Heterotrimeric G-protein G(q/11) localized on pancreatic zymogen granules is involved in calcium-regulated amylase secretion

The heterotrimeric G-protein G(q/11) was identified on pancreatic acinar zymogen granules and its function ill calcium-regulated exocytosis was examined, Western blotting showed alpha(q/11), but not alpha(s) or alpha(o), to be localized to the zymogen granule membrane along with G-protein beta-subunit; all three alpha subunits were present in a plasma membrane fraction and the alpha(q/11) signal was 30-fold more enriched in the plasma membrane as compared with granule membrane. Neither CCK receptors nor ru subunits of the sodium pump, both plasma membrane markers were present on granule membranes, Immunohistochemistry of pancreatic lobules showed that alpha(q/11) localized to the zymogen granule-rich apical region of acinar cells together with a mush stronger signal at the basolateral plasma membrane. When the substance-P-related peptide GPAnt-2a, an antagonist of G(q/11), was introduced into streptolysin-O permeabilized acini to bypass the plasma membrane, the amylase release induced by 10 mu M free calcium was potentiated in a concentration-dependent manner. By contrast, another substance-P-related peptide, GPAnt-1, an antagonist of G(o) and G(i), showed no effect on calcium-induced amylase release from permeabilized acini, GPAnt-2a peptide also exerted an inhibitory effect on the total. GTPase activity of the purified zymogen granules and a larger inhibitory effect on the GTPase activity of the G(q/11) protein immunopurified from zymogen granules, GPAnt-1, however, did not inhibit GTPase activity of either zymogen granules or immunopurified G(q)/(11). These results suggest that GPAnt-2a peptide augmented calcium-induced amylase release from permeabilized acini by inhibiting GTPase activity of the G(q/11) protein on zymogen granules. We conclude that G(q/11) protein on zymogen granules plays a tonic inhibitory role in calcium-regulated amylase secretion from pancreatic acini.