Effect of Histamine and Its Methyl Derivatives on Cyclic AMP Metabolism in Gastric Mucosa and Its Blockade by an H2 Receptor Antagonist

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Abstract
In a cell-free system prepared from guinea pig gastric mucosa, histamine and Nα-methyl-histamine produced dose-dependent stimulation of cyclic AMP formation and 1,4-methylhistamine had a minimal stimulatory effect. N-methyl-N′-(2-[5-methylimidazole-4-yl-methylthio]-ethyl) -thiourea (metiamide), a new H2 receptor inhibitor, selectively blocked the stimulation of adenylate cyclase by histamine and its active methyl derivative but had no substantial effect on the basal adenylate cyclase activity or adenylate cyclase stimulated by sodium fluoride. Metiamide inhibited the histamine stimulation of adenylate cyclase at 1/100 the concentration of the histamine. Histamine, its methyl derivatives, and metiamide did not influence the activity of cyclic AMP phosphodiesterase from gastric mucosa. Therefore, histamine stimulates gastric mucosal adenylate cyclase via interaction with the H2 receptor without influencing cyclic AMP breakdown, and N-methylation of histamine on the side chain preserves or even increases its stimulating ability. On the other hand, N-methylation in the ring nearly abolishes the ability of histamine to interact with the H2 receptor.