Liver inositol, 1,4,5-trisphosphate-binding sites are the Ca2(+)-mobilizing receptors.
Ins(1,4,5)P3 is the intracellular messenger that in many cells mediates the effects of Ca2(+)-mobilizing receptors on intracellular Ca2+ stores. An Ins(1,4,5)P3 receptor from cerebellum has been purified and functionally reconstituted, but the relationship between this protein and the high-affinity Ins(1,4,5)P3-binding sites of peripheral tissues is unclear. We compared the Ins(1,4,5)P3-binding sites of liver and cerebellum by measuring inhibition of specific Ins(1,4,[32P]5)P3 binding by various ligands under equilibrium conditions, and find that each ligand binds with similar affinity in the two tissues. Earlier studies in which Ins(1,4,5)P3 binding and Ca2+ mobilization were measured under different conditions demonstrated large differences between KD values for binding and EC50 values (concn. giving half-maximal effect) for Ca2+ release. We show here that, when measured under identical conditions, KD and EC50 values for four agonists are similar. Schild analysis of inhibition of Ins(1,4,5)P3 binding by ATP demonstrates a competitive interaction between the two at the liver Ins(1,4,5)P3-binding site, and this partly accounts for earlier discrepancies in binding and Ca2(+)-release data. We conclude that the high-affinity Ins(1,4,5)P3-binding site of hepatocytes is likely to be the receptor that mediates Ca2+ mobilization, and that this receptor is at present indistinguishable from that in cerebellum.
Nunn
D L
DL
Department of Pharmacology, University of Cambridge, U.K.
Taylor
C W
CW
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002531:Cerebellum / Q000378:metabolism*; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D007295:Inositol Phosphates / Q000378:metabolism; D007700:Kinetics; D008024:Ligands; D008099:Liver / Q000378:metabolism*; D008970:Molecular Weight; D051381:Rats; D011956:Receptors, Cell Surface / Q000378:metabolism*; D015398:Signal Transduction
D007295:Inositol Phosphates; D008024:Ligands; D011956:Receptors, Cell Surface; D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate; D002118:Calcium