Supplemental oxygen administration during mechanical ventilation reduces diaphragm blood flow and oxygen delivery

Endothelial-Specific Reduction in Arf6 Impairs Insulin-Stimulated Vasodilation and Skeletal Muscle Blood Flow Resulting in Systemic Insulin Resistance in Mice

BACKGROUND

Much of what we know about insulin resistance is based on studies from metabolically active tissues such as the liver, adipose tissue, and skeletal muscle. Emerging evidence suggests that the vascular endothelium plays a crucial role in systemic insulin resistance; however, the underlying mechanisms remain incompletely understood. Arf6 (ADP ribosylation factor 6) is a small GTPase that plays a critical role in endothelial cell function. Here, we tested the hypothesis that the deletion of endothelial Arf6 will result in systemic insulin resistance.

METHODS

We used mouse models of constitutive endothelial cell-specific Arf6 deletion (Arf6f/- Tie2Cre+) and tamoxifen-inducible Arf6 knockout (Arf6f/f Cdh5CreER+). Endothelium-dependent vasodilation was assessed using pressure myography. Metabolic function was assessed using a battery of metabolic assessments including glucose and insulin tolerance tests and hyperinsulinemic-euglycemic clamps. We used a fluorescence microsphere-based technique to measure tissue blood flow. Skeletal muscle capillary density was assessed using intravital microscopy.

RESULTS

Endothelial Arf6 deletion impaired insulin-stimulated vasodilation in white adipose tissue and skeletal muscle feed arteries. The impairment in vasodilation was primarily due to attenuated insulin-stimulated nitric oxide bioavailability but independent of altered acetylcholine-mediated or sodium nitroprusside-mediated vasodilation. Endothelial cell-specific deletion of Arf6 also resulted in systematic insulin resistance in normal chow-fed mice and glucose intolerance in high-fat diet-fed obese mice. The underlying mechanisms of glucose intolerance were reductions in insulin-stimulated blood flow and glucose uptake in the skeletal muscle and were independent of changes in capillary density or vascular permeability.

CONCLUSIONS

Results from this study support the conclusion that endothelial Arf6 signaling is essential for maintaining insulin sensitivity. Reduced expression of endothelial Arf6 impairs insulin-mediated vasodilation and results in systemic insulin resistance. These results have therapeutic implications for diseases that are associated with endothelial cell dysfunction and insulin resistance such as diabetes.

Pulmonary hypertension alters blood flow distribution and impairs the hyperemic response in the rat diaphragm

Pulmonary hypertension (PH) is characterized by pulmonary vascular remodeling, respiratory muscle and cardiac impairments, and exercise intolerance. Specifically, impaired gas exchange increases work of the diaphragm; however, compromised contractile function precludes the diaphragm from meeting the increased metabolic demand of chronic hyperventilation in PH. Given that muscle contractile function is in part, dependent upon adequate blood flow (Q ˙ ), diaphragmatic dysfunction may be predicated by an inability to match oxygen delivery with oxygen demand. We hypothesized that PH rats would demonstrate a decreased hyperemic response to contractions compared to healthy controls. Methods: Sprague-Dawley rats were randomized into healthy (HC, n = 7) or PH (n = 7) groups. PH rats were administered monocrotaline (MCT) while HC rats received vehicle. Disease progression was monitored via echocardiography. Regional and total diaphragm blood flow and vascular conductance at baseline and during 3 min of electrically-stimulated contractions were determined using fluorescent microspheres. Results: PH rats displayed morphometric and echocardiographic criteria for disease (i.e., acceleration time/ejection time, right ventricular hypertrophy). In all rats, total costal diaphragm Q ˙ increased during contractions and did not differ between groups. In HC rats, there was a greater increase in medial costal Q ˙ compared to PH rats (55% ± 3% vs. 44% ± 4%, p < 0.05), who demonstrated a redistribution of Q ˙ to the ventral costal region. Conclusion: These findings support a redistribution of regional diaphragm perfusion and an impaired medial costal hyperemic response in PH, suggesting that PH alters diaphragm vascular function and oxygen delivery, providing a potential mechanism for PH-induced diaphragm contractile dysfunction.