Ca2+-transport

Epithelial calcium channel 1 (ECaCl), synonym TRJPV5, is a member ofthe TRP family of ion channels, implicated in vitamin D-dependent transcellular Ca2+ transport in epithelial cells ofthe kidney, placenta and the intestine. 

The site level at which [Ca2+]i regulates NCX activity (CBD) is different from the one required for Ca2+ transport. Submicromolar concentrations (0.1-0.3 pM) of intracellular Ca2+ are needed to activate the antiporter through these Ca2+ binding site. The location of such regulatory site has been identified in the 134-amino acid-length region, situated in the center of the intracellular f loop [2], (Table 1)... 

TRPV5 and TRPV6, also known as the epithelial Ca2+ channel or ECaC (TRPV5) and Ca2+transporter 1 or Ca2+ transporter-like (TRPV6), are the only two Ca2+-selective TRP channels identified so far. They may function in vitamin D-dependent transcellular transport of Ca2+in kidney, intestine and placenta. TRPV6 is also expressed in pancreatic acinar cells, and in prostate cancer, but not in healthy prostate or in benign prostate hyperplasia. 

Figure 4 Schematic representation of the Ca2+-transporting systems affecting cellular calcium homeostasis during hormonal stimulation, oq = oq-adrenergic receptor VP = vasopressin receptor PLC = phospholipase C PI = phosphatidylinositol PIP = phospha-tidylinositol-4-phosphate PIP2 = phosphatidylinositol-4,5-biphosphate IP3 = inositol-1,4,5-triphosphate DG = diacylglycerol PKC = protein kinase C. (Modified from Refs. 125 and 285.)...

PS Reinach, N Holmbeg, R Chiesa. (1991). Identification of calmodulin-sensitive Ca2+-transporting ATPase in the plasma membrane of bovine corneal epithelial cell. Biochim Biophys Acta 1068 1-8. 

The organization of lipids around the plasma membrane Ca2+-transport ATPase of erythrocytes has been also determined by FRET. Taking advantage of the intrinsic fluorescence of the ATPase due to tryptophan residues and labeling different types of lipids with pyrene, it was demonstrated that the transporter is preferentially surrounded by negatively charged lipids such as phosphoinositides [167],... 

Verbist, J., Gadella, T. W. J., Raeymaekers, L., Wuytack, F., Wirtz, K. W. A. and Casteels, R. (1991). Phosphoinositide-protein Interactions of the plasma-membrane Ca2+-transport ATPase as revealed by fluorescence energy-transfer. Biochim. Biophys. Acta 1063, 1-6. 

The primary plasma-membrane Ca2+ transporter (PMCA) is a P-type pump with high affinity for Ca2+ (Km = 100-200 nmol/1) but relatively low transport capacity [19]. The stoichiometry of PMCA is one Ca2+ transported for each ATP hydrolyzed. These pumps probably do not carry out bulk movements of Ca2+ but are most effective in maintaining very low concentrations of cytosolic Ca2+ in resting cells. A distinguishing characteristic of the PMCAs is that, in addition to binding Ca2+ as a substrate, they are further activated by binding... 

As both fast and slow adaptation mechanisms are regulated by Ca2+, the stereocilia mechanisms that control the free concentration of this ion also play central roles in transduction. Entering Ca2+ is thought to be buffered very rapidly by the mobile buffers parvalbumin 3, calbindin, and calretinin [22,23]. Even before bound Ca2+ can diffuse out of stereocilia, it is pumped back out into the endo-lymph by isoform 2a of the plasma-membrane Ca2+-ATPase (PMCA2) [24,25] (see also Ca2+ transport in Ch. 5). 

FIG. 5. Maintenance of the superficial buffer barrier depends on NCX-assisted Ca2+ transport from the SR lumen to the extracellular space. (A) Rate of loss of SR Ca2+ content, measured as a caffeine transient, into Ca2+ free perfusate at room temperature. (B) Rate of decline in [Ca2+ I from an elevated level, measured as fura 2 fluorescence ratio, into Ca2+ free superfusate, which is either Na+ free or contains 10 /rM CPA or is Na+ free and contains CPA. (C) This cartoon represents a model for maintained buffering by the superficial SR of Ca2+ entry. Ca2+ taken up by SERCA is subsequently released into the SR-PM junctional space from where it is extruded by the NCX. 

Inesi G, Sagara Y 1994 Specific inhibitors of intracellular Ca2+ transport ATPases. J Membr Biol... 

Prishchepa LA, Burdyga TV, Kosterin SA 1996 Two components of sodium azide-insensitive Mg2+, ATP-dependent Ca2+ transport in ureteral smooth muscle membrane structures (translated from Russian). Biokhimiia 61 1250—1256 Rose JG, Gillenwater JY 1973 Pathophysiology of ureteral obstruction. Am J Physiol 225 830-837... 

Raeymaekers L, Wuytack F 1996 Calcium Pumps. In Barany M (ed) Biochemistry of smooth muscle contraction. Academic Press, New York, p 241-253 Shull GE 2000 Gene knockout studies of Ca2+-transporting ATPases. Eur J Biochem 267 5284-5290... 

Work on vascular smooth muscle has not led to any real consensus as to the precise differences in SR function between normal and hypertensive animals, although there is evidence, summarized in a review by Raeymaekers Wuytack (1993) for diminished SR Ca2+ transport in hypertensive animals. More recent work has also provided evidence for increased Ca2+ influx from the extracellular space in vascular smooth muscles from various rat models of hypertension (Nomura et al 1997, Arii et al 1999). 

Figure 11.6 Schematic representation of Ca2+ transport in and out of mitochondria, showing all the Ca2+ transporters and activation of matrix dehydrogenases. PTP—permeability transition pore. (From Carafoli, 2003. Copyright 2003, with permission from Elsevier.)...

Figure 11.7 The microdomain concept of mitochondrial Ca2+ transport. Ca2+ penetrating from outside the cell or released from the ER generates local hotspots of high Ca2+ concentration in the vicinity of mitochondria sufficient to activate their low-affinity Ca2+ uptake system. (From Carafoli, 2002. Copyright (2002) National Academy of Sciences, USA.)...

Janik F, Wolf HU. 1992. The Ca2+-transport-ATPase of human erythrocytes as an in vitro toxicity test system—acute effects of some chlorinated compounds. J Appl Toxicol 12 351-358. 

Activation of neutrophils with PAF occurs through a G-protein-linked receptor, and the subsequent transmembrane signalling involves the stimulation of inositol phosphate metabolism. Within 30 s of addition of PAF (0.01-100 nM), intracellular Ca2+ levels increase and Ca2+ transport from the external medium is enhanced. It seems that phospholipase C-dependent and -independent activation pathways are involved in Ca2+ mobilisation. This indirectly suggests that two receptors may be involved in PAF activation. The first of these is pertussis-toxin-insensitive and may be linked to a... 

Vanderkooi et al.(m) examined the phosphorescence from tryptophan in sarcoplasmic reticulum vesicles and the purified Ca transport ATPase at room temperature in deoxygenated solutions. The phosphorescence decay is multiexponential the lifetime of the long-lived component of phosphorescence is 22 ms. Addition of ATP or vanadate decreased the phosphorescence yield. The Ca2+-ATPase of the sarcoplasmic reticulum alternates between two conformations, called Ei and E2, during Ca2+ transport. The observations were interpreted to indicate that either the binding of vanadate or phosphate to the phosphorylation site of the ATPase or the induced shift in the conformation from the i to the E2 state produced the phosphorescence quenching. 

J. M. Vanderkooi, A. Ierokomas, H. Nakamura, and A. Martonosi, Fluorescence energy transfer between Ca2+ transport ATPase molecule in artifical membranes, Biochemistry 16, 1262-1267 (1977). 

Ton, V.K. and Rao, R. Functional expression of heterologous proteins in yeast Insights into Ca2+ signalling and Ca2+-transporting ATPases. Am J Physiol 287 C580-C589, 2004. 

Figure 6.12 The cellular toxicity of TBT caused by damage to the thiols of the Ca2+ pump. This leads to dramatic mobilization of calcium from the ER. The filled circles represent ATP-dependent Ca2+ transporters. Abbreviations TBT, tri-n-butyltin chloride ER, endoplasmic reticulum.

Thus, an initial drop in ATP is followed by increases in Ca2+, which inhibits ATP synthase and increases ROS and reactive nitrogen species (RNS) formation via xanthine oxidase. These inhibit thiol-dependent Ca2+ transport. The reactive molecules can also inhibit the electron transport chain (by reacting with Fe at the active sites) and enzymes in glycolysis, notably glyceraldehyde 3-phosphate dehydrogenase, leading to further losses of ATP. The depleted ATP exacerbates the intracellular Ca2 increase as a result of reduced transport out and sequestration into the endoplasmic reticulum. 

The depletion of GSH and NADPH will allow the oxidation of protein sulfydryl groups, which may be an important step in the toxicity. Thus, GSH and protein sulfydryl groups, such as those on Ca2+-transporting proteins, are important for the maintenance of intracellular calcium homeostasis. Thus, paracetamol and NAPQI cause an increase in cytosolic calcium, and paracetamol inhibits the Na+/K+ ATPase pump in isolated hepatocytes. 

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