ISOLATION AND IDENTIFICATION OF LYSOPHOSPHATIDYLCHOLINES AS ENDOGENOUS MODULATORS OF THROMBOXANE RECEPTORS

Inhibition of thromboxane receptor radioligand binding to human platel et membranes has been employed as the basis for a radioreceptor assay designed to measure thromboxane receptor binding activity in samples o f biological fluids. This method was used during phase 1 clinical eval uation of the thromboxane receptor antagonist SQ 30,741. Frequently, b aseline plasma samples as well as plasma samples from placebo-treated subjects showed significant inhibition of radioligand binding in the r adioreceptor assay, suggesting the presence of endogenous thromboxane receptor ligands. This receptor binding activity was stable and could be monitored in blood from normal volunteers using a modification of t he radioreceptor assay. In order to identify the substance responsible for the observed activity, the activity present in pooled bovine bloo d was isolated and evaluated by a combination of FAB/MS, H-1-NMR, C-13 -NMR and co-injection with reference standards on HPLC. Several endoge nous thromboxane receptor ligands were identified as L-alpha-lysophosp hatidylcholine (LPC) species. One major species, palmitoyl-LPC. contra cted isolated rat aortic spirals, and these contractions could be dela yed or prevented, but not reversed by the thromboxane receptor antagon ist SQ 29,548. Palmitoyl-LPC slightly potentiated aortic contractions induced by the thromboxane receptor agonist, U-46,619, and diminished in a concentration-dependent manner the antagonism by SQ 29,548 of con tractile responses to U-46,619. These findings are consistent with a p otential for LPC species to bind and activate thromboxane receptors.