title: CCK-X receptors in the endothermic mako shark (Isurus oxyrinchus).
abstract: By mapping the distribution of cholecystokinin (CCK) receptor types onto an established phylogenetic hypothesis of vertebrate relationships, we tested two hypothesis about the evolution of CCK receptors: (1) A single CCK receptor type, CCK-X, is the ancestral receptor, while CCK-A and CCK-B receptors represent derived receptor types; (2) the evolution of two separate CCK receptors is functionally related to the evolution of endothermy. Specifically, we localized and characterized 125I-CCK-binding sites in the gut and brain of mako shark (Isurus oxyrinchus), a warm-blooded chondrichthyean fish. Competitive inhibition studies of 125I-CCK binding showed that the CCK receptor in the mako shark brain, gallbladder, pyloric stomach, and intestine binds sulfated CCK-8 and sulfated gastrin-17 (gastrin-17-II) with much higher affinity (K(i) ranging from 0.05 to 2.02 nM) than unsulfated gastrin-17 (gastrin-17-I, K(i) ranging from 4.63 to 62.17 nM). These results indicate that the mako shark expresses a single CCK-X receptor in all tissues. Additional competitive inhibition studies showed that the mako CCK-X receptor has very low affinities for the following nonpeptide agonist and antagonists: A71623, L364,718, A57696, A65186.72, Cam-1481, and SR 27897B (specific for some mammalian CCK-A receptors) and L365,260 and CI-988 (specific for some mammalian CCK-B receptors), confirming the pharmacological differences between the CCK-X receptor and the CCK-A and -B receptors. Based on the mapped phylogenetic distribution of CCK receptor types, we conclude that CCK-X is the ancestral receptor type and that two receptor types, e.g. CCK-A and CCK-B, are not part of the suite of characters necessary for evolution of endothermy in fishes.
authors.affiliation: Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, USA.
authors.forename: A S
authors.initials: AS
authors.lastname: Oliver
authors.forename: S R
authors.initials: SR
authors.lastname: Vigna
mesh_terms: D000818:Animals; D001665:Binding Sites; D001833:Body Temperature Regulation / Q000502:physiology*; D001923:Brain Chemistry / Q000502:physiology*; D004064:Digestive System / Q000737:chemistry*; D019143:Evolution, Molecular; D005704:Gallbladder / Q000737:chemistry*; D007422:Intestines / Q000737:chemistry; D011706:Pyloric Antrum / Q000737:chemistry; D043886:Receptor, Cholecystokinin A; D043887:Receptor, Cholecystokinin B; D011949:Receptors, Cholecystokinin / Q000032:analysis* / Q000187:drug effects; D012754:Sharks / Q000378:metabolism*
chemical_list: C104399:CCK-X receptor; D043886:Receptor, Cholecystokinin A; D043887:Receptor, Cholecystokinin B; D011949:Receptors, Cholecystokinin