Cyclic ADP‐ribose and inositol 1,4,5‐trisphosphate mobilizes Ca2+ from distinct intracellular pools in permeabilized lacrimal acinar cells

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Abstract
In permeabilized lacrimal acinar cells, cyclic ADP‐ribose (cADP‐ribose) and inositol 1,4,5‐trisphosphate (Ins(1,4,5)P3) release Ca2+ in a dose dependent manner from distinct thapsigargin‐sensitive Ca2+ pools. Ryanodine specifically blocks the Ca2+ response to cADP‐ribose, whereas heparin strongly reduces the response to Ins(1,4,5)P3 application. GTP causes a rapid Ca2+ release by a ryanodine‐ and heparin‐insensitive mechanism and potentiates Ins(1,4,5)P3 but not cADP‐ribose evoked Ca2+ release. It is estimated that cADP‐ribose can release 16 μmol Ca2+/I cells, whereas Ins(1,4,5)P3 can mobilize 55 μmol Ca2+/I cells. The results suggest that cADP‐ribose and Ins(1,4,5)P3 release Ca2+ from distinct internal stores and that a third Ca2+ pool exists which can selectively interact with the Ins(1,4,5)P3‐sensitive Ca2+ store by a GTP‐mediated process.