Calcium, storage

Calsequestrin is the major calcium storage protein of the sarcoplasmic reticulum in skeletal and cardiac muscles. It is highly acidic and has a large capacity for Ca2+. Calsequestrin functions to localize calcium near the junctional face of the terminal cistemae from which calcium can be released into the cytosol via the ryanodine receptor. 

The smooth muscle cell does not respond in an all-or-none manner, but instead its contractile state is a variable compromise between diverse regulatory influences. While a vertebrate skeletal muscle fiber is at complete rest unless activated by a motor nerve, regulation of the contractile activity of a smooth muscle cell is more complex. First, the smooth muscle cell typically receives input from many different kinds of nerve fibers. The various cell membrane receptors in turn activate different intracellular signal-transduction pathways which may affect (a) membrane channels, and hence, electrical activity (b) calcium storage or release or (c) the proteins of the contractile machinery. While each have their own biochemically specific ways, the actual mechanisms are for the most part known only in outline. 

The functions of the calcium-storage capacity of the ER are at least threefold the association of Ca2+ with Ca2+-binding proteins in the ER is part of a chaperone function that is essential for normal protein synthesis the rapid rate of Ca2+ uptake by endoplasmic pumps provides shortterm cytoplasmic Ca2+ buffering that resists untoward and transient changes in [Ca2+] and, finally, many signaling pathways employ elevated [Ca2+] to activate physiological processes. Extensive Ca2+ release from ER is coupled to activation of Ca2+ entry across the plasma membrane, a process known as capacitative calcium entry, which is discussed below. 

When the receptor interacts with its associated G protein, the conformation of the guanine-nucleotide-binding site is altered. The subunits then dissociate, and a phosphatidylinositol-specific phospholipase C (PI-PLC) is activated [5]. The subsequent hydrolysis of phosphatidylinositol bisphosphate then produces inositol triphosphate (IP3) and diacylglycerol (DAG), which are known to be secondary messengers. For example, the water soluble IP3 is released into the cell where its ultimate targets are the calcium storage organelles from which Ca2+ is released [3]. The presence of DAG in cells is known to activate the cellular enzyme protein kinase C (PKC) [6, 7], which phosphorylates a number of cellular... 

A 60 kg adult contains 1000-1200 g of calcium more than 99% is in the bones and teeth. About 1 g is in the plasma and extracellular fluid and 6-8 g in the tissues, sequestered in calcium storage vesicles. The calcium concentration in the blood is about 2.5 mmol/L, about 50% as the free ion and the rest bound to plasma proteins. 

Calcium Storage in the Absence of Calcium Precipitating Anions... 

It has been postulated that proteoglycans play an important role on mineral growth253,2S5, s04, s05). Also, a specifically mineral-generative role for these compounds has been suggested506-508. Evidence has been found for calcium storage in... 

Calsequestrin is a calcium-storage protein found in the sacroplasmic reticulum, which binds about 50 calcium ions per monomer (molecular weight 40 000) with binding constants in the range 103-105 dm3 mol. Release and uptake of Ca2+ during muscle contradion and relaxation involve this store. Calsequestrin from rabbit skeletal muscle has a random coil conformation in the absence of calcium. Binding of Ca2+ is associated with a change to a more compact structure.267... 

We will consider the structural and mechanistic features of these enzymes by examining the Ca2+ ATPase found in the sarcoplasmic reticulum (SR Ca2+ ATPase) of muscle cells. This enzyme, which constitutes 80% of the sarcoplasmic reticulum membrane protein, plays an important role in muscle contraction, which is triggered by an abrupt rise in the cytosolic calcium level. Muscle relaxation depends on the rapid removal of Ca + from the cytosol into the sarcoplasmic reticulum, a specialized compartment for calcium storage, by the SR Ca + ATPase. This pump maintains a Ca2+ concentration of approximately 0.1 iM in the cytosol compared with 1.5 mM in the sarcoplasmic reticulum. 

Grondowitz M. J. and Broce A. B. (1983) Calcium storage in face fly (Dipter Muscidae) larvae for puparium formation. Ann. Entomol. Sci. Am. 76, 418-424. 

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