Altered uterine environment in the spontaneously hypertensive rat.

1. In order to determine the role of the uterine environment in the development of hypertension in the spontaneously hypertensive rat (SHR) we examined the patterns of fetal and placental growth and the composition of amniotic fluid of SHR and normotensive Wistar-Kyoto (WKY) rats during the final trimester of pregnancy. 2. SHR and WKY embryos and amniotic fluid were collected at days 15 and 20 of pregnancy, and fetal and placental weight and amniotic fluid volume were recorded. The sodium and potassium concentration of amniotic fluid was also measured. 3. Fetal and placental weights were significantly lower in SHR compared to WKY at 15 and 20 days of gestation. Amniotic fluid volume was significantly lower in SHR at 15 days, but significantly higher at 20 days. Amniotic fluid sodium concentration was significantly lower in SHR at 15 days but not different at 20 days when compared to WKY. Potassium concentration was lower in SHR at both ages. 4. Thus the reduced placental weight early in the gestational period of the SHR may play a role in its underdevelopment in utero and hence its reduced birthweight. The SHR foetus is also bathed in a hypokalaemic amniotic fluid of increased volume. How this might influence fetal development and later blood pressure is unclear. Also whether these changes are of maternal or fetal origin remain to be determined.