Central but not systemic lipid infusion augments the counterregulatory response to hypoglycemia

Abstract

This study tests the hypothesis that lipids could act as an alternative fuel source in the brain during insulin-induced hypoglycemia. Male Sprague-Dawley rats were subjected to hyperinsulinemic (5 mU.kg(-1).min(-1)) hypoglycemic (approximately 50 mg/dl) clamps. In protocol 1, intralipid (IL), a fat emulsion, was infused intravenously to prevent the fall in free fatty acid levels that occurs in response to hyperinsulinemic hypoglycemia. Intravenous lipid infusion did not alter the counterregulatory responses to hypoglycemia. To test whether IL could have central effects in mediating the counterregulatory response to hypoglycemia, in protocol 2 the brains of precannulated rats were intracerebroventricularly (icv) infused with IL or artificial cerebrospinal fluid (aCSF) as control. Unexpectedly, the epinephrine and glucagon response to hypoglycemia was significantly augmented with icv IL infusion. To determine whether central IL infusion could restore defective counterregulation, in protocol 3 rats were made recurrently hypoglycemic (RH) for 3 days and on the 4th day underwent hyperinsulinemic hypoglycemic clamps with icv IL or aCSF infusion. RH rats had the expected impaired epinephrine response to hypoglycemia, and icv IL infusion again significantly augmented the epinephrine response in RH rats to normal. With regard to our experimental model of hypoglycemic counterregulation, we conclude that 1) systemic lipid infusion did not alter the counterregulatory response to hypoglycemia, 2) the icv infusion of lipids markedly increased CSF FFA levels and paradoxically augmented the epinephrine and glucagon responses, and 3) the blunted sympathoadrenal response in recurrently hypoglycemic rats was completely normalized with the icv lipid infusion. It is concluded that, in the setting of insulin-induced hypoglycemia, increased brain lipids can enhance the sympathoadrenal response.

Fig. 1.

Plasma glucose levels (A), glucose infusion rates (B), insulin levels (C), and free fatty acid (FFA) levels (D) in the basal period and during a hyperinsulinemic (5 mU·kg⁻¹·min⁻¹) hypoglycemic (∼50 mg/dl) clamp in rats treated with intravenous saline (iv + SAL; ▴) or intravenous intralipid (iv + IL; ▵).

Fig. 2.

Plasma epinephrine (A), norepinephrine (C), glucagon (E), and corticosterone levels (G) and their respective areas under curve (AUC; B, D, F, and H) are shown during a hyperinsulinemic (5 mU·kg⁻¹·min⁻¹) hypoglycemic (∼50 mg/dl) clamp in rats treated with iv + SAL (▴ and filled bars) or iv + IL (▵ and open bars).

Fig. 3.

Plasma glucose levels (A), glucose infusion rates (B), insulin levels (C), and FFA levels (D) in the basal period and during a hyperinsulinemic (5 mU·kg⁻¹·min⁻¹) hypoglycemic (∼50 mg/dl) clamp in rats treated with intracerebroventricular artificial cerebrospinal fluid (icv + aCSF; •) or intracerebroventricular intralipid (icv + IL; ○).

Fig. 4.

Plasma epinephrine (A), norepinephrine (C), glucagon (E), and corticosterone levels (G) and their respective AUC (B, D, F, and H) are shown during a hyperinsulinemic (5 mU·kg⁻¹·min⁻¹) hypoglycemic (∼50 mg/dl) clamp in rats treated with icv + aCSF (• and filled bars) or icv + IL (○ and open bars). *P < 0.05

Fig. 5.

Rats underwent 3 days of recurrent hypoglycemia (RH) with insulin injections (∼5 U/kg) to reduce blood glucose to ∼50 mg/dl. On the 4th day, a hyperinsulinemic (5 mU·kg⁻¹·min⁻¹) hypoglycemic (∼50 mg/dl) clamp was performed in rats treated with icv aCSF (RH + icv + aCSF; ▪) or icv IL (RH + icv + IL; □). Plasma glucose levels (A), glucose infusion rates (B), insulin levels (C), and FFA levels (D) are shown.

Fig. 6.

Rats underwent 3 days of RH (as outlined in Fig. 5). Plasma epinephrine (A), norepinephrine (C), glucagon (E), and corticosterone levels (G) and their respective AUC are shown during a hyperinsulinemic (5 mU·kg⁻¹·min⁻¹) hypoglycemic (∼50 mg/dl) clamp in rats treated with RH + icv + aCSF (▪ and filled bars) or RH + icv + IL (□ and open bars). *P < 0.05.