Bioactivity of prolactin isoforms: lactation and recovery of menses in nursing women

To assess whether plasma prolactin (PRL) characteristics relate to lactogen esis and absence or presence of menstrual cycles, we measured bioactive PRL (BIO;PRL) using the Nb2 assay, immunoreactive PRL (TR-PRL) by radioimmunoa ssay, calculated equations describing the BIO-PRL-IR-PRL relationship and s eparated charged PIiL isoforms (by chromatofocusing) in five amenorrhoeic a nd five cycling nursing women at 6 months postpartum and in 10 cycling non- nursing women, Plasma samples were drawn before and 30 min after a suckling episode at 0800, 1600 and 2400 h in nursing women and at the same hours ih non-nursing women. BIO-PRL and IR-PRL concentrations were highest in ameno rrhoeic nursing women, intermediate in cycling nursing women and lowest in cycling non-nursing women. The BIO-PRL-IR-PRL relationship shows that a giv en amount of IR-PRL corresponds to equivalent amounts of BIO-PRL in cycling nursing and cycling nonnursing women, and to a larger extent in amenorrhoe ic nursing women. IR-PRL was present In plasma as several charge isoforms, Bioactive isoforms eluting at PH 6.0-5.1 were found in amenorrhoeic and cyc ling nursing women, reaching similar concentrations after suckling. Bioacti ve isoforms eluting at pH 7.0-6.1 were found only in amenorrhoeic nursing w omen. We speculate that isoforms eluting at pH 6.0-5.1 may play a role in l actation and isoforms eluting at pH 7.0-6.1, in lactational amenorrhoea.