Summary
The injection of PAP (6 μg/kg, i. v.) induced, in rats, haemoconcentration accompanied
by an increase in the platelet number, as attested by the counts of platelets in blood
samples diluted in formalin-free EDTA solution. This increase was significant at 15
min, peaked from 1 to 4 h and returned to basal levels 24 h after the lipid administration.
The release of platelets induced by PAP was inhibited dose-dependently by specific
PAP receptor antagonists such as WEB 2086 (0.5-2 mg/kg), BN 52021 and 48740 RP (5-25
mg/kg). Furthermore, platelet mobilization was clearly impaired in splenectomized
animals stimulated by PAP, whereas thrombocytopenia and haemoconcentration by the
same stimulus were intact. It was also noted that a second injection of PAP, 24 h
after the initial stimulation with the lipid, failed to induce an increase in platelet
counts, indicating autodesensitization. Desensitization to PAP or pretreatment with
PAP antagonists clearly prevented the increase in the platelet counts after stimulation
by adrenaline (15 μg/kg). These findings suggest that, in rats, PAP can induce release
of platelets by a spleen-dependent mechanism and that this lipid may be relevant to
the thrombocytosis triggered by adrenaline.
Keywords
PAF-acether (PAF) - Thrombocytosis - Adrenaline - Spleen
