© 1997 by European Society of Cardiology
Copyright © 1997, European Society of Cardiology
Non-specific inhibition by human lipoproteins of endothelium dependent relaxation in rat aorta may be attributed to lipoprotein phospholipids
Alfred and Baker Medical Unit, Baker Medical Research Institute and Alfred Hospital, Commercial Road, Prahran, Vic. 3181, Australia
* Corresponding author. Tel.: +61 (3) 9276 3320; fax: +61 (3) 9276 3488.
Objective: In vitro incubation of low-density lipoprotein (LDL) is reported to attenuate endothelium dependent relaxation mediated by acetylcholine (ACh) while not affecting endothelium-independent relaxation. This study was designed to examine the effects of other lipid-carrying lipoproteins as well as to study their effects on responses mediated by endothelium dependent agonists other than ACh. Methods: The effects of human LDL, very-low-density lipoprotein (VLDL) and high density lipoprotein (HDL) on endothelium-dependent relaxation by ACh, histamine and the calcium ionophore, A23187 [GenBank] , and endothelium-independent relaxation by sodium nitroprusside (SNP) were investigated in rat isolated aortic rings. The effects of combined LDL and HDL incubation on responses mediated by ACh were also examined. Control experiments included experiments examining the effects of bovine serum albumin on responses mediated by ACh. Thiobarbituric-acid-reactive substances (TBARS) measured before and after organ bath incubation indicated little oxidation of the lipoproteins used.Results: Maximal responses to ACh were inhibited by LDL, VLDL and HDL (0.02 and 0.2 mg protein/ml), to histamine by LDL (0.2 mg protein/ml), VLDL (0.02 and 0.2 mg protein/ml) and HDL (0.02 and 0.2 mg protein/ml) and to A23187 [GenBank] by LDL (0.2 mg protein/ml), VLDL (0.2 mg protein/ml) and HDL (0.02 and 0.2 mg protein/ml). A small but significant correlation (r=0.54, P=0.01) was observed between the level of inhibition of the endothelium-dependent responses and lipoprotein phospholipid concentration in the organ bath but not between the level of inhibition and cholesterol (free and esterified) or triglyceride concentrations. Responses to SNP were unaffected by LDL, VLDL and HDL. Combined incubation of tissues with LDL (0.2 mg protein/ml) and HDL (0.2 mg protein/ml) significantly increased maximal responses to ACh (pre-lipoproteins 81.8±5.7 vs plus-LDL/HDL 100±0.0; P<0.05). Bovine serum albumin had no effect on the maximal responses to ACh. Conclusions: We conclude that inhibition by human lipoproteins of endothelium-dependent agonists occurs with LDL, HDL and VLDL and suggest that this may be due to the phospholipid content of each lipoprotein. However, combined incubation of HDL with LDL negates this effect and an increased maximal response to ACh is reported.
KEYWORDS Vascular reactivity; Nitric oxide; Human, lipoproteins; Phospholipids; Aorta; Rat
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