In today's society, food availability grossly exceeds our body's caloric demands. Excessive caloric intake causes weight gain and induces insulin resistance, a common characteristic of obesity and major risk factor for type 2 diabetes (T2DM) and cardiovascular disease.
The primary targets of insulin action are skeletal muscle, adipose tissue and the liver, but recent data point to the vascular endothelium as an important target. Insulin directly targets the endothelial cell where it activates phosphoinositide 3-kinase, resulting in Akt-mediated phosphorylation of endothelial nitric oxide synthase (eNOS). This leads to NO production - a potent vasodilator in the human body. Simultaneously insulin also activates the mitogen-activated protein kinase pathway in endothelial cells, which enhances the generation of the vasoconstrictor endothelin-1 via extracellular signal-regulated kinases 1/2 signaling. Via these two pathways insulin can regulate vascular tone.
In healthy individuals, insulin signaling in the endothelial cell leads to capillary recruitment in skeletal muscle tissue via vasodilatation of terminal arterioles. It has been proposed that insulin in this matter regulates the delivery of insulin and glucose to skeletal muscle by increasing endothelial surface area. In obese individuals and patients with T2DM, insulin-mediated capillary recruitment in skeletal muscle tissue is impaired and insulin-dependent glucose uptake is diminished. Whether these two processes are linked or occur in parallel remains unknown.
Interestingly, studies in rodents demonstrated that during obesity induced by high fat feeding, insulin resistance develops in the vasculature before these responses are detected in muscle, liver, or adipose tissue. Therefore, insulin signaling in endothelium might change in response to a positive energy balance to prevent nutrient overload in muscle and optimize nutrient storage in adipose tissue. Conversely, it has been hypothesized that early reversal of endothelial insulin resistance could prevent peripheral insulin resistance, assuming a cause-and-effect relationship between these processes. The most compelling evidence for this hypothesis came from studies in endothelial cell specific insulin receptor substrate-2 (IRS-2) knock-out mice. Kubota et al. demonstrated that impaired insulin signaling in endothelial cells, due to reduced IRS-2 expression and insulin-induced eNOS phosphorylation, caused attenuation of insulin-induced capillary recruitment and insulin delivery, which reduced glucose uptake by skeletal muscle. Moreover, restoration of insulin-induced eNOS phosphorylation in endothelial cells by infusion of beraprost sodium - a stable prostaglandin analogue - completely reversed the reduction in capillary recruitment and insulin delivery in tissue-specific knockout mice lacking IRS-2 in endothelial cells and fed a high-fat diet. As a result, glucose uptake by skeletal muscle was restored in these mice.
These data suggest that pharmacological stimulation of tissue perfusion may hold promise as a therapeutic strategy to increase whole body glucose disposal and thus prevent or reduce hyperglycaemia. In humans however, data linking improvement of capillary recruitment by pharmacological agents to restoration of whole-body glucose uptake are lacking. Low dose iloprost infusion - another stable prostaglandin analogue - has been shown to improve insulin-stimulated whole-body glucose uptake, but the mechanistic role of microvascular response was not assessed. Overall, it remains to be demonstrated whether improving capillary recruitment by endothelial insulin signaling or direct stimulation of smooth muscle tissue may serve as an attractive preventive or therapeutic approach to bypass cellular resistance to glucose disposal.
In conclusion, vascular insulin resistance leads to blunted capillary recruitment in the skeletal muscle and may lead to diminished glucose uptake due to a decreased capillary surface area for nutrient exchange. Up till now however it remains unclear if these processes are interrelated or occur in parallel. Evidence from animal studies suggest that vascular insulin resistance precedes diminished whole-body glucose uptake and myocellular impairments. This indicates a potential cause-effect relationship. In humans, however, this was never demonstrated. On the other hand, decreased capillary recruitment of skeletal muscle tissue could also protect muscle tissue from nutrient overload and shunt excess calories towards adipose tissue. Presently, it is unknown whether insulin redistributes blood flow from skeletal muscle to adipose tissue during hypercaloric conditions. Finally, it is unknown if stimulation of tissue perfusion with a pharmacological agent can restore whole-body glucose uptake is therefore an effective strategy in prevention or treatment of insulin resistance.
