Glycemia in blood, brain and subcutaneous tissue measured by a continuous glucose monitoring system

Introduction

Continuous glucose monitoring system (CGMS) technology provides the opportunity to measure glycemia in different tissues [1]. The aim of our study was to determine the lag-time between blood, brain and adipose tissue during rapid glucose changes.

Methods

Fifteen male hereditary hypertriglyceridemic rats underwent the experimental protocol. After intraperitoneal anesthesia, the internal jugular vein and carotid artery were catheterized. A CGMS sensor (Medtronic) was inserted into the brain by micromanipulators and to the abdominal subcutaneous tissue. At the beginning of the experiment (-120 minutes), basal glycemia was measured and calibration of the sensors was started. Thereafter, insulin infusion was started (50 mU/kg/minute) and 20% glucose at a variable rate of infusion. Blood glucose was measured every 5 minutes with manual correction of the glucose infusion rate to maintain the glycemia level of 6 mmol/l. At a time of -10 minutes, the calibration procedure was finished and actual glycemia was recorded to sensors. At a time of 0 minutes, a bolus of glucose 0.5 g/kg was administered; and at a time of 50 minutes, a bolus of insulin 5 IU/kg was administered. Moreover glucose and insulin infusion were stopped at this time. The experiment was finished at time 130 minutes and animals were euthanized.

Results

After an intravenous glucose bolus of 0.5 g/kg, glycemia rose rapidly to 14 mmol/l in 5 minutes. On the contrary, the glucose content in the brain and subcutaneous tissue was increased in a slower manner, with a maximum in about 50 minutes (brain) and 60 minutes (subcutaneous tissue). Intravenous insulin bolus of 5 U/kg was followed by lowering blood glucose concentration to a minimum of 4.5 mmol/l. The brain and subcutaneous tissue glucose content decreased slowly to a minimum of 4.2 mmol/l (brain) and 5.5 mmol/l (subcutaneous tissue). The median glucose lag-time blood versus brain and blood versus subcutaneous tissue was 10 (10; 15) minutes and 15 (15; 25) minutes, respectively (P = 0.01).

Conclusions

Contrary to a previous study, which showed no changes in glucose dynamics after a bolus of glucose between brain, adipose tissue and muscle, our data showed that glucose in the brain follows blood excursions during acute glycemic changes more closely compared with subcutaneous tissue [2].