Angiotensin-converting enzyme-independent angiotensin formation in a humanmodel of myocardial ischemia: Modulation of norepinephrine release by angiotensin type 1 and angiotensin type 2 receptors

Angiotensin II (Ang II) promotes norepinephrine (NE) release from cardiac s ympathetic nerve endings. We assessed in a human model in vitro whether loc ally formed Ang II contributes to NE release in myocardial ischemia. Surgic al specimens of human right atrium were incubated in anoxic conditions. Aft er 70 min of anoxia, NE release (carrier-mediated; caused by NE transporter reversal) was 8-fold greater than normoxic release. Angiotensin-converting enzyme inhibition with enalaprilat failed to reduce anoxic NE release. In contrast, prevention of chymase- dependent Ang II formation with chymostati n, Bowman-Birk inhibitor, or alpha(1)-antitrypsin significantly inhibited a noxic, but not exocytotic, NE release. Two mast-cell stabilizers, cromolyn and lodoxamide, markedly reduced NE release, implicating cardiac mast cells as a major source of chymase. Angiotensin type 1 receptor (AT(1)R) blockad e with EXP3174 inhibited NE release, whereas angiotensin type 2 receptor (A T(2)R) blockade with PD123319 did not. Interestingly, PD123319 reversed the inhibitory effect of EXP3174. Furthermore, synergisms were uncovered betwe en EXP3174 and an AT(2)R agonist, and between EXP3174 and a Na+/H+ exchange r inhibitor. Thus, angiotensin- converting enzyme-independent Ang II format ion via chymase is important for carrier-mediated ischemic NE release in th e human heart. Locally generated Ang II promotes NE release by acting predo minantly at AT(1)Rs, which are likely coupled to the Na+/H+ exchanger. Effe cts of Ang II at AT(2)Rs, seemingly opposite to those resulting from AT(1)R activation, are uncovered when AT(1)Rs are blocked. Because NE release is associated with coronary vasoconstriction and arrhythmias, and mast-cell de nsity and chymase content increase in the ischemic heart, the notion that c hymase-generated Ang II plays a major role in carrier-mediated NE release m ay have important clinical implications.