On the existence of functional angiotensin II receptors on vascular sympathetic nerve terminals in the human forearm

Objective: The existence of presynaptic angiotensin II receptors, modulating the release of the sympathetic neurotransmitter norepinephrine, was examined in the perfused human forearm model. Design and methods: In three groups out of a total of 20 healthy volunteers, intra-arterial infusions of tracer amounts of tritiated norepinephrine were given to measure forearm spillover and total plasma appearance rate of norepinephrine during intra-arterial infusions of angiotensin II (0.02, 0.2 and 2 ng/kg per min), methoxamine (0.08, 0.4 and 2 μg/kg per min) to produce vasoconstriction by stimulating α₁-adrenoceptors and saralasin (0.5 ng/kg per min) to block angiotensin II receptors. Postganglionic sympathoneural activity was stimulated by intravenous infusion of sodium nitroprusside (about 1.3 ng/kg per min) or attenuated by intravenous infusion of trimethaphan (about 1 mg/min). Results: Angiotensin II failed to increase the spillover and total plasma appearance rate of norepinephrine, when given without additional treatment or during sodium nitroprusside or trimethaphan administration. In contrast, the spillover of norepinephrine even decreased during angiotensin II administration, both before and during intravenous sodium nitroprusside administration, probably because of angiotensin ll-induced forearm vasoconstriction. Similar vasoconstrictor doses of angiotensin II and methoxamine produced similar changes in spillover and total plasma appearance rate of norepinephrine. The highest dose of angiotensin II increased diastolic blood pressure by 9% and decreased the pulse rate by 6%. Saralasin affected the spillover and total plasma appearance rate of norepinephrine neither before nor during intravenous sodium nitroprusside infusion. Conclusion: The present results fail to support the view that angiotensin II receptors exert stimulatory modulatory effects on norepinephrine release from sympathetic nerves in the human forearm, at rest or during changes in sympathoneural outflow.