MULTIPLE MECHANISMS OF REGULATION OF THE INOSITOL 1,4,5-TRISPHOSPHATERECEPTOR BY CALCIUM

Ca2+ mobilisation by inositol 1,4,5-trisphosphate (InsP(3)) is a compl ex phenomenon which involves positive and negative feedback regulation by cytosolic Ca2+. It has been shown that Ca2+ increased the affinity of [H-3]-lnsP(3) binding to liver membranes and inhibited [H-3]-InsP( 3) binding to cerebellar membranes. We investigated the effects of Ca2 + on the [H-3]-InsP(3) binding to receptor solubilised and rapidly pur ified by immunoprecipitation. The InsP(3) binding to the purified live r receptor was insensitive to the addition of Ca2+, indicating that Ca 2+ did not interact directly with the receptor. The loss of the Ca2+ e ffect on liver receptor affinity was reproduced by alkaline treatment of liver membranes, which is known to extract the peripheral membrane proteins. This suggests that Ca2+ regulates the liver InsP(3) receptor by interacting with a membrane-associated protein. Ca2+ inhibited the binding of [H-3]-lnsP(3) to purified cerebellar receptors as was foun d with the membrane fraction. The treatment of the purified cerebellar receptor with media of high ionic strength or at alkaline pH did not abolish the effect of Ca2+ on the receptor. This indicates that the in hibitory effect of Ca2+ on [H-3]-InsP(3) binding to cerebellar membran es occurs either via direct interaction with the receptor or via an in tegral protein strongly associated with the receptor. In conclusion, t he mechanisms of regulation of InsP(3)-induced Ca2+ release by Ca2+ in volve different molecular support in cerebellum and in liver. This may reflect different regulation dependent on the receptor type.