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Mean arterial pressure (MAP) increases after birth, however, the mechanisms remain unclear. Systemic angiotensin II (ANG II) infusions increase MAP in newborn sheep, but the direct effects of ANG II on peripheral vascular resistance (PVR) are minimal. Thus, its systemic pressor effects may reflect release of other pressor agents, e.g. alpha-agonists and/or AVP, suggesting they contribute to postnatal regulation of MAP and PVR. To address this, we performed studies in conscious sheep at 7-14, 15-21, and 22-35 d postnatal, infusing phenylephrine (PE) or AVP systemically or intra-arterially into the hindlimb while measuring MAP, heart rate (HR), and femoral blood flow (FmBF). Basal MAP and FmBF rose, whereas HR and femoral vascular resistance (FmVR) fell (p &lt; or = 0.03) during the first month postnatal. Although systemic PE and AVP dose dependently increased MAP and FmVR and decreased FmBF and HR (p &lt; 0.001, ANOVA) at all ages, responses were not age dependent. Notably, increases in FmVR exceeded increases in MAP, and responses to PE appeared to exceed AVP (p &lt; 0.05). Hindlimb infusions of both agents decreased FmBF and increased FmVR dose dependently (p &lt; 0.001, ANOVA) at all ages without altering MAP or HR. These responses also were not age dependent. Unlike ANG II, PE and AVP directly increase PVR in newborn sheep. Moreover, FmVR increases more than MAP at all doses, suggesting these agonists may contribute to postnatal MAP regulation and could mediate the effects of systemic ANG II on postnatal MAP.

Effects of Systemic and Local Phenylephrine and Arginine Vasopressin Infusions in Conscious Postnatal Sheep