Characterization of gastrin/CCK receptors on gastric carcinoid tumor membrane of Mastomys natalensis.

Recently, we identified the specific binding site for gastrin on the gastric carcinoid tumor of Mastomys (Praomys) natalensis. In this study, precise characterization of the gastrin binding site on these tumors was performed. Both 125I-human gastrin I (gastrin) and 125I-CCK-8 bound specifically to the cell membrane, and Scatchard analysis revealed a high affinity binding site for each ligand with similar Kd and Bmax values. The specific binding of both 125I-gastrin and 125I-CCK-8 was displaced in a concentration-dependent manner by various related peptides with a relative potency order of CCK-8 > or = gastrin < des(SO3)CCK-8. In addition, L364,718 as well as L365,260 displaced the binding of both ligands with similar potencies. Furthermore, not only gastrin but also CCK-8 increased [Ca2+]i in these tumor cells, the action of both being inhibited by L364,718 as well as by L365,260 (10(-7) M). These results suggest that the carcinoid tumor of Mastomys possesses a high affinity gastrin/CCK binding site coupled to the increase of [Ca2+]i.