Glucose-dependent insulinotropic peptide (GIP) has been reported to have opposing effects on splanchnic blood flow. GIP infusion in dogs results in an increase in portal vein circulation but a drop in hepatic artery blood flow. In an effort to evaluate whether these different responses were related to intrinsic differences in GIP effects we isolated canine hepatic artery (HAEC) and portal vein (PVEC) endothelial cells. We report that there are differences in GIP activation of the signal transduction pathways in these two cell types. GIP stimulates secretion of Endothelin-1 (ET-1), a potent vasoconstrictor, from HAEC (EC50: 0.28 nM) but not from PVEC. This effect could be abolished by preventing a rise in intracellular calcium demonstrating the calcium dependency of GIP-induced ET-1 secretion from HAEC. The GIP effect was specific, since a GIP receptor antagonist blocked it. In contrast, GIP stimulated nitric oxide production from PVEC (EC50: 0.09 nM) but not from HAEC. Taken together our data demonstrate distinct differences in GIP effects on HAEC from those on PVEC. We conclude that differences in GIP stimulation of ET-1 vs. nitric oxide production in different vascular beds may account for some of the observed differences in its physiological effects.