Extracellular-free Ca++ accounts for the sex difference in the aggregation of human platelets in citrated platelet-rich plasma.

It has been shown in Poiseuille flow, that the ADP-induced aggregation of human platelets in citrated plasma from female donors is significantly greater than from male donors over a range of mean tube shear rate, G, from 41.9 s-1 to 1920 s-1 and mean transit time, t, from 0.2 to 86 s. The present work verifies the sex difference at G = 335 s-1 and t = 43 s and deals with the effect of free Ca2+ on it. An inverse correlation between the extent of single platelet aggregation and donor hematocrit, and between hematocrit and the plasma ionized calcium concentration, [Ca2+], as well as a positive correlation between the extent of single platelet aggregation and [Ca2+] was found. This indicated that the sex difference is due to hematocrit-dependent differences in the [Ca2+] that result when a fixed volume of the chelating agent citrate is used to anticoagulate blood. When the initial citrate concentration was adjusted to compensate for the variable volume dilution of citrate in plasma among donors and the [Ca2+] of males raised above that of females, the sex difference was reversed. Again, aggregation correlated with [Ca2+]. At the physiological [Ca2+] in both heparinized PRP and hirudinized PRP, the rate of aggregation and aggregate size were much greater than in citrated plasma but no sex difference was detected.