Angiotensin II mediates uterine vasoconstriction through alpha-stimulation

Area of Science:

• Reproductive physiology
• Cardiovascular pharmacology
• Renal and vascular physiology

Background:

• Intravenous angiotensin II (ANG II) elevates uterine vascular resistance (UVR), but direct uterine infusions do not, suggesting indirect mechanisms.
• Type 2 ANG II (AT(2)) receptors are prevalent in uterine vasculature, hinting at a role for systemic type 1 ANG II (AT(1)) receptors and alpha-adrenergic pathways.

Purpose of the Study:

• To investigate the mechanisms by which ANG II affects uterine vascular resistance (UVR) and systemic blood pressure.
• To differentiate the roles of systemic versus uterine AT(1) receptors and alpha-adrenergic activation in ANG II's vascular effects.

Main Methods:

• Comparison of systemic pressor and UVR responses to intravenous phenylephrine and ANG II in ewes.
• Utilized systemic and uterine alpha-receptor blockade, and AT(1) receptor blockade.
• Evaluated responses in pregnant and nonpregnant states.

Main Results:

• Systemic alpha-receptor blockade inhibited phenylephrine effects on mean arterial pressure (MAP) and UVR.
• ANG II-induced UVR increases were significantly reduced (>65%) by alpha-receptor blockade, indicating involvement of an alpha-agonist release.
• Systemic AT(1) receptor blockade abolished all ANG II responses, while combined uterine AT(1) blockade and systemic alpha-blockade led to sustained MAP and UVR increases.

Conclusions:

• ANG II-mediated pressor responses are primarily driven by systemic AT(1) receptor activation.
• Increased UVR in response to ANG II results from AT(1) receptor-mediated release of an endogenous alpha-agonist and subsequent uterine autoregulatory adjustments.
• Uterine AT(2) receptor blockade did not influence these responses.