Inositol 1:2-cyclic,4,5-trisphosphate is only a weak agonist at inositol 1,4,5-trisphosphate receptors

The activity of inositol 2,4,5-trisphosphate and inositol 1:2-cyclic,4,5-trisphosphate relative to inositol 1,4,5-trisphosphate was examined by two assays; firstly, in a binding assay using rat cerebellar membranes, and secondly, in a Ca2+-mobilization assay using permeabilized Swiss 3T3 cells. In both assays the first two phosphates have a potency at least an order of magnitude less than inositol 1,4,5-trisphosphate. The possible reasons for differences between these results and previous data are discussed, as are the implications for any putative physiological role for the cyclic trisphosphate.