Inositol tetrakisphosphate mobilizes calcium from cerebellum microsomes.
Ca2+ accumulated by rat cerebellum microsomes in the presence of MgATP was released by added inositol tetrakisphosphate [Ins(1,3,4,5)P4]. The concentrations of D-myo-inositol-1,4,5-trisphosphate [D-Ins(1,4,5)P3], D-Ins(1,3,4,5)P4, and DL-Ins(1,3,4,5)P4 required for half-maximal release were 0.15, 4.6, and 7.5 microM, respectively. Maximal concentrations of InsP4 released only 70% of the Ca2+ released by maximal concentrations of Ins(1,4,5)P3. Inositol pentakisphosphate and D-myo-inositol-1,3,4-trisphosphate were relatively inactive. Additional Ca2+ was released when Ins(1,4,5)P3 (or a nonhydrolyzable analog) was added after completion of InsP4-mediated Ca2+ release but not when this sequence of additions was reversed. This indicates that InsP4 releases Ca2+ from part of the InsP3-releasable compartment. No evidence for synergism between InsP4 and InsP3 was obtained and responses to suboptimal concentrations of both inositol phosphates were approximately additive. Heparin was a potent inhibitor of InsP4-mediated Ca2+ release. Inhibition by heparin was competitive with respect to InsP4 concentration and the Ki for heparin was 0.6 microgram/ml (approximately 100 nM), irrespective of whether InsP4 or InsP3 was used as an agonist. A 3-phosphatase capable of converting [3H]Ins(1,3,4,5)P3 to [3H]Ins(1,4,5)P3 could not be detected in cerebellum microsomes. Hence, we conclude that, in vitro, receptors capable of recognizing Ins(1,3,4,5)P4 can also be coupled directly to a Ca2+ release system.
Joseph
S K
SK
Department of Biochemistry and Biophysics, University of Pennsylvania School of Medicine, Philadelphia 19104.
Hansen
C A
CA
Williamson
J R
JR
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002451:Cell Compartmentation / Q000187:drug effects; D002531:Cerebellum / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D006493:Heparin / Q000494:pharmacology; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000378:metabolism / Q000494:pharmacology*; D008861:Microsomes / Q000378:metabolism*; D010716:Phosphatidylinositols / Q000494:pharmacology; D051381:Rats; D011956:Receptors, Cell Surface / Q000378:metabolism; D013329:Structure-Activity Relationship; D013403:Sugar Phosphates / Q000494:pharmacology*
D007295:Inositol Phosphates; D010716:Phosphatidylinositols; D011956:Receptors, Cell Surface; D013403:Sugar Phosphates; D006493:Heparin; D002118:Calcium