Blood-to-Brain Glucose Transport, Cerebral Glucose Metabolism, and Cerebral Blood Flow Are Not Increased After Hypoglycemia

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Abstract
Recent antecedent hypoglycemia has been found to shift glycemic thresholds for autonomic (including adrenomedullary epinephrine), symptomatic, and other responses to subsequent hypoglycemia to lower plasma glucose concentrations. This change in threshold is the basis of the clinical syndromes of hypoglycemia unawareness and, in part, defective glucose counterregulation and the unifying concept of hypoglycemia-associated autonomic failure in type 1 diabetes. We tested in healthy young adults the hypothesis that recent antecedent hypoglycemia increases blood-to-brain glucose transport, a plausible mechanism of this phenomenon. Eight subjects were studied after euglycemia, and nine were studied after ∼24 h of interprandial hypoglycemia (∼55 mg/dl, ∼3.0 mmol/l). The latter were shown to have reduced plasma epinephrine (P = 0.009), neurogenic symptoms (P = 0.009), and other responses to subsequent hypoglycemia. Global bihemispheric blood-to-brain glucose transport and cerebral glucose metabolism were calculated from rate constants derived from blood and brain time-activity curves—the latter determined by positron emission tomography (PET)—after intravenous injection of [1-11C]glucose at clamped plasma glucose concentrations of 65 mg/dl (3.6 mmol/l). For these calculations, a model was used that includes a fourth rate constant to account for egress of [11C] metabolites. Cerebral blood flow was measured with intravenous [15O]water using PET. After euglycemia and after hypoglycemia, rates of blood-to-brain glucose transport (24.6 ± 2.3 and 22.4 ± 2.4 μmol · 100 g−1 · min−1, respectively), cerebral glucose metabolism (16.8 ± 0.9 and 15.9 ± 0.9 μmol · 100 g−1 · min−1, respectively) and cerebral blood flow (56.8 ± 3.9 and 53.3 ± 4.4 ml · 100 g−1 · min−1, respectively) were virtually identical. These data do not support the hypothesis that recent antecedent hypoglycemia increases blood-to-brain glucose transport during subsequent hypoglycemia. They do not exclude regional increments in blood-to-brain glucose transport. Alternatively, the fundamental alteration might lie beyond the blood-brain barrier.