Angiotensin II Relaxes Microvessels Via the AT 2 Receptor and Ca 2+ -Activated K + (BK Ca ) Channels

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Abstract
Angiotensin II (Ang II) is one of the most potent vasoconstrictor substances, yet paradoxically, Ang II may dilate certain vascular beds via an undefined mechanism. Ang II–induced vasoconstriction is mediated by the AT1 receptor, whereas the relative expression and functional importance of the AT2 receptor in regulating vascular resistance and blood pressure are unknown. We now report that Ang II induces relaxation of mesenteric microvessels and that this vasodilatory response was unaffected by losartan, an AT1 receptor antagonist, but was inhibited by PD123,319, a selective antagonist of AT2 receptors. In addition, reverse transcriptase–polymerase chain reaction studies revealed high amounts of AT2 receptor mRNA in smooth muscle from these same microvessels. Ang II–induced relaxation was inhibited by either tetraethylammonium or iberiotoxin, suggesting involvement of the large-conductance, calcium- and voltage-activated potassium (BKCa) channel. Subsequent whole-cell and single-channel patch-clamp studies on single myocytes demonstrated that Ang II increases the activity of BKCa channels. As in our tissue studies, the effect of Ang II on BKCa channels was inhibited by PD123,319, but not by losartan. In light of these consistent findings from tissue physiology, molecular studies, and cellular/molecular physiology, we conclude that Ang II relaxes microvessels via stimulation of the AT2 receptor with subsequent opening of BKCa channels, leading to membrane repolarization and vasodilation. These findings provide evidence for a novel endothelium-independent vasodilatory effect of Ang II.

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