Endothelin-Induced Increased Nitric Oxide Mediates Augmented Distal Nephron Acidification as a Result of Dietary Protein

Abstract

Tested was the hypothesis that enhanced nitric oxide (NO) production that is stimulated by increased renal endothelin activity mediates decreased distal nephron HCO 3 secretion that is induced by dietary protein. Munich-Wistar rats that ate minimum electrolyte diets with 50% casein-provided protein (HiPro) compared with controls that ate 20% protein for 3 wk had higher urine excretion of endothelin-1 (80 ؎ 15.7 versus 29 ؎ 3.9 fmol/kg body wt per d; P < 0.02) and of the NO metabolites NO 2 /NO 3 (21.2 ؎ 1.9 versus 14.9 ؎ 0.8 mol/kg body wt per d; P < 0.03). Bosentan, an endothelin A/B receptor antagonist, reduced HiPro rats' urine excretion of net acid (5859 ؎ 654 versus 8017 ؎ 1103 mol/d; P < 0.03, paired t test) and NO 2 /NO 3 (18.1 ؎ 1.1 versus 22.9 ؎ 2.0 mole/kg body wt per d; P < 0.05, paired t test). N-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, also decreased urine net acid excretion (6621 ؎ 717 versus 8449 ؎ 1086 mol/d; P < 0.05, paired t test) but was not additive to bosentan. L-NAME increased in situ late distal nephron HCO 3 delivery in HiPro rats (18.8 ؎ 1.7 versus 9.6 ؎ 1.4 pmol/mm per min; P < 0.001) that was mediated by increased distal nephron HCO 3 secretion (؊7.2 ؎ 0.7 versus ؊3.5 ؎ 0.4 pmol/mm per min; P < 0. (3), the mechanism by which endothelin reduces distal nephron HCO 3 secretion is not known. In addition to increasing renal production of endothelin, increased dietary protein increases urine excretion of nitric oxide (NO) metabolites (4). Furthermore, endothelin increases renal NO production (5), and NO increases acidification in the proximal (6) and distal (7) nephron. Consequently, we tested the hypothesis that increased renal endothelin production that is induced by increased dietary protein reduces distal nephron HCO 3 secretion through increased NO. Materials and Methods Animals and Diet ProtocolMale and female Munich-Wistar rats (Harlan Sprague-Dawley, Houston, TX; 200 to 228 g) ate standard rat chow (Prolab RMH 2500 with 23% protein, Purina, Indianapolis, IN) for 1 wk, then ate a custom minimum electrolyte diet with protein as purified high nitrogen casein (ICN Nutritional Biochemicals, Cleveland, OH) for 3 wk and drank distilled H 2 O ad libitum. Rats that were on the high-protein diet (HiPro) ate custom diet with 50% protein, and controls ate 20%. In preliminary studies, similarweight rats ate 24.6 Ϯ 0.9 and 27.1 Ϯ 1.2 g/d, respectively (n ϭ 4, P ϭ 0.15), so all rats received 24 g/d diet to ensure similar diet intake and complete ingestion of drug mixed with diet in rats that were given drugs. Some received bosentan (Actelion, Allschwil, Switzerland), a nonpeptide endothelin A/B receptor antagonist (8), mixed with study diet at 100 mg/kg body wt per d. This oral dose blocks action of pressor doses of intravenous big ET-1 for Ͼ24 h (8). Preliminary studies determined that the minimal chronic dose of the NO synthase (NOS) inhibitor N-nitro-l-arginine methyl ester (L-NAME) that reduced urine net acid excretion (NAE) in paired HiPro rats (6451 Ϯ 785 versus 848...

Dietary protein induces endothelin-mediated kidney injury through enhanced intrinsic acid production

“…Higher urine NAE in 50 than 20% CAS was due to higher ammonium (NH 4 þ ) excretion and lower HCO 3 excretion but titratable acidity (TA) excretion was similar as shown previously. 5,7 Urine NAE was lower in 50% CAS eating NaHCO 3 (36587470 and 35697387 mm/day at 12 and 96 weeks, respectively) than 50% CAS eating either NaCl (77547816 and 78727984 mm/day, Po0.002 vs NaHCO 3 ) or Na 2 SO 4 (79637902 and 81977996 mm/day, Po0.003 vs NaHCO 3 ). Lower NAE in NaHCO 3 animals was predominantly due to lower NH 4 þ excretion compared with the NaCl-and Na 2 SO 4 eating 50% CAS (19397207 vs 46247527 and 50997628 mm/day, respectively, Po0.001).…”

“…3,4 High protein diet (HiPro) as CAS induces endothelin-mediated kidney acidification that mediates acid excretion in this setting of increased intrinsic acid production. [5][6][7] Endothelin has been implicated as a mediator of GS and TII 8,9 and so might contribute to GS and TII associated with HiPro. Consequently, GS and TII observed with HiPro might be a long-term detrimental consequence of mechanisms that achieve the short-term benefit of increased acid excretion in response to increased intake of acidproducing protein.…”

“…Male Munich-Wistar rats are at higher risk for GS and TII with aging compared with other strains, 10 and it was in this strain that HiPro-induced, endothelin-mediated augmented kidney acidification was shown. [5][6][7] These studies tested the following hypotheses: (1) HiPro-induced GS and TII are mediated through increased intrinsic acid production and (2) enhanced endothelin action mediates GS and TII in this setting.…”

Nutritional and Non-nutritional Management of the Nephrotic Syndrome

“…As a consequence, urinary albumin excretion increases when patients or animals with the nephrotic syndrome are fed a high protein diet [22,23,57e59], although a component of increased albuminuria may be a consequence of the added dietary acid load associated with protein intake acting through an endothelin mediated mechanism. [60,61]. The increase in albuminuria is not a consequence of increased plasma albumin levels.…”

“…Accelerated rates of renal ammonia production that result from increased dietary acid loads which accompany high protein diets may also lead to renal injury, possibly by the activation of complement [85,86]. Higher hydrogen ion concentrations enhance generation of endothelin-1 in the kidney, which also promotes progressive renal injury as well as increasing proteinuria [60,61] (see Chapter 18). The reduction in urinary protein excretion that follows dietary protein restriction potentially can have a salutary effect on progressive renal injury through any or all of these mechanisms.…”