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 C a2+ influx, Ca2+ mobilization in response to carbachol and Ag was comp ared in transfected rat basophilic RBL-2H3(ml) cells that expressed bo th Fcepsilon and ml muscarinic receptors. Although the stimulants acti vated phospholipase C via different coupling mechanisms, a G protein f or carbachol or a tyrosine kinase for Ag, they released Ca2+ from the same intracellular pool and used the same or very similar mechanisms f or influx of Ca2+ as indicated by the similar patterns of inhibition o f uptake of Ca-45(2+) by various cations. With both stimulants, influx and sustained increases in free cytosolic Ca2+ ([Ca2+]i) were associa ted with relatively small increases in inositol 1,4,5-trisphosphate (I P3). Blockade of Ca2+ influx resulted in rapid decline in [Ca2+]i to b asal levels; resumption of influx caused a substantial ''spike'' in [C a2+]i before [Ca2+]i reequilibrated at the same former steady-state le vels but without perturbing levels of IP3. Thus, the refilling and dis charge of Ca2+ from IP3-sensitive stores might occur synchronously on resumption of influx, or asynchronously during sustained influx and el evation of [Ca2+]i. Together, the results suggested that influx of Ca2 + in response to stimulation via Fcepsilon 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 pathwa y.