Role of Brain in Counterregulation of Insulin-Induced Hypoglycemia in Dogs

The role of the brain in directing counterregulation during hypoglycemia induced by insulin infusion was assessed in overnight-fasted conscious dogs. Concomitant brain and peripheral hypoglycemia was induced in one group of dogs (n = 5) by infusing insulin peripherally at a rate of 3.5 mU · kg⁻¹ · min⁻¹. In another group (n = 4), insulin was infused as described above to induce peripheral hypoglycemia, and brain hypoglycemia was minimized by infusing glucose bilaterally into the carotid and vertebral arteries to maintain the brain glucose level at a calculated concentration of 85 mg/dl. Glucose was also infused peripherally as needed so that the peripheral glucose levels in both of the protocols were similar (45 ± 2 mg/dl with and 48 ± 3 mg/dl without brain glucose infusion, both P < .05). The responses (in terms of change of area under the curve) of epinephrine, norepinephrine, cortisol, and pancreatic polypeptide when brain glycemia was controlled during insulin infusion were only 14 ± 6, 39 ± 12, 17 ± 8, and 9 ± 4%, respectively, of those present during insulin infusion without concomitant brain glucose infusion (all P < .05). Of particular interest was the glucagon response that occurred when head hypoglycemia was minimized; the glucagon level was only 21 ± 8% of that present when marked brain hypoglycemia accompanied insulin infusion (P < .05). During hypoglycemia resulting from insulin infusion, endogenous glucose production (EGP), as assessed with [3-³H]glucose, rose from 2.6 ± 0.1 to 4.4 ± 0.5 mg · kg⁻¹ min⁻¹ (P < .05). In contrast, EGP decreased from 2.7 ± 0.2 to 2.0 ± 0.3 mg · kg⁻¹ · min⁻¹ When brain hypoglycemia was minimized. In an additional set of studies, when insulin was infused at 3.5 mU · kg⁻¹ · min⁻¹ and glucose was infused peripherally to maintain both the head and peripheral glucose concentrations at 88 ± 6 mg/dl, EGP decreased from 2.6 ± 0.1 to 1.2 ± 0.2 mg · kg⁻¹ · min⁻¹. These results suggest that under marked hyperinsulinemic conditions the brain is the primary director of glucagon release and that it is responsible for ∼ 75% of the life-sustaining glucose production.