Hyperglycemia suppresses the sympatho-adrenal response to hypoxia, but not to handling stress

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We hypothesized that the ability of prior hyperglycemia to suppress the sympatho-adrenal response would depend on the type of stress. To test this hypothesis, hyperglycemia was induced in chronically catheterized rats, before submitting them to either hypoxia (7.5% O 2) or handling stress. Central venous blood samples were drawn for the determination of plasma glucose, epinephrine (EPI), norepinephrine (NOR) and insulin concentrations. Hypoxia caused significant increases in plasma EPI and NOR concentrations (ΔEPI=+2.95±0.68 nmol/l, ΔNOR=+12.45±1.29 nmol/l). Hyperglycemia, antecedent to hypoxia, dose dependently reduced the sympatho-adrenal response. In contrast, the sympatho-adrenal response to handling stress was not affected by even marked antecedent hyperglycemia (ΔEPI=+2.48±0.46 nmol/l, ΔNOR=+3.12±0.69 nmol/l at glucose=20.7±0.6 mmol/l; vs. ΔEPI=+2.48±0.58 nmol/l, ΔNOR=+2.97±0.11 nmol/l at glucose=6.77±0.17 mg/dl). Thus, antecedent hyperglycemia suppresses the hypoxia-induced activation of both the sympathetic nerves and the adrenal medulla, but not the activation induced by handling. We conclude that the ability of hyperglycemia to suppress sympathetic activation depends on the stress producing the activation. We therefore speculate that hypoxic stress has a metabolic component to its central activation that handling stress does not.