Effect of angiotensin II on noradrenaline release in the human forearm

The aim was to test the hypothesis that in normal humans angiotensin II would stimulate local release of noradrenaline under basal conditions or during a sympathetic stimulus provided by lower body negative pressure (LBNP).

Nine healthy volunteers received intra-arterial infusions of angiotensin II, 5 ng.min-1, into the non-dominant forearm. Forearm blood flow (strain gauge plethysmography) and regional noradrenaline spillover (using the tracer methodology of Esler) were measured during angiotensin II alone, LBNP alone, and LBNP plus angiotensin II.

Angiotensin II and LBNP decreased forearm blood flow comparably: from 3.1(SD 1.5) to 2.4 (0.9) ml.100 g-1.min-1 during angiotensin II, p < 0.05; and from 3.3(1.5) to 2.5(1.0) ml.100 g-1.min-1 during LBNP, p < 0.05 (p = NS, A-II v LBNP). Angiotensin II had no effect on forearm venous noradrenaline or regional noradrenaline spillover. LBNP increased venous noradrenaline outflow from the forearm, from 1.6(0.40) to 2.1(0.6) nmol.min-1 (p < 0.05), while regional noradrenaline spillover tended to increase, rising from 1.5(0.8) to 2.0(1.0) nmol.100 ml-1.min-1. Angiotensin II did not enhance forearm blood flow or noradrenaline responses to LBNP.

In the human forearm, mildly vasoconstrictor infusions of angiotensin II do not increase local release of noradrenaline, either alone or during mild LBNP. At least under these conditions, angiotensin II would not appear to be a potent influence on local sympathetic activity.