Insulin Plays a Permissive Role for the Vasoactive Effect of GIP Regulating Adipose Tissue Metabolism in Humans

Context and Objective: Glucose-dependent insulinotropic polypeptide (GIP) in combination with hyperinsulinemia increases blood flow and triglyceride (TAG) clearance in subcutaneous (sc) abdominal adipose tissue in lean humans. The present experiments were performed to further investigate the role of insulin for the vasoactive effect of GIP in adipose tissue metabolism and whether the vasodilatory effect of GIP is dependent on C-peptide. Methods: Six lean healthy subjects were studied. The sc abdominal adipose tissue metabolism was assessed by Fick's principle during GIP infusion (1.5 pmol/kg/min) in combination with 1) euglycemic-high insulinemic clamp (Eu-gluc-Hi(insu)), raising plasma insulin concentrations to postprandial levels, 2) hyperglycemic-euinsulinemic clamp (Hy(gluc)-Eu-insu), and 3) hyperglycemic-hyperinsulinemic clamp, raising plasma insulin concentrations to supraphysiological levels. During the hyperglycemic clamps, endogenous insulin and C-peptide secretion were inhibited by infusion of the somatostatin analogue octreotide. Results: During GIP infusion, Eu-gluc-Hi(insu), and hyperglycemic-hyperinsulinemic clamps, sc abdominal adipose tissue blood flow (ATBF) was similar and increased from 2.1 +/- 0.2 and 2.2 +/- 0.4 ml min(-1) (100 g tissue)(-1) to 7.1 +/- 0.6 and 7.6 +/- 0.1 ml min(-1) (100 g tissue)(-1), respectively (P = .01). ATBF remained virtually constant (2.7 +/- 0.4 ml min(-1) [100 g tissue](-1)) during Hy(gluc)-Eu-insu and GIP infusion. In addition, adipose tissue TAG clearance increased significantly (P = .03), whereas free fatty acid output (P = .01), glycerol output (P = .02) and free fatty acid/glycerol release ratio (P = .04) decreased during the Eu-gluc-Hi(insu) clamp compared to Hy(gluc)-Eu-insu clamp with GIP. Conclusion: In healthy lean humans, insulin is permissive for GIP to induce an increase in blood flow and TAG clearance in sc abdominal adipose tissue. This effect is independent of C-peptide.