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(Circulation. 1998;97:2323-2330.)
© 1998 American Heart Association, Inc.

Clinical Investigation and Reports

Inhibition of Neutral Endopeptidase Causes Vasoconstriction of Human Resistance Vessels In Vivo

Charles J. Ferro, BSc, MRCP; James C. Spratt, BSc, MRCP; William G. Haynes, MD, MRCP; ; David J. Webb, MD, FRCP

From the Clinical Pharmacology Unit and Research Centre, University of Edinburgh, Western General Hospital, Edinburgh, UK.

Correspondence to Professor D.J. Webb, Clinical Pharmacology Unit and Research Centre, University of Edinburgh, Western General Hospital, Edinburgh EH4 2XU, UK. E-mail d.j.webb{at}.ed.ac.uk

Background—Neutral endopeptidase (NEP) degrades vasoactive peptides, including the natriuretic peptides, angiotensin II, and endothelin-1. Systemic inhibition of NEP does not consistently lower blood pressure, even though it increases natriuretic peptide concentrations and causes natriuresis and diuresis. We therefore investigated the direct effects of local inhibition of NEP on forearm resistance vessel tone.

Methods and Results—Four separate studies were performed, each with 90-minute drug infusions. In the first study, 10 healthy subjects received a brachial artery infusion of the NEP inhibitor candoxatrilat (125 nmol/min), which caused a slowly progressive forearm vasoconstriction (12±2%; P=0.001). In a second two-phase study, 6 healthy subjects received, 4 hours after enalapril (20 mg) or placebo, an intra-arterial infusion of the NEP inhibitor thiorphan (30 nmol/min). Thiorphan caused similar degrees of local forearm vasoconstriction (P=0.6) after pretreatment with both placebo (13±1%, P=0.006) and enalapril (17±6%, P=0.05). In a third three-phase study, 8 healthy subjects received intra-arterial thiorphan (30 nmol/min), the endothelin ET_A antagonist BQ-123 (100 nmol/min), and both combined. Thiorphan caused local forearm vasoconstriction (13±1%, P=0.0001); BQ-123 caused local vasodilatation (33±3%, P=0.0001). Combined thiorphan and BQ-123 caused vasodilatation (32±1%, P=0.0001) similar to BQ-123 alone (P=0.98). In a fourth study, 6 hypertensive patients (blood pressure >160/100 mm Hg) received intra-arterial thiorphan (30 nmol/min). Thiorphan caused a slowly progressive forearm vasoconstriction (10±2%, P=0.0001).

Conclusions—Inhibition of local NEP causes vasoconstriction in forearm resistance vessels of both healthy volunteers and patients with hypertension. The lack of effect of ACE inhibition on the vasoconstriction produced by thiorphan and its absence during concomitant ET_A receptor blockade suggest that it is mediated by endothelin-1 and not angiotensin II. These findings may help to explain the failure of systemic NEP inhibition to lower blood pressure.

Key Words: natriuretic peptides • vasoconstriction • endothelin • angiotensin II • human

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