NONPARALLEL SECRETION OF PEPSINOGEN AND ACID BY GASTRIC OXYNTOPEPTIC CELLS OF THE TOAD (BUFO-MARINUS)

Pepsinogen and HCI secretion in the amphibian.stomach are performed by a single cell type, the oxyntopeptic cell. These functions were studi ed in gastric mucosae of toads (Bufo marinus) mounted in Ussing-type c hambers. HCI and peptic activity of luminal fluid were measured by tit ration and proteolysis of albumin, respectively. Distribution of pepsi nogen in the gastric mucosa was heterogeneous, activity being highest in the proximal part of the stomach. Zymogen granules in the oxyntopep tic cell were more abundant in the deeper cells of the glands and in t he fundus. On stimulation, the granules were released into the lumen o f the glands by exocytosis. Histamine, forskolin, or carbachol alone e ach induced an increase in HCI and pepsinogen secretion. Carbachol aft er maximal histamine or forskolin stimulation produced an extra increa se in both secretions that was greater for pepsinogen response. Simila rly, joint addition of carbachol and histamine was more potent than hi stamine alone for both parameters; however, the effect was greater on pepsinogen release. Pretreatment with cimetidine blocked HCI and pepsi nogen responses to carbachol but did not affect responses to forskolin . Addition of omeprazole to forskolin-stimulated mucosae uncoupled the two secretions, inducing a total inhibition of HCI secretion with a s light reduction in pepsinogen secretion. Thus pepsinogen release, simi lar to HCl secretion, is sensitive to cAMP and Ca2+-dependent secretag ogues. However, the action of Ca2+ would require the previous elevatio n of cAMP induced by the different secretagogues. In such a case, the increase in intracellular Ca2+ WoUld result in a nonparallel activatio n of the two secretions.