The type 2 ryanodine receptor of neurosecretory PC12 cells is activated by cyclic ADP-ribose. Role of the nitric oxide/cGMP pathway

Of two neurosecretory PC12 cell clones that respond to NO donors and 8- bromo-cGMP with similar increases in cADP-ribose and that possess molecularly similar Ca²⁺ stores, only one (clone 16A) expresses the type 2 ryanodine receptor, whereas the other (clone 27) is devoid of ryanodine receptors. In PC12-16A cells, activation of the NO/cGMP pathway induced slow [Ca²⁺](i) responses, sustained by release from Ca²⁺ stores. In contrast, PC12-27 cells were insensitive to NO donors. Likewise, in PC12-16A cells preincubated with NO donors, Ca²⁺ stores were partially depleted, as revealed by a test with thapsigargin, whereas those in clone 27 were unchanged. The NO-induced Ca²⁺ release was increased synergistically by caffeine, and the corresponding store depletion was magnified by ryanodine. The specificity for the NO/cGMP pathway was confirmed by the effects of two blockers of cGMP- dependent protein kinase I, while the role of cADP-ribose was demonstrated by the effects of its antagonist, 8-amino-cADP-ribose, administered to permeabilized cells. These results demonstrate in neurosecretory cells a ryanodine receptor activation pathway similar to that known in sea urchin oocytes. The signaling events described here could be of great physiological importance, especially in the nervous system.