Calcium wave propagation in pancreatic acinar cells -: Functional interaction of inositol 1,4,5-trisphosphate receptors, ryanodine receptors, and mitochondria

In pancreatic acinar cells, inositol 1,4,5-trisphosphate (InsP(3))-dependent cytosolic calcium ([Ca2+](i)) increases resulting from agonist stimulation are initiated in an apical "trigger zone," where the vast majority of InsP(3) receptors (InsP(3)R) are localized. At threshold stimulation, [Ca2+](i) signals are confined to this region, whereas at concentrations of agonists that optimally evoke secretion, a global Ca2+ wave results. Simple diffusion of Ca2+ from the trigger zone is unlikely to account for a global [Ca2+](i) elevation. Furthermore, mitochondrial import has been reported to limit Ca2+ diffusion from the trigger zone. As such, there is no consensus as to how local [Ca2+](i) signals become global responses. This study therefore investigated the mechanism responsible for these events. Agonist-evoked [Ca2+](i) oscillations were converted to sustained [Ca2+](i) increases after inhibition of mitochondrial Ca2+ import. These [Ca2+], increases were dependent on Ca2+ release from the endoplasmic reticulum and were blocked by 100 mu M ryanodine. Similarly, "uncaging" of physiological [Ca2+](i) levels in whole-cell patch-clamped cells resulted in rapid activation of a Ca2+-activated current, the recovery of which was prolonged by inhibition of mitochondrial import. This effect was also abolished by ryanodine receptor (RyR) blockade. Photolysis of D-myo InsP(3) P-4(5)-1-(2-nitrophenyl)-ethyl ester (caged InsP(3)) produced either apically localized or global [Ca2+](i) increases in a dose-dependent manner, as visualized by digital imaging. Mitochondrial inhibition permitted apically localized increases to propagate throughout the cell as a c\iave, but this propagation was inhibited by ryanodine and was not seen for minimal control responses resembling [Ca2+](i) puffs. Global [Ca2+](i) rises initiated by InsP(3) were also reduced by ryanodine, limiting the increase to a region slightly larger than the trigger zone. These data suggest that, while Ca2+ release is initially triggered through InP3R release by RyRs is the dominant mechanism for propagating global waves. In addition, mitochondrial Ca2+ import controls the spread of Ca2+ throughout acinar cells by modulating RyR activation.