Changes in platelet intracellular free calcium in normal pregnancy.

Objective: To determine if platelet intracellular free calcium concentration (p.(Ca2+)i) changes with gestation in normotensive, uncomplicated pregnancy.
Design: A prospective, longitudinal study of primigravid pregnant women compared to a large control group consisting of nulliparous nonpregnant volunteers.
Setting: The antenatal clinic at University Hospital, Queen's Medical Centre, Nottingham.
Subjects: Two groups of women were studied. Thirty-one nulliparous, nonpregnant women not using oral contraception were investigated in the early follicular phase of their menstrual cycles. Also, 24 primigravid women with normotensive, uncomplicated pregnancies were investigated on eight separate occasions during their pregnancies.
Results: A significant increase in mean p.(Ca2+)i was demonstrated by 28 weeks gestation in the pregnant women as compared to the nonpregnant control group and the same individuals when investigated at 12 weeks postpartum (P < 0.05; ANOVA). This change was maximal at 36 weeks gestation (P < 0.001; ANOVA); concentrations had returned to those not significantly different from nonpregnant women by six weeks postpartum.
Conclusions: Basal platelet (Ca2+)i increases significantly by the third trimester of normal, primigravid pregnancy as compared to pregnancy values and postnatal values. These data mirror the previously described observations of platelet behaviour noting increased activity at this gestation of pregnancy. It may be that the increased basal p.(Ca2+)i indicates that smaller transient signals have to be generated to induce platelet activity, such as shape change, aggregation and exocytosis.