Roland C. Blantz

Angiotensin II effects upon the glomerular microcirculation and ultrafiltration coefficient of the rat.

The effects of both synthetic and biologically produced angiotensin II (AII) upon the process of glolerular filtration were examined in the plasma-expanded (2.5% body wt) Munich-Wistar rat, by micropuncture evaluation of pressures, nephron plasma flow (rpf) and filtration rate (sngfr). Plasma expansion was chosen as a control condition because (a) response to AII was uniform and predictable, (b) endogenous generation of AII was presumably suppressed, and (c) the high control values for rpf permitted accurate determination of values for the glomerular permeability coefficient (LpA) before and during AII infusion. With subpressor quantities of synthetic Asn-1, Val-5 AII (less than 5 ng/100 g body wt/min), sngfr fell from 47.7 in the control group to 39.8 nl/min/g kidney (P less than 0.005). The rpf fell to 60% of control values (P less than 0.001). Measurement of glomerular capillary (PG) and Bowmans space (Pt) hydrostatic pressures in surface glomeruli with a servo-nulling device permitted evaluation of the hydrostatic pressure gradient (deltaP = PG - Pi). DeltaP increased from 38.1 +/- 1.2 in control to 45.9 +/- 1.3 mm Hg after Asn-1, Val-5 AII and essentially neutralized the effect of decreased rpf in sngfr. The sngfr then fell as a result of a decreased in LpA from 0.063 +/- 0.008 in control to 0.028 +/- 0.004 nl/s/g kidney/mm Hg after Asn-1, Val-5 AII (P less than 0.02). Lower doses of Asp-1, Ile-5 AII (less than 3 ng/100 g body wt/min) had no effect on sngfr, rpf, deltaP, and afferent and efferent vascular resistance, but significantly elevated systemic blood pressure, suggesting peripheral effects on smooth muscle at this low dose. LpA was 0.044 +/- 0.007 nl/s/g kidney/mm Hg after low-dose Asp-1, Ile-5 AII, and 0.063 +/- 0.008 in the control group (0.02 greater than P greater than 0.1). Higher, equally pressor doses of native AII (5 ng/100 g body wt/min) produced effects almost identical to similar quantites of synthetic Asn-1, Val-5 AII upon rpf, deltaP, sngfr, and renal vascular resistance. LpA again fell to 0.026 +/- 0.004 nl/s/g kidney/mn Hg, a value almost identical to that after the synthetic AII. Paired studies with Asp-1, Ile-5 AII also demonstrated a consistent reduction in LpA.

Inhibition of constitutive nitric oxide synthase (NOS) by nitric oxide generated by inducible NOS after lipopolysaccharide administration provokes renal dysfunction in rats.

Excess NO generation plays a major role in the hypotension and systemic vasodilatation characteristic of sepsis. Yet the kidney response to sepsis is characterized by vasoconstriction resulting in renal dysfunction. We have examined the roles of inducible nitric oxide synthase (iNOS) and endothelial NOS (eNOS) on the renal effects of lipopolysaccharide administration by comparing the effects of specific iNOS inhibition, -N6-(1-iminoethyl)lysine (L-NIL), and 2,4-diamino6-hydroxy-pyrimidine vs. nonspecific NOS inhibitors (nitro- -arginine-methylester). cGMP responses to carbamylcholine (CCh) (stimulated, basal) and sodium nitroprusside in isolated glomeruli were used as indices of eNOS and guanylate cyclase (GC) activity, respectively. LPS significantly decreased blood pressure and GFR (112+/-4 vs. 83+/-4 mmHg; 2.66+/-0.29 vs. 0. 96+/-0.22 ml/min, P < 0.05) and inhibited the cGMP response to CCh. GC activity was reciprocally increased. L-NIL and 2, 4-diamino-6-hydroxy-pyrimidine administration prevented the decrease in GFR (2.71+/-0.28 and 3.16+/-0.18 ml/min, respectively), restored the normal response to CCh, and GC activity was normalized. In vitro application of L-NIL also restored CCh responses in LPS glomeruli. Neuronal NOS inhibitors verified that CCh responses reflected eNOS activity. L-NAME, a nonspecific inhibitor, worsened GFR (0.41+/-0.15 ml/min), a reduction that was functional and not related to glomerular thrombosis, and eliminated the CCh response. No differences were observed in eNOS mRNA expression among the experimental groups. Selective iNOS inhibition prevents reductions in GFR, whereas nonselective inhibition of NOS further decreases GFR. These findings suggest that the decrease in GFR after LPS is due to local inhibition of eNOS by iNOS, possibly via NO autoinhibition.

Nitric oxide and angiotensin II. Glomerular and tubular interaction in the rat.

Nitric oxide (NO) has been proposed to modulate the renal response to protein as well as basal renal hemodynamics. We investigated whether NO and angiotensin II (AII) interact to control glomerular hemodynamics and absolute proximal tubular reabsorption (APR) during glycine infusion and in unstimulated conditions. In control rats, glycine increased single nephron GFR and plasma flow with no change in APR. The NO synthase blocker, NG-monomethyl L-arginine (LNMMA), abolished the vasodilatory response to glycine, possibly through activation of tubuloglomerular feedback due to a decrease in APR produced by LNMMA + glycine. Pretreatment with an AII receptor antagonist, DuP 753, normalized the response to glycine at both glomerular and tubular levels. In unstimulated conditions, LNMMA produced glomerular arteriolar vasoconstriction, decreased the glomerular ultrafiltration coefficient, and reduced single nephron GFR. These changes were associated with a striking decrease in APR. DuP 753 prevented both glomerular and tubular changes induced by LNMMA. In conclusion, NO represents a physiological antagonist of AII at both the glomerulus and tubule in both the basal state and during glycine infusion; and inhibition of NO apparently enhances or uncovers the inhibitory effect of AII on proximal reabsorption.

Glomerular Hyperfiltration and the Salt Paradox in Early Type 1 Diabetes Mellitus: A Tubulo-Centric View

ABSTRACT. Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. This review proposes a “tubulo-centric” view of glomerular function in early type I diabetes mellitus. The following are particularly discussed ( 1 ) the primary role of an increase in reabsorption by the proximal tubule in early glomerular hyperfiltration, ( 2 ) the role of sodium-glucose cotransport and tubular growth under these conditions, and ( 3 ) the primary role of reabsorption by the proximal tubule for the paradoxical relationship between dietary salt and glomerular filtration rate. Finally, an outline is presented of potential therapeutic implications for the prevention of diabetic kidney disease. E-mail: volker.vallon@uni-tuebingen.de

Agmatine, a bioactive metabolite of arginine. Production, degradation, and functional effects in the kidney of the rat.

Until recently, conversion of arginine to agmatine by arginine decarboxylase (ADC) was considered important only in plants and bacteria. In the following, we demonstrate ADC activity in the membrane-enriched fraction of brain, liver, and kidney cortex and medulla by radiochemical assay. Diamine oxidase, an enzyme shown here to metabolize agmatine, was localized by immunohistochemistry in kidney glomeruli and other nonrenal cells. Production of labeled agmatine, citrulline, and ornithine from [3H]arginine was demonstrated and endogenous agmatine levels (10(-6)M) in plasma ultrafiltrate and kidney were measured by HPLC. Microperfusion of agmatine into renal interstitium and into the urinary space of surface glomeruli of Wistar-Frömter rats produced reversible increases in nephron filtration rate (SNGFR) and absolute proximal reabsorption (APR). Renal denervation did not alter SNGFR effects but prevented APR changes. Yohimbine (an alpha 2 antagonist) microperfusion into the urinary space produced opposite effects to that of agmatine. Microperfusion of urinary space with BU-224 (microM), a synthetic imidazoline2 (I2) agonist, duplicated agmatine effects on SNGFR but not APR whereas an I1 agonist had no effect. Agmatine effects on SNGFR and APR are not only dissociable but appear to be mediated by different mechanisms. The production and degradation of this biologically active substance derived from arginine constitutes a novel endogenous regulatory system in the kidney.

Ischemic acute renal failure and antioxidant therapy in the rat. The relation between glomerular and tubular dysfunction.

The effects of antioxidant therapy with probucol were evaluated in rats subjected to 1 h renal ischemia and to 24 h reperfusion. Probucol exerted significant antioxidant effects in renal cortical tubules in vitro when exposed to a catalase-resistant oxidant. At 24 h probucol treatment (IP) improved single nephron glomerular filtration rate (SNGFR) (28.1 +/- 3.3 nl/min) in comparison to untreated ischemic (I) rats (15.2 +/- 3.0), primarily as a result of improving SNGFR in a population of low SNGFR, low flow and/or obstructed nephrons. However, absolute proximal reabsorption remained abnormally low in IP rats at 24 h (5.9 +/- 0.8 nl/min), and cell necrosis was greater than in I rats. Kidney GFR remained low in IP rats due to extensive tubular backleak of inulin measured by microinjection studies. Evaluations after 2 h of reperfusion revealed a higher SNGFR in IP (36 +/- 3.1 nl/min) than I rats (20.8 +/- 2.7 nl/min). Absolute proximal reabsorption was essentially normal (11.6 +/- 1.3 nl/min) in IP rats, which was higher than IP rats at 24 h and the concurrent I rats. Administration of the lipophilic antioxidant, probucol, increased SNGFR and proximal tubular reabsorption within 2 h after ischemic renal failure. Although SNGFR remained higher than I rats at 24 h, absolute reabsorption fell below normal levels and tubular necrosis was more extensive in IP rats. Early improvement in nephron filtration with antioxidants may increase load dependent metabolic demand upon tubules and increase the extent of damage and transport dysfunction.

Acetaminophen : Acute and chronic effects on renal function

Acetaminophen (APAP) is normally metabolized in the liver and kidney by P450 enzymes. No toxicity is observed with therapeutic doses of APAP. However, after ingestion of large quantities of APAP (>2,000 mg/kg), highly reactive quinones, metabolites of APAP, are generated; these react with glutathione and sulfhydryl groups on critical proteins, resulting in cellular dysfunction and hepatic and renal toxicity. The P450 metabolizing enzymes differ somewhat in character between the liver and kidney. Factors that enhance renal toxicity include chronic liver disease, possibly gender, concurrent renal insults, and conditions that alter the activity of P450-metabolizing enzyme systems. Acute renal toxicity is characterized by cellular injury primarily confined to the proximal tubule and significant reductions in glomerular filtration rate. However, there is little evidence that chronic administration of APAP contributes to chronic renal disease and analgesic nephropathy. The only report on this subject suggests that combination therapy with aspirin is required for medullary damage in rats. No evidence exists for the development of chronic analgesic nephropathy with APAP alone. Epidemiologic studies in healthy individuals have failed to demonstrate a significant correlation between APAP use and chronic renal disease and classic analgesic nephropathy. Therefore, large doses of APAP can produce both renal and hepatic failure, but little evidence exists for production of classic analgesic nephropathy with the use of APAP alone.

Effect of Mannitol on Glomerular Ultrafiltration in the Hydropenic Rat

The effect of mannitol upon glomerular ultrafiltration was examined in hydropenic Munich-Wistar rats. Superficial nephron filtration rate (sngfr) rose from 32.0+/-0.9 nl/min/g kidney wt to 42.0+/-1.6 (P < 0.001) in eight rats. Hydrostatic pressure gradients acting across the glomerular capillary (DeltaP) were measured in glomerular capillaries and Bowmans space with a servo-nulling device, systemic (piA) and efferent arteriolar oncotic pressures (piE) were determined by microprotein analysis. These data were applied to a computer-based mathematical model of glomerular ultrafiltration to determine the profile of effective filtration pressure (EFP = DeltaP - pi) and total glomerular permeability (L(p)A) in both states. Filtration equilibrium obtained in hydropenia (L(p)A >/= 0.099+/-0.006 nl/s/g kidney wt/mm Hg) and sngfr rose because EFP increased from a maximum value of 4.2+/-1.1 to 12.8+/-0.5 mm Hg after mannitol (P <0.01). This increase was due to both increased nephron plasma flow and decreased piA. Computer analysis of these data revealed that more than half (>58%) of this increase was due to decreased piA, consequent to dilution of protein. Since EFP was disequilibrated after mannitol, L(p)A could be calculated accurately (0.065 +/- 0.003 nl/s/g kidney wt/mm Hg) and was significantly lower than the minimum estimate in hydropenia.Therefore, sngfr does increase with mannitol and this increase is not wholly dependent upon an increase in nephron plasma flow since the major factor increasing EFP was decreased piA.

Determinants of kidney oxygen consumption and their relationship to tissue oxygen tension in diabetes and hypertension

The high renal oxygen (O2) demand is associated primarily with tubular O2 consumption (Qo2) necessary for solute reabsorption. Increasing O2 delivery relative to demand via increased blood flow results in augmented tubular electrolyte load following elevated glomerular filtration, which, in turn, increases metabolic demand. Consequently, elevated kidney metabolism results in decreased tissue oxygen tension. The metabolic efficiency for solute transport (Qo2/TNa) varies not only between different nephron sites, but also under different conditions of fluid homeostasis and disease. Contributing mechanisms include the presence of different Na+ transporters, different levels of oxidative stress and segmental tubular dysfunction. Sustained hyperglycaemia results in increased kidney Qo2, partly due to mitochondrial dysfunction and reduced electrolyte transport efficiency. This results in intrarenal tissue hypoxia because the increased Qo2 is not matched by a similar increase in O2 delivery. Hypertension leads to renal hypoxia, mediated by increased angiotensin receptor tonus and oxidative stress. Reduced uptake in the proximal tubule increases load to the thick ascending limb. There, the increased load is reabsorbed, but at greater O2 cost. The combination of hypertension, angiotensin II and oxidative stress initiates events leading to renal damage and reduced function. Tissue hypoxia is now recognized as a unifying pathway to chronic kidney disease. We have gained good knowledge about major changes in O2 metabolism occurring in diabetic and hypertensive kidneys. However, further efforts are needed to elucidate how these alterations can be prevented or reversed before translation into clinical practice.

Role of Nitric Oxide in Inflammatory Conditions

Nitric oxide (NO) plays an important regulatory/modulatory role in a variety of inflammatory conditions. NO is a small, short-lived molecule that is released from a variety of cells in response to homeostatic and pathologic stimuli. It may act as a vasodilator and a platelet inhibitor and may interfere with adhesion molecules to prevent neutrophil adhesion. NO release may also lead to the formation of highly reactive species such as peroxynitrite and stable nitrosothiols and may cause mitochondrial damage and nitration of protein tyrosine residues. In addition, NO inhibits cell proliferation via inhibition of polyamine synthesis and cell uptake and may well act as a ‘brake’ on the proliferative response following cytokine exposure. All three isoforms of nitric oxide synthases are found in the kidney during inflammation. The site of NO release impacts significantly on its net function and structural impact. NO plays a protective role in many forms of immune injury, such as nephrotoxic serum-induced glomerulonephritis, autoimmune tubular interstitital nephritis, and experimental allergic encephalomyelitis. NO overproduction in sepsis, after cytokine exposure, inducible NO synthase transcription, and local inflammation can autoinhibit endothelial NO synthase, leading to selective renal and mesenteric vasoconstriction.