Guanine nucleotides activate multiple signaling pathways in permeabilized gastric chief cells. Evidence for GTP gamma S-induced calcium-independent pepsinogen secretion

Nonhydrolyzable guanine nucleotide analogues were used to evaluate the role of guanine nucleotide binding (G) proteins in regulating pepsinogen secretion from streptolysin O-permeabilized chief cells from guinea pig stomach. In the presence of 100 nM calcium, 100 microM guanosine 5'-(beta,gamma-imido)triphosphate or guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) caused a 2- to 4-fold increase in pepsinogen secretion. GTP gamma S stimulated secretion in the absence of calcium (up to 10 mM EGTA). With or without added calcium, GTP analogues caused a 2- to 3-fold increase in cAMP, whereas guanosine 5'-O-2-(thio)diphosphate and calcium alone had no effect on cAMP levels. GTP analogue-induced activation of phospholipase C was evidenced by a calcium-independent increase in cytidine diphospho-1,2-diacylglycerol levels (50% above basal). Phorbol ester- and GTP gamma S-stimulated phosphorylation of a 72-kDa acidic protein was abolished by an inhibitor of protein kinase C (CGP 41251). However, GTP gamma S-induced pepsinogen secretion was only partially inhibited by adding CGP 41251 or a protein kinase C inhibitor peptide. These results indicate that guanine nucleotides activate major signaling pathways in gastric chief cells. Nevertheless, GTP gamma S can induce pepsinogen secretion independently of changes in calcium, cAMP, or activation of protein kinase C.