The activity of the NCX exchangers is also activated by calcium ion release from the sarcoplasmic reticulum (SR). The initial
release is called local calcium release (LCR) and occurs simultaneously with the influx of calcium through the CaT channels.
The second release called calcium-induced calcium release (CICR) is during depolarization; this represents a 'tick' of the calcium clock.
The illustration at the left indicates these activities.
The previous series of tutorials described the structure and function of the various membrane-bound channels and exchangers that produce the pacemaker action potential. That series of voltage-regulated activities has been named the 'membrane clock'. Research during the past few decades has verified that the cyclical release of calcium from the sarcoplasmic reticulum, the 'calcium clock', also plays an important role in modifying this action potential.
The histology of pacemaker cells is quite different from that of myocardial contractile cells. Pacemaker cells are spindle or stellate shaped and extremely small with a diameter of only 4nm. They are densely covered with caveloli ... small flask-shaped indentations ... that accounts for 50% of their surface area. They have few and unorganized myofilaments, numerous mitochondria and a sarcoplasmic reticulum (SR) found throughout the cytoplasm. The sections of SR that abut the sarcolemma (cell membrane) are referred to as 'junctional' while the remaining sections are referred to as 'network'.
The portion of the main diagram at the right represent the sarcoplasmic reticulum (SR) as a mushroom. Calcium ions are pumped into it by the activity of sarcoplasmic reticulum calcium ATPase (SERCA) located in the organelle's network membrane. These ions diffuse toward and accumulate beneath ryanodine receptor channels (RYR2) located in the junctional membrane. When the RYR2 channels open the ions diffuse into the cytoplasm just beneath the cell membrane. From there they are either removed by NCX exchangers or returned (purple arrows) to the lumen of this organelle by SERCAs.
Phospholamban (pink) is a two-helix protein closely associated with SERCA. The illustration shows that it can assume two conformations, one in which a helix is bent over a portion of SERCA blocking the calcium ion pathway and the other unbent shape that does not obstruct the pathway. The double arrow represents the conversion of one conformation to the other.
Phosphorylation by Ca2+-calmodulin-dependent protein kinase (CaMKII) and PKA favors the 'open' conformation and increase the pumping rate filling the lumen faster. The concentration of CaMKII increases during depolarization; the concentration of PKA is controlled by the autonomic nervous system.
Ryanodine receptors are embedded in the SR membrane at the junctional region that is located just beneath the cell membrane. They
are channels that open to release Ca2+ into the cytoplasm. RYR2s are arranged in groups called
'calcium release units' (CRUs). There are ~10,000 CRUs per cell, with 100 RYRs each, giving a total of ~1,000,000 RYRs per cell.
These channels are ~10X larger than most other channels. Each consists of four separate proteins referred to as 'monomers' (light blue) that are arranged around a pore. Several helices pass through the SR membrane with a large (light blue oval) cytoplasmic foot containing several binding sites as shown in the right illustration.
There are two other membrane-embedded proteins associated with each monomer; triadin (orange) and junctin (purple). Each consists of one alpha helix and a positively-charged C terminal extending into the cytoplasm (pairs of short wavy lines). Within the lumen of the SR, negatively-charged calsequestrin (CASQ2) molecules (long wavy red lines) bond to these positive sites as shown in the illustration. These groupings are called a calcium-sensing receptors.
The calcium-sensing receptors cause the channels to open when luminal Ca2+ concentrations increase (intrinsic control). This occurs when Ca2+ bind to calsequestrin (CASQ2) molecules detaching them from the calcium-sensing receptors as shown in the right illustration. Calcium released by this mechanism is called a local calcium release (LCR). Extrinsic control occurs when binding sites on the feet open the channels in response to high concentrations of Ca2+ and PKA in the cytoplasm. Calcium released by this mechanism is called calcium-induced calcium release (CICR).
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SERCAs pump continually so that
Ca2+ is always filling the SR and diffusing toward the RYR2s in junctional regions.
As more and more Ca2+ attaches to the calcium-sensing receptors the CASQ2 molecules
will detach and open the channel (right illustration above). This causes a local calcium release (LCR) from the SR.
This loss of luminal Ca2+ is restored as Ca2+
unbinds from CASQ2s allowing them to reattach to the RYR2s and close the channels (left illustration above).
Individual LCRs occur rapidly, each starting before the preceding one has ended, causing the stair-step pattern shown in the illustration at the left. This is thought to occur because binding of LCR Ca2+ to the RYR2 feet makes them more likely to respond to internal Ca2+ supplies.
Calcium-induced calcium release (CICR) occurs when the influx of Ca2+ through CaL channels attaches to RYR2 binding sites on RYR2 'feet' holding those channels open until ~66% of the remaining Ca2+ in the SR has been released. The channels close when sufficient cytoplasmic Ca2+ have been removed by (1) the NCX exchangers (2) SERCA or, (3) membrane-bound calcium pumps. The activity levels of each of these depends on the concentration of Ca2+ in the cytoplasm.
The intrinsic control mechanism of the sarcoplasmic reticulum causes the release of 'spurts' of Ca2+ (LCRs) that increase the activation of membrane-bound NCX exchangers. Additionally, the rapid influx of Ca2+ through the membrane-bound CaL channels opens the RYR2 channels of the SR causing a second Ca2+ release (CICR). In this manner the activities of membrane-bound components (membrane clock) and activities of reticulum-bound components (calcium clock) interact; the clocks are doubly coupled together.
Updated:1/30/2016
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