Inhibition of inositol trisphosphate-induced calcium release by cyclicADP-ribose in A7r5 smooth-muscle cells and in 16HBE14o- bronchial mucosal cells

Ca²⁺ release from intracellular stores occurs via two families of intracellular channels, each with their own specific agonist: Ins(1,4,5)P₃ for the Ins(1,4,5)P₃ receptor and cyclic ADP-ribose (cADPR) for the ryanodine receptor. We now report that cADPR inhibited Ins(1,4,5)P₃-induced Ca²⁺ release in permeabilized A7r5 cells with an IC₅₀ of 20 μM, and in permeabilized 16HBE14o- bronchial mucosal cells with an IC₅₀ of 35 μM. This inhibition was accompanied by an increase in specific [³H]Ins(1,4,5)P₃ binding. 8-Amino-cADPR, but not 8-bromo-cADPR, antagonized this effect of cADPR. The inhibition was prevented by a whole series of inositol phosphates (10 μM) that did not affect Ins(1,4,5)P₃-induced Ca²⁺ release, and by micromolar concentrations of PP_i and various nucleotide di- or triphosphates. We propose that cADPR must interact with a novel regulatory site on the Ins(1,4,5)P₃ receptor or on an associated protein. This site is neither the Ins(1,4,5)P₃-binding domain, which prefers Ins(1,4,5)P₃ and only binds nucleotides and PP_i in the millimolar range, nor the stimulatory adenine nucleotide binding site.