Agonist-Induced Release of Neuropeptide Y by Platelets

Abstract

Neuropeptide Y (NPY) is a multi-functional peptide localized in a number of tissues besides the brain, and its expression is high in strains of mice with autoimmune disease. Functionally, along with its neurotransmitter role, NPY has multiple effects on both vascular tissues and tissues involved in immune responses. Previous studies demonstrated that immunoreactive NPY (i-NPY) exists in large quantities in platelets and bone marrow of some species, and suggested its release during platelet aggregation by collagen. Because NPY has been hypothesized to have a role in a number of pathophysiological states in which platelets are involved, including immune responses, hypertension and vascular smooth muscle proliferation, we sought to further characterize platelet i-NPY content and release by a diverse group of physiological platelet agonists. Platelet rich plasma and gel filtered platelets were prepared from citrated whole blood of male Sprague Dawley rats. Platelet content and concentration of i-NPY were quantified by RIA. Platelet aggregation responses (turbidometric method) and release of i-NPY were measured in rat platelet preparations stimulated by a thromboxane agonist (U44069), collagen, thrombin and ADP. All agents induced dose-dependent aggregation, although ADP and U44069 were weak agonists in citrated platelet rich plasma, ADP only inducing primary aggregation. Total i-NPY content in platelet rich plasma of Sprague-Dawley rats was 32.1±3.8 pmol/ml; more than 20 pmol/ml was released into supernatant of aggregating platelets stimulated with high dose thrombin or collagen during secondary, irreversible aggregation. Although U44069 and ADP both stimulated i-NPY release in a dose-dependent manner, release was substantially lower than with the other agonists. We conclude that i-NPY release from rat platelets is associated mainly with secondary, irreversible aggregation, and can be produced by a variety of platelet stimulating agents as part of the platelet release reaction. The present study and other recent studies demonstrate wide variability in platelet i-NPY content between and within species. Investigation of the genetic regulation of i-NPY content in platelets could offer new insights into the relationship between NPY and pathophysiology of the immune and cardiovascular systems.