these effects resemble those produced by -ow extracellular K concentration: the resting potential first increases and then decreases, the action potential is prolonged and eventually, low potential oscillations occur in the plateau range. In a few preparations, action potential shortening occurs. 2. The threshold current for initiating action potentials by an intracellular electrode is reversibly increased by salicylate. 3. The activation curve, soo(Em), for the pace-maker K current, iK2, shifted in a hyperpolarizing direction. The magnitude of the shift is about -5 mV in 5 mM-salicylate and -30 mV in 50 mM-salicylate. 4. The apparent reversal potential for iK2 is shifted in a negative direction. The magnitude of this shift at a given salicylate concent;ation varies with the K concentration. In an extracellular K concentration of 2.7 mM an average shift of -18 mV occurs in 10 mM-salicylate; in 8 mM, the average shift is only -1 mV. 5. It is proposed that most of these effects may be produced by an increase in surface negative potential produced by the binding of salicylate to the cell membrane. This would produce the hyperpolarizing shift of activation curves for ionic current and, by increasing surface K activity, may lead to stimulation of the Na-K pump to produce an increase in the K gradient across the cell membrane.

The interaction of ouabain and salicylate on sheep cardiac muscle

1. The action potential duration in sheep ventricular fibres is rapidly diminished on exposure to 10(-6) M-ouabain. However, if 10--20 mM-sodium salicylate is added to the ouabain solution, glycoside-induced shortening is prevented, and a substantial increase in duration then occurs. Sodium salicylate also reverses the shortening effect of ouabain if applied after the glycoside has been allowed to act alone. 2. Sodium salicylate alone produces a much smaller prolongation than in the presence of ouabain. Alone and in the presence of ouabain it eventually increases the threshold and produces inexcitability. 3. Three other surface charge agents have been compared with salicylate: aminonaphthalene sulphonate, sodium dodecylsulphate and salicylamide, were unable to counter the actions of ouabain at the concentrations used. Since they also produced no changes in excitability it is likely that they did not bind significantly to the cell membrane. 4. In Purkinje fibres the reversal potential for the pacemaker current, iK2, is initially shifted in a negative direction in the presence of 10(-6) M-ouabain and 10 mM-salicylate instead of the positive shift expected with ouabain alone at this concentration. 5. In guinea-pig ventricle, salicylate alone reduces the duration of the action potential. This effect is rapidly reversible. Toxic levels of ouabain also reduce the action potential duration but this effect takes several hours to reverse. By contrast, the effects of salicylate and ouabain applied together are readily reversible. 6. It is suggested that the mechanism of these effects may depend on the ability of a surface negative charge agent like salicylate to increase the surface K+ concentration at the membrane and so protect the sodium-potassium pump from inhibition by large doses of ouabain.