Insulin regulates its own delivery to skeletal muscle by feed-forward actions on the vasculature

Abstract

Insulin, at physiological concentrations, regulates the volume of microvasculature perfused within skeletal and cardiac muscle. It can also, by relaxing the larger resistance vessels, increase total muscle blood flow. Both of these effects require endothelial cell nitric oxide generation and smooth muscle cell relaxation, and each could increase delivery of insulin and nutrients to muscle. The capillary microvasculature possesses the greatest endothelial surface area of the body. Yet, whether insulin acts on the capillary endothelial cell is not known. Here, we review insulin's actions at each of three levels of the arterial vasculature as well as recent data suggesting that insulin can regulate a vesicular transport system within the endothelial cell. This latter action, if it occurs at the capillary level, could enhance insulin delivery to muscle interstitium and thereby complement insulin's actions on arteriolar endothelium to increase insulin delivery. We also review work that suggests that this action of insulin on vesicle transport depends on endothelial cell nitric oxide generation and that insulin's ability to regulate this vesicular transport system is impaired by inflammatory cytokines that provoke insulin resistance.

Figures

Fig. 1.

Rate of blood flow and type of endothelium in several tissues involved in insulin secretion/action in the postprandial setting.

Fig. 2.

Insulin action at each level of the arterial tree and how this action is impacted adversely by insulin resistance with clinical consequences.

Fig. 3.

Endothelial cell stained for cavelolin-1 (left), the insulin receptor (middle), and the overlay (right).

Fig. 4.

Working model for transendothelial insulin transport.