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 45Ca2+ 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 microM 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.