Neutral endopeptidase 24.11 is important for the degradation of both endogenous and exogenous glucagon in anesthetized pigs

Abstract

Glucagon has a short plasma t1/2in vivo, with renal extraction playing a major role in its elimination. Glucagon is degraded by neutral endopeptidase (NEP) 24.11 in vitro, but the physiological relevance of NEP 24.11 in glucagon metabolism is unknown. Therefore, the influence of candoxatril, a selective NEP inhibitor, on plasma levels of endogenous and exogenous glucagon was examined in anesthetized pigs. Candoxatril increased endogenous glucagon concentrations, from 6.3 ± 2.5 to 20.7 ± 6.3 pmol/l [COOH-terminal (C)-RIA, P < 0.05]. During glucagon infusion, candoxatril increased the t1/2determined by C-RIA (from 3.0 ± 0.5 to 17.0 ± 2.5 min, P < 0.005) and midregion (M)-RIA (2.8 ± 0.5 to 17.0 ± 3.0 min, P < 0.01) and reduced metabolic clearance rates (MCR; 19.1 ± 3.2 to 9.4 ± 2.0 ml·kg−1·min−1, P < 0.02, C-RIA; 19.2 ± 4.8 to 9.0 ± 2.3 ml·kg−1·min−1, P < 0.05, M-RIA). However, neither t1/2nor MCR determined by NH2-terminal (N)-RIA were significantly affected ( t1/2, 2.7 ± 0.4 to 4.5 ± 1.6 min; MCR, 30.3 ± 6.4 to 28.5 ± 9.0 ml·kg−1·min−1), suggesting that candoxatril had no effect on NH2-terminal degradation but leads to the accumulation of NH2-terminally truncated forms of glucagon. Determination of arteriovenous glucagon concentration differences revealed that renal glucagon extraction was reduced (but not eliminated) by candoxatril (from 40.4 ± 3.8 to 18.6 ± 4.1%, P < 0.02, C-RIA; 29.2 ± 3.1 to 14.7 ± 2.2%, P < 0.02, M-RIA; 26.5 ± 4.0 to 19.7 ± 3.5%, P < 0.06, N-RIA). Femoral extraction was reduced by candoxatril when determined by C-RIA (from 22.7 ± 2.4 to 8.0 ± 5.1%, P < 0.05) but was not changed significantly when determined using M- or N-RIAs (10.0 ± 2.8 to 4.7 ± 3.7%, M-RIA; 10.5 ± 2.5 to 7.8 ± 4.2%, N-RIA). This study provides evidence that NEP 24.11 is an important mediator of the degradation of both endogenous and exogenous glucagon in vivo.