Cardiovascular responses to forskolin in the ovine fetus

Six near-term ewes were instrumented to measure regional blood flows in the maternal and fetal subthoracic structures and allowed to recover for 5 days. Control blood flows were measured and 10(-3) molar forskolin was infused in the fetal hindlimb vein at 1 ml/min. After 10 min of infusion, maternal and fetal regional blood flows were measured. The fetal blood pressure was 44 +/- 3 mmHg in the control state and 40 +/- 4 mmHg after forskolin, P less than 0.056. The fetal renal vascular resistance changed from 24.4 +/- 2.4 to 17.5 +/- 1.7 mmHg.ml-1.min.g, P less than 0.005. The placenta had a control resistance of 27.7 +/- 5.0 and 25.6 +/- 5.1 mmHg.ml-1.min.g after forskolin, P less than 0.05. The placental membranes showed vasodilation: control resistance was 261 +/- 49 and 168 +/- 39 mmHg.ml-1.min.g after forskolin, P less than 0.02. The generalized vasodilation of the fetal circulation was paralleled in the maternal circulation. Forskolin, a lipid soluble diterpene, apparently had a placental clearance close to the theoretical maximum. Vasodilation was seen in the maternal renal, placental and uterine vasculatures. Maternal blood pressure was unchanged. Maternal placental vascular resistance was 47.4 +/- 3.0 mmHg.ml-1.min.g in the control state and 40.6 +/- 3.3 mmHg.ml-1.min.g after forskolin, P less than 0.02. Forskolin is a vasodilator in both the fetal and maternal circulations. The maintenance of a relatively normal blood pressure in the face of regional vasodilation shows that forskolin may have a positive inotropic effect on the fetal heart. These results indicate that neither the fetal nor the maternal ovine placental vasculature is maximally dilated in the control state.