Ranitidine, a histamine H2-receptor antagonist, competitively inhibits histamine from binding to H2-receptors on parietal cells. It suppresses the normal secretion of acid by parietal cells and the meal-stimulated secretion of acid. This is accomplished by two mechanisms: histamine released by ECL cells in the stomach is blocked from binding on parietal cell H2 receptors which stimulate acid secretion, and other substances that promote acid secretion (such as gastrin and acetylcholine) have a reduced effect on parietal cells when the H2 receptors are blocked.