LUMINAL CALCIUM REGULATES THE INOSITOL TRISPHOSPHATE RECEPTOR OF RAT BASOPHILIC LEUKEMIA-CELLS AT A CYTOSOLIC SITE

Hormones, growth factors, and other stimuli can generate Ca2+ spikes and waves by activation of the phosphoinositide (PI) pathway. The sources of these Ca2+ signals are inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ stores. Here we use a rapid perfusion apparatus to measure the release of Ca-45(2+) from permeabilized rat basophilic leukemia (RBL) cells to investigate the regulation of IP3-mediated Ca2+ release by cytosolic and luminal Ca2+. At 200 nM IP3, Ca2+ release was potentiated by an increase in the cytosolic Ca2+ concentration. This potentiation by Ca2+ was nearly absent at 500 nM IP3. Previous studies in smooth muscle cells and neurons showed an inhibition of Ca2+ release above 300 nM Ca2+. In contrast, no such inhibition was observed in RBL cells. When assayed at low cytosolic Ca2+ concentrations, IP3-mediated release was steeply dependent upon luminal Ca2+ concentration. At high luminal Ca2+ concentration, 1 mu M IP3 released most of the stored Ca2+ even in the complete absence of cytosolic Ca2+ However, at high cytosolic Ca2+ concentrations (890 nM), IP3-mediated release was no longer steeply dependent upon the luminal Ca2+ concentration. Furthermore, high concentrations of BAPTA inhibited IP3-mediated release in the absence of cytosolic Ca2+ This suggests that a rapid and local luminal Ca2+ feedback is generated by luminal Ca2+ ions binding to cytosolic sites of the same channel or closely associated channels. This ''luminal Ca2+ feedback'' can be initiated by an increase in the concentration either of IP3, of cytosolic Ca2+, or of luminal Ca2+. It is likely that ''luminal Ca2+ feedback'' is exploited by cells in both the initiation and termination of Ca2+ spikes.