Alpha and Angiotensin Receptor Tone in the Near-Term Sheep Fetus 1

Summary

We have described the effects of alpha and angiotensin receptor blockade on the mean arterial blood pressure and umbilical vascular resistance of the near-term sheep fetus. In nine control fetuses the mean arterial pressure was 46 ± 1 mm Hg and the umbilical vascular resistance was 0.291 ± 0.01 mm Hg × min/ml × kg. Alpha receptor blockade decreased the mean fetal arterial pressure to 44 ± 2 mm Hg (NS). The umbilical vascular resistance decreased to 0.245 ± 0.04 mm Hg × min/ml × kg (N = 5). This change was not significant. The effect of angiotensin receptor blockade on the umbilical vascular resistance was tested in seven near-term fetuses. The mean fetal arterial pressure dropped to 43 ± 3 mm Hg (NS). The umbilical vascular resistance was 0.285 ± 0.03 mm Hg × min/ml × kg. This change was not significant. Alpha and angiotensin receptor blockade did not abolish the umbilical vasoconstriction that was seen after the injection of prostaglandin E₂.

We conclude that in the near-term sheep fetus neither alpha receptors nor angiotensin II receptors play a significant role in maintaining the tone of the umbilical vascular bed and that the prostaglandin E₂ induced umbilical vasoconstriction is not due to the activation of alpha or angiotensin receptors.