Showing papers by "David A. Eisner published in 1986"

A study of intracellular calcium oscillations in sheep cardiac Purkinje fibres measured at the single cell level.

Abstract: Previous work has shown that an elevation of intracellular calcium concentration [( Ca2+]i) produces spontaneous oscillations of [Ca2+]i. However the fact that the oscillations are unsynchronized between different cells has made it difficult to study them. We have therefore injected only one cell in a Purkinje fibre with aequorin in order to avoid these problems. The addition of strophanthidin (10 microM) produced an increase of mean aequorin light over the course of several minutes. During this period spontaneous oscillations of light developed and, with time, their frequency and magnitude increased. The oscillations could first be seen at levels of [Ca2+]i of less than 1 microM. The amplitude of the oscillations of [Ca2+]i could be up to 10 microM and was modulated at a slow rate (about 0.3-0.5 Hz). This suggests that, even within one cell, different regions may oscillate at different frequencies. Elevating [Ca+]o, removing extracellular Na+, or depolarization increased the magnitude of the aequorin light oscillations. Converting the records to [Ca2+]i showed that this increase in the magnitude of the aequorin oscillations was accompanied by a real increase of mean [Ca2+]i and of the magnitude of the oscillations [Ca2+]i. The frequency of the oscillations increased up to a point but saturated at a maximum value of 3-4 Hz. Since previous experiments have used the mean aequorin light to estimate mean [Ca2+]i, we have calculated the error produced in this calculation by the presence of [Ca2+]i oscillations. We estimate that the error is greatest at low levels of Ca2+ loading when the frequency of the oscillations is low. However, at higher Ca2+ loads, when the frequency is above 2 Hz, the error is probably less than 10%. If oscillations were produced by removal of external Na+ after the application of strophanthidin, then either ryanodine or caffeine abolished the oscillations. Furthermore, in both cases, the resulting steady level of [Ca2+]i was similar to the mean level before the addition of the drugs. In another series of experiments we examined the effects of these drugs on oscillations produced by the application of strophanthidin. Caffeine produced a transient increase in both the frequency of the oscillations and mean [Ca2+]i before abolishing the oscillations and decreasing [Ca2+]i to below the level in the absence of caffeine. In contrast ryanodine gradually decreased both the mean [Ca2+]i and the frequency until the oscillations were abolished. During this period of slowing of the oscillations their magnitude was often increased.

Metabolic consequences of increasing intracellular calcium and force production in perfused ferret hearts.

Abstract: 31P nuclear magnetic resonance was used to measure the relative concentrations of phosphorus-containing metabolites in Langendorff-perfused ferret hearts. Intracellular concentrations of inorganic phosphate ([Pi]i), phosphocreatine [( PCr]i), ATP ([ATP]i) and [H+] (pHi) were determined. Exposure of the heart to strophanthidin (10-40 microM) produced an increase in developed pressure over 5-10 min. In the presence of strophanthidin, [ATP]i was unchanged, [PCr]i showed a small fall, [Pi]i showed a small rise and there was a small acidosis. Exposure of the heart to a solution in which all the Na had been replaced by K (0 Na(K) solution) produced an increase of resting pressure which then decayed. During this contracture [PCr]i fell transiently and [Pi]i rose transiently with approximately the same time course as the contracture. However, [ATP]i remained constant throughout. The exposure to the 0 Na(K) solution also produced an intracellular acidosis. The changes in [PCr]i and [Pi]i and the intracellular acidosis were all increased during perfusion with 0 Na(K) if the heart had previously been exposed to strophanthidin. The efflux of lactate from the heart was increased during the exposure to the 0 Na(K) solution. The magnitude of this increase was enhanced by prior exposure to strophanthidin. The increase of intracellular lactate (calculated from this efflux) was sufficient to account for the observed intracellular acidification. An increase of lactate efflux could also be measured when an isolated papillary muscle was exposed to the 0 Na(K) solution. The intracellular acidification produced by Na removal was substantially decreased after prevention of glycolysis either by substrate depletion or by the application of iodoacetate. Elevation of the extracellular calcium concentration ([Ca2+]o) produced a large increase of developed pressure which was accompanied by a small transient increase of [Pi]i, a decrease of [PCr]i and a small intracellular acidosis. There was also an increase of lactate efflux. After exposure to strophanthidin the same increase of [Ca2+]o decreased developed pressure. The associated rise in [Pi]i and fall in [PCr]i were increased but there was no significant acidosis under these conditions. In addition to other explanations (Allen, Eisner, Pirolo & Smith, 1985a), it is likely that the fall of force is partly accounted for by the rise of [Pi]i.

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

Abstract: 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.

The role of intracellular Ca ions in the therapeutic and toxic effects of cardiac glycosides and catecholamines.

Abstract: Many inotropic maneuvers act by increasing the intracellular calcium concentration [( Ca2+]i). The present report illustrates this with respect to the positive inotropic effects of cardiac glycosides and catecholamines. It is shown that the increased contractility produced by cardiac glycosides is accompanied by an increase in intracellular Na concentration and, furthermore, that the relationship between contraction and Na is very steep. This steep dependence, which may result from a Na-Ca exchange which exchanges several Na ions per Ca, means that maneuvers that have only small effects on Na will have significant effects on contraction. Cardiac glycosides also produce abnormal pacemaker activity and cardiac arrhythmias. These originate from a transient inward current activated by oscillations of [Ca2+]i, which result from spontaneous oscillatory release of Ca ions from the sarcoplasmic reticulum. The local anesthetic group of antiarrhythmic agents abolishes the transient inward current. Catecholamines also increase systolic [Ca2+]i and, in high enough concentrations, can produce oscillations of [Ca2+]i. This tendency of glycosides and catecholamines to produce arrhythmogenic oscillations of [Ca2+]i is a major limitation to their use. Therefore, inotropic agents that act by means other than increasing [Ca2+]i may be of great efficacy.

Measurement of intracellular calcium during the development and relaxation of tonic tension in sheep Purkinje fibres.

Abstract: The photoprotein aequorin was micro-injected into several cells in a sheep Purkinje fibre. The intracellular Ca concentration [( Ca2+]i) was measured from the resulting light emission. Inhibition of the Na-K pump with strophanthidin resulted in the development of tonic tension which increased on depolarization. This increase was accompanied by an increase of aequorin light. Increasing external Ca concentration [( Ca2+]o) or the magnitude of the depolarization increased both light and tension. If the depolarizing pulse was maintained for several minutes then both tonic tension and aequorin light slowly decayed. The relationship between tension and light was unaffected during this decay. On repolarization the light decayed to below the level before the depolarization before slowly increasing. During this period a test depolarization produced increases of aequorin light and tension which were smaller than control. The application of ryanodine (1-10 microM) abolished all components of tension other than the tonic component. Under these conditions the time course of the increase of tonic tension and aequorin light on depolarization was sufficiently slow to be measured. In most (five out of six) experiments the relationship between light and tension during this development of tonic tension was found to be similar to that during the subsequent spontaneous decay. However, in one experiment the decay of force was greater than could be accounted for by the fall of [Ca2+]i. It is concluded that most of the spontaneous relaxation of tonic tension can be attributed to a fall of [Ca2+]i rather than to other explanations such as an intracellular acidification or increase of inorganic phosphate concentration.

Electrophysiological effects of cardiac glycosides

Abstract: In this paper we review the effects of cardiac glycosides on the electrophysiological properties of mammalian cardiac muscle. Cardiac glycosides produce an initial prolongation of the action potential due to inhibition of the electrogenic Na-K pump current. This is then followed by a gradual decrease of action potential duration to less than control length. The origin of this decrease is uncertain. Cardiac glycosides also produce a transient depolarization which follows the action potential and which can produce a spontaneous action potential. This transient depolarization is produced by a transient inward current which results from a Ca-activated conductance. The triggering increase of [Ca2+]i originates from the spontaneous release of Ca ions from the sarcoplasmic reticulum (s. r.). Both this spontaneous release and the resulting transient inward current can be abolished by inhibitors of s. r. function such as caffeine and ryanodine. As well as these oscillations of [Ca2+]i which follow repolarization, there are also spontaneous oscillations in the absence of stimulation. These spontaneous oscillations, which also result from release of Ca from the sarcoplasmic reticulum, interfere with the systolic rise of Ca produced by stimulation. Thus the Ca oscillations are not only responsible for the cardiac arrhythmias produced by digitalis but also compromise the magnitude of the positive inotropic effect.