Protection of atrial natriuretic factor against degradation: diuretic and natriuretic responses after in vivo inhibition of enkephalinase (EC 3.4.24.11) by acetorphan.

Abstract

Atrial natriuretic factor (ANF) might be beneficial in several cardiovascular disorders, but its poor oral absorption and rapid inactivation in vivo have so far prevented its use in therapeutics. We have assessed the role of enkephalinase (membrane metallo-endopeptidase, EC 3.4.24.11) in the in vivo inactivation of ANF in mice and healthy human volunteers by evaluating the effects of acetorphan, a potent inhibitor.In mice, the degradation of 125I-labeled ANF was markedly delayed, as shown by the levels of the intact peptide in the plasma and the kidney, a major target organ. The effect of acetorphan was due to the inhibition of enkephalinase activity, since it occurred at an EDs5 very close to this drug's ID50 for the inhibition of the specific binding of radioactive material to the kidney or lung peptidase that was measured after administration of [3H]acetorphan. The effects ofacetorphan were also studied in eight healthy human volunteers by using a randomized double-blind, placebo-controlled design. Oral administration of acetorphan elicited a lasting elevation of plasma ANFlike immunoreactivity, with a time course parallel to that of the inhibition of plasma enkephalinase activity. These effects were accompanied by significant increases in urinary volume and sodium excretion, two well-established renal responses to ANF peptides. These results indicate that enkephalinase plays a critical role in ANF degradation in vivo and that its inhibition enhances the levels of circulating endogenous ANF, which, in turn, results in diuresis and natriuresis. Enkephalinase inhibition may constitute another therapeutic approach to the treatment of cardiovascular diseases, such as congestive heart failure or essential hypertension, on which ANF is postulated to have a beneficial effect.

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

“…10,17 However, a reduction in blood pressure has not been clearly demonstrated in normotensive subjects, 10,18,20,21 and two studies have even reported an increase in blood pressure 22,23 despite the potent vasodilator actions of the natriuretic peptides. 12,13,16 Also, several studies on hypertensive patients 19,[32][33][34][35] have not demonstrated a reduction in blood pressure.…”

“…Therefore, increasing the circulating concentrations of ANP through inhibition of NEP is an attractive therapeutic approach to a number of cardiovascular diseases such as hypertension and heart failure. 16 However, although NEP inhibitors increase circulating ANP concentrations in humans and cause the expected natriuresis, 10,17-19 they do not generally lower blood pressure in normotensive subjects. 10,18,20,21 Indeed, both candoxatril 22 and candoxatrilat 23 have been reported as raising blood pressure in normotensive subjects.…”

“…16 However, although NEP inhibitors increase circulating ANP concentrations in humans and cause the expected natriuresis, 10,17-19 they do not generally lower blood pressure in normotensive subjects. 10,18,20,21 Indeed, both candoxatril 22 and candoxatrilat 23 have been reported as raising blood pressure in normotensive subjects. Although NEP inhibitors have been reported to lower blood pressure in patients with essential hypertension, 24 -31 this finding has not been universal.…”

“…NEP is inhibited by several agents, including candoxatrilat 8 ; thiorphan 9 and its prodrug, sinorphan 10 ; and phosphoramidon. 1 ANP has potent natriuretic 11 and vasodilator properties 12,13 and inhibits activity of the renin-angiotensin-aldosterone system by reducing both renin 14 and aldosterone 15 release.…”

A Three-dimensional Model of the Neprilysin 2 Active Site Based on the X-ray Structure of Neprilysin

“…The physiological implication of NEP in the inactivation of these messenger peptides was confirmed in vivo by the design and use of selective and potent inhibitors, e.g. thiorphan (21)(22)(23)(24). These functions of NEP and the design of specific inhibitors thereof are of considerable interest, because the inhibitors of NEP have recently been introduced in human gastroenterologic (25) and cardiovascular (26) therapeutic fields.…”