Physiological shear flow enhances pinocytosis in human platelets

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 Ca²⁺ levels measured using Fluo-4. The pharmacological inhibition of intracellular Ca²⁺ signaling with prostaglandin E1 and chelation of extracellular Ca²⁺ with ethylene glycol tetraacetic acid suppressed both shear-induced Ca²⁺ elevation and pinocytosis, suggesting Ca²⁺ 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 Ca²⁺ 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.