Variations in the molecular forms of prolactin during the menstrual cycle, pregnancy and lactation

Size heterogeneity of immunoreactive prolactin (PRL) was studied in serum samples obtained from eight normoprolactinemic women during the menstrual cycle and five additional patients at pregnancy and lactation. Gel filtration of sera from women with normal ovarian function tested at day 10-12th of their menstrual cycle showed two predominant PRL forms, approximately 22K and 26K mol wt. In addition two polymeric variants, 50K ("big" PRL) and 100K ("big-big" PRL) were found in less proportion, accounting for approximately 34% of the total PRL immunoreactivity detected in the sera. It was also noted a low mol wt form eluting around the region of 16K mol wt. In pregnant women the major PRL form was the 22K and its proportion showed a gradual increase as progression of gestation. The polymeric PRL forms were found in substantially less amount as gestation progressed. After parturition, in nursing mothers the 22K form remained prominent and in greater concentrations than the 26K monomeric variant. Large and low mol wt PRL forms were constantly detected in sera from women during the lactation period. From these data we confirmed that PRL circulates at various molecular forms and the relative proportion of these molecular variants exhibit changes according to the physiological state. In our study the predominant form was the 22K PRL (nonglycosylated) and it was of interest to discover the presence of a low mol wt PRL which elutes in the 16K area. The significance of this latter finding is not clear at the present.