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W J Lederer

Publications and source records attributed to W J Lederer.

At least 145 records · Page 8Linked to original sources
PubMed1986

The arrhythmogenic current ITI in the absence of electrogenic sodium-calcium exchange in sheep cardiac Purkinje fibres.

Sheep cardiac Purkinje fibres were voltage clamped with a two-microelectrode technique. Under conditions that are known to elevate intracellular calcium (0 mM-external potassium), membrane currents were examined. In the above conditions, a brief depolarizing pulse leads to an oscillatory inward current (ITI) which peaks at about 300 ms after the repolarization. An after-contraction is also observed, the peak of which occurs about 80 ms after the peak of ITI. This result is in accord with the results of Kass, Lederer, Tsien & Weingart (1978a). We replaced external sodium with an isotonic CaCl2 solution to remove the sodium-calcium exchange mechanism as a possible current carrier for ITI. In the steady state under these conditions an oscillatory membrane current and after-contraction are seen following repolarization. This current was identified as ITI on the basis of its temporal relation to both the repolarization step and the after-contraction. In isotonic CaCl2, ITI has a reversal potential of -37 mV. Because of this fact ITI cannot be explained by an electrogenic sodium-calcium exchange mechanism alone. The reversal potential suggests that ITI arises from a channel which is permeable to both potassium and calcium. Fluctuations of membrane current and of tension were recorded in the steady state at different holding potentials. Power spectral analysis showed that the current fluctuations were at a minimum at a holding potential of -37 mV. Tension fluctuations were, however, relatively constant over the range of membrane potentials examined (-17 to -70 mV). The peak power of the current fluctuations occurred at about 1.5 Hz (at a holding potential of -70 mV). This peak shifted towards higher frequencies with increasing depolarization. A similar shift in frequency was observed for the tension fluctuations. Cross-correlations between membrane current and tension were calculated for various steady membrane potentials. This analysis shows that the current fluctuations are associated with the tension fluctuations, each with a principal period of about 0.5 s. This analysis also shows that at potentials more negative than the reversal potential of ITI, increasing tension is associated with increasing inward current and that the tension fluctuations follow current fluctuations by about 70 ms. At potentials positive to the reversal potential of ITI, increasing tension was associated with increasing outward current. This analysis therefore indicates that the fluctuations in membrane current reverse at a potential similar to the reversal potential of ITI.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Effects of membrane potential on intracellular calcium concentration in sheep Purkinje fibres in sodium-free solutions.

1. The intracellular Ca2+ concentration [( Ca2+]i) was measured in voltage-clamped sheep cardiac Purkinje fibers while recording tension simultaneously. 2. When [Na+]i was elevated (by Na+-K+ pump inhibition) depolarization produced an increase of tonic tension. 3. Replacement of external Na+ by Li+ or choline produced a contracture which then relaxed spontaneously. Following this relaxation, depolarization either had no effect on tonic tension or produced a small decrease. 4. When external Na+ was replaced by Ca2+, depolarization (over the range -120 to -20 mV) produced a decrease of tonic tension and [Ca2+]i. Hyperpolarization increased tonic tension and [Ca2+]i. 5. An after-contraction and accompanying increase of [Ca2+]i were produced by repolarization in both Na+-free and Na+-containing solution. This eliminates the possibility that the stimulus for the after-contraction is the increase of [Ca2+]i during the depolarization and suggests that the stimulus may be the change of membrane potential. 6. The increase of [Ca2+]i on hyperpolarization seen in Na+-free solutions persisted in the presence of ryanodine. 7. These results show, in contrast to previous work, that in Na+-free solutions tonic tension is still sensitive to membrane potential. The results support the hypothesis that, in Na+-containing solutions, the increase of tonic tension on depolarization results from a voltage-dependent Na+-Ca2+ exchange. The reduction of tonic tension on depolarization in Na+-free solutions may be due to the decrease of the electrochemical gradient for Ca2+ to enter the cell.

Aequorin↗
PubMed1985

The effects of intracellular Na on contraction and intracellular pH in mammalian cardiac muscle.

Intracellular Na and pH were measured with recessed-tip ion-selective microelectrodes in voltage-clamped sheep cardiac Purkinje fibers. Intracellular Na activity (aiNa) was elevated by inhibiting the Na/K pump. This produced an increase of twitch tension that had a steep dependence on the increase of aiNa. These effects of aiNa on twitch tension are probably mediated by an Na-Ca exchange. An increase of aiNa also produced a component of tonic tension that appears to be produced directly by the Na-Ca exchange. The dependence of tonic tension and aiNa on membrane potential suggests that this exchange process may be voltage-sensitive. The increase of aiNa is associated with an intracellular acidification that appears to be secondary to an increase of [Ca2+]i produced by Na-Ca exchange. Therefore, as well as affecting [Ca2+]i, Na-Ca exchange can under some circumstances influence pHi indirectly, and this complicates the interpretation of changes in tension, since protons and Ca ions have opposite effects on contractile force.

Animals↗
PubMed1985

Na-Ca exchange: stoichiometry and electrogenicity.

This review discusses the evidence concerning the stoichiometry of Na-Ca exchange. In particular we consider whether the Na-Ca exchange has been shown to transport more than two Na+ ions per Ca2+ ion and therefore whether it generates an electric current. The first part of this review discusses both direct and indirect evidence concerning the stoichiometry of the exchange and its possible voltage dependence. We find that, although there is some evidence suggesting that more than two Na+ ions may exchange for each Ca2+ ion, most of the available evidence is equivocal and cannot fix the stoichiometry precisely. Furthermore, using a simple and explicit circulating carrier model for the Na-Ca exchange, we show that the effect of membrane potential on the Na-Ca exchange may be considerably more complicated than is generally believed. In particular we find that both electrogenic and electroneutral exchanges will be affected by membrane potential. We therefore conclude that the demonstration of the voltage dependence of the Na-Ca exchange does not necessarily imply that it is electrogenic. Additionally, this analysis shows that, apart from a restricted range near thermodynamic equilibrium, it is impossible to predict either the magnitude or the direction of the effects of membrane potential on the exchange. In the second part of the review we consider whether any known membrane currents may be attributed to Na-Ca exchange. We show, in contrast to previous suggestions, that the Na-Ca exchange can theoretically produce a current that appears to be activated by intracellular Ca and that has a reversal potential. However, the experimental demonstration that a given current is produced by Na-Ca exchange is hampered by the existence of other Ca- and Na-dependent currents. In conclusion, we feel that there is no evidence that allows any particular membrane current to be unambiguously identified with the Na-Ca exchange.

Action Potentials↗
PubMed1985

Lidocaine's negative inotropic and antiarrhythmic actions. Dependence on shortening of action potential duration and reduction of intracellular sodium activity.

The mechanisms by which lidocaine brings about negative inotropic effects and antiarrhythmic actions in the heart have been examined. Using sheep cardiac Purkinje fibers, we studied the effects of "therapeutic" concentrations of lidocaine (20 microM lidocaine = 5.4 micrograms/ml) on electrical activity, intracellular sodium activity, and tension. For the preparation with a normal, physiological level of intracellular sodium activity (5-9 mM), the application of lidocaine leads to the following actions: action potential duration is decreased, intracellular sodium activity falls, and twitch tension is reduced. If intracellular sodium activity is first elevated (e.g., by sodium pump inhibition) so that arrhythmogenic transient depolarizations (and the underlying transient inward current are seen) then lidocaine has the following actions: The action potential duration is reduced. There is a reduction of the magnitude of the arrhythmogenic transient depolarization (or the underlying membrane current transient inward current). The magnitude of the aftercontraction that accompanies the transient depolarization (or transient inward current) is reduced. If, after sodium pump inhibition, intracellular sodium activity is still rising-then, the application of lidocaine leads to a reduction of the rate of rise of intracellular sodium. From these results, parallel voltage-clamp experiments, and recent work by others, we conclude that lidocaine leads to the reduction of the arrhythmogenic transient depolarization and a reduction of twitch tension by decreasing the inward sodium current. These actions are mediated by a reduction in action potential duration and a reduction of intracellular sodium activity and of intracellular calcium activity (by the sodium-calcium exchange mechanism).

Action Potentials↗
PubMed1985

Ryanodine block of calcium oscillations in heart muscle and the sodium-tension relationship.

The control of tension is examined in cardiac Purkinje fibers. We show, in accordance with earlier results (14, 15), that tension produced by this preparation is a steep power function of intracellular sodium. With the aid of ryanodine, a pharmacological agent that blocks spontaneous and spatially asynchronous calcium release from the sarcoplasmic reticulum (SR), we investigate the influence that such calcium fluctuations have on tension. We find that even when we control for alterations of intracellular pH, the presence of such fluctuations reduces the dependence of tension on intracellular sodium. We present a simple model that can explain how the presence of oscillations of intracellular calcium leads to a reduction of the slope of the tension-calcium relationship. We show, furthermore, that when the oscillations are spatially asynchronous, this reduction of slope is even greater. The modeling takes account of the known relationships between tension and calcium and tension and sarcomere length. We conclude that the effect of ryanodine to steepen the tension-sodium relationship can be explained by ryanodine's blocking calcium release from the SR, thereby abolishing oscillations of intracellular calcium.

Alkaloids↗
PubMed1984

Sodium pump stoicheiometry determined by simultaneous measurements of sodium efflux and membrane current in barnacle.

Ouabain-sensitive Na efflux, membrane potential and membrane current were measured in single, perfused muscle cells taken from the giant barnacle, Balanus nubilus. This preparation permits control of the intracellular and extracellular solution composition as well as control of the membrane potential while measuring ion fluxes across the plasma membrane. The addition of ouabain (10(-4) M) to the extracellular solution produces a rapid depolarization of membrane potential (1-4 mV) and a simultaneous and proportional reduction of Na efflux (10-40 pmol/s). Ouabain-induced changes in membrane potential or Na efflux do not depend on the presence of extracellular Na. Under voltage control, the application of ouabain (10(-4) M) produces a rapid monotonic fall in outward current (1-3 microA) and a simultaneous fall in Na efflux (10-40 pmol/s). The fraction of ouabain-dependent Na efflux that appears as outward current is constant in any given preparation as the Na-pump turnover rate varies. Over a limited range, changes in membrane potential do not affect ouabain-sensitive Na efflux. The ouabain-sensitive Na efflux and membrane current are not altered by the presence of 50 mM-internal tetraethylammonium (TEA) ions. We conclude that the Na pump is electrogenic in barnacle muscle and that 49 +/- 10% of the extruded Na+ leaves the intracellular compartment as uncompensated charge. We find that the transport stoicheiometry of Na:K, calculated from the ouabain-dependent changes in membrane current and Na efflux, is between 3:2 and slightly more than 2:1.

Animals↗
PubMed1984

The quantitative relationship between twitch tension and intracellular sodium activity in sheep cardiac Purkinje fibres.

Tension was measured in voltage clamped sheep cardiac Purkinje fibres while simultaneously measuring the intracellular Na activity (aiNa) with a recessed-tip, Na-selective micro-electrode. Inhibiting the Na-K pump either by exposing the preparation to a K-free solution or by adding the cardioactive steroid strophanthidin increased both aiNa and twitch tension and resulted in the development of tonic tension, after-contractions and a transient inward current (ITI). The increase of twitch tension was present at lower aiNa than that required to produce the other phenomena. The relationship between the magnitude of the twitch tension and aiNa was always non-linear. Twitch tension increased steeply with aiNa at first but the relationship flattened off at higher aiNa and tension eventually decreased. Over the steep range, the relationship between tension and aiNa could be represented as: twitch tension = b (aiNa)y where y had a mean value of 3.2. Changing membrane potential or [Ca2+]o changed b but had little effect on y. Mn (2 mmol/l) greatly decreased twitch tension but, at least initially, had little effect on tonic tension. The steep relationship between twitch tension and aiNa was seen, irrespective of whether the Na-K pump was inhibited either by exposure to K-free solution or to strophanthidin and whether the relationship was measured either when aiNa was increasing or after it had reached a steady state. The steep dependence of twitch tension on aiNa observed in the present work means that manoeuvres which produce even small changes of aiNa will have significant effects on contraction.

Animals↗
PubMed1983

Ca2+ ions can affect intracellular pH in mammalian cardiac muscle.

Although intracellular pH (pHi) has important effects on both the mechanical and electrical properties of cardiac muscle, the control of pHi in the heart is still poorly understood. One important determinant of pHi appears to be the transmembrane Na+ gradient. It has therefore been suggested that Na+-H+ exchange assists in the control of pHi in heart as has been proposed for other excitable cells. However, pHi and the intracellular Ca2+ concentration ([Ca2+]i) are interdependent in a variety of tissues and it has been shown recently that pHi can affect [Ca2+]i in cardiac muscle. As [Ca2+]i in cardiac muscle is also strongly influenced by the transmembrane Na+ gradient it is possible that the apparent Na+-dependence of pHi is secondary to changes in [Ca2+]i. Previous work in cardiac muscle has not been able to separate the effects of Na+-H+ exchange and [Ca2+]i on pHi (refs 4,5). Here we demonstrate in cardiac muscle that an increase in [Ca2+]i produces an intracellular acidification which cannot be ascribed to Na+-H+ exchange.

Animals↗
PubMed1983

Piezoelectric translator. A simple and inexpensive device to move microelectrodes and micropipettes small distances rapidly.

A device is described that is capable of rapidly moving microelectrodes and micropipettes over distances up to 15 mu. This piezoelectric transLator uses the diaphragm from virtually any available piezoelectric buzzer in combination with simple physical support and drive electronics. All of the necessary details for the construction of this small device are presented. Each finished unit is about 2 cm long with a diameter of 2 cm and can be readily adapted to existing manipulators. The translator has been found useful in aiding the independent penetration by one or more microelectrodes of single cells or of more complicated multicellular preparations (including those that lie behind a connective tissue layer). This new device offers fine control of microelectrode motion that cannot be obtained by the other methods used to aid microelectrode and micropipette penetration of cell membranes (e.g. capacitance overcompensation--"ringing in"' or "tickling"--or tapping the manipulator base). Finally, the device described in this paper is extremely simple and inexpensive to build.

Cytological Techniques↗
PubMed1983

The control of tonic tension by membrane potential and intracellular sodium activity in the sheep cardiac Purkinje fibre.

Intracellular Na activity (aiNa) was measured with recessed-tip, Na-selective micro-electrodes in voltage-clamped sheep cardiac Purkinje fibres. Tension was measured simultaneously. aiNa was increased reversibly either by exposing the preparation to K-free, Rb-free solution of by adding the cardioactive steroid strophanthidin. An increase of aiNa produced an increase of tonic tension which was larger at depolarized membrane potentials. At sufficiently negative membrane potentials, changes of aiNa (over the range 6-30 mM) had no effect on tonic tension. Therefore, both an increase of aiNa and a depolarization are required to increase tonic tension. It is concluded that either a low level of aiNa or a large negative membrane potential is sufficient to maintain a low intracellular Ca concentration. Tonic tension was measured as a function of aiNa. At a given membrane potential the relationship can be described empirically by an equation of the form: tonic tension = b(aiNa)y, where y is a constant and b depends on membrane potential. In five experiments y was found to be 3.7 +/- 0.7 (mean +/- S.E.M.) over a range of potentials from -60 to -10 mV. Tonic tension was measured as a function of membrane potential. At a given aiNa the relationship can be described approximately as: tonic tension = k exp (aV), where a is a constant and k depends on aiNa. In five experiments a was found to be 0.06 +/- 0.01 mV-1 (mean +/- S.E.M.). A depolarization of 10 mV increases tonic tension by the same amount as does an increase of aiNa that is equivalent to a 3.7 mV change of the Na equilibrium potential, ENa. Hence ENa is nearly 3 times more effective than membrane potential in controlling tonic tension. During a prolonged depolarization (several minutes) the initial increase of tonic tension decays gradually. This is associated with a fall of aiNa. The relationship between tonic tension and aiNa is similar to that seen when aiNa is increased by inhibiting the Na pump. It is concluded that the fall of aiNa is responsible for the decay of tonic tension. The changes of tonic tension reported in this paper are consistent with the effects of aiNa and membrane potential on a voltage-dependent Na-Ca exchange. The possibility that a voltage-dependent Ca channel contributes to tonic tension is also discussed.

Animals↗
PubMed1983

The role of intracellular sodium activity in the anti-arrhythmic action of local anaesthetics in sheep Purkinje fibres.

The effects of lidocaine have been examined on the arrhythmogenic transient inward current (ITI) in voltage-clamped sheep cardiac Purkinje fibres. Tension and intracellular Na activity (aiNa) were measured simultaneously. The addition of lidocaine (200-300 microM) produced an immediate decrease of inward holding current and a gradual fall of aiNa. The relative magnitudes of the changes of current and aiNa were shown to be consistent with the outward shift of current representing principally a reduction of inward Na current. The Na pump was inhibited by reducing the external Rb concentration in a K-free solution. This produced an after-contraction and transient inward current (ITI) along with a rise of aiNa. The subsequent addition of lidocaine decreased the magnitude of ITI and the after-contraction while decreasing aiNa. Tetrodotoxin (TTX) had qualitatively similar effects to lidocaine on inward holding current, aiNa, ITI and the after-contraction. When aiNa was changed by (i) lidocaine, (ii) TTX or (iii) small changes of external Rb concentration, a hysteresis was seen in the relationship between aiNa and ITI or after-contraction. The hysteresis was similar to that previously found between aiNa and contraction (Eisner, Lederer & Vaughan-Jones, 1981). Despite this hysteresis, neither lidocaine nor TTX affected the relationship between magnitudes of ITI and the after-contraction. It is suggested that the fall of aiNa is a major factor in the reduction of ITI by lidocaine. These results are discussed in relation to the anti-arrhythmic actions of lidocaine.

Action Potentials↗
PubMed1983

Effects of extracellular sodium on calcium efflux and membrane current in single muscle cells from the barnacle.

The actions of extracellular sodium (Nao) on membrane potential, membrane current, membrane conductance and Ca efflux were examined in single muscle cells from the giant barnacle, Balanus nubilus. The intracellular compartment was perfused to facilitate the control of intracellular constituents including calcium ions (Ca2i+). Changing Nao has no large effect on Ca efflux when free intracellular calcium activity, [Ca2+]i, is low (about 0.1 microM). However, increasing [Ca2+]i leads to the development of Nao-dependent Ca efflux as well as to an augmentation in Nao-independent Ca efflux. Reducing Nao (using Li+ as a substitute cation) leads to a depolarization of the membrane when [Ca2+]i is low (about 0.1 microM). Increasing [Ca2+]i causes the membrane to depolarize. With [Ca2+]i at about 10.0 microM, reduction of Nao produces a hyperpolarization of the membrane. Significant Nao-dependent inward current is seen when [Ca2+]i is high. This current is large with respect to the Nao-dependent changes in Ca efflux (about 1 microA per p-mole/sec). The Ca2i+-activated, Nao-dependent changes in Ca efflux and membrane current are not sensitive to La3o+. However, Lao3+ does inhibit a fraction of the Cai2+-activated changes in membrane current and Ca efflux which are not dependent on Nao. Over a limited range of membrane potential Ca efflux is not voltage-dependent. Possible relationships between the Nao-dependent changes in Ca efflux and Nao-dependent changes in membrane potential or current are discussed. We find that these changes cannot be readily interpreted in terms of a single transport mechanism.

Animals↗
PubMed1982

The effects of sodium pump activity on the slow inward current in sheep cardiac Purkinje fibres.

The effects of Na pump activity on the slow inward current, Isi, magnitude and twitch tension were investigated in sheep cardiac Purkinje fibres. A two-microelectrode voltage-clamp method was used, tension being measured simultaneously. Na pump activity was lowered either by reducing the extracellular K concentration, [K]O, or by applying the cardiotonic steroid strophanthidin. Reduction of [K]O from 4 to 0 mM leads to time-dependent increases in Isi magnitude and twitch tension. The increases of Isi and tension could be reversed by adding Tl, Rb, Cs or NH4 ions to the K-free superfusate. The actions of these ions are attributed to the known ability of these cations to activate the external site of the Na pump. This conclusion is supported by the observation that such activator cations do not reverse the increases in Isi and tension produced by strophanthidin. We conclude that the effects of low [K]O on Isi are mediated by Na pump inhibition. Similarly the Na pump inhibition produced by strophanthidin increases Isi and tension, although, in this case, other mechanisms may also contribute. Measurements of the activity of the electrogenic Na pump show that elevated intracellular Na ion concentration secondary to Na pump inhibition and not the instantaneous Na pump turnover rate mediates the increase in Isi magnitude.

Animals↗