Abstract
The relationship between maternal plasma volume (PV) expansion and fetal growth is well established, but the underlying mechanisms remain unclear. Here, we examined the influence of maternal body weight and fetoplacental mass on gestational PV increment in the rat. Because IGF-I and IGF-II have growth-promoting and vasoactive properties, their relationship to PV expansion and fetoplacental growth was also studied. In normal rats, the gradual expansion of PV (+35% at day 22, i.e., term) was accompanied by a rise in circulating IGF-II (+45%) and a considerable drop in IGF-I (-73%). Increased maternal body weight induced by an obesogenic diet did not influence PV and circulating IGFs compared with rats on the standard diet. Combining the results from both diets, circulating IGF-II was the principal correlate of PV. A second experiment examined the effect of fetoplacental mass reduction by surgically removing half of the gestational sacs at day 16. This procedure reduced maternal PV and circulating IGF-II at term by 14% and 20%, respectively. We then investigated the effect of a constant infusion of IGF-II (1 mgxkg(-1)xday(-1)) from day 16, which raised circulating IGF-II by 38% and found increased PV (+19%) and a larger placental trophospongial area (+29%) at term. Our results indicate that the placenta, the primary source of IGF-II synthesis in pregnancy, drives PV expansion, and that IGF-II is among the regulatory factors of the gestational PV increment. Further studies should clarify whether IGF-II directly affects vascular function and/or indirectly promotes the secretion of placenta-derived vasoactive substances.