Inositol 1,4,5-trisphosphate-induced Ca2+ release is inhibited by mitochondrial depolarization

Abstract

We investigated the consequences of depolarizing the mitochondrial membrane potential (Δψ(mit)) on Ca2+ signals arising via inositol 1,4,5-trisphosphate receptors (InsP3R) in hormone-stimulated HeLa cells. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) or a mixture of antimycin A + oligomycin were found to rapidly depolarize Δψ(mit). Mitochondrial depolarization enhanced the number of cells responding to a brief application of a Ca2+-mobilizing hormone and prolonged the recovery of cytosolic Ca2+ after washout of the hormone; effects consistent with the removal of a passive Ca2+ buffer. However, with repeated application of the same hormone concentration both the number of responsive cells and peak Ca2+ changes were observed to progressively decline. The inhibition of Ca2+ signalling was observed using different Ca2+-mobilizing hormones and also with a membrane-permeant Ins(1,4,5)P3 ester. Upon washout of FCCP, the Ca2+ signals recovered with a time course similar to the re-establishment of Δψ(mit). Global measurements indicated that none of the obvious factors such as changes in pH, ATP concentration, cellular redox state, permeability transition pore activation or reduction in Ca2+-store loading appeared to underlie the inhibition of Ca2+ signalling. We therefore suggest that local changes in one or more of these factors, as a consequence of depolarizing Δψ(mit), prevents InsP3R activation.