LYSOPHOSPHATIDYLCHOLINE INHIBITS RELAXATION OF RABBIT ABDOMINAL-AORTA MEDIATED BY ENDOTHELIUM-DERIVED NITRIC-OXIDE AND ENDOTHELIUM-DERIVED HYPERPOLARIZING FACTOR-INDEPENDENT OF PROTEIN-KINASE-C ACTIVATION

Hypercholesterolemia is associated with increased oxidized LDL and impaired endothelium-dependent relaxation (EDR). An inhibitory component of oxidized LDL is lysophosphatidylcholine (LPC). To determine the effect and mechanism(s) of action of LPC on EDR mediated by endothelium-derived nitric oxide (EDNO) and endothelium-derived hyperpolarizing factor (EDHF), rabbit abdominal aortic rings were suspended for measurement of isometric tension and studied under three conditions: control; with 25 mmol/L K+ buffer to isolate relaxation mediated by EDNO; and in rings treated with N-omega-nitro-L-arginine methyl ester (L-NAME, 30 mu mol/L) to isolate relaxation mediated by EDHF. Incubation with LPC (10 and 30 mu mol/L) for 30 minutes inhibited EDR in a concentration-dependent manner. LPC (30 mu mol\L) significantly inhibited maximal relaxation to acetylcholine in control, 25 mmol/L K+-, and L-NAME-treated rings (77.1+/-7.8%, 42.1+/-8.9%, and 3.4+/-7.7%) compared with untreated rings (99.0+/-0.9%, 90.9+/-2.2%, and 54.7+/-4.7%, P<.05). Inhibition of relaxation was specific to endothelium-dependent responses in that relaxation to direct smooth muscle vasodilators (papaverine, 8-bromo-cGMP, and sodium nitroprusside) were unaltered by LPC. The inhibition by LPC (30 mu mol/L) was not due to cytotoxicity, because EDR returned to normal levels after repeated washing with physiological salt solution containing 0.1% albumin. Coincubation with protein kinase C inhibitors, staurosporine (20 nmol/L) or calphostin C (1 mu mol/L), had no effect on the EDR inhibition by LPC (30 mu mol/L). Furthermore, LPC continued to inhibit EDR in rings in which protein kinase C was downregulated by incubation for 18 hours with 1 mu mol/L phorbol 12-myristate 13-acetate (PMA). The inhibition of EDR to the receptor-independent agonist A23187 by LPC (30 mu mol/L) but not by PMA (30 nmol/L) further supports a lack of effect of LPC on protein kinase C. Thus, the inhibitory effect of LPC on EDR is not limited to EDNO but also inhibits relaxation mediated by EDHF. Also, the inhibition of relaxation to EDNO and EDHF is not mediated by activation of protein kinase C.