G protein-coupled receptors mediate coronary flow- and agonist-induced responses via lectin-oligosaccharide interactions.

Blood flow acts parallel to the coronary luminal endothelial surface layer (LESL) and modulates multiple parenchymal functions via the release of paracrine agents. Evidence indicates that the LESL may be a flow-sensing organelle and that perhaps through flow-induced lectin (L)·oligosaccharide (O) complex formation (L·O) participates in this process. LESL integrins and selectins are both lectinic and flow sensitive, but the L properties of flow-sensitive G protein-coupled receptors (GPCRs) are unknown. Therefore, we investigated the presence of L in the LESL and hypothesized that if flow-sensitive GPCRs are L, flow and O will determine their response to receptor activation. The LESL protein fraction isolated from guinea pig hearts was passed through an affinity chromatography column made of three sugars, mannose, galactose, and N-acetylglucosamine, and the lectinic fraction was eluted. Immune dot blot was used to identify L proteins in the LESL fraction. Our results indicate the following. 1) Two-dimensional SDS-PAGE (2D-SDS-PAGE) of the LESL lectinic fraction revealed at least 167 Ls. 2) Among these Ls, we identified three selectins and the GPCRs: angiotensin II, bradykinin (B2-R), adenosine A1 and A2, prolactin, endothelin, α1-adrenergic (α1A-R), thromboxane A2, ß1-adrenergic, ß3-adrenergic, and insulin receptors; the first six GPCRs are known to be flow sensitive. 3) The amplitude of receptor-induced vascular responses by α1A-R and B2-R activation (phenylephrine or bradykinin, respectively) was a function of flow and O (hyaluronidate). Our results support a novel mechanism of GPCR-mediated responses to flow via L·O interaction.

Cardiac ischemia and ischemia/reperfusion cause wide proteolysis of the coronary endothelial luminal membrane: possible dysfunctions.

BACKGROUND: Ischemia and ischemia-reperfusion (I/R) are common clinical insults that disrupt the molecular structure of coronary vascular endothelial luminal membrane (VELM) that result in diverse microvasculature dysfunctions. However, the knowledge of the associated biochemical changes is meager. We hypothesized that ischemia and I/R-induced structural and functional VELM alterations result from biochemical changes. First, these changes need to be described and later the mechanisms behind be identified. METHODS: During control conditions, in isolated perfused rat hearts VELM proteins were labeled with biotin. The groups of hearts were: control (C), no flow ischemia (I; 25 min), and I/R (I; 25 min, reperfusion 30 min). The biotinylated luminal endothelial membrane proteins in these three different groups were examined by 2-D electrophoresis and identified. But, it must be kept in mind the proteins were biotin-labeled during control. RESULTS: A comparative analysis of the protein profiles under the 3 conditions following 2D gel electrophoresis showed differences in the molecular weight distribution such that MW(C) > MW(I) > MW(I/R). Similar analysis for isoelectric points (pH(i)) showed a shift toward more acidic pHi under ischemic conditions. Of 100 % proteins identified during control 66% and 88% changed their MW-pH(i) during ischemia and I/R respectively. Among these lost proteins there were 9 proteins identified as adhesins and G-protein coupled receptors. GENERAL SIGNIFICANCE: I and I/R insults alter MW-pH(i) of most luminal glycocalyx proteins due to the activation of nonspecific hydrolizing mechanisms; suspect metalloproteases and glycanases. This makes necessary the identification of hydrolyzing enzymes reponsible of multiple microvascular dysfunctions in order to maintain the integrity of vascular endothelial membrane. VELM must become a target of future therapeutics.

Luminal endothelial lectins with affinity for N-acetylglucosamine determine flow-induced cardiac and vascular paracrine-dependent responses.

Coronary blood flow applied to the endothelial lumen modulates parenchymal functions via paracrine effectors, but the mechanism of flow sensation is unknown. We and others have demonstrated that coronary endothelial luminal membrane (CELM) oligosaccharides and lectins are involved in flow detection, and we proposed that cardiac effects of coronary flow result from a reversible flow-modulated lectin-oligosaccharide interaction. Recently, glycosylated and amiloride-sensitive Na(+)/Ca(++) channels (ENaCs) have been proposed to be involved in the flow-induced endothelial responses. Because N-acetylglucosamine (GlcNac) is one of the main components of glycocalyx oligosaccharides (i.e., hyaluronan [-4GlcUAbeta1-3GlcNAcbeta1-](n)), the aim of this article is to isolate and define CELM GlcNac-binding lectins and determine their role in cardiac and vascular flow-induced effects. For this purpose, we synthesized a 460-kDa GlcNac polymer (GlcNac-Pol) with high affinity toward GlcNac-recognizing lectins. In the heart, intracoronary administration of GlcNac-Pol upon binding to CELM diminishes the flow-dependent positive inotropic and dromotropic effects. Furthermore, GlcNac-Pol was used as an affinity probe to isolate CELM GlcNac-Pol-recognizing lectins and at least 35 individual lectinic peptides were identified, one of them the beta-ENaC channel. Some of these lectins could participate in flow sensing and in GlcNac-Pol-induced effects. We also adopted a flow-responsive and well-accepted model of endothelial-parenchymal paracrine interaction: isolated blood vessels perfused at controlled flow rates. We established that flow-induced vasodilatation (FIV) is blocked by endothelial luminal membrane (ELM) bound GlcNac-Pol, nitro-l-arginine methyl ester and indomethacin, amiloride, and hyaluronidase. The effect of hyaluronidase was reversed by infusion of soluble hyaluronan. These results indicate that GlcNac-Pol inhibits FIV by competing and displacing intrinsic hyaluronan bound to a lectinic structure such as the amiloride-sensitive ENaC. Nitric oxide and prostaglandins are the putative paracrine mediators of FIV.