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 a nd waves by activation of the phosphoinositide (PI) pathway. The sourc es of these Ca2+ signals are inositol 1,4,5-trisphosphate (IP3)-depend ent Ca2+ stores. Here we use a rapid perfusion apparatus to measure th e 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 potentiate d by an increase in the cytosolic Ca2+ concentration. This potentiatio n by Ca2+ was nearly absent at 500 nM IP3. Previous studies in smooth muscle cells and neurons showed an inhibition of Ca2+ release above 30 0 nM Ca2+. In contrast, no such inhibition was observed in RBL cells. When assayed at low cytosolic Ca2+ concentrations, IP3-mediated releas e was steeply dependent upon luminal Ca2+ concentration. At high lumin al Ca2+ concentration, 1 mu M IP3 released most of the stored Ca2+ eve n in the complete absence of cytosolic Ca2+ However, at high cytosolic Ca2+ concentrations (890 nM), IP3-mediated release was no longer stee ply dependent upon the luminal Ca2+ concentration. Furthermore, high c oncentrations of BAPTA inhibited IP3-mediated release in the absence o f cytosolic Ca2+ This suggests that a rapid and local luminal Ca2+ fee dback is generated by luminal Ca2+ ions binding to cytosolic sites of the same channel or closely associated channels. This ''luminal Ca2+ f eedback'' can be initiated by an increase in the concentration either of IP3, of cytosolic Ca2+, or of luminal Ca2+. It is likely that ''lum inal Ca2+ feedback'' is exploited by cells in both the initiation and termination of Ca2+ spikes.