Effect of plasma pH on stability and capacity of aggregation of platelets

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 CO(2) 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 CO(2) (T(CO(2))) and CO(2) tension (P(CO(2))). When human plasma was sheared using a cone-plate viscometer, the pH of the human plasma increased time- and shear rate-dependently. Although T(CO(2)) of human plasma was not affected, P(CO(2)) was decreased by shearing, indicating that the decreased P(CO(2)) 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(CO(2)) decrease and the pH changes alone can contribute to platelet activation by enhancing shear stress-induced P-selectin expression.

Influence of hypercapnia and hypoxia on rabbit platelet aggregation

The influence of changes in pCO2, pH and pO2 on the aggregation of rabbit blood platelets was studied in vitro, with emphasis on hypercapnia, acidosis and hypoxia. Hypercapnia combined with acidosis caused a reduction in rabbit platelet aggregation, as induced by collagen, thrombin and ADP; the effect being most pronounced with collagen and smallest with ADP. Hypoxia reduced thrombin induced platelet aggregation, but had no effect on ADP and collagen induced aggregation. Synergistic activation of rabbit platelets, as induced by the addition of serotonin to platelet rich plasma together with collagen or ADP, seemed to be equally sensitive to changes in pCO2 and pH as activation by the individual agents, and insensitive to changes in pO2.

Effect of blood gases and pH on thromboembolic reactions in rabbit mesenteric microvessels

The influence of changes in systemic blood gas and pH values on the thromboembolic reaction following wall puncture was studied in rabbit mesenteric arterioles and venules (diameter 20-40 microns), using intravital video-microscopy. Under normal circumstances the number of emboli produced was higher in arterioles than in venules (6 and 1, respectively). The initial thrombus growth, the number of emboli produced per vessel and the total duration of the embolisation period were not significantly influenced by changes in blood gas and pH values in both arterioles and venules. Therefore, the observed difference in thromboembolic reaction between arterioles and venules cannot be explained by differences in blood gas and pH values in these microvessels. Since reduced velocity, as a measure of wall shear rate, did not correlate with the thromboembolic reaction in arterioles or venules, fluid dynamics can also not explain the difference, indicating that the thrombogenic or antithrombogenic activity of arteriolar and venular walls differs following injury. A combination of hypercapnia and hypoxia was found to result in a prolongation of the average time period needed to produce a new embolus in both vessel types. This prolongation in embolus production time was largely due to the occurrence of periods, in which the thrombus did not grow, reflecting hampering of the adhesion and aggregation of blood platelets to a growing thrombus under hypercapnic/hypoxic conditions.

Dependence of the antagonism at human platelet 5-HT2 receptors by ketanserin on the reaction pH

The potency for the inhibition of 5-hydroxytryptamine (5-HT)-induced human platelet aggregation by ketanserin, pipamperone, spiperone, cyproheptadine and BW 501 in vitro decreased as the reaction pH increased progressively from 7.4 to 8.6, the largest shift (X 1125) in IC50 value (pH 7.4: 4 X 10(-9) M; pH 8.6: 4.5 X 10(-6) M) being found for ketanserin. With such an alkaline pH-shift, the fraction of the ionized form of the drugs decreased, reduction of the inhibitory capacity and of the ionized fraction being strongly correlated. Ketanserin (40 mg orally, - 15 h) in human volunteers, completely inhibited the 5-HT-induced platelet aggregation measured ex vivo, when tested at a reaction pH of 7.4; without gassing with CO2 5% -O2 95%, the plasma pH became alkaline and the inhibitory potency of the drug was strongly reduced (-60%). This study demonstrates the importance of the reaction pH for the aggregation of human platelets induced by 5-HT and its inhibition by ketanserin.