Liraglutide prevents microvascular insulin resistance and preserves muscle capillary density in high-fat diet-fed rats

Abstract

Muscle microvasculature critically regulates endothelial exchange surface area to facilitate transendothelial delivery of insulin, nutrients, and oxygen to myocytes. Insulin resistance blunts insulin-mediated microvascular recruitment and decreases muscle capillary density; both contribute to lower microvascular blood volume. Glucagon-like peptide 1 (GLP-1) and its analogs are able to dilate blood vessels and stimulate endothelial cell proliferation. In this study, we aim to determine the effects of sustained stimulation of the GLP-1 receptors on insulin-mediated capillary recruitment and metabolic insulin responses, small arterial endothelial function, and muscle capillary density. Rats were fed a high-fat diet (HFD) for 4 wk with or without simultaneous administration of liraglutide and subjected to a euglycemic hyperinsulinemic clamp for 120 min after an overnight fast. Insulin-mediated muscle microvascular recruitment and muscle oxygenation were determined before and during insulin infusion. Muscle capillary density was determined and distal saphenous artery used for determination of endothelial function and insulin-mediated vasodilation. HFD induced muscle microvascular insulin resistance and small arterial vessel endothelial dysfunction and decreased muscle capillary density. Simultaneous treatment of HFD-fed rats with liraglutide prevented all of these changes and improved insulin-stimulated glucose disposal. These were associated with a significantly increased AMPK phosphorylation and the expressions of VEGF and its receptors. We conclude that GLP-1 receptor agonists may exert their salutary glycemic effect via improving microvascular insulin sensitivity and muscle capillary density during the development of insulin resistance, and early use of GLP-1 receptor agonists may attenuate metabolic insulin resistance as well as prevent cardiovascular complications of diabetes.

Keywords: endothelial function; glucagon-like peptide-1; insulin resistance; microvascular recruitment; vasodilation.

Fig. 1.

Liraglutide (Lira) treatment improves insulin responses in high-fat diet (HFD)-fed rats. Each rat was fed either a chow diet (CD) or HFD ± Lira for 4 wk and received a 2-h euglycemic insulin clamp (3 mU·kg⁻¹·min⁻¹) after an overnight fast. A: time course of glucose infusion rate (GIR) and GIR during steady state (90–120 min). B: muscle Akt phosphorylation (p-Akt). C: muscle ERK phosphorylation (p-ERK); n = 5–8/group. *P < 0.05 compared with CD; #P < 0.05 compared with HFD. t-Akt, total Akt; t-ERK, total ERK.

Fig. 2.

Effect of liraglutide (Lira) on insulin-mediated muscle microvascular perfusion and muscle oxygenation during HFD feeding. Each rat was fed either a CD or HFD ± Lira for 4 wk and received a 2-h euglycemic insulin clamp (3 mU·kg⁻¹·min⁻¹) after an overnight fast. A: microvascular blood volume (MBV). B: microvascular blood flow velocity (MFV). C: microvascular blood flow (MBF). D: muscle oxygen saturation; n = 5–7/group. *P < 0.05 compared with respective baseline (0 min). VI, video intensity.

Fig. 3.

Effect of liraglutide (Lira) on insulin-induced nitric oxide (NO) and endothelin-1 (ET-1) production. Each rat was fed either a CD or HFD ± Lira for 4 wk and received a 2-h euglycemic insulin clamp (3 mU·kg⁻¹·min⁻¹) after an overnight fast. A: plasma NO concentrations. B: plasma ET-1 concentrations; n = 4–8/group. *P < 0.05 compared with respective baseline (0 min).

Fig. 4.

Liraglutide (Lira) prevents HFD-induced endothelial dysfunction and insulin resistance in isolated saphenous artery. Each rat was fed either a CD or HFD ± Lira for 4 wk. Distal saphenous artery was isolated and preconstricted with phenylephrine, and the vasodilatory responses to acetylcholine (ACh; A), sodium nitroprusside (SNP; B), or insulin (C) were measured; n = 6/group. *P < 0.05 compared with CD.

Fig. 5.

Liraglutide (Lira) treatment prevents HFD-induced decrease in muscle capillary density. Rats were fed a HFD either with or without Lira for 4 wk. Control rats were fed a CD for 4 wk. A: representative images of capillary content in gastrocnemious. B: Muscle capillary density (capillaries/muscle fiber); n = 12/group. *P < 0.0002 compared with CD; #P = 0.001 compared with HFD.

Fig. 6.

Effect of liraglutide (Lira) treatment on muscle AMP-activated protein kinase (AMPK) phosphorylation (p-AMPK) and VEGF expression in HFD-fed rats. Each rat was fed either a CD or HFD ± Lira for 4 wk. A: muscle p-AMPK; n = 6/group. B: VEGF mRNA expression; n = 10–11/group. C: VEGFR1 and -2 mRNA expressions; n = 9–10/group. *P < 0.05 compared with CD; #P < 0.05; compared with HFD. t-AMPK, total AMPK.