REFRACTORINESS OF PLATELETS AFTER REVERSIBLE STIMULATION BY ADP, PAF AND AA

It has been observed that platelets which have been, stimulated by thrombin, ADP or platelet-activating factor (PAF) show an inhibited response to a subsequent stimulation by the same agonist. We performed a crossover stimulation of human platelets (PRP) with the four agonists ADP, PAF, arachidonic acid(AA) and collagen (COL); after a first stimulation with a dose giving a reversible aggregation (0,1-0,4×10^-6M ADP, 0,1-0,5×10^-8 M PAF) or a shape change (0,05-0, 1mM AA., 0,4 ug/ml COL) the platelets were again stimulated after 5 min with the same or a higher dose, usuallyresulting in an ir_K8 reversible aggregation (1,0-3,0×10^-6 M ADP, 1,0-5,0×10^-8 M PAF, 0,3-0,5mM AA, 1,0 ug/ml COL).

ADP-, PAF- and AA-stimulation were inhibited by a prestimulation with the same agonist (ADP/ADP 60% inhibition, PAF/PAF 100% inhibition, AA/AA 10% inhibition) ; ADP and PAF did not or only slightly inhibit each other. Prestimulation with AA changed a reversible ADP- or PAF-induced aggregation into an irreversible one, and a reversible AA-stimulation was increased to irreversible by ADP-prestimulation. COL-aggregation was not influenced by any prestimulation nor did a COL-presti-mulation influence the second stimulation by any of the four agonists.

The refractoriness of platelets for ADP and PAF after prestimulation with the same agonist may be explained by receptor internalization; in the case of ADP the effect lasted for at least one hour. The enhancement of ADP- and PAF-aggregation by an AA-prestimulation can be explained by other investigations which have shown that the arachidonic acid pathway acts as a positive feedback mechanism in platelets.