Abstract
By mapping the distribution of cholecystokinin (CCK) receptor types onto an established phylogenetic hypothesis of vertebrate relationships, we tested two hypotheses 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 characterized125I-CCK-binding sites in the gut and brain of mako shark (Isurus oxyrinchus), a warm-blooded chondrichthyean fish. Competitive inhibition studies of125I-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 (Kiranging from 0.05 to 2.02 nM) than unsulfated gastrin-17 (gastrin-17-I,Kiranging from 4.63 to 62.17 nM). These results indicated 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 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.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.