Calcium transport across cellular membranes

Molecular and cellular organization of transepithelial calcium transport Calcium transport across the intestinal and renal epithelium proceeds on a transcellular as well as on a paracellular route. Transfer across polarized cells is a vectorial multi-step process (Bronner 1990, 1991), which encompasses (i) calcium entry across the brush-border membrane via two types of calcium channels (Muller et al. 2000, Peng etal. 2000, Slepchenko and Bronner 2001) ... 

As mentioned above, palytoxin produces multiple pharmacological actions at the cellular, tissue, and organism level. Alterations in the Na" and gradients by the toxin will initiate a complex chain of cellular events it could alter the active transport of various ions as well as the transport processes across intracellular membranes. One important parameter that can be altered by palytoxin is the cytosolic free calcium concentration, which will produce alterations in numerous cellular processes. 

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]. 

Cells constantly, either actively or passively, transport chemicals across the plasma membrane and communicate with surrounding cells, generating chemical diffusion gradients around them. Measurement of such activities within gradients enables a better understanding of the cellular transport mechanisms. For example, calcium is regulated to nanomolar values in the cytosol by a complex interaction of cellular and plasma membrane pumps, reporters, channel activity, regional sequestration, and chemical... 

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