Vascular renin-angiotensin system and neurotransmission in hypertensive persons

The existence of a vascular renin-angiotensin system and its role in modulating sympathetic activity were evaluated in forearm arterioles of hypertensive individuals. Isoproterenol (0.03, 0.01, 0.3 microgram/100 ml/min for 5 minutes each; n = 5) was infused into the brachial artery, and active and inactive renin, angiotensin II, and norepinephrine forearm balance (venous-arterial differences corrected for forearm blood flow by strain-gauge plethysmography) were measured. Isoproterenol caused vasodilation and a dose-dependent active and inactive renin, angiotensin II, and norepinephrine outflow, an effect blunted by propranolol (10 micrograms/100 ml/min). To evaluate the role of local angiotensin II on beta-mediated norepinephrine overflow, the experiment was repeated with captopril (2.5 micrograms/100 ml/min for 10 minutes; n = 5), which abolished angiotensin II release and significantly reduced norepinephrine overflow. To test whether angiotensin II facilitates both prejunctional norepinephrine release and its postjunctional action, we evaluated the effect of exogenous angiotensin II, infused into the brachial artery at low concentrations (0.001 microgram/100 ml/min), on forearm vasoconstriction and norepinephrine release induced by endogenous sympathetic activation (application of a lower body negative pressure: -10 and -20 mm Hg for 5 minutes, n = 10) and on the vasoconstrictor effect of local norepinephrine (0.0015, 0.005, 0.015, 0.05, 0.15 micrograms/100 ml/min for 3 minutes each; n = 6). Although angiotensin II increased the vasoconstricting effect and the norepinephrine release induced by lower body negative pressure, it failed to affect norepinephrine-mediated vasoconstriction. Our data indicate the existence in hypertensive individuals of a vascular renin-angiotensin system that seems to modulate sympathetic activity through the presynaptic facilitation of norepinephrine release.