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 of nitric oxide (NO) and platelet activating factor (PAF) from resting and calcium-ionophore (A23187)-activated rat peritoneal mast cells (RPMC) was studied using a platelet aggregation bioassay. RPMC spontaneously released NO, which inhibited TRAP-6-, ADP-, and PAF-stimulated platelet aggregation. This effect of NO was abolished by the addition of an NO binding agent, oxyhemoglobin (oxyHb), to the platelet suspension. The RPMC-induced suppression of platelet aggregation was completely inhibited by the NO-synthase inhibitor 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 pretreatment of the RPMC with L-NAME or with the inhibitor of the constitutive NO-synthase 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 PAF secretion.