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CCK-A- and CCK-B-like receptors in the gallbladder and stomach of the alligator (Alligator mississippiensis).

Autoradiographic analysis of radioligand binding was used to localize and characterize cholecystokinin (CCK) receptors expressed in the stomach and gallbladder of the American alligator (Alligator mississippiensis). Alligator gallbladder smooth muscle expresses a CCK-A receptor subtype, and stomach oxyntic mucosa expresses a distinct receptor subtype, termed CCK-B/X because of its similarities to both CCK-B and CCK-X receptors. Both the gallbladder and the stomach binding sites have very low affinities for a panel of nonpeptide receptor agonists and antagonists that are selective for mammalian CCK-A and CCK-B receptors. These results suggest that CCK receptor subtypes diverged from the ancestral CCK-X receptor in an early amniote, prior to the divergence of mammals and reptiles in vertebrate phylogeny, and that CCK-A receptors may have evolved before CCK-B receptors. Our findings support the hypothesis that the evolution of gastrin preceded the evolution of two separate CCK receptor subtypes.