Antagonism of vasoconstriction induced by platelet-activating factor in guinea-pig perfused hearts by selective platelet-activating factor receptor antagonists.

Platelet-activating factor (Paf) is a potent coronary vasoconstrictor in rat, guinea-pig, dog and pig. The present study investigated the mechanism and duration of Paf in guinea-pig isolated, Krebs-perfused hearts. Dose-related and sustained decreases in cardiac contractility and increases in coronary perfusion pressure were elicited by bolus doses of Paf (0.3-100 pmol). Platelet-activating factor (30 pmol) induced increases in the production of immunoreactive thromboxane B2 (TXB2), leukotriene B4 (LTB4) and LTC4, but not 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha). In addition, the release of leukotriene-like material following Paf was observed using on-line superfusion bioassay. The coronary vasoconstrictor actions of Paf were partially antagonized by the leukotriene receptor antagonist, FPL 55712 (1.9 microM), or by indomethacin (2.8 microM). The combined use of these compounds did not result in further significant inhibition. The Paf receptor antagonists, BN 52021 (30 microM) and L 652, 731 (10 microM), antagonized both the increase in coronary perfusion pressure and the decrease in cardiac contractility induced by Paf (10-100 pmol) in a surmountable and relatively selective manner. The effects of a bolus dose of 100 pmol Paf were sustained in excess of 18 min. Exogenous Paf underwent little metabolism on passing through the coronary circulation with only 2% being converted to lyso-Paf and approximately 4% being retained by the heart after 18 min of perfusion. These results suggest that the coronary vasoconstrictor actions of Paf are partially dependent on the release of vasoactive arachidonic acid metabolites. The extraordinary potency and the long-lasting action of Paf indicate a potential role for this pro-inflammatory mediator in disorders of the coronary circulation.