Endoplasmic reticulum calcium ions

The cytosolic concentration of free Ca2+ is generally at or below 100 mi, far lower than that in the surrounding medium, whether pond water or blood plasma. The ubiquitous occurrence of inorganic phosphates (Pj and I l ,) at millimolar concentrations in the cytosol necessitates a low cytosolic Ca2+ concentration, because inorganic phosphate combines with calcium to form relatively insoluble calcium phosphates. Calcium ions are pumped out of the cytosol by a P-type ATPase, the plasma membrane Ca2+ pump. Another P-type Ca2+ pump in the endoplasmic reticulum moves Ca2+ into the ER lumen, a compartment separate from the cytosol. In myocytes, Ca2+ is normally sequestered in a specialized form of endoplasmic reticulum called the sarcoplasmic reticulum. The sarcoplasmic and endoplasmic reticulum calcium (SERCA) pumps are closely related in structure and mechanism, and both are inhibited by the tumor-promoting agent thapsigargin, which does not affect the plasma membrane Ca2+ pump. 

Inositol trisphosphate opens a calcium transport channel in the membrane of the endoplasmic reticulum. This leads to an influx of calcium from storage in the endoplasmic reticulum and a 10-fold increase in the cytosolic concentration of calcium ions. Calmodulin is a small calcium binding protein found in all cells. Its affinity for calcium is such that, at the resting concentration of calcium in the cytosol (of the order of 0.1 /xmol per L), little or none is bound to calmodulin. When the cytosolic concentration of calcium rises to about 1 /xmol per L, as occurs in response to opening of the endoplasmic reticulum calcium transport channel, calmodulin binds 4 mol of calcium per mol of protein. When this occurs, calmodulin undergoes a conformational change, and calcium-calmodulin binds to, and activates, cytosolic protein kinases, which in turn phosphorylate target enzymes. 

Calcium ions (Ca ) are important for the mediation of hepatic injury. Cytosolic free calcium is maintained at relatively low concentrations compared to the extracellular levels. The majority of intracellular calcium is sequestered within the mitochondria and endoplasmic reticulum. Membrane associated calcium and magnesium ATPases are responsible for maintaining the calcium gradient (Farrell et ah, 1990). Significant and persistent increases in the intracellular calcium result from nonspecific increases in permeability of the plasma membrane, mitochondrial membranes, and membranes of the smooth endoplasmic reticulum. Calcium pumps in the mitochondrial membrane require NADPH, thus depletion of available NADPH can cause calcium release from mitochondria (Cullen, 2005). 

Similarly, each molecule of IP will continue to keep the endoplasmic reticulum calcium channel open until it is phosphorylated to inactive inositol tetrakisphosphate, thus maintaining a flow of calcium ions into the cytosol. Each molecule of calcium-calmodulin will bind to, and activate, a molecule of protein kinase for as long as the cytosol calcium concentration remains high. It is only as the calcium is pumped back into the endoplasmic reticulum that the calcium concentration falls low enough for calmodulin to lose its bound calcium and be inactivated. Again each molecule of phosphorylated enzyme will catalyse the metabolism of many thousands of mol of substrate per second, until it is dephosphorylated by phosphoprotein phosphatase. 

Mammals synthesize phosphatidylserine (PS) in a calcium ion-dependent reaction involving aminoalcohol exchange (Figure 25.21). The enzyme catalyzing this reaction is associated with the endoplasmic reticulum and will accept phosphatidylethanolamine (PE) and other phospholipid substrates. A mitochondrial PS decarboxylase can subsequently convert PS to PE. No other pathway converting serine to ethanolamine has been found. 

The endoplasmic reticulum (ER) is responsible for the production of the protein and lipid components of most of the cell s organelles. The ER contains a large number of folds, but the membrane forms a single sheet enclosing a single closed sac. This internal space is called the ER lumen. The smooth endoplasmic reticulum (ER) in muscle cells contains the vesicles and tubules that serve as a store of calcium ions. These are released as one step in the muscle contraction process. Calcium pumps, Ca +-ATPases, serve to move the calcium from the cytoplasm to the ER or SR lumen. 

Figure 14-3. Signaling through protein kinase C (PKC). Activated phospholipase C cleaves the inositol phospholipid PIP2 to form both soluble (IP3) and membrane-associated (DAG) second messengers. DAG recruits PKC to the membrane, where binding of calcium ions to PKC fully activates it. To accomplish this, IP3 promotes a transient increase of intracellular concentration by binding to a receptor on the endoplasmic reticulum, which opens a channel allowing release of stored calcium ions. PIP2, phosphatidylinositol 4,5-bisphosphate DAG, diacylglycerol PLC, phospholipase C IP3, inositol trisphosphate.

Inositol triphosphate is water soluble and therefore diffuses into the cytoplasm, where it mobilizes calcium from its stores in microsomes or the endoplasmic reticulum. The Ca ions then activate Ca-dependent kinases (like troponin C in muscle) directly or bind to the ubiquitous Ca-binding protein calmodulin, which activates calmodulin-dependent kinases. These kinases, in turn, phosphorylate cell-specific enzymes. 

Consistent with their role in signaling, calcium ions are unevenly distributed within cells. Mitochondria, endoplasmic reticulum, Golgi, and nuclei may all take up calcium ions. Cytoplasmic Ca2+... 

As mentioned in Box 6-D, mitochondria sometimes take up calcium ions. The normal total concentration of Ca2+ is 1 mM and that of free Ca2+ may be only 0.1 pM 22e f However, under some circumstances mitochondria accumulate large amounts of calcium, perhaps acting as a Ca2+ b u ffer.22 2 The so called ryanodine receptors (Fig. 19-21), prominent in the endoplasmic reticulum, have also been found in heart mitochondria, suggesting a function in control of calcium oscillations.221) On the other hand, accumulation of calcium by mitochondria may be pathological and the activation of Ca2+-dependent proteases may be an initial step in apoptosis.2211 221 ... 

IP3 mobilizes Ca2+ from intra- or extracellular stores. The interior of a cell is kept very low in Ca2+ ions, at a concentration less than 10-9 M., while the outside [Ca2+] is about 10-3 M. This million-fold concentration gradient is the result of cellular calcium-dependent ATPase protein. Ca-ATPase uses up to a third of the ATP synthesized by a cell to maintain the concentration gradient. The stores of Ca2+ available for use inside the cell are found primarily in the endoplasmic reticulum. A large store of Ca2+ exists in the mitochondrial matrix, but this seems to be a final dumping ground —in other words, calcium ions in the mitochondria don t come into the cytoplasm. 

The depolarization that accompanies the action potential induces an increase in membrane permeability to calcium ions. A large inward electrochemical gradient exists for calcium and it moves into the terminal. The calcium that enters the terminal activates enzymes that cause the attachment of some of the vesicles to releasing sites on the terminal membrane, membrane fusion, and the release of the vesicular contents into the synaptic cleft. Transmitter release is terminated by the removal of calcium from the terminal cytoplasm, either via a calcium pump, which pumps it out of the cell, or by uptake into the endoplasmic reticulum or into mitochondria. 

Aluminum in micromolar concentrations was found to inhibit calcium pumping in endoplasmic reticulum. The Ca2+ ATPase activity of rat brain and cerebellum was remarkably reduced and mitochondria showed increased Ca2+ release in the presence of exactly estimated 50 pmol L-1 Al3+ [67]. Aluminum was found to be an important disrupter of intracellular calcium homeostasis, interfering also with the mitochondrial Ca2+ pump, as well as activating an Na+-K+ ATPase - the antiport mechanism of ion exchange in the plasma membrane, which regulates the Ca2+-Na+ antiporter exchange [67]. 

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