Characterisation of inositol 1,4,5-trisphosphate binding sites in rabbit aortic smooth muscle

The present study investigated the characteristics of D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) binding sites in crude membrane preparations of rabbit aortic smooth muscle. A particular aim was to demonstrate if increases in cytoplasmic cyclic guanosine 3':5' monophosphate (cGMP), which mediates the effect of nitrovasodilators, may cause smooth muscle relaxation in part by the displacement of Ins(1,4,5)P3 binding. Negligible Ins(1,4,5)P3 binding was observed at pH < 7, while maximum binding occurred over the pH range 8-9. Saturation analysis of isotopic dilution binding data revealed an apparently homogenous population of Ins(1,4,5)P3 binding sites with a KD of 4.02 +/- 0.53 nM and a Bmax of 27.7 +/- 4.6 fmol/mg protein. Heparin, an Ins(1,4,5)P3 receptor antagonist, inhibited binding with an IC50 of 11.43 +/- 2.81 micrograms/ml. The ability of other polyphosphate compounds to inhibit Ins(1,4,5)P3 binding in this preparation was also examined. D-myo-Inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), adenosine 5'-triphosphate (ATP) and guanosine 5'-triphosphate (GTP) inhibited Ins(1,4,5)P3 binding, although each was significantly less potent that Ins(1,4,5)P3. In contrast, cyclic guanosine 3':5' monophosphate (cGMP) did not significantly alter Ins(1,4,5)P3 binding in rabbit aortic smooth muscle. This observation suggests that competitive inhibition of Ins(1,4,5)P3 receptor binding is not an important consideration in cGMP-mediated vascular smooth muscle cell relaxation.