Calcium transport through membranes

Pumiliotoxin B had calcium-dependent effects on evoked release of acetylcholine from nerve terminals in frog neuromuscular preparations 26). The alkaloid in a dose and stimulus-dependent manner caused repetitive endplate potentials in response to a single stimulation of nerve. This effect on neurotransmitter release was strongly calcium-dependent and probably involved facilitation by pumiliotoxin B of evoked calcium transport through plasma membranes and/or membranes of the endoplasmic reticulum of the nerve terminal. 

Skeletal tissue mineralization (bone formation) is initiated by osteoblasts, which secrete the osteoid matrix (Fig. 9.4). They express type I procollagen in secretory vesicles together with matrix vesicles that pinch off from the membrane. The matrix vesicles are pushed away from the cell surface, possibly by the flow of fluid containing calcium and phosphate ions that are also transported through the cell from the extracellular fluid on the outer surface. Collagen fibers develop further away from the cell surface than from fibroblasts. 

The calcium-transport system is another potential source of binding proteins. Transport into mitochondria, across plasma membranes and through the intestinal mucosa and the renal tubule may be the function of distinct transport systems, each of which may be the source of binding proteins. The possibility of isolating bacterial mutants deficient both in calcium uptake and in calcium-binding proteins offers an interesting approach to the role of binding proteins in calcium transport. 

Alternative models for the calcium effects have been advanced by Hubbard, Jones, and Landau and by Katz and Miledi. These workers believe that the reaction of calcium with sites on the vesicle or on the presynaptic membrane controls the rate of discharge of synaptic vesicles. The effect of depolarization is then either (a) to facilitate the transport of Ca + to these specific absorption sites by opening channels for ionic transport, or (b) to facilitate the transport of the calcium complex through the membrane. To what extent can the calcium effect on MEPP be understood within the framework of the basic electrokinetic theory without such new ad hoc assumptions, namely, by means of an effect of [Ca +] on the free energy of activation AC ... 

In contrast to calcium or iron, there exists in blood no remarkable protein-based transport system for Cl". That anion is dissolved within the plants or animals fluid volumes and contributes to the osmotic condition. At first view, any movement of Cl includes transport of water, and vice versa. That is visible in the relationship between Cl and water in the gastrointestinal tract (Meyer 1996). However, living cells require suitable osmotic conditions both outside and inside, and so a system which drives ions through membranes is needed. The capacity of such a system is impressively established in fish. Marine fish live in a hyperosmotic environment which withdraws water from the animal this exosmosis is counteracted by a high oral water intake (Evans 1998, Pelis etal. 2001). Cl uptake by the mucosa cells occurs in relation to the osmotic gradient, and chloride extrusion... 

All these studies on chromatographic size exclusion separations on neutral separation media and selectivity of transportation of ions through membranes were carried out with very dilute electrolyte solutions. Only Rona and Schmuckler [135] examined Dead Sea concentrated brine on a Bio-Gel P-2 (crosshnked polyacrylamide) column and obtained a Hthium-enriched fraction free of calcium and magnesium. Bio-Gel P, however, is known to retain cations and probably enters hydrogen-bond interactions between the anions and the amide hydrogen. The elution order of chlorides was thus different from that expected for pure SEC, namely, K, Na, Li, Mg, and Ca, all, however, emerging before the hold-up (dead) volume of the column. 

Calcium is absorbed in the whole small intestine with the bulk preferentially from duodenum and the upper part of jejunum. The absorption consists of a passive diffusion as well as an active transport through the mucosa. A schematic description of the cellular and paracellular pathway for calcium absorption from the intestinal lumen to blood across the intestinal epithelium can be seen in Fig. 1. The higher rate of calcium transport in duodenum as compared to that in jejunum and ileum is probably not due to greater cellularity of duodenum but to a greater calcium transport by each duodenal cell. The molecular basis for calcium entry across the brushborder is not known in detail, but initial calcium binding may be an early step in the activation of a calcium channel or to increase membrane fluidity [3]. 

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