Calcium- and voltage-activated plateau currents of cardiac Purkinje fibers.

We have used the two-microelectrode voltage-clamp technique to investigate the components of membrane current that contribute to the formation of the early part of the plateau phase of the action potential of calf cardiac Purkinje fibers. 3,4-Diaminopyridine (50 microM) reduced the net transient outward current elicited by depolarizations to potentials positive to -30 mV but had no consistent effect on contraction. We attribute this effect to the blockade of a voltage-activated transient potassium current component. Ryanodine (1 microM), an inhibitor of sarcoplasmic reticulum calcium release and intracellular calcium oscillations in Purkinje fibers (Sutko, J.L., and J.L. Kenyon. 1983. Journal of General Physiology. 82:385-404), had complex effects on membrane currents as it abolished phasic contractions. At early times during a depolarization (5-30 ms), ryanodine reduced the net outward current. We attribute this effect to the loss of a component of calcium-activated potassium current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. At later times during a depolarization (50-200 ms), ryanodine increased the net outward current. This effect was not seen in low-sodium solutions and we could not observe a reversal potential over a voltage range of -100 to +75 mV. These data suggest that the effect of ryanodine on the late membrane current is attributable to the loss of sodium-calcium exchange current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. Neither effect of ryanodine was dependent on chloride ions, which suggests that chloride ions do not carry the ryanodine-sensitive current components. Strontium (2.7 mM replacing calcium) and caffeine (10 mM), two other treatments that interfere with sarcoplasmic reticulum function, had effects in common with ryanodine. This supports the hypothesis that the effects of ryanodine may be attributed to the inhibition of sarcoplasmic reticulum calcium release.

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Initial Contractile Response of Isolated Rat Heart Cells to Halothane, Enflurane, and Isoflurane

Anesthesiology ◽

10.1097/00000542-199701000-00018 ◽

1997 ◽

Vol 86 (1) ◽

pp. 137-146 ◽

Cited By ~ 12

Author(s):

David M. Wheeler ◽

Todd R. Rice ◽

William H. duBell ◽

Harold A. Spurgeon

Keyword(s):

Sarcoplasmic Reticulum ◽

Intracellular Calcium ◽

Calcium Content ◽

Calcium Transient ◽

Temporary Increase ◽

Heart Cells ◽

Patch Clamp Technique ◽

Intracellular Calcium Transient ◽

Sarcoplasmic Reticulum Calcium ◽

In Cells

Background In several beating cardiac muscle preparations, a short-lived increase in twitch tension or amplitude has been observed when they were exposed abruptly to solutions containing halothane or enflurane. As exposure to the anesthetics was continued, the expected negative inotropic effect became evident after the short-lived increase in twitch. No such increase in twitch has been reported during exposure to isoflurane. It has been hypothesized that this short-lived increase in twitch is caused by an enhancement of calcium release from the sarcoplasmic reticulum, but other mechanisms have not been excluded. Methods Freshly isolated, single rat ventricular cells were stimulated to beat at room temperature and abruptly exposed to solutions containing halothane (0.25-0.64 mM), enflurane (0.69-1 mM), or isoflurane (0.31-0.54 mM). During these exposures, twitch amplitude was measured and intracellular calcium concentration was followed using the calcium-sensitive dye indo-1. In some experiments, the whole-cell patch-clamp technique was used to measure membrane current. In addition, in several cells the sarcoplasmic reticulum calcium content was assessed through the response to brief pulses of caffeine. Results Both the twitch amplitude and the intracellular calcium transient were increased temporarily in cells abruptly exposed to halothane or enflurane. No such behavior was found with isoflurane. After continued exposure to all three agents, both the twitch amplitude and the calcium transient were less than control. During the beats exhibiting an increase in twitch, no alteration in the relation between cell length (twitch amplitude) and the intracellular calcium transient was found compared with control conditions. In addition, the temporary increase in twitch amplitude occurred in cells contracting under voltage-clamp control when halothane was introduced, and it was not associated with any increase in the calcium current. The sarcoplasmic reticulum calcium content at the time of the halothane-induced increase in twitch also was not increased. Conclusions The short-lived increase in twitch after abrupt exposure to halothane or enflurane is related to increased intracellular calcium during the beat and not to any changes in myofilament sensitivity to calcium. Because these results eliminate most alternative explanations for this phenomenon, the authors conclude that halothane, and probably also enflurane, increases the fraction of calcium released from the sarcoplasmic reticulum with each heart beat. Isoflurane appears to lack this action.

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Effects of caffeine, tetracaine, and ryanodine on calcium-dependent oscillations in sheep cardiac Purkinje fibers.

The Journal of General Physiology ◽

10.1085/jgp.86.6.877 ◽

1985 ◽

Vol 86 (6) ◽

pp. 877-889 ◽

Cited By ~ 26

Author(s):

C J Nieman ◽

D A Eisner

Keyword(s):

Sarcoplasmic Reticulum ◽

Intracellular Calcium ◽

Calcium Concentration ◽

Oscillation Amplitude ◽

Membrane Current ◽

Inward Current ◽

Purkinje Fibers ◽

Calcium Dependent ◽

Cardiac Purkinje Fibers ◽

High Concentrations

Membrane current and tension were measured in voltage-clamped sheep cardiac Purkinje fibers. Elevating the intracellular calcium concentration ([Ca2+]i) results in oscillations of membrane current and tension both at rest and during stimulation. During stimulation, an oscillatory transient inward current and an after contraction follow repolarization. We have examined the effects on the oscillations of changing the extracellular calcium concentration ([Ca2+]o) and of adding various drugs. In agreement with previous work, high concentrations of drugs that affect the sarcoplasmic reticulum, namely caffeine (10-20 mM), tetracaine (1 mM), and ryanodine (10 microM), abolish the oscillations. However, at lower concentrations, these three drugs have different effects on the oscillations. Caffeine (1-2 mM) decreases the oscillation amplitude but increases the frequency. Tetracaine (100-500 microM) has little effect on the magnitude of the oscillations but decreases their frequency. Ryanodine, at all concentrations used (0.1-10 microM), eventually abolishes the oscillations but, in doing so, decreases the magnitude, leaving the frequency unaffected. When [Ca2+]o was changed in order to vary [Ca2+]i, both the frequency and the magnitude of the oscillations always changed in the same direction. This suggests that these three drugs have effects in addition to just changing [Ca2+]i.

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The effects of chloride ions in excitation-contraction coupling and sarcoplasmic reticulum calcium release in twitch muscle fibre

Journal of Muscle Research and Cell Motility ◽

10.1007/bf00121162 ◽

1994 ◽

Vol 15 (5) ◽

pp. 563-571 ◽

Cited By ~ 4

Author(s):

Bruno Allard ◽

Oger Rougier

Keyword(s):

Sarcoplasmic Reticulum ◽

Muscle Fibre ◽

Calcium Release ◽

Chloride Ions ◽

Excitation Contraction Coupling ◽

Contraction Coupling ◽

Sarcoplasmic Reticulum Calcium

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The correlation between the increase in slow outward current and in contraction induced by caffeine, ryanodine, and rapid cooling in voltage-clamped frog muscle fibers

Canadian Journal of Physiology and Pharmacology ◽

10.1139/y91-130 ◽

1991 ◽

Vol 69 (6) ◽

pp. 859-864 ◽

Cited By ~ 1

Author(s):

Mohammad Nasri-Sebdani ◽

Flavien Traoré ◽

Aklesso Mouzou ◽

Christian Cognard ◽

Daniel Potreau ◽

...

Keyword(s):

Sarcoplasmic Reticulum ◽

Calcium Release ◽

Muscle Fibers ◽

Outward Current ◽

Frog Muscle ◽

Rapid Cooling ◽

Key Words Skeletal Muscle ◽

Parallel Increase ◽

The One ◽

Sarcoplasmic Reticulum Calcium

The effects of caffeine, ryanodine, and rapid cooling were tested on the depolarization-induced contraction and the apamin-insensitive slow outward current (Iso) of voltage-clamped (double mannitol gap) single frog muscle fibers. Subthreshold caffeine concentrations (0.5–2 mM) induced a monotonic increase in contractile and Iso amplitude. Whatever the concentration, the increase in contraction was roughly twice the one in current. Similar results were obtained upon rapid cooling (20–4 °C) in the presence of 0.5 mM caffeine. In the absence of external Na+ (choline-substituted) 10−5 M ryanodine induced a delayed increase (≈30 min) in contraction and in current, shortly before the development of a drastic and irreversible contracture. Here again, the increase in contraction was twice that in current. In the presence of 5 mM tetraethylammonium (TEA) and (or) 25 nM charybdotoxin, 2 mM caffeine still induced a strong facilitating effect on contraction but the parallel increase in current was strongly reduced. The linear relationship between the increase in current and contractile amplitude has a slope ≈0.5 (whatever the drug used to increase contractility); it is ≈0.1 in the presence of TEA and (or) charybdotoxin. In conclusion, provided the changes in contractile amplitude are caused by parallel changes in depolarization-induced sarcoplasmic reticulum Ca2+ release, about 50% of the apamin-insensitive Iso is controlled by internal Ca2+ release. The main part of this current corresponds to the TEA- and charybdotoxin-sensitive component of Iso.Key words: skeletal muscle, voltage clamp, Ca2+-dependent K+ current, sarcoplasmic reticulum calcium release, caffeine, ryanodine, charybdotoxin.

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Ryanodine modification of cardiac muscle responses to potassium-free solutions. Evidence for inhibition of sarcoplasmic reticulum calcium release.

The Journal of General Physiology ◽

10.1085/jgp.82.3.385 ◽

1983 ◽

Vol 82 (3) ◽

pp. 385-404 ◽

Cited By ~ 111

Author(s):

J L Sutko ◽

J L Kenyon

Keyword(s):

Sarcoplasmic Reticulum ◽

Cardiac Muscle ◽

Calcium Release ◽

Outward Current ◽

Slow Inward Current ◽

Papillary Muscles ◽

Variable Effect ◽

Purkinje Fibers ◽

Steady State Current ◽

Inward Currents

To test whether ryanodine blocks the release of calcium from the sarcoplasmic reticulum in cardiac muscle, we examined its effects on the aftercontractions and transient depolarizations or transient inward currents developed by guinea pig papillary muscles and voltage-clamped calf cardiac Purkinje fibers in potassium-free solutions. Ryanodine (0.1-1.0 microM) abolished or prevented aftercontractions and transient depolarizations by the papillary muscles without affecting any of the other sequelae of potassium removal. In the presence of 4.7 mM potassium and at a stimulation rate of 1 Hz, ryanodine had only a small variable effect on papillary muscle force development and action potential characteristics. In calf Purkinje fibers, ryanodine (1 nM-1 microM) completely blocked the aftercontractions and transient inward currents without altering the steady state current-voltage relationship. Ryanodine also abolished the twitch in potassium-free solutions, but it enhanced the tonic force during depolarizing voltage-clamp steps. This latter effect was dependent on the combination of ryanodine and potassium-free solutions. The slow inward current was not blocked by 1 microM ryanodine, but ryanodine did appear to abolish an outward current that remained in the presence of 0.5 mM 4-aminopyridine. Our observations are consistent with the hypothesis that ryanodine, by inhibiting the release of calcium from the sarcoplasmic reticulum, prevents the oscillations in intracellular calcium that activate the transient inward currents and aftercontractions associated with calcium overload states.

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Regulation of twitch tension in sheep cardiac Purkinje fibers during calcium overload

AJP Heart and Circulatory Physiology ◽

10.1152/ajpheart.1987.253.6.h1540 ◽

1987 ◽

Vol 253 (6) ◽

pp. H1540-H1547 ◽

Cited By ~ 4

Author(s):

J. R. Berlin ◽

M. B. Cannell ◽

W. J. Lederer

Keyword(s):

Sarcoplasmic Reticulum ◽

Calcium Current ◽

Calcium Release ◽

Calcium Overload ◽

Purkinje Fibers ◽

Cellular Processes ◽

Cardiac Purkinje Fibers ◽

Sodium Pump Inhibition ◽

Spontaneous Fluctuations ◽

Oscillatory Period

The dependence of twitch tension on the interval (delta t) between depolarizations was examined in voltage-clamped sheep cardiac Purkinje fibers under conditions of calcium overload. During the development of calcium overload (produced by sodium-pump inhibition), twitch amplitude changes from a monotonic function of delta t to an oscillatory one. We investigated the cellular processes underlying this oscillatory relationship. Measurable calcium current was blocked by D 600 (25 microM), but neither the twitch nor the oscillatory dependence of twitch tension on delta t was abolished. Caffeine (2 mM), applied to modify sarcoplasmic reticulum function, decreased the oscillatory period of the twitch/interval relationship as it increased the frequency of spontaneous fluctuations of resting tension. Our results suggest that the oscillatory relationship between twitch amplitude and delta t is not caused by changes in the calcium current per se but rather by fluctuations in the amount of releasable calcium in the sarcoplasmic reticulum. Additionally, we conclude that the calcium current may not be a necessary prerequisite for depolarization to trigger calcium release from the sarcoplasmic reticulum under conditions of calcium overload.

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