Prolactin molecular heterogeneity. Response to thyrotropin-releasing hormone stimulation of concanavalin A-bound and -unbound immunoassayable prolactin during human pregnancy

Since the milieu of pregnancy stimulates physiologic hyperprolactinemia, we questioned whether prolactin secreted during normal pregnancy contains a large-molecular weight component that binds to concanavalin A and whether this large-molecular weight prolactin contributes to the thyrotropin-releasing hormone (TRH)-releasable pool. Serum was collected from pregnant patients (n = 28) undergoing TRH stimulation tests. This serum was passed through a concanavalin A column and eluted with 0.2M alpha-methylmannoside. Concanavalin A-bound prolactin, as determined by radioimmunoassay, ranged from 10% to 30% of the total immunoassayable prolactin. An increase in the basal serum concentration of both concanavalin A-bound and -unbound prolactin occurred as pregnancy progressed. However, throughout gestation, only the concanavalin A-unbound prolactin increased after TRH stimulation. The concanavalin A-bound prolactin was found to have a molecular weight of 60,000 by means of Sephadex G-100 permeation chromatography.