Gastric Acid Secretion: Substrate-Dependency and Intracellular Mechanisms of Action of Secretagogues and Inhibitors

Abstract

The principal objectives of this study were: a) to establish which substrates, or combination of substrates, could provide by their metabolism, energy to support acid secretion by the rat parietal cell; b) to establish the putative involvement of protein kinase C in the acidsecretory process, and to determine the principal protein substrates for this enzyme within the parietal cell; c) to investigate the mechanism of action of a probable physiological inhibitor of gastric acid secretion, epidermal growth factor. Acid secretion was estimated by direct measurement in vivo, and by the accumulation of the weak base, aminopyrine, in vitro. The effects of metabolic substrates were investigated using a preparation of isolated parietal cells maximally stimulated with secretagogues. Concentrations of glucose, oleate, lactate, D-3-hydroxybutyrate, L-isoleucine, acetoacetate, and L-valine similar to those found in plasma could enhance acid secretion above the level found in the absence of exogenous’ substrates. Supraphysiological concentrations of acetate, butyrate and L-leucine, but not L-glutamine also supported acid secretion. Supraphysiological concentrations of lactate and L-isoleucine produced a greater secretory activity than 5mM-glucose. Addition of combinations of certain substrates at physiological concentrations to 5mM-glucose, increased acid-secretory activity. Thus, maximal rates of acid secretion by isolated cells require a combination of substrates in addition to glucose. 12-0-tetradecanoylphorbol-13-acetate, exerted a specific inhibitory action on acid secretion in vivo, most likely by activation of protein kinase C in the parietal cell. An enzyme exhibiting many of the characteristics of protein kinase C phosphorylated an 89Kda protein present in a 100,000g cytosolic fraction of an homogenate derived from a preparation highly enriched with parietal cells. Epidermal growth factor inhibited acid secretion stimulated by histamine, but had no effect on basal secretion or on that stimulated by the other secretagogues tested. The inhibitory action of epidermal growth factor on acid secretion by rat parietal cells probably did not involve prostaglandin production or activation of protein kinase C. A site of action close to the generation or hydrolysis of cAMP is suggested.