Differential responses to systemic and local angiotensin II infusions in conscious postnatal sheep

Angiotensin II (ANG II) increases blood pressure (MAP) via specific ANG II receptors (AT) and is considered important in regulating MAP after birth. In adult animals, AT(1) receptors predominate in vascular smooth muscle (VSM) and mediate vasoconstriction. In newborn sheep, AT(2) receptors, which do not mediate vasoconstriction, predominate in vascular smooth muscle until 2 wk postnatal when they are replaced by AT(1). Thus, the mechanisms whereby ANG II increases MAP after birth are unclear. We examined the effects of ANG II on femoral vascular resistance (FmVR) and blood flow (FmBF) in serial studies of newborn sheep (n = 7) at 7-14 d, 15-21 d, and 22-35 d. Animals had femoral catheters implanted for systemic ANG II infusions and cardiovascular monitoring, and a flow probe was implanted on the contralateral artery proximal to the superficial saphenous artery, which contained a catheter for intra-arterial ANG II infusions. Studies were performed using a range of systemic and intra-arterial ANG II doses. Systemic ANG II increased MAP dose-dependently at all ages (p < 0.001); however, responses were not age dependent. FmBF rose dose dependently at 7-14 d (p < 0.001) and was unchanged at older ages. FmVR was unaffected at 7-14 d, but values increased dose dependently at 15-21 d and 22-3 5d (p < 0.001), although never exceeded relative increases in MAP. Local ANG II did not alter MAP, FmBF, or FmVR at any age. Although systemic ANG II increases MAP and FmVR dose dependently after birth, ANG II-induced vasoconstriction is attenuated. Furthermore, intra-arterial ANG II does not alter FmVR in the absence of systemic responses, suggesting incomplete vascular smooth muscle AT(1) expression, stimulation of local ANG II antagonists, or ANG II-mediated release of another vasoconstrictor.