A calcium spark is the microscopic release of calcium (
Ca2+) from a store known as the
sarcoplasmic reticulum (SR), located within
muscle cells. This release occurs through an
ion channel
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ...
within the
membrane of the
SR, known as a
ryanodine receptor (RyR), which opens upon activation. This process is important as it helps to maintain Ca
2+ concentration within the
cell. It also initiates
muscle contraction
Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such a ...
in
skeletal
A skeleton is the structural frame that supports the body of an animal. There are several types of skeletons, including the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside ...
and
cardiac muscle
Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle ...
s and
muscle relaxation in
smooth muscles. Ca
2+ sparks are important in physiology as they show how Ca
2+ can be used at a subcellular level, to signal both local changes, known as local control, as well as whole cell changes.
Activation
As mentioned above, Ca
2+ sparks depend on the opening of ryanodine receptors, of which there are three types:
*
Type 1 – found mainly in skeletal muscle
*
Type 2 – found mainly in the heart
*
Type 3 – found mainly in the brain
Opening of the channel allows Ca
2+ to pass from the
SR, into the cell. This increases the local Ca
2+ concentration around the RyR, by a factor of 10. Calcium sparks can either be evoked or spontaneous, as described below.
Evoked
Electrical impulses, known as
action potentials
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...
, travel along the cell membrane (sarcolemma) of
muscle cells. Located in the sarcolemma of smooth muscle cells are receptors, called dihydropyridine receptors (DHPR). In skeletal and cardiac muscle cells, however, these receptors are located within structures known as T-tubules, that are extensions of the plasma membrane penetrating deep into the cell (see figure 1). These DHPRs are located directly opposite to the
ryanodine receptors, located on the
sarcoplasmic reticulum and activation, by the action potential causes the DHPRs to change shape.
In
cardiac
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...
and
smooth muscle
Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...
, activation of the DHPR results in it forming an
ion channel
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ...
. This allows Ca
2+ to pass into the
cell, increasing the local Ca
2+ concentration, around the RyR. When four Ca
2+ molecules bind to the RyR, it opens, resulting in a larger release of Ca
2+, from the SR . This process, of using
Ca2+ to activate release of
Ca2+ from the
SR is known as
calcium-induced calcium release.
However, in skeletal muscle the DHPR touches the RyR. Therefore, the shape change of the DHPR activates the RyR directly, without the need for Ca
2+ to flood into the cell first. This causes the RyR to open, allowing Ca
2+ to be released from the SR.
Spontaneous
Ca
2+ sparks can also occur in cells at rest (i.e. cells that have not been stimulated by an action potential). This occurs roughly 100 times every second in each cell
and is a result of Ca
2+ concentration being too high. An increase in Ca
2+ within the SR is thought to bind to Ca
2+ sensitive sites on the inside of the RyR causing the channel to open. As well as this, a protein called
calsequestrin (found within the SR) detaches from the RyR, when calcium concentration is too high, again allowing the channel to open (see
sarcoplasmic reticulum for more details). Similarly, a decrease in Ca
2+ concentration within the SR has also proven to lower RyR sensitivity. This is thought to be due to the calsequestrin binding more strongly to the RyR, preventing it from opening and decreasing the likelihood of a spontaneous spark.
Calcium after release
There are roughly 10,000 clusters of
ryanodine receptors within a single cardiac cell, with each cluster containing around 100 ryanodine receptors.
During a single spontaneous spark, when
Ca2+ is released from the SR, the Ca
2+ diffuses throughout the
cell. As the RyRs in the heart are activated by Ca
2+, the movement of the Ca
2+ released during a spontaneous spark, can activate other neighbouring RyRs within the same cluster. However, there usually isn't enough Ca
2+ present in a single spark to reach a neighbouring cluster of
receptor
Receptor may refer to:
*Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse
*Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...
s.
The calcium can, however, signal back to the DHPR causing it to close and preventing further influx of calcium. This is known as
negative feedback
Negative feedback (or balancing feedback) occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by othe ...
.
An increase in Ca
2+ concentration within the
cell or the production of a larger spark, can lead to a large enough calcium released that the neighbouring cluster can be activated by the first. This is known as spark-induced spark activation and can lead to a Ca
2+ wave of calcium release spreading across the cell.
During evoked Ca
2+ sparks, all clusters of
ryanodine receptor
Ryanodine receptors (RyR for short) form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons.
There are three major isoforms of the ryanodine receptor, which are found in different tissu ...
s, throughout the
cell are activated at almost exactly the same time. This produces an increase in Ca
2+ concentration across the whole cell (not just locally) and is known as a whole cell Ca
2+ transient. This Ca
2+ then binds to a protein, called
troponin
image:Troponin Ribbon Diagram.png, 400px, Ribbon representation of the human cardiac troponin core complex (52 kDa core) in the calcium-saturated form. Blue = troponin C; green = troponin I; magenta = troponin T.; ; rendered with PyMOL
Troponin, ...
, initiating contraction, through a group of proteins known as myofilaments.
In
smooth muscle
Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...
cells, the Ca
2+ released during a spark is used for muscle relaxation. This is because, the Ca
2+ that enters the
cell via the DHPR in response to the
action potential
An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...
, stimulates both muscle contraction and calcium release from the SR. The Ca
2+ released during the spark, then activates two other
ion channels
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of i ...
on the membrane. One
channel allows
potassium ions to exit the
cell, whereas the other allows
chloride ions to leave the
cell. The result of this movement of
ions, is that the membrane voltage becomes more negative. This deactivates the
DHPR (which was activated by the positive membrane potential produced by the action potential), causing it to close and stopping the flow of Ca
2+into the cell, leading to relaxation.
Termination
The mechanism by which SR Ca
2+ release terminates is still not fully understood. Current main
theories are outlined below:
Local depletion of SR Ca2+
This theory suggests that during a calcium spark, as calcium flows out of the SR, the concentration of Ca
2+ within the SR becomes too low. However, this was not thought to be the case for spontaneous sparks as the total release during a Ca
2+ spark is small compared to total SR Ca
2+ content and researchers have produced sparks lasting longer than 200 milliseconds, therefore showing that there is still enough Ca
2+ left within the
SR after a 'normal' (200ms) spark.
However local depletion in the junctional SR may be much larger than previously thought (see
). During the activation of a large number of ryanodine receptors however, as is the case during electrically evoked Ca
2+ release , the entire
SR is about 50% depleted of Ca
2+ and this mechanism will play an important role in repriming of release.
Stochastic attrition
Despite the complicated name, this idea simply suggests that all ryanodine receptors in a cluster, and the associated dihydropyridine receptors happen to randomly close at the same time. This would not only prevent calcium release from the SR, but it would also stop the stimulus for calcium release (i.e. the flow of calcium through the DHPR).
However, due to the large numbers of RyRs and DHPRs in a single cell, this theory seems to be unrealistic, as there is a very small probability that they would all close together at exactly the same time.
Inactivation/adaptation
This theory suggests that after activation of the RyR and the subsequent release of Ca
2+, the
channel closes briefly to recover. During this time, either the channel cannot be reopened, even if calcium is present (i.e. the RyR is inactivated) or the channel can be reopened, however more calcium is required to activate it than usual (i.e. the RyR is in an adaptation phase). This would mean that one-by-one the RyRs would close, thus ending the spark.
Sticky cluster theory
This theory suggests that the above three theories all play a role in preventing calcium release.
[Sobie, E.A., Dilly, K.W., Cruz, J. dos S., Lederer, J.W. and Jafri, S.M. (2002) 'Termination of cardiac ca(2+) sparks: An investigative mathematical model of calcium-induced calcium release', 83(1)]
Discovery
Spontaneous Ca
2+ sparks were discovered in
cardiac muscle cells, of rats, in 1992 by Peace Cheng and Mark B. Cannell in Jon Lederer's laboratory at the University of Maryland, Baltimore, U.S.A.
Initially the idea was rejected by the scientific journal,
Nature
Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
, who believed that the sparks were only present under laboratory conditions (i.e. they were artifacts), and so wouldn't occur naturally within the body. However they were quickly recognised as being of fundamental importance to muscle
physiology
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemic ...
, playing a huge role in excitation-contraction coupling.
The discovery was made possible due to improvements in
confocal microscopes. This allowed for the detection of the release of Ca
2+, which were highlighted using a substance known as
fluo-3
Fluo-3 is a fluorescence indicator of intracellular calcium (Ca2+), developed by Roger Y. Tsien and colleagues. It is used to measure Ca2+ inside living cells in flow cytometry, and confocal laser scanning microscopy using visible light excitati ...
, which caused the Ca
2+ to glow. Ca
2+ “sparks” were so called because of the spontaneous, localised nature of the Ca
2+ release as well as the fact that they are the initiation event of
excitation-contraction coupling.
Detection and analysis
Because of the importance of Ca
2+ sparks in explaining the gating properties of
ryanodine receptor
Ryanodine receptors (RyR for short) form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons.
There are three major isoforms of the ryanodine receptor, which are found in different tissu ...
s in situ (within the body), many studies have focused on improving their detectability
in the hope that by accurately and reliably detecting all Ca
2+ spark events, their true properties can finally help us to answer the unsolved mystery of spark termination.
See also
*
Calcium-induced calcium release
*
Confocal microscopy
Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM) or laser confocal scanning microscopy (LCSM), is an optical imaging technique for increasing optical resolution and contrast of a micrograph by means of using a ...
*
Ryanodine receptor
Ryanodine receptors (RyR for short) form a class of intracellular calcium channels in various forms of excitable animal tissue like muscles and neurons.
There are three major isoforms of the ryanodine receptor, which are found in different tissu ...
References
{{Reflist
External links
Software
SparkMaster - Automated Ca2+ Spark Analysis with ImageJ- Free software for Ca
2+ spark analysis in confocal linescan images
Cell biology