The rise in concentration of free Ca2+ and of pH provides sequential, synergistic signals for secretion in antigen-stimulated rat basophilic leukemia (RBL-2H3) cells.

Ag stimulation of rat basophilic leukemia (RBL-2H3) cells results in hydrolysis of inositol phospholipids, a transient increase in concentration of cytosol Ca2+ [( Ca2+]i), a gradual increase in cytosolic pH (pHi) and the activation of protein kinase C. To determine whether all these changes serve as signals for secretion, studies were conducted with cells permeabilized with streptolysin O in which pHi and [Ca2+]i could be varied independently of each other and enzyme activities could be manipulated. At resting pHi (approximately 7.0) and [Ca2+]i (0.1 microM), the permeabilized cells showed little secretory response to Ag. At resting pHi, elevated levels of Ca2+ (0.33 microM) were required for maximal secretory response to Ag. At a pHi of 7.4, however, 0.1 microM [Ca2+]i was sufficient to sustain near maximal responses to Ag. Therefore, a small increase of [Ca2+]i to 0.33 microM was required to initiate secretion, but once the pHi was elevated secretion could be sustained at near basal levels of [Ca2+]i. Since elevating the [Ca2+]i and pHi, by themselves promoted little secretion, another potentiating signal must have been generated by antigen stimulation. This signal was possibly transduced via hydrolysis of inositol phospholipids and protein kinase C. Even with an elevated [Ca2+]i (0.33 microM) the hydrolysis of the phospholipids and secretion stimulated by Ag were inhibited by guanosine 5'(2-O-thio)diphosphate and neomycin. Furthermore, both protein-kinase C and the secretory response to Ag were lost after permeabilized cells were washed but both were retained if cells were exposed to PMA before permeabilization.