Metabolic Sensors Mediate Hypoglycemic Detection at the Portal Vein

Abstract

The current study sought to ascertain whether portal vein glucose sensing is mediated by a metabolic fuel sensor analogous to other metabolic sensors presumed to mediate hypoglycemic detection (e.g., hypothalamic metabosensors). We examined the impact of selectively elevating portal vein concentrations of lactate, pyruvate, or beta-hydroxybutyrate (BHB) on the sympathoadrenal response to insulin-induced hypoglycemia. Male Wistar rats (n = 36), chronically cannulated in the carotid artery (sampling), jugular vein (infusion), and portal vein (infusion), underwent hyperinsulinemic-hypoglycemic ( approximately 2.5 mmol/l) clamps with either portal or jugular vein infusions of lactate, pyruvate, or BHB. By design, arterial concentrations of glucose and the selected metabolite were matched between portal and jugular (NS). Portal vein concentrations were significantly elevated in portal versus jugular (P < 0.0001) for lactate (5.03 +/- 0.2 vs. 0.84 +/- 0.08 mmol/l), pyruvate (1.81 +/- 0.21 vs. 0.42 +/- 0.03 mmol/l), or BHB (2.02 +/- 0.1 vs. 0.16 +/- 0.03 mmol/l). Elevating portal lactate or pyruvate suppressed both the epinephrine (64% decrease; P < 0.01) and norepinephrine (75% decrease; P < 0.05) responses to hypoglycemia. In contrast, elevating portal BHB levels failed to impact epinephrine (P = 0.51) or norepinephrine (P = 0.47) levels during hypoglycemia. These findings indicate that hypoglycemic detection at the portal vein is mediated by a sensor responding to some metabolic event(s) subsequent to the uptake and oxidation of glucose.