CHOLECYSTOKININ-B RECEPTOR LIGANDS OF THE DIPEPTOID SERIES ACT AS AGONISTS ON RAT STOMACH HISTIDINE-DECARBOXYLASE

Background & Aims: The effect of gastrin on the enterochromaffin-like cells in the rat stomach is mediated by cholecystokinin (CCK)-B receptors and manifested as activation of histidine decarboxylase (HDC). The dipeptoids PD 136450, PD 135158, and PD 134308 are thought to be selective CCK-B receptor antagonists. The effects of the dipeptoids and of gastrin-17 and sulfated CCK-8 on rat stomach HDC activity were examined. Methods: Drugs were infused intravenously or subcutaneously to fasted rats, and the HDC activity was determined. Results: The dipeptoids activated HDC with maximal responses (50%-60% of the maximal response to gastrin) at 1 mu mol . kg(-1). h(-1). Rat gastrin-17 activated HDC maximally at 3 nmol . kg(-1). h(-1), and sulfated CCK-8 produced maximal response at 20 nmol . kg(-1). h(-1). The CCK-B receptor antagonist L-365,260 inhibited the HDC activation induced by gastrin, sulfated CCK-8, or the dipeptoids. The dipeptoids did not inhibit the gastrin-induced HDC activation. Conclusions: Gastrin, sulfated CCK-8, and the dipeptoids activated rat stomach HDC. L-365,260 but not devazepide inhibited the HDC activation. Thus, the dipeptoids, which failed to inhibit the gastrin-induced HDC activation, act as CCK-B receptor agonists and not as antagonists. It is important to recognize this to ensure appropriate interpretation of data obtained with these drugs.