Characterization of the coronary vascular responses to platelet-activating factor in the isolated perfused heart

Platelet-activating factor (PAF) is a potent phospholipid mediator with diverse in vivo and in vitro coronary vascular effects. In the present study, the coronary vascular responses to bolus injections of PAF were compared in rat hearts perfused under constant flow and under constant pressure. Low levels of PAF (1 pmol) produced vasodilatation only, while higher PAF concentrations (100 pmol) produced initial vasodilatation which was followed by a vasoconstriction under both experimental conditions. To determine species differences in PAF action, the effect of PAF was tested on perfused guinea pig hearts. Unlike in perfused rat hearts, only a dose-dependent vasoconstrictor response was observed in perfused guinea pig hearts following a bolus injection of 1 fmol to 10 pmol of PAF. The results from repeated injections of PAF indicated that depletion of vasoactive mediators induced by PAF or receptor desensitization may explain a failure of a second injection of PAF to initiate a vasoconstrictor response. After PAF injection, the coronary vascular response to leukotriene was not altered, indicating that the reduced vasoconstrictor effect of a second injection of PAF cannot be due to a reduced ability of the smooth muscle to constrict. The study demonstrates that similar coronary vascular responses to PAF are observed in perfused rat hearts under either constant flow rate or constant pressure and that some of the variable coronary vascular responses reported may be due to the difference between animal species.