Calcium and Cell Steady States
Since the realisation that calcium was a major trigger of activity both in extracellular and intracellular systems a change has come about in our knowledge of calcium binding in different biological zones. We know now that there is a wide diversity of calcium controlled systems. Triggering is not just a matter of a simple switch in calcium ion levels associated with a cell followed by simple binding to a target. Each eukaryotic cell has a complex shape and a complicated set of compartments, reticula and organelles. Within different zones of the cell cytoplasmic membrane and of the different compartment membranes there are differentially placed transport and receptor systems which use calcium. Again within the compartment and cytoplasmic aqueous solutions there is a variety of response systems, binding units, which act in rather different ways to the calcium. This is not just a matter of the calcium binding constants but is reflected in the effect of calcium on the organisation of protein components in the cell. The extent of these organised reactions is not known but includes contraction elements, themselves related to cell shape changes, exocytosis and perhaps cell division. The extent of the findings, see Table I, leads me to suggest that calcium is more than a trigger : it is also an internal coordinator of the cell integrating its compartmentalised activities through differential, kinetically controlled, i.e. energy-linked, homeostatic devices while the cell is at rest. Rest state homeostasis needs a re-examination.
KeywordsCalcium Concentration Calcium Level Phosphate Metabolism Calcium Site Cell Steady State
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