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Phosphoramidon inhibition of the in vivo conversion of big endothelin‐1 to endothelin‐1 in the human forearm
Author(s) -
Plumpton Christopher,
Haynes William G.,
Webb David J.,
Davenport Anthony P.
Publication year - 1995
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1995.tb16669.x
Subject(s) - phosphoramidon , chemistry , medicine , endothelin receptor , endocrinology , endothelin 1 , radioimmunoassay , basal (medicine) , endothelins , venous blood , receptor , insulin
1 The vasoconstrictor peptide, endothelin‐1 (ET‐1) and a biologically inactive C‐terminal fragment (CTF) are generated from an intermediate big ET‐1 by a putative ET converting enzyme, sensitive to phosphoramidon. We have developed a procedure using selective solid‐phase extraction and specific radioimmunoassays to measure the levels of immunoreactive (IR) big ET‐1 and the products of conversion (ET‐1 and CTF) in human plasma. These techniques have been used to determine the levels of the three peptides in venous plasma following local infusions of ET‐1 and big ET‐1, both alone and together with phosphoramidon 2 Infusion of ET‐1 into the brachial artery (5 pmol min −1 ) significantly increased (P<0.05) IR ET levels from a basal level of 2.3 pM to 55.2 pM in plasma from the infused arm after 60 min of infusion. This corresponded with a marked decrease in forearm blood flow from a basal level of 2.6 ml dl −1 min −1 to 1.7 ml dl −1 min −1 . The levels of IR big ET‐1 and CTF were unchanged. Co‐infusion of phosphoramidon (30 nmol min −1 ) with ET‐1 had no significant effect on the plasma IR levels of ET, big ET‐1, CTF, or blood flow 3 Big ET‐1 (50 pmol min −1 ) significantly increased ( P < 0.05) venous concentrations of all three IR peptides after 60 min compared to basal (ET: from 2.2 to 7.7 pM, big ET‐1: from 0 to 386.0 pM, CTF: from 0.2 to 37.0 pM). Forearm blood flow decreased significantly ( P < 0.05) from a basal level of 3.0 ml dl −1 min −1 to 1.6 ml dl −1 min −14 When phosphoramidon was co‐infused with big ET‐1, both the rise in IR ET and associated vasoconstriction were abolished. However, IR CTF was still detected, suggesting that either some conversion by phosphoramidon‐insensitive enzyme(s) was occurring, and/or that CTF was being protected from further degradation by phosphoramidon 5 These data show that in the human forearm the activity of a phosphoramidon‐sensitive ET converting enzyme is at least in part responsible for the vasoconstrictor properties of exogenous big ET‐1. Furthermore, because measurable levels of newly synthesized ET‐1 are likely to be rapidly reduced in the blood/plasma through receptor binding, assay of IR big ET‐1 and CTF may be a more sensitive measure of ET‐1 generation in disease.