RELEASE OF C1-INHIBITOR FROM PLATELET ALPHA GRANULES REGULATES COMPLEMENT ACTIVATION IN AN In Vitro MODEL OF VENTRICULAR ASSIST

Abstract

Introduction: Ventricular Assist Devices (VAD) have improved the survival of patients with end-stage heart failure. However, long-term use of VADs are complicated by thromboembolism. Complement activation may play a role in these complications. Previous data has shown lower levels of complement activation in VADs treated with Aggrastat, a platelet GP IIb/IIIa inhibitor. Complement is regulated by the release of C1 esterase inhibitor (C1 INH) from platelet alpha granules by a receptor-mediated process. Platelet factor 4 (PF4) is a member of the chemokine superfamily of proinflammatory cytokines, which also resides in platelet alpha granules and can be used as a marker for platelet degranulation. The purpose of this study is to confirm that increases in serum concentrations of C1-INH, in response to VAD-induced complement activation, are caused by platelet degranulation in an in vitro VAD model. Methods: Eight in vitro non-pulsatile centrifugal VAD circuits were circulated for 4 days using 450 ml of fresh human whole blood. A highly selective inhibitor of platelet receptor GP IIb/IIIa (Aggrastat) was added to 4 of the circuits while the remaining 4 circuits were untreated. Temperature, ACT, cardiac index, pH, pCO (2), pO2, Ca++, and glucose were maintained at physiologic values. Serum levels of PF4 and C1-INH were measured by ELISA. Figure 1Figure 1Results: PF4 levels of VADs treated with Aggrastat increased to 389% of baseline levels by 72 hrs of VAD circulation while the untreated group increased by 569%. In untreated VADs, PF4 and C1-INH levels show an early rise in serum concentration that peaks at approximately hour 5 of circulation. Discussion: Treatment of in vitro VADs with Aggrastat reduced serum levels of PF4 suggesting that GP IIb/IIIa inhibition may attenuate the release of platelet alpha granule contents during degranulation. The early rise in both PF4 and C1-INH suggests that fluctuations in serum concentrations of C1-INH are attributed to platelet degranulation. Therefore, the early rise in CI-INH may be a regulatory response towards higher levels of complement activation in VADs without Aggrastat. Accordingly, lower levels of complement activation in VADs treated with Aggrastat may require less regulation causing a delayed rise of serum C1-INH. Figure 2Figure 2