| Co-Investigator(Kenkyū-buntansha) |
SAITO Hiroyuki Third Department of Internal Medicine, Asahikawa Medical College, Lecuturer, 医学部, 助手 (20311532)
YOKOTA Kinnichi Third Department of Internal Medicine, Asahikawa Medical College, Assistant Professor, 医学部, 講師 (10250573)
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| Research Abstract |
We examined first the effect of intracisternal injection of orexin-A on gastric acid secretion in rats. Intracisternal injection of orexin-A dose-dependently stimulated gastric acid output by the pylorus-ligation method. In contrast, intraperitoneal administration of orexin-A failed to stimulate acid, secretion, suggesting that orexin-A acts in the brain to stimulate gastric acid secretion. The stimulation of gastric secretion was not observed in the vagotomyzed rats, indicating that the vagus nerve mediates the orexin-induced acid secretion Orexin-A is a neuropeptide consisting 33 amino acids with two intrachain disulfide bonds, namely Cys6-Cys12 and Cys7-Cys14. In contrast, orexin-B, a peptide containing 28 amino acids without disulfide bond, which has no stimulatory action of gastric acid. Intracisternal injection of orexin-A but not orexin-B or orexin-A (15-33) that does not contain both disulfide bonds stimulated gastric acid secretion in pylonis-ligated conscious rats. The abilit
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y of the stimulation of gastric acid output was less in three alanine-substituted orexin-A, [Ala 6, 12]orexin-A, [Ala 7, 14]orexin-A and [Ala 6, 7, 12, 14]orexin-A, than orexin-A, Orexins-induced calcium increase was measured in CHO-K1 cells expressing OX1R or OX2R. Orexin-A induced a transient increase in [Ca2+]i in CHO-K1/OX1R cells in a dose-dependent manner. EC50 values for OX1R of orexin-A, orexhi-B or orexin-A (15-33) was 0.068, 0.69 or 4.1 nM, respectively, suggesting that peptides containing no disulfide bonds have lower potency for the receptor. Agonistic activity for OX1R of the three orexin-A analogues with modification of one or both disulfide bonds was significantly reduced as compared with that of orexin-A. EC50 values for OX2R of orexin-A and orexin-B was almost equal but potency for the receptor of orexin-A (15-33) and three alanine substituted orexin-A was less than that of orexin-A. A significant inverse relationship between gastric acid output and EC50 values for OX1R but not OX2R was observed. These results suggested that the orexin-A-induced acid stimulation requires OX1R activation and that disulfide bonds in orexin-A may have a key role in the receptor activation. Less
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