Toll like receptor 4 is essential for a functional endothelium in mesenteric arteries.

It is well established that Toll like receptors (TLR) are important sensing systems for the innate immunity. The detection of either Gram positive or Gram negative bacteria are determined by TLR2 and TLR4 respectively. Activation of TLRs by bacterial components reduces the ability of vessels to contract, which contributes to hypotension associated with sepsis. However, TLR4 may also have a physiological role in the endothelium, as recent data from our lab demonstrated that the ability of acetylcholine to induce endothelium dependent vasodilatation is impaired in vessels from TLR4-/- mice. Endothelium dependent vasodilatation is mediated by the release of nitric oxide and Endothelium Derived Hyperpolarising Factor (EDHF), here we investigated whether TLR4 or TLR2 deficiency influences these pathways.
First order mesenteric arteries from either wild type TLR2-/-or TLR4-/- mice were removed and mounted and normalised in isometric wire myographs, force was measured using a computerised system. Concentration response curves to U46619 (3x10-9-3x10-7M) were performed on each of the tissues. Wild type and TLR4-/- arteries were pre-incubated with L-NAME plus indomethacin or charybdotoxin plus apamin to inhibit vasodilation via the release of nitric oxide and prostacyclins or EDHF pathways respectively. Arteries were then pre-contracted with an EC80 concentration of U46619 and vasodilatation responses to acetylcholine (3x10-8-10-4M) assessed. Arteries were fixed with 4% paraformaldehyde, stained by DAPI, and viewed whole with a confocal microscope.

Contraction induced by U46619 was similar between wild type, TLR2-/- and TLR4-/- mice. Acetylcholine induced vasodilation was reduced in vessels from TLR4-/-, however, the residual vasodilator response was equally inhibited by L-NAME plus indomethacin, and charybdotoxin plus apamin. A complete endothelial cell layer was shown by confocal imaging stained with DAPI.
This data suggests that TLR4 is important for a functional endothelium in resistance arteries and identifies a novel role for this receptor in physiological health of the cardiovascular system. The link between these observations and innate immunity remains to be established.