Control of inositol 1,4,5-trisphosphate-induced Ca2+ release by cytosolic Ca2+.
The synergistic action of cytosolic Ca2+ and inositol 1,4,5-trisphosphate (InsP3) in releasing intracellular Ca2+ stores has been suggested to be responsible for the complex intracellular Ca2 signals observed during hormonal stimulation of many cell types. However, the ability of cytosolic Ca2+ to potentiate Ca2+ release has recently been questioned because of the observed inhibitory effects of Ca2+ chelators used in previous studies. In the present study, EGTA and BAPTA [1,2-bis-(2-amino-phenoxy)ethane- NNN'N'-tetra-acetic acid] poorly inhibited InsP3-induced Ca2+ release from permeabilized A7r5 smooth-muscle cells. Additionally, stimulatory effects of cytosolic and luminal Ca2+ were observed either in the complete absence of Ca2+ chelator or at constant Ca(2+)-free chelator concentration. These data suggest that potentiation of InsP3-induced Ca2+ release by Ca2+ in A7r5 cells reflects an interaction between Ca2+ and InsP3 receptors, rather than a decrease in chelator-dependent inhibition. The EC50 for activation of InsP3-induced Ca2+ release by cytosolic Ca2+ was unaffected by ATP, or by changing InsP3 concentration, although InsP3-induced Ca2+ release became less sensitive to the inhibitory effects of cytosolic Ca2+ as the InsP3 concentration was elevated. Increasing H+ or Mg2+ concentration shifted the Ca(2+)-activation curve towards higher Ca2+ concentrations. These data suggest that, in addition to the InsP3-binding site, the affinity of the Ca(2+)-binding site(s) on InsP3 receptors can be modulated by intracellular cations.
Bootman
M D
MD
Babraham Institute Laboratory of Molecular Signalling, Department of Zoology, University of Cambridge, U.K..
Missiaen
L
L
Parys
J B
JB
De Smedt
H
H
Casteels
R
R
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001011:Aorta; D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism*; D004357:Drug Synergism; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004622:Embryo, Mammalian; D006863:Hydrogen-Ion Concentration; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D008274:Magnesium / Q000494:pharmacology; D009131:Muscle, Smooth, Vascular / Q000502:physiology / Q000648:ultrastructure; D051381:Rats
D004533:Egtazic Acid; D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate; D008274:Magnesium; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium