Pentagastrin stimulated gastric acid and pepsin secretions show parallel rates of tachyphylaxis in the conscious cat. The responses to histamine show only slight tachyphylaxis. Somatostatin 10 μg.kg -1.hr -1 inhibits pentagastrin but not histamine stimulated acid secretion and inhibits pentagastrin stimulated pepsin secretion. The inhibition of pentagastrin stimulated acid and pepsin secretion by Somatostatin delays the tachyphylaxis of these responses, but the rates of tachyphylaxis when they do subsequently occur are identical. Metiamide 10 mg.kg -1.hr -1 equally inhibits histamine and pentagastrin stimulated acid secretion but does not inhibit pentagastrin stimulated pepsin secretion. Inhibition of acid secretion during metiamide infusion neither prevents nor delays acid nor pepsin tachyphylaxis. It is suggested that tachyphylaxis of acid and pepsin secretion is a gastrin receptor phenomenon and that Somatostatin occupies or modifies the behavior of these receptors, preventing tachyphylaxis. Metiamide, however, exerts its action only on the histamine H 2 receptor and not the gastrin receptor mechanism, and this apparently does not prevent or delay acid tachyphylaxis.
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