Acute Nitric Oxide Synthase Inhibition Accelerates Transendothelial Insulin Efflux In Vivo

Endothelial β1-integrins are necessary for microvascular function and glucose uptake

Microvascular insulin delivery to myocytes is rate limiting for the onset of insulin-stimulated muscle glucose uptake. The structural integrity of capillaries of the microvasculature is regulated, in part, by a family of transmembrane adhesion receptors known as integrins, which are composed of an α and a β subunit. The integrin β1 (itgβ1) subunit is highly expressed in endothelial cells (ECs). EC itgβ1 is necessary for the formation of capillary networks during embryonic development, and its knockdown in adult mice blunts the reactive hyperemia that manifests during ischemia reperfusion. In this study, we investigated the contribution of EC itgβ1 in microcirculatory function and glucose uptake, with an emphasis on skeletal muscle. We hypothesized that loss of EC itgβ1 would impair microvascular hemodynamics and glucose uptake during insulin stimulation, creating "delivery"-mediated insulin resistance. An itgβ1 knockdown mouse model was developed to avoid the lethality of embryonic gene knockout and the deteriorating health resulting from early postnatal inducible gene deletion. We found that mice with (itgβ1fl/flSCLcre) and without (itgβ1fl/fl) inducible stem cell leukemia cre recombinase (SLCcre) expression at 10 days post cre induction have comparable exercise tolerance and pulmonary and cardiac functions. We quantified microcirculatory hemodynamics using intravital microscopy and the ability of mice to respond to the high metabolic demands of insulin-stimulated muscle using a hyperinsulinemic-euglycemia clamp. We show that itgβ1fl/flSCLcre mice compared with itgβ1fl/fl littermates have 1) deficits in capillary flow rate, flow heterogeneity, and capillary density; 2) impaired insulin-stimulated glucose uptake despite sufficient transcapillary insulin efflux; and 3) reduced insulin-stimulated glucose uptake due to perfusion-limited glucose delivery. Thus, EC itgβ1 is necessary for microcirculatory function and to meet the metabolic challenge of insulin stimulation.NEW & NOTEWORTHY The microvasculature is an important site of resistance to muscle glucose uptake. We show that microvasculature integrins determine the exchange of glucose between the circulation and muscle. Specifically, a 30% reduction in the expression of endothelial integrin β1 subunit is sufficient to cause microcirculatory dysfunction and lead to insulin resistance. This emphasizes the importance of endothelial integrins in microcirculatory function and the importance of microcirculatory function for the ability of muscle to consume glucose.

Sex and sex steroids as determinants of cardiovascular risk

There are considerable sex differences regarding the risk of cardiovascular disease (CVD), including arterial hypertension, coronary artery disease (CAD) and stroke, as well as chronic renal disease. Women are largely protected from these conditions prior to menopause, and the risk increases following cessation of endogenous estrogen production or after surgical menopause. Cardiovascular diseases in women generally begin to occur at a later age than in men (on average with a delay of 10 years). Cessation of estrogen production also impacts metabolism, increasing the risk of developing obesity and diabetes. In middle-aged individuals, hypertension develops earlier and faster in women than in men, and smoking increases cardiovascular risk to a greater degree in women than it does in men. It is not only estrogen that affects female cardiovascular health and plays a protective role until menopause: other sex hormones such as progesterone and androgen hormones generate a complex balance that differentiates heart and blood vessel function in women compared to men. Estrogens improve vasodilation of epicardial coronary arteries and the coronary microvasculature by augmenting the release of vasodilating factors such as nitric oxide and prostacyclin, which are mechanisms of coronary vasodilatation that are more pronounced in women compared to men. Estrogens are also powerful inhibitors of inflammation, which in part explains their protective effects on CVD and chronic renal disease. Emerging evidence suggests that sex chromosomes also play a significant role in shaping cardiovascular risk. The cardiovascular protection conferred by endogenous estrogens may be extended by hormone therapy, especially using bioidentical hormones and starting treatment early after menopause.

Endoplasmic reticulum as a therapeutic target in type 2 diabetes: Role of phytochemicals

Type 2 diabetes mellitus (T2DM) is a metabolic disorders characterized by insulin resistance and β-cell dysfunction with an increasing worldwide incidence. Several studies have revealed that long-term glucotoxicity results in β-cell failure and death through induction of endoplasmic reticulum (ER) stress. Owing to the chronic progression of T2DM and the low effectiveness of antidiabetic drugs in long-term use, medicinal plants and their secondary metabolites seem to be the promising alternatives. Here we have provided a comprehensive review regarding the role of phytochemicals to alleviate ER stress in T2DM. Ginsenoside compound K, baicalein, quercetin, isopulegol, kaempferol, liquiritigenin, aspalathin, and tyrosol have demonstrated remarkable improvement of T2DM via modulation of ER stress. Arctigenin and total glycosides of peony have been shown to be effective in the treatment of diabetic retinopathy through modulation of ER stress. The effectiveness of grape seed proanthocyanidins and wolfberry is also shown in the relief of diabetic neuropathy and retinopathy. Resveratrol is involved in the prevention of atherosclerosis via ER stress modulation. Taken together, the data described herein revealed the capability of herbal constituents to prevent different complications of T2DM via a decrease in ER stress which open new doors to the treatment of diabetes.

Capillary Endothelial Insulin Transport: The Rate-limiting Step for Insulin-stimulated Glucose Uptake

The rate-limiting step for skeletal muscle glucose uptake is transport from microcirculation to muscle interstitium. Capillary endothelium poses a barrier that delays the onset of muscle insulin action. Defining physiological barriers that control insulin access to interstitial space is difficult because of technical challenges that confront study of microscopic events in an integrated physiological system. Two physiological variables determine muscle insulin access. These are the number of perfused capillaries and the permeability of capillary walls to insulin. Disease states associated with capillary rarefaction are closely linked to insulin resistance. Insulin permeability through highly resistant capillary walls of muscle poses a significant barrier to insulin access. Insulin may traverse the endothelium through narrow intercellular junctions or vesicular trafficking across the endothelial cell. Insulin is large compared with intercellular junctions, making this an unlikely route. Transport by endothelial vesicular trafficking is likely the primary route of transit. Studies in vivo show movement of insulin is not insulin receptor dependent. This aligns with single-cell transcriptomics that show the insulin receptor is not expressed in muscle capillaries. Work in cultured endothelial cell lines suggest that insulin receptor activation is necessary for endothelial insulin transit. Controversies remain in the understanding of transendothelial insulin transit to muscle. These controversies closely align with experimental approaches. Control of circulating insulin accessibility to skeletal muscle is an area that remains ripe for discovery. Factors that impede insulin access to muscle may contribute to disease and factors that accelerate access may be of therapeutic value for insulin resistance.

Recent advances in understanding glucose transport and glucose disposal

Deficient glucose transport and glucose disposal are key pathologies leading to impaired glucose tolerance and risk of type 2 diabetes.  The cloning and identification of the family of facilitative glucose transporters have helped to identify that underlying mechanisms behind impaired glucose disposal, particularly in muscle and adipose tissue.  There is much more than just transporter protein concentration that is needed to regulate whole body glucose uptake and disposal.  The purpose of this review is to discuss recent findings in whole body glucose disposal.  We hypothesize that impaired glucose uptake and disposal is a consequence of mismatched energy input and energy output.  Decreasing the former while increasing the latter is key to normalizing glucose homeostasis.

Transendothelial Insulin Transport is Impaired in Skeletal Muscle Capillaries of Obese Male Mice

OBJECTIVE

The continuous endothelium of skeletal muscle (SkM) capillaries regulates insulin's access to skeletal myocytes. Whether impaired transendothelial insulin transport (EIT) contributes to SkM insulin resistance (IR), however, is unknown.

METHODS

Male and female C57/Bl6 mice were fed either chow or a high-fat diet for 16 weeks. Intravital microscopy was used to measure EIT in SkM capillaries, electron microscopy to assess endothelial ultrastructure, and glucose tracers to measure indices of glucose metabolism.

RESULTS

Diet-induced obesity (DIO) male mice were found to have a ~15% reduction in EIT compared with lean mice. Impaired EIT was associated with a 45% reduction in endothelial vesicles. Despite impaired EIT, hyperinsulinemia sustained delivery of insulin to the interstitial space in DIO male mice. Even with sustained interstitial insulin delivery, DIO male mice still showed SkM IR indicating severe myocellular IR in this model. Interestingly, there was no difference in EIT, endothelial ultrastructure, or SkM insulin sensitivity between lean female mice and female mice fed a high-fat diet.

CONCLUSIONS

These results suggest that, in male mice, obesity results in ultrastructural alterations to the capillary endothelium that delay EIT. Nonetheless, the myocyte appears to exceed the endothelium as a contributor to SkM IR in DIO male mice.

Perfusion controls muscle glucose uptake by altering the rate of glucose dispersion in vivo

These studies test, using intravital microscopy (IVM), the hypotheses that perfusion effects on insulin-stimulated muscle glucose uptake (MGU) are 1) capillary recruitment independent and 2) mediated through the dispersion of glucose rather than insulin. For experiment 1, capillary perfusion was visualized before and after intravenous insulin. No capillary recruitment was observed. For experiment 2, mice were treated with vasoactive compounds (sodium nitroprusside, hyaluronidase, and lipopolysaccharide), and dispersion of fluorophores approximating insulin size (10-kDa dextran) and glucose (2-NBDG) was measured using IVM. Subsequently, insulin and 2[¹⁴C]deoxyglucose were injected and muscle phospho-2[¹⁴C]deoxyglucose (2[C¹⁴]DG) accumulation was used as an index of MGU. Flow velocity and 2-NBDG dispersion, but not perfused surface area or 10-kDa dextran dispersion, predicted phospho-2[¹⁴C]DG accumulation. For experiment 3, microspheres of the same size and number as are used for contrast-enhanced ultrasound (CEU) studies of capillary recruitment were visualized using IVM. Due to their low concentration, microspheres were present in only a small fraction of blood-perfused capillaries. Microsphere-perfused blood volume correlated to flow velocity. These findings suggest that 1) flow velocity rather than capillary recruitment controls microvascular contributions to MGU, 2) glucose dispersion is more predictive of MGU than dispersion of insulin-sized molecules, and 3) CEU measures regional flow velocity rather than capillary recruitment.

The vascular endothelium plays a role in insulin action

The endocrine system relies on the vasculature for delivery of hormones throughout the body, and the capillary microvasculature is the site where the hormones cross from the blood into the target tissue. Once considered an inert wall, various studies have now highlighted the functions of the capillary endothelium to regulate transport and therefore affect or maintain the interstitial environment. The role of the capillary may be clear in areas where there is a continuous endothelium, yet there also appears to be a role of endothelial cells in tissues with a sinusoidal structure. Here we focused on the most common endocrine disorder, diabetes, and several of the target organs associated with the disease, including skeletal muscle, liver and pancreas. However, it is important to note that the ability of hormones to cross the endothelium to reach their target tissue is a component of all endocrine functions. It is also a consideration in organs throughout the body and may have greater impact for larger hormones with target tissues containing a continuous endothelium. We noted that the blood levels do not always equal interstitial levels, which is what the cells are exposed to, and discussed how this may change in diseases such as obesity and insulin resistance. The capillary endothelium is, therefore, an essential and understudied aspect of endocrinology and metabolism that can be altered in disease, which may be an appropriate target for treatment.