Physiological shear flow enhances pinocytosis in human platelets

Abstract

Abstract Pinocytosis, the uptake of extracellular fluid, is an important yet poorly understood function of platelets. Although platelets are continuously exposed to shear flow in the circulation, the mechanism by which shear modulates pinocytosis in human platelets remains unclear. Here, we investigated the effects of physiological shear rate on platelet pinocytosis using a rotational viscometer combined with flow cytometric quantification using pHrodo dextran. Exposure to 500–1500 s -1 shear rates enhanced platelet pinocytosis and increased intracellular Ca 2+ levels measured using Fluo-4. The pharmacological inhibition of intracellular Ca 2+ signaling with prostaglandin E1 and chelation of extracellular Ca 2+ with ethylene glycol tetraacetic acid suppressed both shear-induced Ca 2+ elevation and pinocytosis, suggesting Ca 2+ dependence. Notably, shear exposure within this physiological range did not induce classical platelet activation markers, including integrin αIIbβ3 activation or P-selectin expression, suggesting that shear-induced pinocytosis occurs independently of canonical activation pathways. Under plasma-free conditions, platelet pinocytosis was markedly enhanced, even in the absence of shear, without concomitant Ca 2+ elevation, suggesting the involvement of distinct plasma-dependent regulatory mechanisms. Together, these findings demonstrate that physiological shear promotes calcium-dependent platelet pinocytosis through activation-independent pathways, and highlight shear modulation as a potential strategy for drug loading into platelets in blood cell-based drug delivery systems.