Luminal Ca2+ increases the sensitivity of Ca2+ stores to inositol 1,4,5-trisphosphate.
Ca2+ within intracellular stores has been proposed to act with cytosolic inositol 1,4,5-trisphosphate (InsP3) to cause opening of the integral Ca2+ channel of the InsP3 receptor, leading to mobilization of intracellular Ca2+ stores [FEBS Lett. 263:5-9 (1990)]. We have tested that suggestion in saponin-permeabilized rat hepatocytes by manipulating the Ca2+ content of the stores and then determining their sensitivity to InsP3, while keeping the cytosolic Ca2+ concentration constant. Stores depleted of Ca2+ by incubation with ionomycin were significantly less sensitive to InsP3, an effect thought likely to result from the decrease in luminal free Ca2+ concentration rather than from direct effects of ionomycin on InsP3 binding or Ca2+ permeability. The luminal free Ca2+ concentration of stores loaded in the presence of pyrophosphate appeared to be substantially reduced, and again there was a significant inverse correlation between the estimated free Ca2+ concentration of the stores and their sensitivity to InsP3. By following the kinetics of 45Ca2+ uptake into empty stores in the presence of inositol trisphosphorothioate, a stable InsP3 analogue, we demonstrated that stores respond to inositol trisphosphorothioate only after their luminal free Ca2+ concentration exceeds a critical level. We conclude that InsP3 and luminal Ca2+ together regulate Ca2+ mobilization from intracellular stores, and we discuss some of the implications of this interaction for the complex Ca2+ signals evoked by extracellular stimuli.
Nunn
D L
DL
Department of Pharmacology, University of Cambridge, UK.
Taylor
C W
CW
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics / Q000494:pharmacology; D002132:Calcium Radioisotopes; D000252:Calcium-Transporting ATPases / Q000502:physiology; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D011756:Diphosphates / Q000494:pharmacology; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D007424:Intracellular Fluid / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000166:cytology / Q000378:metabolism; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains
D002132:Calcium Radioisotopes; D011756:Diphosphates; D015544:Inositol 1,4,5-Trisphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium