Ž . European Journal of Pharmacology 370 1999 307–312 Enzymatic inactivation of major circulating forms of atrial and brain natriuretic peptides Junko Ozaki a , Hiromasa Shimizu a , Yukiya Hashimoto a , Hiroshi Itoh b , Kazuwa Nakao b , Ken-ichi Inui a, ) a Department of Pharmacy Kyoto UniÕersity Hospital, Faculty of Medicine, Kyoto UniÕersity, Sakyo-ku, Kyoto 606-8507, Japan b Second DiÕision Department of Medicine, Kyoto UniÕersity Hospital, Faculty of Medicine, Kyoto UniÕersity, Sakyo-ku, Kyoto 606-8507, Japan Received 17 September 1998; received in revised form 12 February 1999; accepted 19 February 1999 Abstract Ž . We compared the enzymatic inactivation of major circulating forms of atrial natriuretic peptide ANP and brain natriuretic peptide Ž . Ž . BNP . Both ANP and BNP induced a significant increase in cyclic GMP cGMP formation in cultured epithelial cell line derived from porcine kidney, LLC-PK . The cGMP formation stimulated by ANP in LLC-PK cells was significantly decreased by pre-treatment of the 1 1 peptide with rat renal brush-border membranes, and the inactivation of ANP was inhibited by neutral endopeptidase inhibitors, phosphoramidon and S-thiorphan. BNP exhibited greater resistance to enzymatic inactivation than did ANP. In addition, phosphoramidon Ž y1 y1 . potentiated the natriuresis with a low dose 7.5 pmol min kg of ANP but not of BNP in rats. These results suggest that enzymatic degradation of natriuretic peptides is highly dependent on peptide structure, and that the affinity of BNP to neutral endopeptidase is less than that of ANP. q 1999 Elsevier Science B.V. All rights reserved. Ž . Ž . Ž . Keywords: ANP Atrial natriuretic peptide ; BNP Brain natriuretic peptide ; Kidney epithelial cell line LLC-PK ; Neutral endopeptidase; 1 Phosphoramidon; Thiorphan 1. Introduction The vasodilating and natriuretic actions of atrial natri- Ž . uretic peptide ANP are therapeutic for some cardio- vascular disorders including hypertension and congestive heart failure. Clinical application of ANP is, however, limited because of its short biological half-life and poor Ž bio-availability following oral administration Wilkins et . al., 1990 . On the other hand, endogenous circulating ANP levels are elevated in several diseases for which ANP is considered of clinical benefit. An alternative to administer- ing exogenous ANP is to enhance the activity of the endogenous circulating peptide by inhibiting its metabolism Ž . by neutral endopeptidase EC 3.4.24.11 . Endogenous ANP increases in cardiac failure rats produced by aortocaval fistula or myocardial infarction as compared to control rats, and treatment with the neutral endopeptidase in- ) Corresponding author. Tel.: q81-75-751-3577; Fax: q81-75-751- 4207; E-mail: inui@kuhp.kyoto-u.ac.jp hibitors, thiorphan and phosphoramidon, has significant Ž natriuretic effect in cardiac failure rats only Wilkins et al., . 1990; Yasuhara et al., 1994 . However, the circulating ANP concentration is increased only slightly by the admin- istration of neutral endopeptidase inhibitors. The findings suggest that the natriuretic response to neutral endopepti- dase inhibitors in cardiac failure rats cannot be explained simply in terms of an increase in circulating ANP levels, and that ANP filtered at the glomerulus is mainly responsi- ble for the ANP-potentiating natriuretic effect of neutral Ž endopeptidase inhibitors. Wilkins et al., 1990; Yasuhara et . al., 1994; Yamaguchi et al., 1998 . Neutral endopeptidase is involved in the metabolism of several other peptides including kinins, enkephalins, neu- Ž .Ž rotensins, and brain natriuretic peptide BNP Kenny and . Stephenson, 1988; Norman et al., 1991 . It should be noted that circulating BNP levels are also elevated in disease states in which the renal responses to neutral endopepti- Ž dase inhibitors are more pronounced Mukoyama et al., . 1990a; Hama et al., 1995 . BNP was initially isolated from 0014-2999r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0014-2999 99 00115-6