The Hyperinsulinemic Isoglycemic Hyperlactemic Clamp

The Cori Cycle (glucose-lactate interchange) dominates carbohydrate fluxes. Glucose-to-lactate flux follows peripheral disposal; lactate-to-glucose occurs during gluconeogenesis. As glucose flux can be assessed by the change in an exogenous glucose infusion rate (GIR) required to maintain a constant concentration (glucose clamp), so too can lactate flux be assessed from an exogenous lactate infusion rate (LIR) required to maintain isolactemia. Simultaneous assessment of both fluxes will enable estimation of lactate-derived gluconeogenesis (GNG), and its contribution to the rate of glucose appearance (Ra) without using radiolabeled tracers, the mainstay of flux measurement. Anesthetized Sprague-Dawley rats were intubated, catheterized via artery and vein, and partially pancreatectomized to eliminate endogenous glucagon secretion. Establishment of a hyperinsulinemic (100mU/kg/h), hyperglycemic (7mM) clamp was followed by a hyperlactemic (4mM) clamp via independent variable infusions of glucose/Na-lactate in response to 5-min measures of plasma concentrations (YSI 2950D Analyzer with glucose/lactate electrodes). Hyperlactemia (∼10x resting) rendered it the dominant gluconeogenic substrate. After at least 30 min of isolactemia, glucagon was infused for 60 min (0, 1, 3, 10 or 50 µg/kg/h) to enable dose/concentration response analysis. Glucagon administration dose-dependently reduced the GIR and increased the LIR necessary to maintain isoglycemia/isolactemia (respective ED50’s 1.33, 0.98 µg/kg/h). δ-GIR of up to -12.2 mmol/kg/h potentially represented increased glucose Ra. δ-LIR of up to +8.6 mmol/kg/h potentially represented GNG. Since 2 lactate moieties are required per glucosyl, GNG contributed ∼35% of glucagon-stimulated glucose Ra (∼65% being glycogenolysis).

In conclusion, the hyperinsulinemic isoglycemic hyperlactemic clamp enables a quantitative assessment of hepatic processes, complementing that of peripheral processes revealed with the glucose clamp.