Metabolism of cyclic ADP-ribose in opossum kidney renal epithelial cells

We have previously shown that NAD⁺ inhibits renal Na⁺-P(i) symport; however, the biochemical mechanism of NAD⁺ in this action is not clarified. We now propose that NAD⁺ acts indirectly by first being converted to cyclic ADP-ribose (cADPR), a potent stimulator of intracellular Ca²⁺ mobilization. In permeabilized opossum kidney (OK) cells, a cell line often employed as a model for study of proximal tubular epithelial transport, cADPR is synthesized from β-NAD⁺ in a substrate concentration (0.01-1 mM) and time- dependent manner. That cADPR was generated from β-NAD⁺ by OK cells was verified by coelution with authentic cADPR on anion exchange high-performance liquid chromatography and by homologous desensitization of the Ca²⁺ release bioassay to authentic cADPR. cADPR synthesized by permeabilized OK cells was not influenced by the addition of parathyroid hormone. The OK cell also contains the enzyme activity necessary to catalyze catabolism of cADPR. Identification of these two key enzyme activities of cADPR metabolism in OK cells is consistent with a possible role of cADPR in regulation of the Na⁺- P(i) symporter by NAD⁺ in response to metabolic stimuli.