Plasma membrane cytosolic calcium concentration

The inner mitochondrial membrane may function primarily as a calcium sink, taking up excess calcium in the cytosol that results from hormonal activation of the cell. At cytosolic Ca + concentrations greater than 0.6 /rmol/L, the mitochondrial calcium pump is activated and stores calcium in the mitochondrial matrix as a nonionic, rapidly exchangeable, phosphate salt. At low cytosolic calcium concentrations, the inner mitochondrial membrane allows Ca + to leak into the cytosol. The capacity of the active influx pathway (the pump) is much greater than that of the passive efflux route (the leak). The mitochondrial pump-leak system may serve to fine-tune the cytosolic calcium concentration while the plasma membrane is the principal safeguard against entry of toxic amounts of calcium into the cell. 

The rise and fall in cytosolic calcium are the principal mechanisms that initiate contraction and relaxation of vascular and other smooth muscles. The cytosolic calcium concentration is regulated by the release and reuptake of calcium from/into intracellular stores and by its flux across the plasma membrane. A large part of the calcium influx is controlled by voltage-dependent opening and closing of calcium channels. Smooth muscle cells contain two types of voltage-dependent calcium channels, the T- and L-type channels, which are the products of different genes (Hofmann et al., 1994). 

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. 

Fig. 3. The interrelated changes in the metabolism of phosphatidylinositol 4,5-bisphosphate (PIP2) and Ca2+ during activation of the cell by a typical Ca2+-dependent hormone. R, receptor G, guanine regulatory protein PLC, phospholipase C DG, diacylglycerol CK, protein kinase C [Ca2+]sm, the Ca2+ concentration in a cellular domain just beneath the plasma membrane (striped area) Insl,4,SP3, inositol 1,4,5-trisphosphate Insl,3,4,5P4, inositol 1,3,4,5-tetrakisphosphate [Ca2+]c, cytosolic Ca2+ concentration CaM, calmodulin arrows (=>), fluxes of Ca2+ across membranes s=>, energy-dependent fluxes CaY, a calcium pool in specialized compartment of the endoplasmic reticulum. See text for discussion.

The mobilization of calcium results not only in the observed transient rise in intracellular free calcium and enhanced cellular efflux, but also in a net loss of calcium from the cell (Fig. 1). Thus, total cell calcium declines with All stimulation of adrenal and vascular smooth muscle cells [44]. Furthermore, total cell calcium remains low throughout the duration of exposure to All, suggesting that the continued formation of small amounts of 1,4,5-IP3 prevents refilling of the ER pool. Upon the removal of All and the immediate reduction in IP3 concentration, total cell calcium rapidly recovers to prestimulation levels without a detectable change in cytosolic free calcium, as measured by calcium-sensitive dyes. This observation has been taken as evidence that the IP3-releasable ER pool is in direct communication with the plasma membrane and that extracellular calcium refills the pool without entering the bulk cytosol (see Ref. 45). The location of this pool within the cell (cytosolic vs. adjacent to the plasma membrane) remains a matter of controversy (see Rasmussen arid Barrett, Chapter 4). 

This obvious dependence on extracellular calcium is somewhat unexpected because (1) the sustained enhancement of calcium influx rate is adequately balanced by an increase in calcium efflux rate so that (2) the calcium concentration in the bulk cytosol is maintained near the basal value. This apparent paradox may be resolved by a model [54] which postulates that during the sustained phase of cellular response the high rate of calcium cycling across the plasma membrane raises the calcium concentration in a region just below the plasma membrane, often called the submembrane domain (see Rasmussen and Barrett, Chapter 4). Because the elevated calcium level in this domain is not conducted into the bulk cytosol, it cannot activate calcium-dependent response elements in the cytosol. Rather it regulates the activity of calcium-sensitive, plasma membrane-associated enzymes such as the calcium pump and PKC, the previously described phospholipid-dependent, calcium-activated protein kinase. 

The CaR is expressed in many cells such as parathyroid cells and C cells in the thyroid gland. It is also expressed in the kidneys, osteoblasts, in the gastrointestinal mucosa, and hematopoietic cells in bone marrow. It has been found that CaR is expressed in different amounts on the cell face of many cell types and in diverse species. CaR is a member of the G protein-coupled receptor family with seven hydrophobic transmembrane helices in the plasma membrane. It has a large N-terminal domain with about 600 amino acids located in the extracellular environment and is essential for sensing extracellular calcium concentration. CaR has a large cytosolic C-terminal domain with about 200 amino acids that is subjected to phosphorylation. A dimeric structure has been proposed based on the known crystal structure of the metabotropic glutamate receptor type 1. 

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). 

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