Viscosity in POAG: effects of shear rate and cross-linking

Abstract

Purpose : Previous studies from our laboratory have demonstrated increased number of hemorrhages and superactivated platelets (SAPs) in both primary open-angle glaucoma (POAG) and Alzheimer’s disease. Notably inhibitors of the innate immune receptor toll-4 block SAPs in both diseases. To determine the possible causes of the platelet activation and hemorrhages, viscosity of platelet rich plasma (PRP) from POAG and controls was measured at low shear rates with varying agonist concentrations.

Methods : Blood samples were obtained from control (n=3) and POAG (n=3) patients after obtaining informed consent from subjects and Institutional Review Board approval. PRP was isolated by centrifugation. PRP in the presence of calcium was then stimulated with graded amounts of thrombin with and without the presence of selected toll-4 inhibitors—resveratrol (R), quercetin (Q), and a μ-opiate antagonist (N)—to prevent increases in viscosity and fibrin clot formation. Low shear rates between 50, 250 and 500 seconds^-1 were applied for 6 minutes at 37^○C using a cone-plate rheometer (Brookfield). The rheometer detects the resistance to rotation (shear stress) as a function of speed (shear rate). Viscosity was measured in centipoises (cP) at 1 second intervals.

Results : At a low shear rate of 50 seconds ^-1, there is a significant (p<0.02) increase in viscosity and responsiveness to thrombin in POAG vs. control PRP as shown in Table 1. POAG PRP clots at lower thrombin concentrations (2.3 fold; p<0.02).

Conclusions : The results showed a significant difference in responsiveness to thrombin in POAG vs. control PRP at 50 seconds^-1. Notably, the toll-4 inhibitors prevent fibrin clot formation. The mechano-transduction at low shear rate is most likely mediated by an integrin. Notably, the fibrin cross-link is due to coagulation factor XIII, a transglutaminase. Thus, our preliminary results suggest a marked difference in PRP response to thrombin between control and POAG PRP. POAG patients may be more likely to have clots and hemorrhages at low shear rate, and thus, this observation may be an important factor in the occurrence of hemorrhages in POAG and other microvascular pathologies such as Alzheimer’s disease.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.

 

Table 1