1‐O‐hexadecyl‐2‐acetyl‐sn‐glycero‐3‐phospho (N,N,N trimethyl) hexanolamine: an analogue of platelet‐activating factor with partial agonist activity

Abstract

During studies of the role of platelet‐activating factor (PAF) in macrophage superoxide anion generation (O2−1), we identified an agonist action of the putative PAF receptor antagonist 1‐O‐hexadecyl‐2‐acetyl‐sn‐glycero‐3‐phospho (N,N,N‐trimethyl) hexanolamine (hexanolamine PAF) in guinea‐pig macrophages. The 1‐O‐octadecyl form of this compound has specific antagonist actions at PAF receptors. The agonist properties of hexanolamine PAF were examined in rabbit washed platelets (aggregation) and in guinea‐pig peritoneal macrophages (O2−generation). Hexanolamine PAF induced significant platelet aggregation (50% of the PAF maximum). However, the omission of bovine serum albumin (BSA) from the Tyrode buffer resulted in a diminution of the response of washed platelets during storage for 24 h at 4°C (7% of PAF maximum), whereas the maximum response to PAF was unaffected by storage for this period, irrespective of the presence of BSA. Platelet aggregation induced by hexanolamine PAF was not accompanied by a detectable increase in intracellular calcium [Ca2+]i, whereas the aggregation response to PAF was preceded by a large rise in [Ca2+]i. Hexanolamine PAF induced O2−generation in adherent macrophages, with a maximum response 45% of that to PAF. Hexanolamine PAF (100 nm), at a concentration equi‐effective with PAF (1 nm) for stimulation of O2−generation in macrophages, induced an increase in [Ca2+]i which was significantly less than that induced by PAF. PAF concentration‐response curves were constructed in platelets or macrophages following pretreatment with hexanolamine PAF (0.1 and 1 μm). The interaction between PAF and the putative partial agonist (hexanolamine PAF) had the characteristics expected of a partial agonist interacting with a full agonist. Platelet aggregation induced by hexanolamine PAF was antagonized non‐competitively by the PAF receptor antagonist, WEB 2086, whereas antagonism of PAF‐induced aggregation by WEB 2086 was competitive. Macrophage O2−generation induced by hexanolamine PAF or PAF was antagonized by WEB 2086. These data indicate that hexanolamine PAF is a partial agonist at PAF receptors in macrophages and platelets. The inability of hexanolamine PAF to increase [Ca2+]i in platelets suggests that PAF receptors may be coupled to platelet aggregation by both Ca2+‐dependent and ‐independent pathways. 1991 British Pharmacological Society