Insulin acts centrally to enhance reflex tachycardia in conscious rats

Systemic and central insulin treatment for 10 days were compared in conscious rats given insulin either by daily subcutaneous injection or by continuous intracerebroventricular infusion. To measure changes in baroreflex sensitivity, heart rate responses elicited reflexly by elevating blood pressure with phenylephrine or lowering it with sodium nitroprusside were recorded before and after insulin treatment. Although reflex bradycardia and basal mean pressures and heart rates were unaffected, reflex tachycardia was consistently more pronounced in rats treated with insulin (whether given systemically or centrally) than in vehicle-treated controls. Plasma glucose was reduced and plasma insulin was elevated but only in insulin-treated rats injected subcutaneously and not in those infused intracerebroventricularly. Inasmuch as reflex tachycardia was still enhanced when insulin was infused intracerebroventricularly despite the absence of hypoglycemia and hyperinsulinemia, our results are compatible with the interpretation that insulin acts directly on the brain to enhance reflex tachycardia. This interpretation furthermore implies that insulin, even after chronic systemic treatment, could reach the brain and alter baroreflex regulation.