Effect of Vagal Cooling on the Counterregulatory Response to Hypoglycemia Induced by a Low Dose of Insulin in the Conscious Dog

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Abstract
We previously demonstrated, using a nerve-cooling technique, that the vagus nerves are not essential for the counterregulatory response to hypoglycemia caused by high levels of insulin. Because high insulin levels per se augment the central nervous system response to hypoglycemia, the question arises whether afferent nerve fibers traveling along the vagus nerves would play a role in the defense of hypoglycemia in the presence of a more moderate insulin level. To address this issue, we studied two groups of conscious 18-h–fasted dogs with cooling coils previously placed on both vagus nerves. Each study consisted of a 100-min equilibration period, a 40-min basal period, and a 150-min hypoglycemic period. Glucose was lowered using a glycogen phosphorylase inhibitor and a low dose of insulin infused into the portal vein (0.7 mU · kg−1 · min−1). The arterial plasma insulin level increased to 15 ± 2 μU/ml and the plasma glucose level fell to a plateau of 57 ± 3 mg/dl in both groups. The vagal cooling coils were perfused with a 37°C (SHAM COOL; n = 7) or a −20°C (COOL; n = 7) ethanol solution for the last 90 min of the study to block parasympathetic afferent fibers. Vagal cooling caused a marked increase in the heart rate and blocked the hypoglycemia-induced increase in the arterial pancreatic polypeptide level. The average increments in glucagon (pg/ml), epinephrine (pg/ml), norepinephrine (pg/ml), cortisol (μg/dl), glucose production (mg · kg–1 · min–1), and glycerol (μmol/l) in the SHAM COOL group were 53 ± 9, 625 ± 186, 131 ± 48, 4.63 ± 1.05, −0.79 ± 0.24, and 101 ± 18, respectively, and in the COOL group, the increments were 39 ± 7, 837 ± 235, 93 ± 39, 6.28 ± 1.03 (P < 0.05), −0.80 ± 0.20, and 73 ± 29, respectively. Based on these data, we conclude that, even in the absence of high insulin concentrations, afferent signaling via the vagus nerves is not required for a normal counterregulatory response to hypoglycemia.

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