Shear stress-induced pH increase in plasma is mediated by a decrease in PCO2: The increase in pH enhances shear stress-induced P-selectin expression in platelets

To investigate shear stress-induced platelet activation, the cone-plate viscometer or the Couette rotational viscometer has been widely used. In a previous report, it was shown that shearing platelet-rich plasma using a Couette rotational viscometer could lead to an increase in pH by CO₂ release. However, any clear mechanism has not been provided. In this study, we examined whether shearing cell free plasma only using a cone-plate viscometer can also induce pH increase and studied the underlying mechanism of shear-induced pH increase by directly measuring total CO₂ (T_CO2) and CO₂ tension (P_CO2). When human plasma was sheared using a cone-plate viscometer, the pH of the human plasma increased time- and shear rate-dependently. Although T_CO2 of human plasma was not affected, P_CO2 was decreased by shearing, indicating that the decreased P_CO2 is associated with a pH increase of plasma. In addition, the pH of bicarbonate-containing suspension buffer was also shown to be increased by shearing; suggesting that the platelet studies using suspension buffers containing bicarbonate could be affected similarly. The effects of pH changes on shear stress-induced platelet activation were also investigated in the same in vitro systems. While shear stress-induced platelet aggregation was not affected by the pH changes, P-selectin expression was significantly increased in accordance with the pH increase. In conclusion, shear stress using a cone-plate viscometer induces pH increase in plasma or bicarbonate-containing suspension buffer through a P_CO2 decrease and the pH changes alone can contribute to platelet activation by enhancing shear stress-induced P-selectin expression.