AJP - Cell Physiology, Vol 258, Issue 6 C1077-C1085, Copyright © 1990 by American Physiological Society
Regulation of inositol 1,4,5-trisphosphate-induced Ca2+ release. I. Effect of Mg2+
P. Volpe, B. H. Alderson-Lang and G. A. Nickols
Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77550.
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.
