The role of agonist-induced activation and inhibition for the regulation of purinergic receptor expression in human platelets.

INTRODUCTION: Adenosine diphosphate (ADP) as physiological activator of human platelets mediates its effects via three purinergic receptors: P2Y1, P2Y12 and P2X1. The inhibition of P2Y12 is used pharmacologically to suppress aggregation underlining the physiological significance of this receptor. Since the regulation of purinergic receptor expression has not thoroughly been investigated yet, this study analyzed the content of purinergic receptors on the platelet surface membrane upon activation and inhibition.

MATERIALS AND METHODS: The surface expression of purinergic receptors was measured by flow cytometry using two different polyclonal antibodies as basal values and after incubation with thrombin receptor activating peptide (TRAP-6) or with inhibitors DEA/NO, MAHMA/NO or Prostaglandin E1 (PGE1). Western blot analysis was used to confirm inhibitory effects.

RESULTS: Both investigated antibodies revealed a significant increase of purinergic receptor expression upon TRAP-6 stimulation. The NO donors, DEA/NO and MAHMA/NO, did not influence basal or TRAP-6 stimulated values. PGE1 did not affect basal receptor expression, but diminished TRAP-6 stimulated purinergic receptor expression in a dose-dependent manner.

CONCLUSIONS: In summary, TRAP-6 induced platelet activation leads to an elevation of purinergic receptor expression. In contrast to other surface ligands, this effect is not suppressed by cGMP-mediated inhibition, but almost completely abrogated by enhanced cAMP-mediated signaling as induced by PGE1.