The role of relaxin in the pregnancy associated reduction in plasma osmolality

Abstract

Recent data suggest that the ovarian peptide relaxin is responsible for the pregnancy-associated fall in plasma osmolality in the rat. In order 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 pregnancies conceived following ovum donation (OD, n = 12), spontaneously (N, n = 12) and following in-vitro fertilization and embryo transfer (IVF, n = 14). These groups were chosen, as relaxin concentrations throughout pregnancy are undetectable (OD), elevated (IVF) or normal (N). Thus, 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 osmolality fell significantly during pregnancy in both the OD and N groups, but not the IVF group. In addition, plasma osmolality was only significantly greater in OD when compared with IVF group at 10 weeks gestation; thereafter there were no significant differences between the groups with regard to plasma osmolality. Similarly, at 10 weeks the plasma concentrations of sodium and potassium were significantly higher in the OD than in either the N or IVF groups. Thus, although relaxin may be important in the initial control of plasma osmolality, other factors, probably derived from or regulated by the feto-placental unit, supersede it as pregnancy advances.