No abstract

Insulin stimulates glucose disposal in skeletal muscle in part by increasing microvascular blood flow, and this effect is blunted during insulin resistance. We aimed to determine whether metformin treatment improves insulin-mediated glucose disposal and vascular insulin responsiveness in skeletal muscle of insulin-resistant rats. Sprague–Dawley rats were fed a normal (ND) or high-fat (HFD) diet for 4 weeks. A separate HFD group was given metformin in drinking water (HFD + MF, 150 mg/kg/day) during the final 2 weeks. After the intervention, overnight-fasted (food and metformin removed) anaesthetised rats underwent a 2-h euglycaemic–hyperinsulinaemic clamp (10 mU/min/kg) or saline infusion. Femoral artery blood flow, hindleg muscle microvascular blood flow, muscle glucose disposal and muscle signalling (Ser473-AKT and Thr172-AMPK phosphorylation) were measured. HFD rats had elevated body weight, epididymal fat pad weight, fasting plasma insulin and free fatty acid levels when compared to ND. HFD-fed animals displayed whole-body and skeletal muscle insulin resistance and blunting of insulin-stimulated femoral artery blood flow, muscle microvascular blood flow and skeletal muscle insulin-stimulated Ser473-AKT phosphorylation. Metformin treatment of HFD rats reduced fasting insulin and free fatty acid concentrations and lowered body weight and adiposity. During euglycaemic-hyperinsulinaemic clamp, metformin-treated animals showed improved vascular responsiveness to insulin, improved insulin-stimulated muscle Ser473-AKT phosphorylation but only partially restored (60%) muscle glucose uptake. This occurred without any detectable levels of metformin in plasma or change in muscle Thr172-AMPK phosphorylation. We conclude that 2-week metformin treatment is effective at improving vascular and metabolic insulin responsiveness in muscle of HFD-induced insulin-resistant rats.

Acute infusion of AngII significantly alters basal haemodynamic and metabolic homeostasis in rats. Both local and systemic AngII infusion attenuated insulin's microvascular actions in skeletal muscle, but only local AngII infusion led to reduced insulin-stimulated muscle glucose uptake. While increased local, tissue production of AngII may be a factor that couples microvascular insulin resistance and hypertension, additional studies are needed to determine the molecular mechanisms responsible for these vascular defects.

The matching of capillary blood flow to metabolic rate of the cells within organs and tissues is a critical microvascular function which ensures appropriate delivery of hormones and nutrients, and the removal of waste products. This relationship is particularly important in tissues where local metabolism, and hence capillary blood flow, must be regulated to avoid a mismatch between nutrient demand and supply that would compromise normal function. The consequences of a mismatch in microvascular blood flow and metabolism are acutely apparent in the brain and heart, where a sudden cessation of blood flow, for example following an embolism, acutely manifests as stroke or myocardial infarction. Even in more resilient tissues such as skeletal muscle, a short-term mismatch reduces muscle performance and exercise tolerance, and can cause intermittent claudication. In the longer-term, a microvascular-metabolic mismatch in skeletal muscle reduces insulin-mediated muscle glucose uptake, leading to disturbances in whole-body metabolic homeostasis. While the notion that capillary blood flow is fine-tuned to meet cellular metabolism is well accepted, the mechanisms that control this function and where and how different parts of the vascular tree contribute to capillary blood flow regulation remain poorly understood. Here, we discuss the emerging evidence implicating pericytes, mural cells that surround capillaries, as key mediators that match tissue metabolic demand with adequate capillary blood flow in a number of organs, including skeletal muscle. K E Y W O R D Scapillary blood flow, microvasculature, pericytes, skeletal muscle | 521 ATTRILL eT AL.

The skeletal muscle microvasculature is a key regulator of peripheral resistance and plays a major role in determining muscle function. In insulin resistance and type 2 diabetes (T2D), blood flow is disrupted contributing to metabolic dysfunction. Pericytes are contractile cells known to regulate capillary diameter and microvascular blood flow in several organs, however little is known about their role in skeletal muscle. In this study, we developed a novel mouse model of T2D to characterize pericytes in healthy, insulin resistant and T2D skeletal muscle.Male Tg(Cspg4-DsRed.T1)1Akik/J mice were allocated to control diet (CD; 6% fat wt/wt, n=5) or high fat diet (HFD; 23% fat wt/wt, n=17) for 17wks to model insulin resistance. To model T2D, we infused streptozotocin (STZ) across 14 days using osmotic mini-pumps to generate moderate (MOD, 200mg/kgSTZ, n=5) and severe (SEV, 250-300mg/kgSTZ, n=7) hyperglycaemia. In week 17, mice underwent a 2hr glucose tolerance test (GTT; 2.0g/kg IP glucose) after which mice were euthanized and cardiac perfused with 4% paraformaldehyde. The gastrocnemius and soleus were excised, processed for staining, and imaged. HFD mice were obese (CD 33.0±3.5, HFD 38.7±4.1g, p=0.011) and had elevated fasting glucose regardless of STZ dose (CD 9.6±0.9, HFD 14.7±5.1mmol/L, p=0.036). HFD mice had elevated plasma insulin (CD 166±18, HFD 242±109pmol/L, p=0.003) which was reduced to CD levels with STZ (MOD 99±44, SEV 126±46pmol/L, p<0.001 vs HFD). HFD and STZ induced a stepwise increase in blood glucose at the end of the 2hr GTT (CD 11.0±0.9, HFD 15.9±3.4, MOD 23.5±3.7, SEV 31.2±2.6mmol/L, p<0.001 for group effect). Although HFD did not change capillary density in the gastrocnemius (CD 1311±190, HFD 1263±200cap/mm2, p=0.641), capillary density was reduced in MOD (1158±315cap/mm2, p=0.007 vs HFD) and increased in SEV (1553±174cap/mm2, p=0.035 vs HFD). There was no change in pericyte density in the gastrocnemius (CD 61±21 vs HFD 63±22 vs MOD 64±15 vs SEV 81±27cells/mm2, p=0.484 for group effect). Similarly, pericyte density in the soleus was unchanged between CD (175±43 cells/mm2), HFD (147±18 cells/mm2) and MOD (118±25 cells/mm2). In contrast, SEV had a ~40% increase in pericyte density compared to all other groups (SEV 242±36cells/mm2, p<0.05). Perhaps most importantly, we observed distinct changes in pericyte morphology in both muscles in the HFD, MOD and SEV mice compared with CD mice. These changes include swelling and fragmentation of pericyte cytoplasmic processes that normally cover 95% of skeletal muscle capillaries.In summary, changes in pericyte density and morphology occur in insulin resistance and T2D. Given microvascular dysfunction is a hallmark of insulin resistance and T2D, this work suggests pericyte damage may contribute to blood flow dysregulation and the progression of disease. Further work is needed to understand the functional consequences of pericyte changes to skeletal muscle health in insulin resistance and T2D. This work has been supported by the Tasmanian School of Medicine and the University of Tasmania. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.