Cyclic AMP accelerates calcium waves in pancreatic acinar cells

Cytosolic Ca2+ (Ca-i(2+)) flux within the pancreatic acinar cell is important both physiologically and pathologically. We examined the role of cAMP in shaping the apical-to-basal Ca2+ wave generated by the Ca2+-activating agonist carbachol. We hypothesized that cAMP modulates intra-acinar Ca2+ channel opening by affecting either cAMP-dependent protein kinase (PKA) or exchange protein directly activated by cAMP (Epac). Isolated pancreatic acinar cells from rats were stimulated with carbachol (1 mu M) with or without vasoactive intestinal polypeptide (VIP) or 8-bromo-cAMP (8-Br-cAMP), and then Ca-i(2+) was monitored by confocal laser-scanning microscopy. The apical-to-basal carbachol (1 mu M)-stimulated Ca2+ wave was 8.63 +/- 0.68 mu m/s; it increased to 19.66 +/- 2.22 mu m/s (*P < 0.0005) with VIP (100 nM), and similar increases were observed with 8-Br-cAMP (100 mu M). The Ca2+ rise time after carbachol stimulation was reduced in both regions but to a greater degree in the basal. Lag time and maximal Ca2+ elevation were not significantly affected by cAMP. The effect of cAMP on Ca2+ waves also did not appear to depend on extracellular Ca2+. However, the ryanodine receptor (RyR) inhibitor dantrolene (100 mu M) reduced the cAMP-enhancement of wave speed. It was also reduced by the PKA inhibitor PKI (1 mu M). 8-(4-chlorophenylthio)2'-O-Me-cAMP, a specific agonist of Epac, caused a similar increase as 8-Br-cAMP or VIP. These data suggest that cAMP accelerates the speed of the Ca2+ wave in pancreatic acinar cells. A likely target of this modulation is the RyR, and these effects are mediated independently by PKA and Epac pathways.