Glycogen Is the Major Source of Lactate in the Ventromedial Hypothalamus of Recurrently Hypoglycemic Rats

Abstract

Lactate is an important metabolic substrate for sustaining brain energy demands when glucose supplies are limited. Recurrent hypoglycemia (RH) leads to increased lactate levels in the ventromedial hypothalamus (VMH) which contributes to counterregulatory failure, but the source of this lactate remains unclear. It has been shown that astrocytic glycogen can be broken down into lactate in the brain. Therefore, the present study investigates whether glycogen serves as the major source of lactate in the VMH of RH rats. To address this question, Sprague-Dawley rats were implanted with mini-osmotic pumps and bilateral intracranial guide cannulas that targeted the VMH to deliver either artificial extracellular fluid (aECF) or the glycogen phosphorylase (GP) inhibitor, 1,4-dideoxy-1,4-imino-d-arabinitol (DAB). Subsequently, the rats underwent 3 bouts of hypoglycemia before undergoing a hyperinsulinemic-hypoglycemic clamp on day 4 (n=6/group). A separate group of rats receiving aECF into the VMH were given 3 saline injections as controls before undergoing a hypoglycemic clamp on day 4 (n=6). During the clamps, microdialysis was used to measure VMH lactate levels and VMH glycogen content was determined at the end of the clamp from liquid nitrogen fixed brain samples. VMH lactate levels were elevated in RH rats compared to hypoglycemia-naïve controls (3.6±0.2 vs. 2.3±0.4 mM; P=0.03) and this was associated with impairments in the counterregulatory responses (37% and 45% reduction in glucagon and epinephrine, respectively; P<0.05). Compared to the RH rats, inhibition of GP with DAB prevented the breakdown of VMH glycogen in RH rats (5.3±0.5 vs. 2.4±0.5 μmol/g tissue; P<0.002), prevented the rise in VMH lactate levels (1.5±0.3 vs. 3.6±0.2 mM; P<0.001) and prevented the onset of counterregulatory failure (glucagon and epinephrine responses were similar to controls, P=NS). Our data suggests that astrocytic glycogen likely serves as the primary source of lactate in the VMH of RH rats. Disclosure G. Su: None. O. Chan: None. R. Farhat: None. K. Chapman: None.