Effect of adrenal medullectomy on metabolic responses to chronic intermittent hypoxia in the frequently sampled intravenous glucose tolerance test

Abstract

Obstructive sleep apnea is associated with type 2 diabetes. We have previously developed a mouse model of intermittent hypoxia (IH) mimicking oxyhemoglobin desaturations in patients with sleep apnea and have shown that IH increases fasting glucose, hepatic glucose output, and plasma catecholamines. We hypothesize that adrenal medulla modulates glucose responses to IH and that such responses can be prevented by adrenal medullectomy. We performed adrenal medullectomy or sham surgery in lean C57BL/6J mice, which were exposed to IH or intermittent air (control) for 4 wk followed by the frequently sampled intravenous glucose tolerance test (FSIVGTT) in unanesthetized unrestrained animals. IH was administered during the 12-h light phase (9 AM to 9 PM) by decreasing inspired oxygen from 21 to 6.5% 60 cycles/h. Insulin sensitivity (S_I), insulin independent glucose disposal [glucose effectiveness (S_G)], and the insulin response to glucose (AIR_G) were determined using the minimal model method. In contrast to our previous data obtained in restrained mice, IH did not affect fasting blood glucose and plasma insulin levels in sham-operated mice. IH significantly decreased S_G but did not affect S_I and AIR_G Adrenal medullectomy decreased fasting blood glucose and plasma insulin levels and increased glycogen synthesis in the liver in hypoxic mice but did not have a significant effect on the FSIVGTT metrics. We conclude that, in the absence of restraints, IH has no effect on glucose metabolism in lean mice with exception of decreased S_G, whereas adrenal medullectomy decreases fasting glucose and insulin levels in the IH environment.NEW & NOTEWORTHY To our knowledge, this is the first study examining the role of adrenal catecholamines in glucose metabolism during intermittent hypoxia (IH) in unanesthetized unrestrained C57BL/6J mice. We report that IH did not affect fasting glucose and insulin levels nor insulin sensitivity and insulin secretion during, whereas glucose effectiveness was decreased. Adrenal medullectomy decreased fasting blood glucose and insulin levels in mice exposed to IH but had no effect on glucose metabolism, insulin secretion, and insulin sensitivity.

Keywords: glucose effectiveness; insulin resistance; obstructive sleep apnea; type 2 diabetes.

Fig. 1.

Fasting blood glucose (A) and fasting serum insulin levels (B). *P < 0.05 for the effect of hypoxia.

Fig. 2.

Absolute blood glucose levels (A), change in blood glucose levels from baseline (time 0, B), and plasma insulin levels (C) during the frequently sampled intravenous glucose tolerance test (FSIVGTT). At time 0 an intravenous injection of 1 g/kg of D50 was administered over a 15-s interval and blood glucose and plasma insulin were sampled over a 1-h period. P < 0.05 for the effect of adrenal medullectomy and intermittent hypoxia on glucose levels; P < 0.001 for the effect of intermittent hypoxia on change in glucose levels from baseline.

Fig. 3.

The minimal model of FSIVGTT. The effect of intermittent hypoxia and adrenal medullectomy on insulin sensitivity (S_I), glucose effectiveness (S_G), the acute insulin response to glucose (AIR_G), the disposition index (DI), the areas under curve for blood glucose level adjusted for the baseline (AUC_G) and plasma insulin levels adjusted for the baseline (AUC_I). *P < 0.05 for the overall effect of intermittent hypoxia; †P < 0.001 for the effect of intermittent hypoxia in medullectomized mice.

Fig. 4.

Glucokinase gene expression in the liver.

Fig. 5.

Liver hexokinase and glucokinase activities. A: V_max activity of glucokinase was measured at 100 mM of glucose. B: V_0.5 activity of glucokinase was measured at Km concentration of glucose (8 mM). C: a sum of hexokinases I, II, and III activities was measured at glucose concentration of 0.5 mM.

Fig. 6.

A: liver glycogen levels and phosphorylation of the β-subunit of glycogen synthase kinase 3 (GSK3β). B: representative immunoblots of phosphorylated GSK3β (p-GSK3β), total GSK3β and a housekeeping protein glyceraldehyde 3-phosphate dehydrogenase (GAPDH) run on the same membrane. C: optical density ratio of p-GSK3β to total GSK3β.