Emptying Of Intracellular Ca2 Research Articles

Antigen and carbachol mobilize calcium by similar mechanisms in a transfected mast cell line (RBL-2H3 cells) that expresses ml muscarinic receptors.

Because of unresolved questions about the mechanism of Ag-stimulated Ca2+ influx, Ca2+ mobilization in response to carbachol and Ag was compared in transfected rat basophilic RBL-2H3(ml) cells that expressed both Fc epsilon and ml muscarinic receptors. Although the stimulants activated phospholipase C via different coupling mechanisms, a G protein for carbachol or a tyrosine kinase for Ag, they released Ca2+ from the same intracellular pool and used the same or very similar mechanisms for influx of Ca2+ as indicated by the similar patterns of inhibition of uptake of 45Ca2+ by various cations. With both stimulants, influx and sustained increases in free cytosolic Ca2+ ([Ca2+]i) were associated with relatively small increases in inositol 1,4,5-trisphosphate (IP3). Blockade of Ca2+ influx resulted in rapid decline in [Ca2+]i to basal levels; resumption of influx caused a substantial "spike" in [Ca2+]i before [Ca2+]i reequilibrated at the same former steady-state levels but without perturbing levels of IP3. Thus, the refilling and discharge of Ca2+ from IP3-sensitive stores might occur synchronously on resumption of influx, or asynchronously during sustained influx and elevation of [Ca2+]i. Together, the results suggested that influx of Ca2+ in response to stimulation via Fc epsilon receptors occurred through a pathway, analogous to that observed with other types of stimulants, in which Ca2+ influx follows emptying of intracellular Ca2+ stores by IP3. Also, secretion was highly dependent on this IP3-dependent pathway.

Emptying of intracellular Ca2+ stores releases a novel small messenger that stimulates Ca2+ influx.

Intracellular Ca2+ signals that last more than a few minutes after the onset of stimulation depend critically on influx of extracellular Ca2+. Such Ca2+ influx can be triggered in many cell types by depletion of intracellular Ca2+ stores without detectable elevations of known messengers. The mechanism by which store depletion can control plasma membrane Ca2+ permeability remains controversial. Here we present evidence for a novel soluble mediator. Calcium depletion of a lymphocyte cell line caused the messenger to be released from intracellular organelles into the cytoplasm and to a much lesser extent into the extracellular medium. The messenger caused Ca2+ influx when applied to macrophages, astrocytoma cells, and fibroblasts and was therefore named CIF (for Ca(2+)-influx factor). CIF appears to have hydroxyls (or hydroxyl and amino groups) on adjacent carbons, a phosphate, and a M(r) under 500.