IP3 and cyclic ADP-ribose induced Ca2+ release from intracellular stores of pancreatic acinar cells from rat in primary culture

We have measured Ca2+ concentration changes in intracellular Ca2+ stores ([Ca2+](store)) of rat pancreatic acinar cells in primary culture in response to the Ca2+ mobilizing substances inositol-1,4,5-trisphosphate (IP3) and cyclic ADP-ribose (cADPr) using the Ca2+-sensitive dye mag Fura-2. We found that in this cell model IP3 releases Ca2+ in a quantal manner. Higher Ca2+ concentration in the stores allowed a response to lower IP3 concentrations ([IP3]) indicating that the sensitivity of IP3 receptors to IP3 is regulated by the Ca2+ concentration in the stores. Cyclic ADPr, that modifies 'Ca2+-induced-Ca2+-release' (CICR), was also able to release Ca2+ from intracellular stores of pancreatic acinar cells in primary culture. In comparison to the Ca2+ ionophore ionomycin, which induced a maximal decrease (100%) in [Ca2+](store), a hypermaximal [IP3] (10 muM) dropped [Ca2+](store) by 87% and cADPr had no further effect. Cyclic ADPr reduced [Ca2+](store) by only 56% and subsequent IP3 addition caused further maximal decrease in [Ca2+](store). Furthermore, a maximal [IP3] caused the same decrease in [Ca2+](store) in all regions of the cell, whereas cADPr dropped the [Ca2+](store) between 20 and 80% in different cell regions. From these data we conclude that in primary cultured rat pancreatic acinar cells at least three types of Ca2+ stores exist. One type possessing both cADPr receptors and IP3 receptors, a second type possessing only IP3 receptors, and a third type whose Ca2+ can be released by ionomycin but neither by IP3 nor by cADPr. (C) 2001 Harcourt Publishers Ltd.