Modulation of intracellular calcium concentration using non-excitatory electrical signals applied to the tissue

Abstract

According to a method and device for modulating intracellular calcium concentration in biological tissue, a stimulation probe is applied to the tissue, a non-excitatory stimulation pulse is generated, and the pulse is conveyed to the stimulation probe. In one embodiment concerning cardiac tissue, a stimulation probe is applied to a patient's heart, a signal is received from at least one sensor responsive to the patient's cardiac muscle activity, a non-excitatory stimulation pulse responsive to the signal is generated, and the pulse is conveyed to the stimulation probe.

Description

FIELD OF THE INVENTION

This invention relates generally to invasive devices and methods for treatment of the heart, including devices and methods for stimulation of the heart muscle. More particularly, this invention relates to control of cellular tissue, specifically the modulation of intracellular calcium concentration in cardiac muscle cells.

BACKGROUND OF THE INVENTION

Cardiac insufficiency, characterized inter alia by a reduction in cardiac output, is a common, well-known and well-documented heart malfunction. It develops as a result of congenital defects or as an end-effect of many diseases. Cardiac output, i.e., the output of the heart per unit time, is the product of stroke volume and heart rate. Hence, variations in cardiac output can be produced by changes in cardiac rate or stroke volume. The stroke volume can be influenced, for example, by changing the strength of cardiac contraction, by changing the length of the cardiac muscle fibers, and by changing contractility of cardiac muscle independent of fiber length. The heart rate and rhythm influence the cardiac output both directly and indirectly, since changes in the rate and rhythm also affect myocardial contractility.

The human body normally regulates the cardiac output in response to body needs by changing the heart rate, as during physical exercise, and/or by adapting the stroke volume. Under pathological conditions, however, some of the normal regulatory mechanisms may be damaged. For example, heart tissue damaged due to-myocardial infarct typically cannot sustain normal pumping function, leading to a reduction in stroke volume, and hence of cardiac output. The body may react to such a reduction by increasing the heart rate, thus imposing long term strain on the heart muscles, leading in more severe cases to heart failure. There is thus a need for devices and treatments that can regulate the cardiac output, so as to compensate for the deficiencies in the normal regulation mechanisms.

In response to this need, modern cardiology has developed means to control various parameters associated with the heart's operation. Pharmaceuticals, for example, may be used to influence the conduction velocity, excitability, contractility and duration of the refractory period of the heart tissue. These pharmaceuticals are used to treat arrhythmia, enhance cardiac output and prevent fibrillation. Pharmaceuticals are generally limited in effectiveness in that they affect both healthy and diseased segments of the heart, usually, with a relatively low precision. They frequently also have unwanted side-effects.

A special kind of control can be achieved using implantable electronic devices, which provide excitatory electrical stimulation to the heart to control directly the heart rate and/or rhythm. For example, a pacemaker, an electronic device which is typically implanted in the heart to support the heart's electrical excitation system or to bypass a blocked portion of the conduction system. Another type of cardiac electronic device is a defibrillator, which senses fibrillation in the heart and applies a high voltage impulse to “reset” the heart. While electronic pacemakers can control the heart rate, however, they are limited in their capacity to enhance cardiac output, and they are known to reduce stroke volume in at least some instances. Defibrillators are useful in treating arrhythmia when it occurs (although they are painful to the patient and traumatic to the heart), but they provide no long-term amelioration of cardiac insufficiency.

Thus, none of the treatments known in the art allow effective, long-term regulation of cardiac output. PCT patent application PCT/IL97/00012, published as WO 97/25098, to Ben-Haim et al., which is incorporated herein by reference, describes methods for modifying the force of contraction of at least a portion a heart chamber by applying a non-excitatory electric field to the heart at a delay after electrical activation of the portion. The non-excitatory field is such as does not induce new activation potentials in cardiac muscle cells, but rather modifies the cells' response to the activation.

OBJECTS OF THE INVENTION

It is an object of some aspects of the present invention to provide improved methods and apparatus for controlling calcium concentration in biological tissue.

It is also an object of the present invention to provide methods and apparatus for modulating intracellular calcium concentration in cardiac tissue.

It is a further object of the present invention to provide methods and apparatus for modulating cardiac contractibility.

These and other objects of the invention will become more apparent form the discussion below.

SUMMARY OF THE INVENTION

In preferred embodiments of the present invention, a controller comprises a non-excitatory stimulation probe, including one or more non-excitatory stimulation electrodes, at least one sensor, preferably a sensing electrode; and electronic control circuitry, coupled to the stimulation probe and sensor. The stimulation electrodes and, preferably, the sensor are implemented in the heart. Alternatively, a sensing electrode may be placed on a body surface. The circuitry receives signals from the sensor, indicative of the heart's activity, and responsive thereto, drives the stimulation electrodes to provide non-excitatory electrical stimulation to the heart. A non-excitatory electrical field, current or voltage is passed through biological tissue, such as cardiac tissue, or in its proximity, resulting in either changing trans-membranal calcium ion fluxes or and/or intracellular stores content.

The term “non-excitatory electrical stimulation” (“IDS”) in the context of the present patent application and in the claims, refers to electrical pulses that do not induce new activation potentials to propagate in cardiac muscle cells. Rather, such pulses generally affect the response of the heart muscle to the action potentials, possibly by modulating cell contractility within selected segments of the cardiac muscle.

In any case, the effect of the device on intracellular calcium concentration is preferably regulated by changing the timing of the non-excitatory stimulation pulse relative to the heart's activity, preferably relative to the heart's local electrical activity or ECG signals received by the sensing electrode, and/or by changing other pulse characteristics, such as voltage, current, duration, polarity, waveform and frequency of the waveform. Preferably, the device senses the heart's sinus rhythm and applies and synchronizes the stimulation pulse relative thereto, preferably with a delay before the onset of the stimulation pulse. Additionally, the circuitry may analyze the signals, for example, to determine the QT interval, so as to adjust the stimulation pulses responsive thereto. Alternatively, when the heart's rhythm is irregular, due to ventricular premature beats (VPB's) or other cardiac arrhythmias, the device preferably identifies and analyzes the irregularity, using signal processing methods known in the art, and adjusts or withholds the stimulation pulse accordingly.

In some preferred embodiments of the present invention the control circuitry is contained within a console external to the body, and the electrodes are fed percutaneously into the subject's vascular system, for example, through the femoral artery, and are implanted in the heart. Such embodiments are useful particularly in short-term therapy to regulate and stabilize the subject's hemodynamics following an insult or trauma, for example, open heart surgery or MI.

In alternative preferred embodiments of the present invention, the electronic control circuitry is contained within a miniaturized, implantable case, similar to pacemaker cases known in the art.

In some preferred embodiments of the present invention, the non-excitatory stimulation electrodes known in the art, such as pacing or electrophysiology electrodes. Preferably, the stimulation electrodes comprise large-area carbon electrodes or any other metal electrodes such as titanium nitrate, iridium oxide, most preferably vitreous carbon, or alternatively, pyro-carbon. Both types of carbon materials are known for their compatibility with heart tissue, in-vivo durability and excellent electrical properties, including high electrical conductivity. Thus, they allow a relatively high electrical current to be delivered to a relatively large segment of the heart tissue, without inducing electrical excitation.

In other preferred embodiments of the present invention, the non-excitatory stimulation electrodes are inserted into one of the blood vessels of the heart, preferably into the coronary sinus, or alternatively, into a coronary artery.

In another preferred embodiment of this type, different stimulation pulses are applied to respective ones or groups of the plurality of stimulation electrodes. Preferably, the different stimulation pulses are applied to the respective electrodes with a predetermined delay between the different pulses. The delay may be varied so as to achieve a desired hemodynamic effect, for example, to maximize the increase in stroke volume.

In still other such preferred embodiments, the positions of the plurality of stimulation electrodes and/or characteristics of the stimulation pulses applied thereto are optimized responsive to clinical characteristics of the heart. Preferably, before insertion of the electrodes, a map of the heart is produced, for example, an electrophysiological map, as described in U.S. Pat. No. 5,568,809, or a phase-dependent geometrical map, as described in PCT Patent Application PCT/IL97/00011, both of which are incorporated herein by reference. Preferably, the map includes information regarding the viability of the heart tissue, for example, based on local contractility or electrical activity. The non-excitatory stimulation electrodes are then positioned responsive to the map.

Preferably, applying the IDS signal includes conveying electrical energy to cells of the heart, such that action potentials are generally not generated in the cells responsive to the application of the non-excitatory signal.

Further preferably, the IDS signal is applied to improve hemodynamic performance of the heart. Preferably, the IDS signal is applied in order to increase contractility of the heart or, alternatively or additionally, to increase systolic pressure generated by the heart.

In a preferred embodiment, applying the IDS signal includes sensing physiological variables and applying the signal responsive thereto. Preferably, sensing the variable includes detecting an electrical depolarization wave in the tissue. Alternatively, sensing the variable includes sensing a hemodynamic parameter. Preferably, applying the pacing pulses include controlling application of the pacing pulses responsive to the variable, wherein controlling the application of the pacing pulses includes making a transition from a first stimulation mode to a second stimulation mode responsive to the variable.

There is also provided, in accordance with a preferred embodiment of the present invention, apparatus for stimulating cardiac tissue, including:

• • a plurality of electrodes, which are placed at multiple sites in at least two different chambers of the heart; and
  • an electrical control unit, which applies pacing pulses to two or more of the electrodes at respective pacing sites in the at least two different chambers, and which applies an IDS signal to at least one of the electrodes in a vicinity of one or more of the pacing sites following application of the pacing pulse at the site.

Preferably, the at least one of the electrodes to which the IDS signal is applied includes one of the electrodes to which the pacing pulses are applied.

Further preferably, at least one of the pacing sites is in the left ventricle, and the IDS signal is applied to an electrode in the left ventricle.

Preferably, the control unit applies the IDS signal between during a time period which begins between about 0 and 100 ms after the onset of a pacing pulse applied by the control unit, wherein the time period is set so as to substantially eliminate the possibility that a propagating action potential will be generated responsive to application of the IDS signal. Preferably, the time period begins between about 10 and 50 ms after the onset of the pacing pulse.

Preferably, the IDS signal is applied in order to increase contractility of the heart or, alternatively or additionally, in order to increase systolic pressure generated by the heart.

In a preferred embodiment, the apparatus includes a sensor, which senses a physiological variable, wherein the control unit receives an input from the sensor and applies the IDS signal responsive thereto. Preferably, the sensor detects an electrical depolarization wave in the tissue. Alternatively (or additionally, the sensor senses a hemodynamic: parameter or senses motion. Preferably, the control unit controls application of the pacing pulses responsive to the variable. Further preferably the control unit makes a transition from a first stimulation mode to a second stimulation mode responsive to the variable.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood form the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which:

FIG. 1A is a graph of shortening vs. time, before, during, and after an IDS signal;

FIG. 1B is a graph of calcium concentration vs. time;

FIG. 2A is a graph of calcium concentration vs. time;

FIG. 2B is a graph of shortening vs. time;

FIG. 3 depicts two graphs vs. time, graph (a) representing concentration form vs. time, and graph (b) represents action potential vs. time; and

FIG. 4 depicts two graphs of concentration force vs. time, (a) being a control and (b) representing the use of a drug.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention is designed to modulate intracellular calcium concentration in a biological tissue using a non-excitatory electrical signal (IDS). In particular, the invention relates to the modulation of the intracellular calcium in cardiac muscle cells and thus the modulation of cardiac contractility. According to this invention a non-excitatory electrical field, current or voltage is passed through the tissue or in its proximity, resulting in either changing trans-membranal calcium ion fluxes or an intracellular calcium stores content. In another aspect of the invention the electrical field may interfere/enhance the affinity of intracellular calcium binding elements to calcium. In a further aspect the rise in intracellular calcium concentrations may initiate a cascade of events including, but not limited to, phosphorylation/dephosphorylation, gene transcription, and/or post translation modification.

Systems are disclosed which utilize the application of electrical current to a tissue, effecting tissue contractility by means of modulating intracellular calcium. At least one pair of electrodes is used for applying the signal. Electrode placement is adapted for achieving the maximum desired effect. The electrodes are attached to an either implantable or external device with programming capabilities. This device can be tested and calibrated non-invasively by external mechanisms. In addition, stimulation parameters can be adjusted by a similar programming mechanism.

The characteristics of the electrodes used for the stimulation are important. This invention utilizes both uni-polar and bi-polar electrode configurations.

A novel aspect of this method of modulating intracellular calcium in cells is the ability to adjust the timing and the amount of calcium increase/decrease using temporal electrical current rather than systemic pharmacological agents.

The accompanying figures show changes in intracellular calcium resulting from the application of the IDS signal together with experimental evidence on the effect of the IDS signal on the calcium handling in the cell. FIG. 1A represents the shortening of a single isolated myocyte measured using optical means. Each of the lines represents a single contraction of the myocyte. When the IDS signal was delivered, a marked increase in the myocyte shortening of 30.9±5% (n=10) was observed. The shortening returned to baseline when signal delivery was stopped.

FIG. 1B shows the optical measurements of calcium concentration from a single myocyte without the application of the IDS signal (dark line) and on top of it superimposed the intracellular calcium concentration during the delivery of the IDS signal (gray line). An increase of 26.6% (n=3) in the peak calcium level was observed. The calcium concentration changes were measured using the florescence ration of a calcium sensitive dye Fura-2 at two wavelengths, 340 nm and 380 nm. The same result showing a large increase in calcium concentration was measured from an isolated ferret heart using a Langendorf setting. The ventricle contraction force and contractility increased by up to 50%, and at the same time cellular calcium concentration, measured using Aquarine calcium sensitive dye, was increased by up to 50%.

FIGS. 2A and 2B show the effect of the IDS signal on the shortening and intracellular calcium of myocytes isolated from canine heart with heart failure (generated by repeated ischemia events). Each line represents an average of 20 consecutive beats before (black line) and during (gray line) the application of the signal. FIG. 2A shows an increase of 24.9±3.9% (n=11) in shortening. FIG. 2B shows an increase of 29.7% (n=3) in intracellular calcium measured using Fluo-3 fluorescence.

FIG. 3 depicts initial changes in contraction force and in action potential duration, measured using an intracellular electrode, during the first three beats of IDS signal application to a rabbit papillary muscle. The action potential duration when the IDS signal is applied (gray line in the lower trace) is superimposed on a control contraction (black line in the middle trace). Black lines, in the upper trace, mark the start of application of the IDS signal. The action potential duration immediately changes upon the application of the IDS signal from the first pulse. There is no significant difference between action potential duration in the consecutive beats. The contraction force behaves differently; there is a small increase in the contraction force on the first beat followed by a larger increase in the second beat and gradual changes until a plateau is reached after 5 to 6 beats. The underlying mechanism is that the IDS signal prolongs action potential duration. As a result of action potential prolongation the flow of calcium into the cell increases and generates two effects:

• • (1) immediate increase in the Sarcolemal calcium level causing the initial increase in the contraction force on the first beat; and
  • (2) increase in the stored calcium in the Sarcoplasmatic Reticulum (SR) that is released during the following contractions and induces a larger increase in contraction until a new steady state is reached with higher contraction force.

The initial increase in the contractile force also supports the possible increase in the affinity of intracellular calcium binding elements that cause part of the increase in the contraction force.

FIG. 4 provides additional evidence supporting the hypothesis of calcium entry. The lower trace shows the increase in contraction force of a rabbit papillary muscle as a result of an IDS signal (shown as a black line on the upper trace). The initial increase in contraction on the first beat followed by the gradual change in the following beats is clearly seen. The middle trace shows the change in contraction force of the same muscle after the addition of the Ryanodine to the bath solution. Ryanodine prevents the accumulation of calcium in the Sarcoplasmatic Reticulum and therefore decreases the baseline contraction force. Upon the application of the IDS signal the contraction force increases immediately on the first beat, but no increase occurs during the following beats since the SR mechanism is disabled by the Ryanodine and no accumulation of calcium in the SR can contribute to the additional increase in contraction force exist.

All such variations, applications and subcombinations of elements are considered to be within scope of the present invention. It will thus be appreciated that the preferred embodiments described above are cited by way of example, and the full scope of the invention is limited only by the claims.

Claims

1. A method of intentionally modulating the intracellular calcium properties in cardiac tissue, comprising: (a) applying a stimulation probe to cardiac tissue in a subject's body;(b) selecting a desired effect on intracellular calcium properties in said cardiac tissue;(c) providing pulse parameters of non-excitatory stimulation designed to achieve said desired effect on said intracellular calcium properties;(d) using said parameters to generate an electrical signal comprising said non-excitatory stimulation; and(e) conveying said electrical signal to the stimulation probe during a refractory period for the tissue, thereby applying said non-excitatory stimulation to said cardiac tissue.

2. The method of claim 1, wherein the stimulation probe comprises one or more stimulation probes.

3. The method of claim 1, wherein the desired effect comprises increasing the intracellular calcium concentration.

4. The method of claim 1, wherein said desired effect comprises decreasing the intracellular calcium concentration.

5. The method of claim 1, wherein the desired effect is an increase in intracellular calcium ion flux.

6. The method of claim 1, wherein the desired effect includes a return to a base line value for the intracellular calcium property.

7. The method of claim 1, wherein the desired effect includes a return to a base line level of the intracellular calcium property between heart beats.

8. The method of claim 1, wherein the parameters include applying said non-excitatory stimulation beginning between 0 and 100 milliseconds after local activation.

9. The method of claim 1, wherein said selecting comprises selecting a desired long term consequence.

10. The method of claim 9, wherein said long term consequence comprises gene transcription and/or post translation modification.

11. The method of claim 1, wherein said selecting a desired effect comprises selecting a transient effect.

12. The method of claim 1, wherein said desired effect is release or re-uptake of calcium from a sarcoplasmic reticulum.

13. A method of intentionally modulating intracellular calcium properties in cardiac tissue, comprising: (a) applying a stimulation probe comprising one or more stimulation electrodes to a subject's heart;(b) applying to said subject's body at least one sensor responsive to said subject's cardiac muscle activity;(c) receiving a sensing signal from said at least one sensor;(d) selecting a desired effect on intracellular calcium properties in said subject's heart;(e) determining parameters of a non-excitatory stimulation pulse for modulation of intercellular calcium concentration to achieve said desired effect, depending on said sending signal;(f) generating an electrical signal comprising said non-excitatory stimulation pulse responsive to said sensing signal; and(g) conveying said electrical signal to at least one of said one or more electrodes during a refractory period for the cardiac tissue thereby applying said non-excitatory stimulation pulse to said cardiac tissue.

14. The method of claim 13, wherein said desired effect comprises increasing the intracellular calcium concentration.

15. The method of claim 13, wherein said desired effect comprises decreasing the intracellular calcium concentration.

16. The method of claim 13, wherein the sensor is an electrode introduced into the heart, said sensor measuring said heart's electrical activity, and wherein generating the electrical signal comprises generating an electrical signal synchronized with said electrical activity sensed by said sensing electrode.

17. The method of claim 13, wherein the sensor is an external electrode attached to the subject's body surface, said electrode measuring an ECG signal, and wherein generating the electrical signal comprises generating an electrical signal synchronized with said ECG signal.

18. The method of claim 13, wherein at least one of the one or more of the stimulation electrodes also functions as a sensing electrode.

19. The method of claim 13, wherein generating the electrical signal comprises generating an electrical signal having a predetermined delay relative to the signal.

20. The method of claim 13, wherein applying the stimulation probe comprises applying a probe comprising a plurality of stimulation electrodes, and wherein generating and conveying the electrical signal comprises generating a sequence of electrical signals and applying each electrical signal in said sequence to a different one of said plurality of stimulation electrodes.

21. The method of claim 13, wherein generating and conveying the electrical signal comprises generating and conveying said electrical signal selectively, based on a characteristic of the signals received from the at least one sensor.

22. The method of claim 21, wherein generating and conveying the electrical signal comprises generating and applying said electrical signal at a rate dependent on the heart rate, but not equal to the heart rate.

23. The method of claim 21, wherein generating and conveying the electrical signal comprises detecting a cardiac arrhythmia and adjusting the application of said electrical signal responsive thereto.

24. The method of claim 21, wherein generating and conveying the electrical signal comprises detecting a QT interval in the signals received from the at least one sensor and generating pulses responsive thereto.

25. The method of claim 13, wherein generating the electrical signal comprises varying one or more parameters of the non-excitatory stimulation pulse, selected from the group consisting of the pulse voltage, current, duration, delay, and waveform frequency.

26. The method of claim 13, wherein the non-excitatory stimulation comprises a baseline pulse and a waveform of substantially higher frequency than the baseline pulse superimposed thereon.

27. The method of claim 26, wherein the waveform comprises a square wave.

28. The method of claim 13, further comprising, after step (g), another step (h) generating and conveying another electrical signal of opposite polarity thereto.

29. The method of claim 13, wherein applying the non-excitatory stimulation pulse comprises varying the extent of a portion of the area of the heart segment to which said stimulation pulse is applied.

30. The method of claim 29, wherein varying the extent comprises selectively addressing a net of stimulation electrodes implanted in the heart.

31. The method of claim 13, wherein applying the stimulation probe comprises inserting the one or more stimulation electrodes into multiple chambers of the heart.

32. The method of claim 13, wherein applying the stimulation probe comprises inserting at least one of the one or more stimulation electrodes into a blood vessel of the heart.

33. The method of claim 32, wherein inserting the at least one stimulation electrode comprises inserting the electrode into the coronary sinus.

34. The method of claim 13, wherein generating and conveying the electrical signal comprises generating and conveying electrical signals at selected times of the day.

35. The method of claim 13, wherein generating and conveying the electrical signal comprises generating and conveying electrical signals which increase the subject's cardiac output.

36. The method of claim 13, wherein generating and conveying the electrical signal comprises generating and conveying electrical signals which decrease the subject's cardiac output.

37. The method of claim 13, wherein generating and conveying the electrical signal comprises generating and conveying electrical signals which increase the efficiency of contraction of the heart.

38. The method of claim 1, wherein said desired effect comprises desired effects on calcium concentration and/or trans-membranal calcium ion flux and/or intracellular stores content.

39. The method of claim 38, wherein the property is ion flux.

40. The method of claim 38, wherein the property is intracellular stores content.

41. The method of claim 38, wherein the property is increased.

42. The method of claim 1 wherein the desired effect comprises increase of the property.

43. The method of claim 1 wherein the desired effect comprises decrease of the property.

44. The method of claim 1 wherein the pulse parameters comprise timing and/or voltage, current, duration, polarity, waveform, and frequency of the waveform of the stimulation.

45. The method of claim 1 wherein the parameters are selected for an individual subject.

46. The method of claim 13, wherein the properties include calcium concentration and/or trans-membranal calcium ion flux and/or intracellular stores content.

47. The method of claim 13, wherein the pulse parameters comprise timing and/or voltage, current, duration, polarity, waveform, and frequency of the waveform of the stimulation.

48. The method of claim 13, wherein said desired effect comprises an increase in intracellular calcium ion flux.

49. The method of claim 13, wherein the parameters include applying said non-excitatory stimulation beginning between 0 and 100 milliseconds after local activation.

50. The method of claim 13, wherein said selecting comprises selecting a long term consequence.

51. The method of claim 13, wherein said selecting a desired effect comprises selecting a transient effect.

52. The method of claim 13, wherein said desired effect is release or re-uptake of calcium from a sarcoplasmic reticulum.

53. A method for regulating the intracellular calcium properties in cardiac tissue, comprising: a. applying a stimulation probe to cardiac tissue in a subject's body;b. determining a desired intracellular calcium property of said tissue;c. generating an electrical signal comprising non-excitatory stimulation which has an effect on said intracellular calcium property commensurate with said desired regulation; andd. conveying said non-excitatory stimulation pulse to said cardiac tissue during a refractory period for said tissue.

54. The method of claim 53, wherein steps (c) and (d) are repeated as often as necessary to regulate the intracellular calcium properties in the cardiac tissue by the signal according to the desired regulation.

55. The method of claim 53, wherein the properties include calcium concentration and/or trans-membranal calcium ion flux and/or intracellular stores content.

56. The method of claim 53, wherein the desired regulation comprises increase of the property.

57. The method of claim 53, wherein the desired regulation comprises decrease of the property.

58. The method of claim 53, wherein modulating comprises increase of the property.

59. The method of claim 53, wherein modulating comprises decrease of the property.

60. The method of claim 53, wherein the desired calcium property comprises an increase in intracellular calcium ion flux.

61. The method of claim 53, wherein the conveying comprises applying said non-excitatory stimulation beginning between 0 and 100 milliseconds after local activation.

62. The method of claim 53, wherein said desired effect is release or re-uptake of calcium from a sarcoplasmic reticulum.