Abstract
It has been demonstrated that insulin-mediated recruitment of microvascular blood volume is associated with insulin sensitivity. We hypothesize that insulin rapidly stimulates penetration of red blood cells (RBC) and plasma into the glycocalyx and thereby promotes insulin-mediated glucose uptake by increasing intracapillary blood volume. Experiments were performed in rats; the role of the glycocalyx was assessed by enzymatic degradation using a bolus of hyaluronidase. First, the effect of insulin on glycocalyx accessibility was assessed by measuring the depth of penetration of RBCs into the glycocalyx in microvessels of the gastrocnemius muscle with Sidestream Dark-field imaging. Secondly, peripheral insulin sensitivity was determined using intravenous insulin tolerance tests (IVITT). In addition, in a smaller set of experiments, intravital microscopy of capillary hemodynamics in cremaster muscle and histological analysis of the distribution of fluorescently labeled 40 kDa dextrans (D40) in hindlimb muscle was used to evaluate insulin-mediated increases in capillary blood volume. Insulin increased glycocalyx penetration of RBCs by 0.34±0.44 µm (P<0.05) within 10 minutes, and this effect of insulin was greatly impaired in hyaluronidase treated rats. Further, hyaluronidase treated rats showed a 35±25% reduction in whole-body insulin-mediated glucose disposal compared to control rats. Insulin-mediated increases in capillary blood volume were reflected by a rapid increase in capillary tube hematocrit from 21.1±10.1% to 29.0±9.8% (P<0.05), and an increase in D40 intensity in individual capillaries of 134±138% compared to baseline at the end of the IVITT. These effects of insulin were virtually abolished in hyaluronidase treated animals. In conclusion, insulin rapidly increases glycocalyx accessibility for circulating blood in muscle, and this is associated with an increased blood volume in individual capillaries. Hyaluronidase treatment of the glycocalyx abolishes the effects of insulin on capillary blood volume and impairs insulin-mediated glucose disposal.
Highlights
After insulin has been secreted by the pancreas into the bloodstream, it is distributed by the microcirculation towards the capillaries where it has to cross the endothelium to bind to insulin receptors on target tissues, such as skeletal muscle
Dynamic variations in red blood cell (RBC) column width were measured in each visible useful microvessel and the position of the RBCglycocalyx interface was used to monitor the level of penetration of red blood cells (RBC) into the glycocalyx (Fig. 1)
Subsequent insulin infusion resulted in an increase in outward movement of the RBC-glycocalyx interface of 0.3460.44 mm at both sides within 10 minutes (p,0.05; Fig. 1C, right panel), while there was no significant effect of insulin on RBC-glycocalyx interface when tested for the entire 30 minutes infusion period
Summary
After insulin has been secreted by the pancreas into the bloodstream, it is distributed by the microcirculation towards the capillaries where it has to cross the endothelium to bind to insulin receptors on target tissues, such as skeletal muscle. Insulin has been indicated to rapidly (within 10–15 minutes) recruit capillary blood volume [4,5], an effect which occurs before an increase of total blood flow, and which appears to be important for insulin’s metabolic actions [5]. The endothelial glycocalyx is the 0.5–1.0 mm thick gel-like layer on the luminal side of the vascular endothelial cells It consists of a mesh of polysaccharide structures and absorbed plasma proteins and water [11,12], and has the last decade been indicated to have an important role in protection of the vessel wall, as well as in regulation of microvascular perfusion and nutrient exchange [9,13]. Intravital microscopy studies in rodent cremaster muscle indicate that under control conditions the glycocalyx is to a large extent inaccessible for red blood cells (RBCs) and plasma macromolecules [14,15], but that its accessibility can be rapidly increased in the presence of the agonists
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