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.