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

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.