Different Types of Vasocontractile Responses to Noradrenaline in the Presence of Platelets with Platelet Activating Factor.

Platelet activating factor (PAF) is known to produce a wide variety of hemodynamic effects. The present study was carried out with the aim of elucidating the mechanism of PAF action on vasoconstrictive response to noradrenaline (NA-R) in the presence of autologous platelets. NA-R was examined in isolated perfused arterial segments. PAF action through stimulation of platelets by noradrenaline (NA) was explored during infusion of platelet rich plasma (PRP) with PAF into the perfusion circuit. Consequently, it was revealed that PRP with PAF elicited an initially augmented response, followed by gradually attenuated responses under conditions of low doses of NA (in the range of 5 to 25 ng). However, the augmented responses were observed consistently under a higher dose of NA (50 ng). In addition, the gradually attenuated responses were reversed by adding tetrodotoxin, a Na-spike inhibitor. Thus, it is concluded that PAF action on NA-R through platelets may be related partly to neurotransmitters originating from perivascular autonomic depressant nerves stimulated by some neuroeffector agents.