1910-P: Effects of Glucagon Concentrations on Splanchnic and Leg Glucagon Extraction in Healthy Humans

We have recently developed a novel isotope dilution method using nonradioactive, stable human Glucagon (Phe 6 13C9, 15N; Phe 22 13C9, 15N) tracer to measure glucagon turnover. In this study we combined this method with organ catheterization technique to estimate the effects of increasing circulating glucagon concentrations, spanning the physiological range, on splanchnic and leg glucagon extraction. We present data from the first 6 healthy subjects (age 25±3.6 yrs; BMI 26.7±4.5 kg/m2; fasting glucose 4.6±0.5 mM; HbA1c 5.1±0.3%) completed thus far. After IRB approval and informed consent, subjects were admitted in the morning after an overnight fast to Interventional Radiology, where catheters were placed in the right femoral artery and vein under local anesthesia with aseptic precautions. Under fluoroscopic guidance, a hepatic venous catheter was positioned through the femoral vein. Glucagon tracer was infused intravenously and Indocyanine Green infused via the femoral artery sheath to measure splanchnic and femoral plasma flows. After a 90 min period for basal sampling, exogenous glucagon was infused at 0.65 ng/kg/min for 60 minutes (low) followed by 1.5 ng/kg/min (high) for another 60 minutes. Plasma samples were collected periodically from femoral artery, femoral vein and hepatic vein for concentrations of Glucagon tracer (Tandem Mass Spectrometry), glucagon, glucose and Indocyanine Green (Spectrophotometry) concentrations. Arterial glucagon concentrations were 45.2±9.6 pg/ml (basal), 56.3±13.3 pg/ml (low) and 81.4±15.5 pg/ml (high). While fractional splanchnic glucagon extraction remained relatively unchanged (24.0±12.5% vs. 27.7±15.6% vs. 23.5±10.1%: basal vs. low vs. high), fractional leg glucagon extraction tended to increase (13.5±3.8% vs. 16.0±7.2% vs. 18.5±9.4%: basal vs. low vs. high) with increasing glucagon concentrations. These initial data suggest differing mechanisms of glucagon clearance across the splanchnic and leg tissues in humans.