Biochemical and physical responses of red blood cells with shear exposure

Abstract

Red blood cells (RBC), the essential oxygen-transporters of the human body, are near constantly exposed to deforming forces (i.e., shear stress) as they traverse the circulatory system. Given that the diameter of microcirculatory blood vessels may be as narrow as ~2 μm, RBC with a resting diameter of ~8 μm require an exceptional capacity to deform and change shape, while sustaining cell function. RBC are equipped with unique physical properties to accommodate cellular deformation under mechanical stress; the characteristic bi-concave disc-shape yielding favourable surface area to volume ratio, paired with a flexible, cytoskeletal mesh-network in the cell membrane, support flexibility of RBC. Recent studies, however, support the notion that RBC deformability is actively regulated by pathways involving second messenger molecules, primarily calcium-ions (Ca2+) and nitric oxide (NO). Exposure of RBC to shear stress is thought to increase endogenous production of NO enzymatically via nitric oxide synthase (RBC-NOS), while mechanically-gated ion channels permit entry of Ca2+ under shear. The physiological magnitude of shear stress exerted upon blood is thought to be limited to ~15 Pa; however, when patients require extracorporeal circulatory devices such as rotary blood pumps or dialysis machines, the resulting mechanical forces approximate values of >100 Pa, that are known to impair the physical properties of RBC. Thus, the aim of the current thesis was to explore the physical and biochemical properties of RBC when exposed to either physiological (i.e., 5 Pa was limited. Physiological shear conditioning partly reversed these calcium-induced impairments, while supraphysiological shear conditioning appeared to paradoxically (albeit marginally) increase cell deformability that was previously impaired by calcium. Intriguingly, single RBC that were aspirated into the micropipette showed increases in intracellular calcium during passage of the aperture, indicating that maximal elongation facilitated influx of Ca2+. Moreover, the rate of Ca2+-influx was found to be proportional with the shear stress applied; increased shear stress led to more rapid influx of calcium. Collectively, the present thesis demonstrated that the response of RBC to shear stress is regulated by complex intracellular messaging in addition to classically-described external forces, which becomes “active” primarily when RBC are exposed to shears within the physiological range (i.e., <15 Pa). Upon supraphysiological shear exposure, however, the physical properties of RBC become significantly impaired; potential alterations in the intracellular messaging system that appears to regulate cell deformability post-mechanical damage remain elusive and worthy of future exploration.