The effect of external calcium and pH on inositol trisphosphate-mediated calcium release from cerebellum microsomal fractions.
Binding of D-myo-inositol 1,4,5-trisphosphate (InsP3) to rat cerebellum membranes has previously been shown to be stimulated by alkaline pH and inhibited by low concentrations of Ca2+ [Worley, Baraban, Suppatopone, Wilson & Snyder (1987) J. Biol. Chem. 262, 12132-12136]. In the present study, Scatchard analysis of InsP3 binding to cerebellum microsomes indicates that the effects of Ca2+ and pH are exerted through changes in the apparent affinity of the receptor without effects on maximal binding. The influence of extravesicular Ca2+ and pH on InsP3-mediated 45Ca2+ release was investigated. Extravesicular Ca2+ inhibited InsP3-mediated Ca2+ release. The inhibitory effect of Ca2+ was most marked when a sub-optimal concentration of InsP3 was used. An increase in extravesicular pH produced a decrease in the concentration of InsP3 that yielded half-maximal Ca2+ release. Regulation of the affinity of the InsP3 receptor by Ca2+ and pH can qualitatively account for the observed effects of these factors on InsP3-mediated Ca2+ release. Feedback inhibition of InsP3 binding by Ca2+ could provide a mechanism to generate Ca2+ oscillations, particularly under hormonal conditions that produce sub-optimal elevations of InsP3 concentration.
Joseph
S K
SK
Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia 19104.
Rice
H L
HL
Williamson
J R
JR
D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002531:Cerebellum / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000378:metabolism*; D007700:Kinetics; D008861:Microsomes / Q000378:metabolism*; D051381:Rats; D013403:Sugar Phosphates / Q000378:metabolism*
D007295:Inositol Phosphates; D013403:Sugar Phosphates; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium