Possible role of P-450 metabolite of arachidonic acid in vasodilator mechanism of angiotensin II type 2 receptor in the isolated microperfused rabbit afferent arteriole

Although angiotensin II type 2 (AT₂) receptor has recently been cloned, its functional role is not well understood. We tested the hypothesis that selective activation of AT₂ receptor causes vasodilation in the preglomerular afferent arteriole (Af-Art), a vascular segment that accounts for most of the preglomerular resistance. We microperfused rabbit Af-Arts at 60 mmHg in vitro, and examined the effect of angiotensin II (Ang II; 10^{- 11}-10^-8 M) on the luminal diameter in the presence or absence of the Ang II type 1 receptor antagonist CV11974 (CV; 10^-8 M). Ang II was added to both the bath and lumen of preconstricted Af-Arts. Ang II further constricted Af-Arts without CV (by 74±7% over the preconstricted level at 10^-8 M; P < 0.01, n = 7). In contrast, in the presence of CV, Ang II caused dose- dependent dilation; Ang II at 10^-8 M increased the diameter by 29±2% (n = 7, P < 0.01). This dilation was completely abolished by pretreatment with an AT₂ receptor antagonist PD123319 (10^-7 M, n = 6), suggesting that activation of AT₂ receptor causes vasodilation in Af-Arts. The dilation was unaffected by inhibiting either nitric oxide synthase (n = 7) or cyclooxygenase (n = 7), however, it was abolished by either disrupting the endothelium (n = 10) or inhibiting the cytochrome P-450 pathway, particularly the synthesis of epoxyeicosatrienoic acids (EETs, n = 7). These results suggest that in the Af-Art activation of the AT₂ receptor may cause endothelium-dependent vasodilation via a cytochrome P-450 pathway, possibly by EETs.