inhibited; PA F, platelet-activating factor. Aggregating agents. Adenosine. Arachidonic. 5'-diphosphate acid. PA F. Quinidine. N.I.. N.I.. 7 0 k 4. Disopyramide. N.I..
002
BIOCHEMICAL SOCIETY TRANSACTIONS
Selective inhibition of platelet-activatingfactor by antiarrhythmic drugs in human platelets ANWAR H. GlLANl and SHEIKH A. SAEED Departmerit of f’han?iucokogy, The Ago Khun Uiiiver~sity, Facirlty of Heulth Sciences, Karachi, Pakistan Platelet aggregation is believed to play an important role in the formation of microthrombi and vascular inflammation. It has been demonstrated that several non-steroidal antiinflammatory drugs (”inhibit I) platelet aggregation and this action may contribute to their anti-inflammatory action. On the other hand, platelet cyclo-oxygenase has distinctly different susceptibility to NSID than cyclo-oxygenase from other tissue [ I ] .This opens the possibility for development of a selective inhibitor of fatty acid cyclo-oxygenase in platelets which would have anti-thrombic activity iri vivo. This proposition is complicated, however, by the fact that platelet aggregation can be induced by several physiologically important endogenous substances such as adenosine 5’-diphosphate, arachidonic acid and platelet-activating factor 121. Release of these substances as a result of damage to the platelet membrane, caused by various stimuli, activates platelet aggregation. Intravenous administration of platelet-activating factor in laboratory animals produces cardiac disturbances, contractility impairment and alterations in the electrocardiogram [3]. Antiarrhythmic drugs are a class of compounds known to possess membrane-stabilizing properties and are widely used for the treatment of cardiac arrhythmias. In the present study, we present evidence that antiarrhythmic drugs selectively inhibit aggregation induced by platelet-activating factor. Blood was obtained from healthy volunteers who had not taken any drug for at least 10 days. Blood was drawn from the antecubital vein into siliconized glass tubes containing 3.8% (w/v)trisodium citrate (one-tenth of the blood volume). Platelet-rich plasma was obtained after centrifugation at 260 g for 15 min. The remaining blood sample was centrifuged at 1200 g for 10 min to obtain platelet-poor plasma. The platelet count was determined by phase-contrast microscopy and all aggregation studies were carried out at 37°C with platelet-rich plasma having platelet counts between 2.5 and 3.00 X 10’/ml of plasma. Aggregation was monitored using a Dual Channel Lumi-Aggregometer (Model 400, Chronolog Corporation, Chicago, U.S.A.) using 0.45 ml aliquots of platelet-rich plasma. The final volume was made up to 0.5 ml with sodium chloride (0.9’/0, w/v) or test drug and incubated for 1 min before challenge with the aggregating agent. Aggregation was induced with adenosine 5’-diphosphate (2.2 p ) arachidonic , acid (1.7 mM) or platletactivating factor (0.8 PM). The resulting aggregation was recorded with a Lumi-Aggregometer and expressed as percentage inhibition compared with control at 4 min after challenge. All drugs were tested at three to four concentrations in duplicate. Statistical differences between control and drug-
Table I . C’ompurari~~e effecrs of’ rrnti-rrrrh?lhriiic. drugs plareler rrggregurion indiiced by vurioiis uggreguring ugents
IC,,, values of various antiarrhythmic drugs are shown a s means k S.E.M. ( p ~ of) three to five determination. N.I.. not inhibited; PA F, platelet-activating factor.
Aggregating agents Adenosine
5’-diphosphate Quinidine
N.I. N.I. N.I. N.I.
Disopyramide Lignocaine
Procainamide Propanolol
91 * 6
Phenytoin
N.I.
Arachidonic acid
PA F
N.I. N.I. N.I. N.I. 290 10 N.I.
70k4 I70 k 8 200 k 6 I so 8 0s ks N.I.
*
*
treated platelet preparations were determined by Student‘s Itest. Table 1 summarizes the effects of various antiarrhythmic drugs on platelet aggregation induced by different aggrcgating agents. Propranolol, quinidine, procainamide, disopyramide and lignocaine, in decreasing order of potency, inhibited platelet-activating-factor-induced aggregation with IC5,,(concentration required to inhibit by SO%) values of 65. 70, 150, and 200 ,UM respectively. None of these drugs. except propranolol, inhibited aggregation induced by arachidonic acid or adenosine 5’-diphosphate. Phenytoin. however. had no effect against platelet aggregation. In conclusion, our data demonstrate that all antiarrhythmic drugs, except phenytoin, exert a selective inhibitory effect o n platelet aggregation induced by platelet-activating factor. The mechanism of this effect may be related t o their membrancstabilizing properties and/or lipophilicity. Whether these inhibitory effects on platelet aggregation are clinically relevant will depend on long-term prospective studies; however, there are good reasons t o suggest that antiarrhythmic drugs may represent a new class of platelet-activating factor antagonists. We thank Mr Hazaruddin R. Khan for technical assistance I . Patrono, C., Ciabattoni, G . & Grossi-Belloni, I>. ( 1975) I’rosrtiglandins 9, 557-568 2. Vargaftig, B. B. & Braquet. P. G . (1987) Hr. Mod. h l l . 43.
312-335 3. Braquet, P., Pdubert-Braquet, M., Bessin, P. & Vargaftig. H. H. ( 1987) Adv. I’rosraglundin, Thromhoxcine untl 1.eiiko“ene Kcs. 17,822-827 Received 28 March 1989
