We have previously demonstrated that activation of M3 muscarinic receptors increases inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4) accumulation in colonic smooth muscle. 2. In the present study, we demonstrate the existence of InsP3 and InsP4 binding sites in colonic circular smooth muscle by use of radioligand binding methods. Both [3H]-InsP3 and [3H]-InsP4 bound rapidly and reversibly to a single class of saturable sites in detergent-solubilized colonic membranes with affinities of 5.04 +/- 1.03 nM and 3.41 +/- 0.78 nM, respectively. The density of [3H]-InsP3 binding sites was 335.3 +/- 19.3 fmol mg-1 protein which was approximately 2.5 fold greater than that of [3H]-InsP4 sites (127.3 +/- 9.1 fmol mg-1 protein). 3. The two high affinity inositol phosphate binding sites exhibited markedly different pH optima for binding of each radioligand. At pH 9.0, specific [3H]-InsP3 binding was maximal, whereas [3H]-InsP4 binding was only 10% that of [3H]-InsP3. Conversely, at pH 5.0, [3H]-InsP4 binding was maximal, while [3H]-InsP3 binding was reduced to 15% of its binding at pH 9.0. 4. InsP3 was about 20 fold less potent (KI = 50.7 +/- 8.3 nM) than InsP4 in competing for [3H]-InsP4 binding sites and could compete for only 60% of [3H]-InsP4 specific binding. InsP4 was also capable of high affinity competition with [3H]-InsP3 binding (KI = 103.5 +/- 1.5 nM), and could compete for 100% of [3H]-InsP3 specific binding. 5. [3H]-InsP3 binding in subcellular fractions separated by discontinuous sucrose density gradients followed NADPH-cytochrome c reductase activity, suggesting an intracellular localization for the majority of InsP3 receptors in this tissue, whereas [3H]-InsP4 binding appeared to be equally distributed between plasma membrane and intracellular membrane populations.6. These results suggest the existence of distinct and specific InsP3 and InsP4 binding sites which may represent the physiological receptors for these second messengers; differences in the subcellular distribution of these receptors may contribute to differences in their putative physiological roles.
