1. In animal studies, angiotensin II facilitates adrenergic neurotransmission by both pre- and postsynaptic mechanisms. We have investigated whether this interaction occurs in forearm resistance vessels in man.

2. The effect of arterial infusion of angiotensin II (320 fmol/min) on sympathetic vasoconstriction produced by lower-body negative pressure (15 mmHg) was studied in six subjects, and that on the response to exogenous noradrenaline (37.5–150 pmol/min) was studied in a further eight subjects. Forearm blood flow was measured by strain-gauge plethysmography.

3. The dose of angiotensin II was chosen to produce no alteration in resting blood flow, and those of noradrenaline were selected to provide a reduction in blood flow equivalent to that produced by lower-body negative pressure.

4. Lower-body negative pressure produced no change in heart rate or diastolic blood pressure, but caused an initial 5 mmHg fall in systolic blood pressure (P < 0.01). Blood flow was reduced by 21 ± 6% in both forearms by lower-body negative pressure during saline infusion. During angiotensin II infusion, there was a marked difference in the response to lower-body negative pressure, with blood flow being reduced by 40 ± 7% in the infused arm, but only by 21 ± 4% in the control arm (P < 0.05). Angiotensin II infusion had no effect on resting blood flow or the responses to noradrenaline.

5. We conclude that angiotensin II augments sympathetic vasoconstriction in forearm resistance vessels in man at a concentration that has no direct effect on blood flow. The absence of an effect of angiotensin II on the response to noradrenaline suggests that augmentation of sympathetic vasoconstriction occurs pre-synaptically through facilitation of noradrenaline release.

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