Human platelet alpha-adrenergic receptors and responses during pregnancy: no change except that with differing...

The possibility that platelet aggregation is altered during pregnancy is controversial. We tested for alterations of alpha-adrenergic receptor-induced platelet function during pregnancy compared to that in the early phase of the menstrual cycle. We found no change in alpha-adrenergic receptor concentration or in affinity of the receptor for epinephrine. The potency and sensitivity of epinephrine to inhibit adenylate cyclase, a response mediated by alpha 2-adrenergic receptors, was not different in platelets from the two groups of subjects. The ability of epinephrine to potentiate aggregation by adenosine diphosphate was significantly increased during pregnancy; however, this was an artifact introduced by lower hematocrits during pregnancy that resulted in an increased citrate concentration in the preparations of platelet-rich plasma from these women. The difference was eliminated by normalization of citrate concentration. Thus effects of alpha-adrenergic receptors on platelet function are not different during pregnancy. It is mandatory to recognize artifactual effects of differing hematocrits on platelet aggregation studied in vitro.