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zfGIP stimulates insulin release from clonal β-cells and mouse islets.zfGIP administration improves glucose tolerance in mice.Insulin release from BRIN-BD11 cells is attenuated by a GLP-1R antagonist.zfGIP stimulates cAMP production in CHL cells transfected with GLP-1R.zfGIP stimulates insulin release in INS-1 GIPR knockout cells.The insulinotropic properties of zebrafish GIP (zfGIP) were assessed in vitro using clonal pancreatic β-cell lines and isolated mouse islets and acute effects on glucose tolerance and insulin release in vivo were evaluated in mice. The peptide produced a dose-dependent increase in the rate of insulin release from BRIN-BD11 rat clonal β-cells at concentrations ≥30 nM. Insulin release from 1.1 B4 human clonal β-cells and mouse islets was significantly increased by zfGIP (10 nM and 1 μM). The in vitro insulinotropic activity of zfGIP was decreased after incubating BRIN-BD11 cells with the GLP-1 receptor antagonist, exendin-4(9-39) (p < 0.001) and the GIP receptor antagonist, GIP (6-30) Cex-K40[Pal] (p < 0.05) but the glucagon receptor antagonist [des-His1,Pro4,Glu9]glucagon amide was without effect. zfGIP (10 nM and 1 μM) produced significant increases in cAMP concentration in CHL cells transfected with the human GLP-1 receptor but was without effect on HEK293 cells transfected with the human glucagon receptor. Conversely, zfGIP, but not human GIP, significantly stimulated insulin release from CRISPR/Cas9-engineered INS-1 clonal β-cells from which the GIP receptor had been deleted. Intraperitoneal administration of zfGIP (25 and 75 nmol/kg body weight) to mice together with an intraperitoneal glucose load (18 mmol/kg body weight) produced a significant decrease in plasma glucose concentrations concomitant with an increase in insulin concentrations. The study provides evidence that the insulinotropic action of zfGIP in mammalian systems involves activation of both the GLP-1 and the GIP receptors but not the glucagon receptor.