REGULATION BY GASTRIC-ACID OF THE PROCESSING OF PROGASTRIN-DERIVED PEPTIDES IN RAT ANTRAL MUCOSA

1. Inhibition of gastric acid secretion by proton pump inhibitors like omeprazole increases the synthesis and secretion of the pyloric antra l hormone gastrin. We report here how omeprazole influences the conver sion of the gastrin precursor to its final products, and the abundance of mRNAs encoding proteins associated with progastrin processing in r at antral mucosa. 2. Progastrin processing was studied using a pulse-c hase protocol in antral mucosa, incubated in vitro, from rats treated with omeprazole for up to 5 days. Labelled peptides were detected by o n-line scintillation counting after immunoprecipitation and HPLC. The mRNAs encoding prohormone-processing enzymes were identified by Northe rn blot, polymerase chain reaction or ribonuclease protection assay, a nd their cellular origins identified by immunocytochemistry. 3. Cleava ge of [H-3]- and [S-35]-labelled progastrins at Arg-94-95 or Arg-57-58 , and amidation at Phe-92 were not influenced by pretreatment with ome prazole. In contrast, cleavage of G34 (the thirty-four amino acid amid ated gastrin) at Lys-74-75 to give G17 (the seventeen amino acid amida ted gastrin), and of G34-Gly to G17-Gly (G34 and G17 with COOH-termina l glycine), was increased 3-fold after treatment with omeprazole for e ither 1 or 5 days. 4. Approximately 20% of newly synthesized amidated and Gly-extended gastrins were secreted within 240 min of the labellin g period in omeprazole-treated samples, but secretion of labelled gast rins from control tissue was undetectable over a comparable period. 5. The amidating enzyme, peptidylglycine alpha-amidating mono-oxygenase (PAM), the prohormone convertases PC1/3, PC2, PC5 and the PC2 chaperon e 7B2 were localized to rat antral gastrin cells by immunocytochemistr y. The relative abundance of mRNA species encoding 7B2, PC5 and PAM we re unchanged after treatment with omeprazole for 5 days, whereas gastr in, PC1/3 and PC2 mRNAs are known to increase at this time. 6. The mai n consequence of increased cleavage at Lys-74-75 is the production of G17 and G17-Gly at the expense of G34 and G34-Gly, respectively. The l atter have longer plasma ha,lf-lives, and so their increased cleavage may serve to limit the rise in plasma gastrin concentration after inhi bition of acid secretion. Changes in the abundance of mRNAs encoding p rohormone-processing enzymes cannot account for the rapidity of the ch anges in cleavage of progastrin at Lys residues after omeprazole.