Adenosine A ₃ Receptor Activation Modulates the Capillary Endothelial Glycocalyx

Abstract

Abstract-The endothelial glycocalyx is a dynamic extracellular matrix composed of cell surface proteoglycans, glycoproteins, and adsorbed serum proteins that has been implicated in the regulation and modulation of capillary tube hematocrit, permeability, and hemostasis. High tissue adenosine levels have been shown to adversely affect microvascular function and tissue survival after an ischemic episode, and previous work in this laboratory has shown that adenosine causes arteriolar constriction and degranulation of mast cells via the A 3 receptor (A3AR). We hypothesized that adenosine exerts at least part of its effect through modification of the glycocalyx via the A3AR. We used an in vivo cremaster model (hamster and mouse) in which circulating plasma was labeled with a 70-kDa FITC-dextran, and the capillaries were examined before and after superfusion with varying concentrations of adenosine (or other vasoactive molecules). Measurements of the dextran exclusion from an endothelial cell surface layer and red cell separation from the endothelial cell surface were made for up to 30 minutes. Our data indicate that adenosine causes a rapid and profound decrease in the ability of the glycocalyx to exclude dextran but only affects red blood cell exclusion at pharmacological levels. Knockout mice deficient in the A3AR were completely protected from glycocalyx changes attributable to adenosine. These data show a potential link between a known vasoactive tissue metabolite, adenosine, and regulation of the glycocalyx, which may be important during (patho)physiological changes in microvascular function during inflammatory insults. 2 In addition to its well-known properties as a vasodilator, adenosine is intimately involved in a variety of other inflammatory reactions, with both proinflammatory 3 and antiinflammatory 4 roles. Previous work from this laboratory has shown that adenosine can, on the one hand, inhibit mast cell degranulation at low concentrations by activating the A 2A receptor 5 and, on the other hand, cause mast cell degranulation via the A 3 receptor. 6 Metabolites of adenosine may also play a key role in the inflammatory response, in that inosine accumulates in tissues in even greater concentrations than adenosine under inflammatory conditions 2 and is a highly selective agonist for A 3 receptors (A3AR). The foregoing suggests that the A3AR may be important in mediating pathophysiological changes in the vasculature under circumstances that increase tissue adenosine content.Recent evidence has shown that ischemia/reperfusion injury, a common model of injury and inflammation, damages the endothelial cell glycocalyx and that this might contribute to the pathophysiological changes during reperfusion. 7 The endothelial cell glycocalyx is an as-yet poorly understood matrix that lines the luminal surface of all blood vessels. This matrix is composed of glycosaminoglycans, proteoglycans, and glycoproteins originating from both the endothelial cells and adsorbed from circulating plasma. These molecules form a surfa...

Hyaluronidase treatment of coronary glycocalyx increases reactive hyperemia but not adenosine hyperemia in dog hearts

“…The effect of adenosine on the glycocalyx does not necessarily have to reflect a true physical degradation of this layer. Rather, an increase in glycocalyx porosity might be involved because the decrease in exclusion of dextrans was found to be much more profound, and to occur at lower concentrations, than that of red blood cells (21). In a previous study of the Duling group, systemic administration of hyaluronidase was equally found to increase glycocalyx porosity independently from a change in the physical dimensions of the layer as judged from the exclusion of red blood cells (6).…”

“…These results might be explained by an increased accessibility of the glycocalyx by circulating blood as suggested by intravital microscopic observations of an impaired ability of the glycocalyx to exclude 70-kDa FITC-dextrans and red blood cells during adenosine superfusion in cremaster capillaries (21). The effect of adenosine on the glycocalyx does not necessarily have to reflect a true physical degradation of this layer.…”

“…Pries et al (1997) calculated that effective vessel diameter available for red blood cell and plasma motion had increased by at least 0.35-0.55 m after a single ϳ10 min infusion of heparinase in the rat mesentery bed. Indeed, a glycocalyx with these dimensions is typically reported in capillaries of cremaster tissue (6,21,30,31), and has been suggested to be present in myocardial capillaries as well (26). Pries et al (22) found a 14 -21% decrease in microvascular flow resistance after enzyme treatment.…”

“…Clinical decision-making to intervene is based on intracoronary parameters that are derived during hyperemia, and it is therefore critical that the adenosine-induced hyperemia accurately reflects the coronary hyperemia that can be attained by endogenous stimuli (7). Intravital microscopic studies of cremaster tissue have suggested that adenosine affects capillary perfusion by limiting the effect of the glycocalyx on vascular resistance (3,12,21). The glycocalyx is the carbohydrate-rich matrix that lines the luminal surface of blood vessels and forms the true interface between the endothelium and flowing blood (6,24,26,30).…”

“…Normally, flowing red blood cells and large dextrans are excluded from the glycocalyx in cremaster capillaries (6,30,31). However, it was shown that adenosine superfusion impaired glycocalyx exclusion properties (21). Because an intact glycocalyx strongly reduces the volume available for flow of plasma and red blood cells in capillaries (3,22,30), an impaired glycocalyx exclusion during adenosine is consistent with an increase in functionally perfused capillary volume, and as a result, elevated capillary tube hematocrit (12).…”