Regulation of inositol 1,4,5-trisphosphate-induced Ca2+ release. I. Effect of Mg2+.
Canine cerebellar membranes were fractionated by differential centrifugation into a crude mitochondrial pellet (P2) and a crude microsomal pellet (P3). The effect of Mg2+ on inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release and [3H]IP3 binding was assessed. Mg2+ inhibited IP3-induced Ca2+ release in a concentration-dependent manner. Mg2+ influenced both the extent of IP3-induced Ca2+ release and the apparent affinity for IP3. A 10-fold change of free Mg2+ (from approximately 30 to approximately 300 microM) reduced the extent of Ca2+ release by two- to threefold and shifted the apparent Michaelis constant from approximately 0.5 to approximately 0.9 microM IP3. Thus Mg2+ seemed to be noncompetitive inhibitor of IP3-induced Ca2+ release. Mg2+ also inhibited Ca2+ release elicited by glycerophosphoinositol 4,5-bisphosphate, a poorly metabolized analogue of IP3. Mg2+ and heparin sodium were shown to be additive inhibitors of IP3-induced Ca2+ release. Mg2+ inhibited [3H]IP3 binding under experimental conditions designed to minimize IP3 hydrolysis. Scatchard plots indicated that 0.5 mM free Mg2+ reduced maximum binding from 10.9 to 3.5 pmol IP3 bound/mg protein and increased the dissociation constant from 136 to 227 nM. The modulation of [3H]IP3 binding and IP3-induced Ca2+ release by Mg2+ could be physiologically relevant.
Volpe
P
P
Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77550.
Alderson-Lang
B H
BH
Nickols
G A
GA
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002531:Cerebellum / Q000187:drug effects / Q000378:metabolism*; D004285:Dogs; D006493:Heparin / Q000494:pharmacology; D006868:Hydrolysis; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism / Q000494:pharmacology*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism*; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism*
D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate; D006493:Heparin; D008274:Magnesium; D002118:Calcium