THE ROLE OF RELAXIN IN THE PREGNANCY-ASSOCIATED REDUCTION IN PLASMA OSMOLALITY

Recent data suggest that the ovarian peptide relaxin is responsible fo r the pregnancy-associated fall in plasma osmolality in the rat, In or der to test whether relaxin has the same role during human pregnancy, plasma osmolality, electrolytes, urea and creatinine were measured in samples obtained at 10, 20 and 30 weeks gestation from singleton pregn ancies conceived following ovum donation (OD, n = 12), spontaneously ( N, n = 12) and following in-vitro fertilization and embryo transfer (I VF, n = 14), These groups were chosen, as relaxin concentrations throu ghout pregnancy are undetectable (OD), elevated (IVF) or normal (N). T hus, if relaxin alone is responsible for the fall in plasma osmolality associated with pregnancy in the human, then plasma osmolality would be expected not to fall during pregnancy in the OD group and to show a consistent decline from OD to N to IVF throughout pregnancy, Plasma o smolality fell significantly during pregnancy in both the OD and N gro ups, but not the IVF group, In addition, plasma osmolality was only si gnificantly greater in OD when compared with IVF group at 10 weeks ges tation; thereafter there were no significant differences between the g roups with regard to plasma osmolality, Similarly, at 10 weeks the pla sma concentrations of sodium and potassium were significantly higher i n the OD than in either the N or IVF groups, Thus, although relaxin ma y be important in the initial control of plasma osmolality, other fact ors, probably derived from or regulated by the fete-placental unit, su persede it as pregnancy advances.