Showing papers by "David A. Eisner published in 1983"

Oscillations of intracellular Ca2+ in mammalian cardiac muscle.

Abstract: Contraction of cardiac muscle depends on a transient rise of intracellular calcium concentration ([Ca2+]i) which is initiated by the action potential1. It has, however, also been suggested that [Ca2+]i can fluctuate in the absence of changes in membrane potential. The evidence for this is indirect and comes from observations of (1) fluctuations of contractile force in intact cells2–5, (2) spontaneous cellular movements6, and (3) spontaneous contractions in cells which have been skinned to remove the surface membrane7. The fluctuations in force are particularly prominent when the cell is Ca2+-loaded, and have been attributed to a Ca2+-induced Ca2+ release from the sarcoplasmic reticulum7. In these conditions of Ca2+-loading the normal cardiac contraction is followed by an aftercontraction8 which has been attributed to the synchronization of the fluctuations5. The rise of [Ca2+]i which is thought to underlie the aftercontraction also produces a transient inward current3. This current, which probably results from a Ca2+-activated nonspecific cation conductance9, has been implicated in the genesis of various cardiac arrhythmias. However, despite the potential importance of such fluctuations of [Ca2+]i their existence has, so far, only been inferred from tension measurement. Here we present direct measurements of such oscillations of [Ca2+]i.

Ca2+ ions can affect intracellular pH in mammalian cardiac muscle

Abstract: Although intracellular pH (pHi) has important effects on both the mechanical and electrical properties of cardiac muscle1–3, the control of pHi in the heart is still poorly understood. One important determinant of pHi appears to be the transmembrane Na+ gradient4,5. It has therefore been suggested that Na+–H+ exchange assists in the control of pHi in heart as has been proposed for other excitable cells6–8. However, pHi and the intracellular Ca2+ concentration ([Ca2+]i) are interdependent in a variety of tissues9–11 and it has been shown recently that pHi can affect [Ca2+]i in cardiac muscle5,12. As [Ca2+]i in cardiac muscle is also strongly influenced by the transmembrane Na+ gradient5 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.

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

Abstract: 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.

The effects of low sodium solutions on intracellular calcium concentration and tension in ferret ventricular muscle.

Abstract: Papillary muscles from the right ventricles of ferrets were micro-injected with the photoprotein aequorin. Both tension and the light emitted by the aequorin, which is a measure of the free intracellular Ca concentration [( Ca2+]i), were monitored. Exposure of the papillary muscle to a solution in which all the Na had been replaced by K (0 Na(K) solution) resulted in an increase in tension which subsequently slowly decreased. This contracture was associated with a large increase in [Ca2+]i followed by a decrease to a steady-state-level which was often significantly greater than that in Na-containing solutions. If choline, Li or Tris was used instead of K as a substitute for Na, both the contracture and the associated increase of [Ca2+]i were reduced. The effects of depolarization alone (by raising external K at constant Na concentration) were compared with those of Na removal alone (at constant external K concentration). Na removal contributes more than depolarization to the effects of a Na-free, K-containing solution on the contracture and rise of [Ca2+]i. Increasing intracellular Na concentration [( Na+]i), by exposure to strophanthidin (10 mumol/l), increased the magnitude of both the contracture and [Ca2+]i in 0 Na(K) solutions. Conversely, decreasing [Na+]i by exposure to a solution containing a decreased extracellular Na concentration [( Na+]o), decreased the contracture and [Ca2+]i. When contractures were produced by solutions with various [Na+]o, the size of the resulting contracture and [Ca2+]i were inversely related to [Na+]o. No contracture was seen unless [Na+]o was reduced to below 70 mmol/l. A decrease in the extracellular Ca concentration [( Ca2+]o) from 2 to 0.5 mmol/l or an increase to 8 mmol/l produced, respectively, large decreases and increases of the twitch and accompanying Ca transient. However, if [Ca2+]o was changed at the same time as Na was replaced by K there was little effect on either the contracture or the rise of [Ca2+]i. If [Ca2+]o was changed before replacing Na by K then increasing [Ca2+]o from 2 to 8 mmol/l decreased, and decreasing [Ca2+]o from 2 to 0.5 mmol/l increased, the rise of [Ca2+]i produced by replacing Na by K. The difference between this result and that obtained when [Ca2+]o was changed at the same time as Na was removed may be due to changes of [Na+]i produced by prolonged exposure to an altered [Ca2+]o.(ABSTRACT TRUNCATED AT 400 WORDS)

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

Abstract: 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.

Stimulation and inhibition by ATP and orthophosphate of the potassium: potassium exchange in resealed red cell ghosts

Abstract: The potassium:potassium (K-K) exchange through the sodium pump has been measured as the ouabain-sensitive 86Rb uptake by Na-free ghosts resealed to contain various concentrations of ATP, orthophosphate and K. The exchange is activated by increasing either internal or external K+ (Rb+) ion concentration. The activation curves can be described by simple Michaelis kinetics as: exchange = Vmax [K]/(Kapp + [K]). Increasing ATP concentration increases the apparent affinity for external K ions but decreases the apparent affinity for internal K (Ki+). Increasing [ATP] from 1 microM to 1 mM typically increases the Kapp for Ki+ from less than 1 mM to about 30 mM. Increasing ATP first activates the exchange but, after an optimal concentration is reached, further increase of ATP inhibits. The value of ATP concentration which gives the maximum flux depends on the internal and external K+ concentrations. The higher [Ki], the greater the optimal ATP concentration. Increasing external K (Rb) decreases the optimal ATP concentration. Increasing the concentration of orthophosphate (Pi) activates the exchange at high ATP but inhibits at low ATP concentration. A concentration of Pi which stimulates the exchange at high external K (Rb) can inhibit at low external K (Rb). These findings are in agreement with a consecutive or ping-pong model of the K-K exchange. We suggest that previous experiments have not shown the inhibitory effects of ATP and Pi because of the particular range of concentrations investigated.

Effects of Sodium Pump Inhibition on Contraction in Sheep Cardiac Purkinje Fibers

Abstract: Publisher Summary It is well established that inhibiting the sodium pump increases the force of contraction of cardiac muscle. This effect appears to be mediated via an increase of intracellular sodium activity. One theory for the mechanism of this effect involves a Na/Ca exchange. Increasing would produce an increase of Ca influx and decrease Ca efflux through such an exchange, leading to increased contraction. To investigate the mechanism of the positive inotropic effects of a raised sodium activity one needs to measure sodium activity and tension simultaneously. The chapter investigates the steady-state relationship between sodium activity and tension; it also characterizes the effects of sudden changes of sodium activity. The only feasible way to measure sodium activity with adequate time resolution is with an ion-selective microelectrode. Recessed-tip Na-sensitive microelectrodes are used in this chapter. The experiments are performed on voltage-clamped sheep cardiac Purkinje fibers. The use of the voltage clamp makes it possible to investigate the effects of changes of sodium activity without secondary effects because of the changes of membrane potential. Sodium pump activity is controlled by changing the external rubidium concentration.

Kinetic Evidence in Favor of a Consecutive Model of the Sodium Pump

Abstract: Publisher Summary Current models of the sodium pump suggest that the efflux of sodium ions from the cell is accompanied by the formation of a phosphoenzyme, which is then hydrolyzed in a reaction accelerated by external potassium ions. The potassium ions then become occluded within the enzyme, and their release at the extracellular surface is accelerated by ATP acting at a low-affinity site. If this scheme is correct, the apparent affinity of the overall reaction should depend on the ATP concentration. Experiments on nonsided preparations showed the expected result, as did recent experiments in the squid axon however, work in red cells has been negative. Another prediction of the model is that phosphate, by acting as a product inhibitor, should reduce the apparent affinity for external K ions.The effects of ATP and P i on the external K affinity are worth investigating. The chapter discusses the effects of these ligands on the influx of K into resealed ghosts prepared from human red blood cells.