Acetal phosphatidic acids: novel platelet aggregating agents

1 Palmitaldehyde, olealdehyde and linolealdehyde acetal phosphatidic acids induced rapid shape change and dose-dependent biphasic aggregation of human platelets in platelet-rich plasma; aggregation was reversible at low doses and irreversible at high doses of the acetal phosphatidic acids. The palmitaldehyde congener elicited monophasic dose-dependent aggregation of sheep platelets in platelet-rich plasma.

2 The threshold concentration for palmitaldehyde acetal phosphatidic acid (PGAP)-induced platelet aggregation was 2.5-5 μM for human platelets and 0.25-0.5 μM for sheep platelets. PGAP was 4-5 times as potent versus human platelets as the olealdehyde and linolealdehyde acetal phosphatidic acids, which were equipotent.

3 PGAP-induced irreversible aggregation of ¹⁴C]-5-hydroxytryptamine (¹⁴C]-5-HT)-labelled human platelets in platelet-rich plasma was accompanied by release of 44.0±2.4% (s.e.) of the platelet ¹⁴C]-5-HT; reversible aggregation was not associated with release. In contrast, PGAP-induced release of ¹⁴C]-5-HT-labelled sheep platelets was dose-dependent.

4 The adenosine diphosphate (ADP) antagonist, 2-methylthio-AMP, and the cyclo-oxygenase inhibitor, aspirin, abolished PGAP-induced second phase aggregation and release in human platelets but did not affect the first, reversible, phase of aggregation. Both the first and second phases of PGAP-induced aggregation were abolished by chlorpromazine, by the phospholipase A₂ inhibitor, mepacrine, and by nmolar concentrations of prostaglandin E₁ (PGE₁); these agents abolished the second, but not the first phase of ADP-induced aggregation.

5 The related phospholipids, lecithin, lysolecithin and phosphatidic acid, at <100 μM, neither induced aggregation of human platelets in platelet-rich plasma, nor modified PGAP-induced aggregation; 1-palmityl lysophosphatidic acid elicited aggregation of human platelets at a threshold concentration of 100 μM.

6 It is concluded that the acetal phosphatidic acids induce platelet aggregation per se by direct action at the platelet membrane, and that the acetal function is of primary importance in their potent platelet-stimulating activity. Moreover, as the acetal phosphatidic acids are the major components of the smooth muscle-contracting acidic phospholipid tissue extract `Darmstoff' (Vogt, 1949), their potent platelet-aggregating properties may be of physiological or pathological significance.