ROLE OF PLATELET-ACTIVATING-FACTOR (PAF) IN PLATELET ACCUMULATION IN RABBIT SKIN - EFFECT OF THE NOVEL LONG-ACTING PAF ANTAGONIST, UK-74,505

1 The contribution of platelet-activating factor (PAF) to platelet dep osition and oedema formation induced by exogenous soluble mediators, z ymosan particles and associated with a reversed passive Arthus (RPA) r eaction in rabbit skin was investigated by use of a novel long-acting PAF receptor antagonist, UK-74,505. 2 Oedema formation and platelet ac cumulation were simultaneously measured by i.v. injection of [I-125]-a lbumin and In-111-labelled rabbit platelets. UK-74,505 was either admi nistered i.v. or used to pretreat radiolabelled platelets in vitro bef ore their injection into recipient animals. Platelets pretreated with UK-74,505 were also labelled with the fluorescent calcium indicator, F ura-2, to assess their ex vivo reactivity to PAF at the end of the in vivo experiment. 3 UK-74,505 (0.5 mg kg-1), administered i.v., inhibit ed PAF-induced oedema formation, but did not affect oedema induced by zymosan particles, bradykinin (BK), histamine, formyl-methionyl-leucyl phenylalanine (FMLP), zymosan-activated plasma (ZAP, as a source of C5 a des Arg), leukotriene B4 (LTB4) or interleukin-8 (IL-8). 4 UK-74,505 , administered i.v. also suppressed the small platelet accumulation in duced by exogenous PAF, but had no effect on accumulation induced by I L-8 or ZAP. Although oedema induced by zymosan was not affected by i.v . UK-74,505, zymosan-induced platelet accumulation was significantly a ttenuated by the antagonist. 5 The RPA reaction in rabbit skin was ass ociated with marked oedema formation and platelet accumulation which w ere both inhibited by i.v. UK-74,505. 6 In vitro, UK-74,505 inhibited aggregation and the increase in intracellular calcium concentration in duced by PAF in rabbit washed platelets in a concentration-dependent m anner (IC50 = 1.6 x 10(-8) M and 1.1 x 10(-8) M, respectively). Platel ets pretreated with 10-6 M UK-74,505, and maintained at 37-degrees-C, were unresponsive to PAF, whilst responding normally to thrombin, for up to 4 h. 7 In a second series of in vivo experiments, platelets were labelled with ... In and loaded with Fura-2. The platelets were then pretreated with 10(-6) M UK-74,505, washed, and injected into recipien t rabbits. These platelets, prepared from blood samples taken at the e nd of the in vivo experiments, exhibited an 80% reduction in their res ponse to PAF as measured ex vivo with Fura-2. However, in contrast to the effects of i.v. UK-74,505, platelets pretreated with the antagonis t did accumulate effectively in the RPA reaction, a significant reduct ion only being observed in responses at the lowest antibody dose. In a ddition, pretreatment of platelets had no effect on the small platelet accumulation induced by PAF. 8 These results suggest that PAF is an i mportant mediator of oedema formation and platelet accumulation in the RPA reaction in rabbit skin. However, they question the role of PAF r eceptors on platelets in this model. The results also indicate that PA F may be involved in platelet accumulation induced by zymosan in rabbi t skin.