Polymers of Prolactin and Their Clinical Significance

Abstract

Elevated levels of prolactin are known to cause a range of disturbances of ovarian function. However, in a small number of women, sustained hyperprolactinemia has been found by chance in association with apparently normal ovarian function and normal fertility. In these women the dominant form of circulating prolactin has been found by gel chromatography to be a polymeric form of large molecular size, 'big-big' prolactin (BBPRL), with similar immunoreactivity to monomeric prolactin (PRL) but much lower biological activity. In a few cases an intermediate polymeric form, 'big' prolactin (BPRL), has also been described. The exact nature and biological significance of polymeric forms of prolactin remain unclear. It has been shown that concentrations of BBPRL in the circulation in individual women change more slowly and less profoundly than concentrations of PRL in various physiological and pharmacological situations. Minor changes occur during the menstrual cycle, while increases of PRL and BBPRL occur progressively during pregnancy. In pregnancy PRL rises much more than BBPRL. Acute stimulation with metoclopramide, TRH, or suckling favors the production of PRL, although BBPRL also rises to a small extent. During treatment with bromocriptine the proportion of PRL in the circulation is markedly reduced, while BBPRL falls to a much lesser extent. Further study is required before it can be proved that BBPRL and BPRL have no biological significance. There is much controversy regarding the structure and mechanism of production of polymeric forms of prolactin. There is no evidence of a circulating factor in serum which binds several molecules to form BBPRL.(ABSTRACT TRUNCATED AT 250 WORDS)