Effects of different acute hypoxic regimens on tissue oxygen profiles and metabolic outcomes

Abstract

Obstructive sleep apnea (OSA) causes intermittent hypoxia (IH) during sleep. Both obesity and OSA are associated with insulin resistance and systemic inflammation, which may be attributable to tissue hypoxia. We hypothesized that a pattern of hypoxic exposure determines both oxygen profiles in peripheral tissues and systemic metabolic outcomes, and that obesity has a modifying effect. Lean and obese C57BL6 mice were exposed to 12 h of intermittent hypoxia 60 times/h (IH60) [inspired O₂ fraction (Fi(O₂)) 21-5%, 60/h], IH 12 times/h (Fi(O₂) 5% for 15 s, 12/h), sustained hypoxia (SH; Fi(O₂) 10%), or normoxia while fasting. Tissue oxygen partial pressure (Pti(O₂)) in liver, skeletal muscle and epididymal fat, plasma leptin, adiponectin, insulin, blood glucose, and adipose tumor necrosis factor-α (TNF-α) were measured. In lean mice, IH60 caused oxygen swings in the liver, whereas fluctuations of Pti(O₂) were attenuated in muscle and abolished in fat. In obese mice, baseline liver Pti(O₂) was lower than in lean mice, whereas muscle and fat Pti(O₂) did not differ. During IH, Pti(O₂) was similar in obese and lean mice. All hypoxic regimens caused insulin resistance. In lean mice, hypoxia significantly increased leptin, especially during SH (44-fold); IH60, but not SH, induced a 2.5- to 3-fold increase in TNF-α secretion by fat. Obesity was associated with striking increases in leptin and TNF-α, which overwhelmed effects of hypoxia. In conclusion, IH60 led to oxygen fluctuations in liver and muscle and steady hypoxia in fat. IH and SH induced insulin resistance, but inflammation was increased only by IH60 in lean mice. Obesity caused severe inflammation, which was not augmented by acute hypoxic regimens.

Fig. 1.

A: average distribution of oxygen saturation measured by pulse oximetry (Sp_O2) and tissue oxygen partial pressure (Pti_O2) during normoxic and hypoxic conditions in lean and obese mice. The red curves show normoxic group, the purple curves show infrequent intermittent hypoxia (12 times/h; IH12), the blue curves show frequent intermittent hypoxia (60 times/h; IH60), and the green curves show sustained hypoxia (SH). Sp_O2 and Pti_O2 values were normalized to 100% (%time spent) for each regimen. B: example tracings during frequent intermittent hypoxia in a lean mouse. The top curves show the Sp_O2 in arterial blood, the bottom curves show the Pti_O2 in the different tissues at the same time.

Fig. 2.

Immunohistostaining of the liver with Hypoxyprobe (pimonidazole) counterstained with hematoxylin. Representative immunohistostaining is shown for both lean and obese mice in the normoxic group (air) and the IH60 group. Original magnification: ×200. Hypoxic areas are stained in brown (pimonidazole). Arrows mark hepatic vein. Scale bar is 50 μm.

Fig. 3.

Indexes of insulin resistance after different hypoxic regimens and 12-h fast. A: glucose. B: insulin. C: leptin. D: adiponectin. Values are means ± SE. Values are shown for normoxia (air), IH12, IH60, and SH. *P < 0.05 compared with lean air group. †P < 0.05 compared with obese air group.

Fig. 4.

Lipid peroxidation measured by malondialdehyde (MDA) levels after different hypoxic regimens. A: liver. B: muscle. C: epididymal fat. Values are means ± SE. Values are shown for normoxia (air), IH12, IH60, and SH. *P < 0.05 compared with lean air group. †P < 0.05 compared with obese air group.

Fig. 5.

Secretion of tumor necrosis factor-α (TNF-α) by adipose tissue ex vivo after different hypoxic regimens. Values are means ± SE. Values are shown for normoxia (air), IH12, IH60, and SH. *P < 0.05 compared with lean air group.

Fig. 6.

Results of the real-time PCR for TNF-α in liver tissue. Values are means ± SE. TNF-α levels are shown as mRNA expression levels normalized to 18S ribosomal RNA concentrations and then expressed as a ratio to the lean air group.