Calcium ion fluxes

Dunlop [18] proposed a model for sub-lytic effects in plant cells, based on the same principles, but including four properties postulated to be of particular importance in these systems, namely calcium ion flux, osmo-regulation, cell-cell contact/aggregation and stress protein expression. Of these factors, osmo-regulation (and its inter-relationship with the cell wall) and aggregation patterns, in particular, distinguish plant cells from mammalian cell systems. 

Lead may affect renin release from the kidney by affecting calcium ion fluxes in the juxtaglomerular cells, as discussed previously under Cardiovascular Effects. 

LIVER Use of isolated perfused liver in studies of biological transport processes, 192, 485 measurement of unidirectional calcium ion fluxes in liver, 192, 495 preparation and specific applications of isolated hepatocyte couplets, 192, 501 characterizing mechanisms of hepatic bile acid transport utilizing isolated membrane vesicles, 192, 517 preparation of basolateral (sinusoidal) and canalicular plasma membrane vesicles for the study of hepatic transport processes, 192, 534. 

Ketamine and PCP are use-dependent noncompetitive antagonists of the NMDA receptor that bind to the intrachannel site of the receptor and prevent calcium ion flux (Javitt and Zukin, 1991) in humans they cause a syndrome that is clinically indistinguishable from schizophrenia and that involves not only positive (psychotic) symptoms, but also, negative symptoms (blunted affect, avolition), thought... 

Calcium performs a variety of cellular functions in muscle and nerve that ultimately result in muscular contraction. Excellent descriptions of calcium s function in muscle and nerve are to be found in the reviews by Hoyle (37), Cohen (38), and Robertson (39). At the neuromuscular junction, the excitable cells are very sensitive to changes in extracellular concentrations of calcium. Curtis (40) and Luttgau (41) described a fall in the resting action potential and electrical resistance when the extracellular calcium concentration fell below 10 M. The action potential and electrical resistance returned to normal following addition of calcium to this vitro preparation. The magnitude of the Initial muscle membrane action potential, that which regulates the propagation of further muscle contraction, is also mediated by the extracellular calcium concentration. While the inward flow of sodium ions from the extracellular space remains the dominant factor in the mechanism of muscle membrane depolarization, calcium ion flux appears to mediate the cell s permeability to sodium ions. This effect is particularly true in cardiac tissue (W). 

Praziquantel (available on a named-patient basis) is a highly effective anthelmintic, which induces muscular contraction and spastic paralysis in trematodes and cestodes by increasing calcium ion fluxes. 

Calcium ion fluxes mediated by a variety of channels and ionophores into liposomes and cells have been studied by loading the vesicles with indicator dyes like arsenazo III (129) or quin-2 (130). The significantly higher calcium affinity of the fluorescence indicator fura-2 (131) was a major advancement in the detection of Ca2+ concentrations in small cells and liposomes (132-134). 

From the earliest measurements of tissue calcium, it was clear that total calcium is largely a measure of stored calcium. Through the years, scientists have used a variety of indirect measures of [Ca2+]j. These include shortening of or tension in muscles secretion from secretory cells the activity of Ca2+-dependent enzymes, most notably glycogen phosphorylase and flux of K+, or K+ currents, as a reflection of Ca2+-activated K+ channels. In addition, investigators often use the radioactive calcium ion [45Ca2+] as an indirect indicator of Ca2+ concentrations and Ca2+ movements. 

Fig. 2. Effect of calcium antagonists (CA) on a cardiac cell. Top typical cardiac action potential. The calcium (slow) inward current flows during the characteristic plateau phase (phase 2) of the action potential. This calcium influx is selectively inhibited by CA. Activation of the sarcoplasmic reticulum (SR) and other cellular calcium pools occurs via Ca + and Na+ ions which flow into the cell. The SR and other pools donate activator Ca + ions which stimulate the contractile proteins. The presence of tubular systems (invaginations), which are characteristic of cardiac tissues, results in considerable enlargement of the cellular surface, thus enabling an effective influx of Na+ and Ca + ions. Inhibition of the calcium inward flux by a CA causes diminished activation of the contractile proteins.

Among all these cognitive interactions between autopoietic entities and the envi-romnent, some are particularly important because they are recursive, i.e., they happen repeatedly. For example, throughout a membrane there is a continuous flux of sodium or calcium ions. This active transport is selective in the sense that it happens with certain ions and not with others. Where does this specificity come from The answer lies in the phylogenesis, the history of the living species, where each state... 

Absorption can be enhanced via several mechanisms. These include increased membrane fluidity, chelation of the calcium ions that serve to maintain the dimension of the intercellular space, solubilization of the mucosal membrane, enhancement in water flux, and reduction of the viscosity of the mucus layer adhering to the epithelial cells. A discussion of various types of pentration enhancers and their mechanism (s) of action is given in Chapter 8 (Section 8.7.1). Table 6.4 summarises the oral absorption enhancers that have been tested for oral dmg delivery. 

Mechanism of action Phenytoin stabilizes neuronal membranes to depolarization by decreasing the flux of sodium ions in neurons in the resting state or during depolarization. It also reduces the influx of calcium ions during depolarization and suppresses repetitive firing of neurons. 

One of the best studied systems concerning the role of calcium in egg fertilization is the sea urchin egg [123]. Upon fertilization, two ion fluxes occur in short prere-plicative phase which are obligatory for the subsequent initiation of processes, including DNA synthesis [124], The first is an internal surge of calcium ions at about 0-3 s post fertilization, and second is the efflux of hydrogen ion coupled to an influx of Na+ resulting in the alkalization of egg cytoplasm [125],... 

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