CYCLIC ADP-RIBOSE INDUCES CA2-INSENSITIVE CA2+ POOLS IN CANINE SALIVARY-GLAND CELLS( RELEASE FROM CAFFEINE)

Cyclic ADP-ribose (cADPR), a novel putative messenger of the ryanodine receptor, was examined regarding its ability to mobilize Ca2+ from in tracellular Ca2+ stores in isolated cells of parotid and submandibular glands of the dog. cADPR induced a rapid and transient Ca2+ release i n the digitonin-permeabilized cells of salivary glands. cADPR-induced Ca2+ release was inhibited by ryanodine receptor antagonists ruthenium red, ryanodine, benzocaine, and imperatoxin inhibitor but not by the inositol 1,4,5-trisphosphate (IP3)-receptor antagonist heparin. Thapsi gargin, at a concentration of 3 to 30 mu M, inhibited IP3-induced Ca2 release, while higher concentrations were required to inhibit cADPR-i nduced Ca2+ release. Cross-potentiation was observed between cADPR and ryanodine or SrCl2, suggesting that cADPR sensitizes the Ca2+-induced Ca2+ release mechanism. Cyclic AMP plays a stimulatory role on cADPR- and IP3-induced Ca2+ release in digitonin-permeabilized cells. Calmod ulin also potentiated cADPR-induced Ca2+ release, but inhibited IP3-in duced Ca2+ release. Acetylcholine and ryanodine caused the rise in int racellular free Ca2+ concentration ([Ca2+](i)) in intact submandibular and parotid cells. Caffeine did not produce any increase in Ca2+ rele ase or [Ca2+](i) rise in any preparation. ADP-ribosyl cyclase activity was found in the centrifuged particulate fractions of the salivary gl ands. These results suggest that cADPR serves as an endogenous modulat or of Ca2+ release from Ca2+ pools through a caffeine-insensitive ryan odine receptor channel, which are different from IP3-sensitive pools i n canine salivary gland cells. This system is positively regulated by cyclic AMP and calmodulin.