Calcium cytoplasm

Calcium and Vascular Smooth Muscle Contraction. Calcium acts on a number of sites associated with the control of the cytoplasmic calcium concentration. Vascular smooth muscle contraction can be initiated by the opening of the slow calcium channel aUowing influx of extraceUular calcium through the sarcolemmal membrane into the cytoplasmic compartment. The iatraceUnlar calcium concentration increases to 1 x 10 Af, a threshold concentration necessary to initiate contraction. 

Cobbold, P. H. (1980). Cytoplasmic free calcium and amoeboid movement. Nature 285 441 146. 

Gilroy, S., Hughes, W. A., and Trewavas, A. J. (1989). A comparison between Quin-2 and aequorin as indicators of cytoplasmic calcium levels in higher plant cell protoplasts. Plant Physiol. 90 482—491. 

Knight, M. R., Campbell, A. K., Smith, S. M., and Trewavas, A. J. (1991). Transgenic plant aequorin reports the effects of touch and cold-shock and elicitors on cytoplasmic calcium. Nature 352 524-526. 

Cadherins (Calcium-dependent adhesion proteins) are transmembrane proteins, which consist of an extracellular domain composed of cadherin-repeats, a transmembrane domain, and a cytoplasmic domain that interacts with catenins and/or other cytoplasmic proteins. 

The activation of NFAT is regulated by calcium. In resting cells, NFAT proteins localize in the cytoplasm... 

The NHR contains also the conserved Calcineurin docking site, PxlxIT, required for the physical interaction of NEAT and Calcineurin. Dephosphorylation of at least 13 serines residues in the NHR induces a conformational change that exposes the nuclear localization sequences (NLS), allowing the nuclear translocation of NEAT. Rephosphorylation of these residues unmasks the nuclear export sequences that direct transport back to the cytoplasm. Engagement of receptors such as the antigen receptors in T and B cells is coupled to phospholipase C activation and subsequent production of inositol triphosphate. Increased levels of inositol triphosphate lead to the initial release of intracellular stores of calcium. This early increase of calcium induces opening of the plasma membrane calcium-released-activated-calcium (CRAC) channels,... 

Figure 48-12. Schematic illustration of some aspects of the role of the osteoclast in bone resorption. Lysosomal enzymes and hydrogen ions are released into the confined microenvironment created by the attachment between bone matrix and the peripheral clear zone of the osteoclast. The acidification of this confined space facilitates the dissolution of calcium phosphate from bone and is the optimal pH for the activity of lysosomal hydrolases. Bone matrix is thus removed, and the products of bone resorption are taken up into the cytoplasm of the osteoclast, probably digested further, and transferred into capillaries. The chemical equation shown in the figure refers to the action of carbonic anhydrase II, described in the text. (Reproduced, with permission, from Jun-queira LC, Carneiro J BasicHistology. Text Atlas, 10th ed. McGraw-Hill, 2003.)...

The calcium phosphate method was first used in 1973 to introduce adenovirus DNA into mammalian cells [3]. DNA-Calcium-phosphate complexes are formed by mixing DNA in a phosphate buffer with calcium chloride. These complexes adhere to the cell membrane and enter the cytoplasm by endocytosis. Disadvantages of DEAE-dextran and calcium phosphate transfection are a certain level of cytotoxicity, a complicated transfection procedure, and the fact that not all cell types can be transfected using these methods. 

Fromm and Spanswick [79] found that electrical stimulation of a plant is followed by ion shifts which are most striking in the phloem cells. While their content of potassium and chloride was diminished after stimulation, the amount of cytoplasmic calcium increased slightly (Table 1). These displacements lead to the conclusion that Ca + influx as well as and CP efflux are involved in the propagation of action potentials. The main difference between propagation of action potentials in animals and plants is that in an axon there is the K /Na transmembrane transport but in phloem cells the K /Ca channels are involved in this process [Fig. 22(b)]. 

Fast-twitch muscle fibers develop tension two to three times faster than slow-twitch muscle fibers because of more rapid splitting of ATP by myosin ATPase. This enables the myosin crossbridges to cycle more rapidly Another factor influencing the speed of contraction involves the rate of removal of calcium from the cytoplasm. Muscle fibers remove Ca++ ions by pumping them back into the sarcoplasmic reticulum. Fast-twitch muscle fibers remove Ca++ ions more rapidly than slow-twitch muscle fibers, resulting in quicker twitches that are useful in fast precise movements. The contractions generated in slow-twitch muscle fibers may last up to 10 times longer than those of fast-twitch muscle fibers therefore, these twitches are useful in sustained, more powerful movements. 

When the action potential reaches the synaptic bouton, depolarisation triggers the opening of voltage-operated calcium channels in the membrane (Figure 2.5). The concentration gradient for Ca2+ favours the passive movement of this ion into the neuron. The subsequent rise in cytoplasmic Ca2+ ion concentration stimulates the release of neurotransmitter into the synaptic cleft, which diffuses across this narrow gap and binds to receptors located on the postsynaptic neuronal membrane (Figure 2.5). 

All cell membranes contain transmembrane proteins that form ion channels. These ion channels are usually selectively permeable to particular ions. Some channels, such as GABA-gated ion channels, are permeable to Cl ions and are inhibitory in nature because they make the inside of the nerve or muscle cells more negative as the Cl ions enter. Some ion channels are permeable to the cations Na and K, and an example of this type is the nicotinic acetylcholine-gated channel. Nicotinic channels have an excitatory effect when they open because Na ions enter and K ions leave through these channels. The cell becomes more positive inside and depolarizes. If the cell is a muscle cell, calcium accumulates in the cytoplasm and it contracts. We have found that all over the surface of Ascaris muscle there are GABA receptors (Martin, 1980) as well as nicotinic acetylcholine channels (Martin, 1982 Robertson and Martin, 1993). 

Another feature of mitochondria is that they handle the syntheses of both iron/sulfur and iron haem units. It appears as if handling metal ions, including that of iron, is risky in the cytoplasm. In fact, other metal ions are pumped out of cells or into vesicles and in some cases into mitochondria - especially calcium. 

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