Detailed assessment of big big prolactin in women with hyperprolactinemia and normal ovarian function

Six women with elevated circulating levels of big big PRL (BBPRL) and apparently normal ovarian function were variously studied through a menstrual cycle and menstruation, through pregnancy and suckling, and during stimulation tests with TRH and suppression with bromocriptine. No significant changes in monomeric PRL were demonstrated during the menstrual cycle, but all subjects showed a small and significant rise in BBPRL during the preovulatory phase. High PRL levels were present in day 1 menstrual plasma, but BBPRL was only present in low concentrations. All subjects (n = 5) demonstrated a rise in both PRL and BBPRL during pregnancy, with a consistent tendency for PRL to increase to a proportionately greater extent than BBPRL. One subject exhibited a rise in PRL (by 93%), but not BBPRL, 30 min after suckling. TRH caused a brisk rise in PRL (by 363 +/- 116%) but only a sluggish rise in BBPRL (by 17.5 +/- 7.4%; n = 3). Bromocriptine rapidly suppressed PRL (by 81.8 +/- 34.4%), but only slowly suppressed BBPRL (by 21.0 +/- 8.7% after 6 h; n = 3). Plasma binding studies did not demonstrate any evidence of a circulating specific PRL-binding protein. These data indicate that plasma concentrations of BBPRL may vary under the influence of a number of factors, but are much less sensitive to TRH stimulation, bromocriptine suppression, pregnancy, and suckling than PRL. The occurrence of BBPRL does not seem to be due to a specific circulating binding protein.