Sodium-calcium exchanger
solute carrier family 8 (sodium/calcium exchanger), member 1 | |||||||
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Identifiers | |||||||
Symbol | SLC8A1 | ||||||
Alt. symbols | NCX1 | ||||||
Chr. 2 p23-p21 | |||||||
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solute carrier family 8 (sodium-calcium exchanger), member 2 | |||||||
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Identifiers | |||||||
Symbol | SLC8A2 | ||||||
Chr. 19 q13.2 | |||||||
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solute carrier family 8 (sodium-calcium exchanger), member 3 | |||||||
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Identifiers | |||||||
Symbol | SLC8A3 | ||||||
Chr. 14 q24.1 | |||||||
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The sodium-calcium exchanger (often denoted Na+/Ca2+ exchanger, exchange protein, or NCX) is an
The exchanger is usually found in the plasma membranes and the mitochondria and endoplasmic reticulum of excitable cells.[3][4]
Function
The sodium–calcium exchanger is only one of the systems by which the cytoplasmic concentration of calcium ions in the cell is kept low. The exchanger does not bind very tightly to Ca2+ (has a low affinity), but it can transport the
- control of neurosecretion
- activity of photoreceptor cells
- cardiac muscle relaxation
- maintenance of Ca2+ concentration in the sarcoplasmic reticulum in cardiac cells
- maintenance of Ca2+ concentration in the endoplasmic reticulum of both excitable and nonexcitable cells
- excitation-contraction coupling
- maintenance of low Ca2+ concentration in the mitochondria
The exchanger is also implicated in the cardiac electrical conduction abnormality known as delayed afterdepolarization.[7] It is thought that intracellular accumulation of Ca2+ causes the activation of the Na+/Ca2+ exchanger. The result is a brief influx of a net positive charge (remember 3 Na+ in, 1 Ca2+ out), thereby causing cellular depolarization.[7] This abnormal cellular depolarization can lead to a cardiac arrhythmia.
Reversibility
Since the transport is electrogenic (alters the membrane potential), depolarization of the membrane can reverse the exchanger's direction if the cell is depolarized enough, as may occur in excitotoxicity.[1] In addition, as with other transport proteins, the amount and direction of transport depends on transmembrane substrate gradients.[1] This fact can be protective because increases in intracellular Ca2+ concentration that occur in excitotoxicity may activate the exchanger in the forward direction even in the presence of a lowered extracellular Na+ concentration.[1] However, it also means that, when intracellular levels of Na+ rise beyond a critical point, the NCX begins importing Ca2+.[1][8][9] The NCX may operate in both forward and reverse directions simultaneously in different areas of the cell, depending on the combined effects of Na+ and Ca2+ gradients.[1] This effect may prolong calcium transients following bursts of neuronal activity, thus influencing neuronal information processing.[10][11]
Na+/Ca2+ exchanger in the cardiac action potential
The ability for the Na+/Ca2+ exchanger to reverse direction of flow manifests itself during the cardiac action potential. Due to the delicate role that Ca2+ plays in the contraction of heart muscles, the cellular concentration of Ca2+ is carefully controlled. During the resting potential, the Na+/Ca2+ exchanger takes advantage of the large extracellular Na+ concentration gradient to help pump Ca2+ out of the cell.[12] In fact, the Na+/Ca2+ exchanger is in the Ca2+ efflux position most of the time. However, during the upstroke of the cardiac action potential there is a large influx of Na+ ions. This depolarizes the cell and shifts the membrane potential in the positive direction. What results is a large increase in intracellular [Na+]. This causes the reversal of the Na+/Ca2+ exchanger to pump Na+ ions out of the cell and Ca2+ ions into the cell.[12] However, this reversal of the exchanger lasts only momentarily due to the internal rise in [Ca2+] as a result of the influx of Ca2+ through the L-type calcium channel, and the exchanger returns to its forward direction of flow, pumping Ca2+ out of the cell.[12]
While the exchanger normally works in the Ca2+ efflux position (with the exception of early in the action potential), certain conditions can abnormally switch the exchanger to the reverse (Ca2+ influx, Na+ efflux) position. Listed below are several cellular and pharmaceutical conditions in which this happens.[12]
- The internal [Na+] is higher than usual (like it is when digoxin and other cardiac glycoside medications block the Na+/K+-ATPase pump.)
- The sarcoplasmic reticulum release of Ca2+ is inhibited.
- Other Ca2+ influx channels are inhibited.
- If the action potential duration is prolonged.
Structure
Based on secondary structure and hydrophobicity predictions, NCX was initially predicted to have 9 transmembrane helices.[13] The family is believed to have arisen from a gene duplication event, due to apparent pseudo-symmetry within the primary sequence of the transmembrane domain.[14] Inserted between the pseudo-symmetric halves is a cytoplasmic loop containing regulatory domains.[15] These regulatory domains have C2 domain like structures and are responsible for calcium regulation.[16][17] Recently, the structure of an archaeal NCX ortholog has been solved by X-ray crystallography.[18] This clearly illustrates a dimeric transporter of 10 transmembrane helices, with a diamond shaped site for substrate binding. Based on the structure and structural symmetry, a model for alternating access with ion competition at the active site was proposed. The structures of three related proton-calcium exchangers (CAX) have been solved from yeast and bacteria. While structurally and functionally homologus, these structures illustrate novel oligomeric structures, substrate coupling, and regulation.[19][20][21]
History
In 1968, H Reuter and N Seitz published findings that, when Na+ is removed from the medium surrounding a cell, the efflux of Ca2+ is inhibited, and they proposed that there might be a mechanism for exchanging the two ions. Digitalis, more commonly known as foxglove, is known to have a large effect on the Na/K ATPase, ultimately causing a more forceful contraction of the heart. The plant contains compounds that inhibit the sodium potassium pump which lowers the sodium electrochemical gradient. This makes the pumping of calcium out of the cell less efficient, which leads to a more forceful contraction of the heart. For individuals with weak hearts, it is sometimes provided to pump the heart with heavier contractile force. However, it can also cause hypertension because it increases the contractile force of the heart.
See also
- Sodium–potassium pump
- Active transport
- Cardiac action potential
- Potassium-dependent sodium-calcium exchanger
References
- ^ S2CID 23146698.
- ^ PMID 16371597.
- ^ S2CID 38199890.
- PMID 17190902.
- S2CID 43050133.
- ISBN 0-7817-0104-X.)
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has generic name (help)CS1 maint: multiple names: authors list (link - ^ a b Lilly, L: "Pathophysiology of Heart Disease", chapter 11: "Mechanisms of Cardiac Arrhythmias", Lippencott, Williams and Wilkens, 2007
- S2CID 25625938.
- S2CID 13912728.
- PMID 26674618.
- PMID 16885232.
- ^ S2CID 4337201.
- S2CID 21425718.
- PMID 15163769.
- PMID 8483905.
- PMID 17962412.
- PMID 16774926.
- S2CID 206538351.
- PMID 23685453.
- S2CID 206549290.
- PMID 23798403.
- PMID 5647333.
- PMID 5764407.
External links
- Sodium-calcium+exchanger at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
- Diagram at cvphysiology.com
- Klabunde, RE. 2007. Cardiovascular Physiology Concepts: Calcium Exchange.