Insulin Acts in Ventromedial Hypothalamus to Regulate the Sympathoadrenal Response to Hypoglycemia in Rats

Abstract

We tested the hypothesis that the correction of ventromedial hypothalamus (VMH) insulin deficiency was sufficient to restore the impaired sympathoadrenal response to hypoglycemia in diabetic rats. Sprague-Dawley rats were injected with either vehicle (nondiabetic controls; CON) or streptozotocin (STZ; 65 mg/kg IP). STZ diabetic rats received osmotic mini-pumps to infuse either artificial cerebrospinal fluid (DIAB) or insulin (3mU/day; DIAB+InsVMH) into the VMH bilaterally. Two weeks later, all three groups underwent hyperinsulinemic (50 mU.kg-1.min-1) hypoglycemic (∼50 mg/dl) clamps. As expected, STZ-diabetic rats showed blunted epinephrine response to hypoglycemia as compared to nondiabetic controls. Chronic infusion of insulin into VMH of diabetic rats normalized the epinephrine response to hypoglycemia and lowered the glucose infusion rate required to maintain hypoglycemia. Examining brain insulin action, it was noted that STZ diabetic rats had decreased Akt phosphorylation and decreased expression of insulin dependent glucose transporter 4 (GLUT4) in VMH by 44 and 40% respectively as compared to the nondiabetic controls. Notably, chronic infusion of insulin to the VMH normalized Akt phosphorylation and GLUT4 expression. In summary, insulin acts chronically in the VMH to preserve the sympathoadrenal response to hypoglycemia, possibly by regulating GLUT4 expression. Disclosure R. Agrawal: None. A. Vieira de Abreu: None. G.T. Durupt: None. S.J. Fisher: None.