Calcium entry into the inositol 1,4,5-trisphosphate-releasable calcium pool is mediated by a GTP-regulatory mechanism.
Intracellular Ca2+ release activated by inositol 1,4,5-trisphosphate (InsP3) plays a pivotal role in Ca2+ signaling in cells. A controlling mechanism for InsP3-induced Ca2+ movements is suggested by results showing that the InsP3-releasable Ca2+ pool is directly modified by a specific and sensitive GTP-regulated Ca2+-translocating process. By using saponin-permeabilized N1E-115 neuroblastoma cells or DDT1MF-2 smooth muscle-derived cells, InsP3 releases 30-50% of Ca2+ accumulated through intracellular high-affinity ATP-dependent Ca2+-pumping activity. Oxalate-promoted Ca2+ uptake is reversed by InsP3, indicating oxalate permeability of the InsP3-releasable pool, which is consistent with this compartment being the endoplasmic reticulum. GTP (10 microM) activates release of 50-70% of accumulated Ca2+ from cells. In the presence of 5-10 mM oxalate, GTP induces a biphasic Ca2+ flux response; initially (1-2 min) GTP induces rapid Ca2+ release followed thereafter by a profound increase in Ca2+ uptake. Thus, GTP-activated Ca2+ influx and efflux compete for Ca2+ access to the oxalate-permeable Ca2+ pool. The nonadditive effects of InsP3 and GTP suggest that InsP3 releases Ca2+ from a subcompartment of the GTP-releasable pool. Most significantly, InsP3 is observed to block the GTP-activated uptake phase in the presence of oxalate, indicating that GTP induces Ca2+ entry into the pool from which InsP3 activates release. Hence, the results provide direct evidence that loading of Ca2+ into the InsP3-sensitive Ca2+ pool is controlled by a GTP-regulated Ca2+-translocating mechanism. Such a process could be significant in regulating the extent and duration of the InsP3-induced Ca2+ signal, a crucial step in the inositol phospholipid signaling pathway.
Mullaney
J M
JM
Department of Biological Chemistry, University of Maryland School of Medicine, Baltimore 21201.
Yu
M
M
Ghosh
T K
TK
Gill
D L
DL
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D002462:Cell Membrane / Q000502:physiology; D006160:Guanosine Triphosphate / Q000502:physiology*; D007295:Inositol Phosphates / Q000378:metabolism*; D007425:Intracellular Membranes / Q000502:physiology; D010070:Oxalates / Q000494:pharmacology; D013403:Sugar Phosphates / Q000378:metabolism*
D007295:Inositol Phosphates; D010070:Oxalates; D013403:Sugar Phosphates; D006160:Guanosine Triphosphate; D002118:Calcium