Platelet-activating factor. A possible role in the modulation of the vasomotor tone and blood pressure.

It is well known that the vascular smooth muscle tone is regulated not only by the activity of the sympathetic nervous system, but also by the release of vasoactive factors from the endothelium including nitric oxide (NO), prostacyclin, thromboxane A2 and endothelin (I; 2; 3) that can be induced by both chemical and mechanical stimuli (4). In addition, several studies have reported endothelium-independent vasodilator responses to an unknown endogenous mediator, in rabbit and rat resistance arteries (5;6). PAF is a potent phospholipid mediator released by various cell types including platelets, leukocytes, macrophages, endothelial cells (7) and cultured rat vascular smooth muscle cells (8). This mediator has many biological actions already described (9). Several reports have demonstrated that PAF (chronotropic effect (12; 13) and an increase in vascular permeability (14). The hypotensive effect of PAF has been attributed mainly to the dilation of resistance vessels (15;16), thus suggesting a role for this mediator in the modulation of vascular smooth muscle tone. Here, we provide evidence that PAF is involved in a negative feedback mechanism which modulates vascular tone, and hence blood pressure, in the anaesthetized rabbit.