Modulation of mast cell activity by a peptide agonist of the thrombin receptor: Role of nitric oxide

The effect of a thrombin receptor agonist peptide (TRAP-6) on the release o f nitric oxide (NO) and platelet activating factor (PAF) from resting and c alcium-ionophore (A23187)-activated rat peritoneal mast cells (RPMC) was st udied using a platelet aggregation bioassay. RPMC spontaneously released NO , which inhibited TRAP-6-, ADP-, and PAF-stimulated platelet aggregation. T his effect of NO was abolished by the addition of an NO binding agent, oxyh emoglobin (oxyHb), to the platelet suspension. The RPMC-induced suppression of platelet aggregation was completely inhibited by the NO-synthase inhibi tor L-NAME. TRAP-6 and its high affinity analog haTRAP stimulated the rapid release of NO from RPMC. The effect of TRAP-6 was inhibited by pretreatmen t of the RPMC with L-NAME or with the inhibitor of the constitutive NO-synt hase isoform (cNOS) calmidazolium. TRAP-6 inhibited PAF release from A23187 -activated RPMC via an NO-dependent mechanism. Platelet aggregation induced by PAF release from activated RPMC was also confirmed in experiments using the PAF receptor antagonist ginkgolide B. Thus, TRAP-6 is a rapidly acting modulator of mast cell reactivity; it stimulates NO release and inhibits P AF secretion.