Albumin stimulates uptake of calcium into subcellular stores in rat cortical astrocytes.

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Abstract
1. When albumin from either plasma or serum is applied at low concentrations to cortical astrocytes a decrease in the level of [Ca2+]i is observed. At higher concentrations trains of calcium spikes are seen. 2. Removal of the polar lipids which are normally bound to native albumin abolishes the ability to induce spikes, but the decrease in [Ca2+]i is unaffected. The decrease is abolished by the denaturation of albumin and is not reproduced by a number of other proteins, and is therefore a specific action of albumin. We conclude that native albumin has a dual agonist action: the decrease in [Ca2+]i is induced by the albumin protein molecule, while the spikes are induced by a lipid normally bound to it. 3. The decrease is rapid (fastest tau = 12 s) and the rate is dependent on the concentration of albumin. [Ca2+]i falls from 77 nM to around 34 nM in the presence of saturating levels of albumin, and this level appears to be maintained indefinitely. 4. The decrease is due to an uptake of calcium into subcellular stores, as it is not abolished by removal of external Ca2+ or Na+ but is abolished by thapsigargin and cyclopiazonic acid, which are specific inhibitors of the endoplasmic reticulum Ca(2+)-ATPase. 5. When the state of store filling after albumin application is probed with a pulse of glutamate it can be seen that stores fill with the same time course as the decrease in [Ca2+]i. The low level of [Ca2+]i in albumin must therefore be maintained by a suppression of calcium influx rather than by a continued uptake into stores. 6. The calcium uptake potentiates the efficacy of low concentrations of calcium-releasing agonists such as glutamate and bradykinin by almost an order of magnitude. 7. A possible function for the calcium uptake caused by albumin is to potentiate the production of calcium spike trains by promoting refilling of calcium stores in the intervals between spikes. The uptake may play a role in the response of astrocytes to damage in the CNS.