The influence of pH, temperature, and calcium on platelet aggregation: maintenance of environmental pH and platelet function for in vitro studies in plasma stored at 37 degrees C.

Abstract

Summary. The threshold concentration of ADP required to induce second phase or irreversible platelet aggregation was shown to differ between individuals when certain environmental factors were controlled during platelet handling. The factors shown to be important were plasma pH, temperature, and calcium concentration. PRP exposed to air showed a rise in pH and this was initially associated with an increase in the threshold concentration of ADP required to induce second phase aggregation. When the pH rose above 8.0 a second phase could not be elicited. Plasma pH and platelet; responsiveness could be maintained for up to 4 hr by simply storing the PRP under an atmosphere of 5% CO2–95% air. The second phase of aggregation is due to fibrin polymerization (Ardlie & Han, 1974) which is calcium‐dependent, and the pH effect was shown to be due in part to a decrease in availability of calcium as a consequence of a pH‐dependent increase in binding of calcium, possibly to citrate and albumin. Alkaline pH also inhibited fibrin polymerization independently of calcium. The ADP threshold for secondary aggregation was also influenced by the citrate concentration and it was shown that 0.1 M and not 0.129 M (3.8%) citrate should be used for blood collection for preparation of plasma for studies of platelet function. Platelets stored at room temperature were much more sensitive to ADP and thrombin than when kept at 37° C and it is suggested that this sensitization at lower temperatures may obscure differences in platelet responsiveness between individuals. As the pH became more alkaline the sensitivity of platelets stored at room temperature increased and then subsequently decreased. Since the coagulation pathway is involved in the response of platelets to stimuli (Ardlie & Han, 1974) meaningful clinical studies of platelet function will have to be carried out with PRP and the present report defines some of the optimal conditions for handling and storage of plasma.