Glomerular hemodynamics in mercury-induced acute renal failure

Changes in the renal function after acute mercuric chloride exposure in the rat are associated with renal vascular endothelial dysfunction and proximal tubule NHE3 inhibition

Mercury is an environmental pollutant and a threat to human health. Mercuric chloride (HgCl₂)-induced acute renal failure has been described by several reports, but the mechanisms of renal dysfunction remain elusive. This study tested the hypothesis that HgCl₂ directly impairs renal vascular reactivity. Additionally, due to the mercury toxicity on the proximal tubule, we investigated whether the HgCl₂-induced natriuresis is accompanied by inhibition of Na⁺/H⁺ exchanger isoform-3 (NHE3). We found that 90-min HgCl₂ infusion (6.5 μg/kg i.v.) remarkably increased urinary output, reduced GFR and renal blood flow, and increased vascular resistance in rats. "In vitro" experiments of HgCl₂ infusion in isolated renal vascular bed demonstrated an elevation of perfusion pressure in a concentration- and time-dependent manner, associated with changes on the endothelium-dependent vasodilatation and the flow-pressure relationship. Moreover, by employing "in vivo" stationary microperfusion of the proximal tubule, we found that HgCl₂ inhibits NHE3 activity and increases the phosphorylation of NHE3 at serine 552 in the renal cortex, in line with the HgCl₂-induced diuresis. Changes in renal proximal tubular function induced by HgCl₂ were parallel to increased urinary markers of proximal tubular injury. Besides, atomic spectrometry showed that mercury accumulated in the renal cortex. We conclude that acute HgCl₂ exposure causes renal vasoconstriction that is associated with reduced endothelial vasodilator agonist- and flow-mediated responses and inhibition of NHE3-mediated sodium reabsorption. Thus, our data suggest that HgCl₂-induced acute renal failure may be attributable at least in part by its direct effects on renal hemodynamics and NHE3 activity.

Effect of mercury vapour exposure on urinary excretion of calcium, zinc and copper: relationship to alterations in functional and structural integrity of the kidney

Background: The kidney has a remarkable capacity to concentrate mercury (Hg) and as such is a primary target organ when exposure to Hg occurs, and it is also an organ for Hg excretion. Objective: The present work aims to investigate the effect of occupational Hg vapour exposure on the urinary excretion of calcium (Ca), zinc (Zn) and copper (Cu), and the possible association of this excretion to work duration as well as renal alterations. Methods: 83 non-smoker participants (36 referents, age: 35.69/9.5 years; 27 Hg vapour-exposed workers with 5/10 years work duration, age: 33.09/5.1 years; and 20 Hg vapour-exposed workers with]/11 years work duration, age: 39.509/8.50 years) were included in the present study. Urinary levels of microalbumin (U-Malb) and retinol-binding protein (U-RBP) as well as cytosolic glutathione S-transferase activity (U-GST) were measured to assess the glomerular and proximal tubular reabsorption functions as well as structural integrity of proximal tubules; respectively. In addition, blood Hg (B-Hg), serum levels of Hg (S-Hg) and Ca (S-Ca), and urinary levels of Hg (U-Hg), Ca (U-Ca), Zn (U-Zn), Cu (U-Cu) and creatinine (U-cr) were estimated. Results: In comparison to referents, all investigated parameters showed significant increase (except S-Ca and U-Zn/U-Cu ratio that significantly decreased among the workers as one group, S-Ca and U-Zn/U-Cu ratio that significantly and nonsignificantly decreased; respectively among workers with 5/10 years work duration, S-Ca and U-Zn/U-Cu ratio that significantly decreased among workers with]/11 years work duration). In addition, B-Hg was nonsignificantly increased and S-Ca was significantly decreased; also, both U-Hg and U-Zn/U-Cu were nonsignificantly decreased among workers with]/11 years work duration in comparison to those with 5/10 years work duration. Also, each of U-Hg, U-Ca, U-Zn and U-Cu was related to one another, while each of U-Ca, U-Zn and U-Cu was related to each of U-Malb, U-RBP and U-GST (except U-Zn was not related to U-GST). Conclusion: Hg vapour exposure leads to renal alterations which may parallel the change in proteinuria and enzymuria as well as the increased loss in urine of each of Ca, Zn and Cu. The urinary assessment of these metals may be used as a good indicator for renal dysfunction.

Lespeflan, a bioflavonoid, and amidinotransferase interaction in mercury chloride intoxication

Amidinotransferase (transamidinase, L-arginine: glycine amidinotransferase, EC 2.1.4.1) is an enzyme that catalyses the first step in creatine synthesis primarily in the kidney and pancreas. The kidney is also the primary target organ for the toxic effect of mercury. Therefore, we studied the effect of acute uremic syndrome on enzyme activity induced by mercury chloride. Because of the potential beneficial effect of bioflavonoids, we have investigated the effects of the bioflavonoid lespeflan on acute uremic syndrome and amidinotransferase activity. Male Spraque Dawley rats weighing about 200 g were used in this study. Acute renal failure was induced by intraperitoneally (i.p.) administration of mercury chloride in a dose of 3 mg/kg. One group of animals was given lespeflan (1.0 mL/kg) 1 hr before mercury chloride administration. Urea and creatinine levels in blood plasma were significantly elevated 48 hr after the induction of acute uremic syndrome (p< 0.001). Kidney transamidinase activity was decreased compared to the control group (p<0.001). Pretreatment by lespeflan potentiates the inhibitory effect of mercury chloride on enzyme activity. We discussed mechanisms of transamidinase inhibition and point thiol group of cysteine forming thiol-conjugates on enzyme inhibition both by mercury and lespeflan.

Decreased expression of brain nitric oxide synthase in macula densa cells and glomerular epithelial cells of rats with mercury chloride-induced acute renal failure

To elucidate the mechanisms responsible for the development of HgCl(2)-induced acute renal failure (ARF), we examined the expression of brain type (b) nitric oxide synthase (NOS), which is involved in the generation of the vasodilator nitric oxide (NO), in the renal cortex of rats at 20 h after exposure to 7.5 mg/kg HgCl(2). Both blood urea nitrogen and serum creatinine were significantly increased in rats exposed to HgCl(2) relative to control rats, indicating the induction of ARF resulting from HgCl(2) exposure. Histopathological analysis demonstrated that, in addition to necrosis of proximal tubule epithelial cells, necrosis of macula densa cells and swelling of glomerular epithelial cells were observed in the renal cortex of rats with HgCl(2)-induced ARF. Consequently, the number of pars maculata segments was decreased by 42% in rats with HgCl(2)-induced ARF compared to control rats. The primary sites of bNOS mRNA and protein expression were macula densa cells and glomerular epithelial cells in the renal cortex of control rats and rats with HgCl(2)-induced ARF. The abundance of the bNOS mRNA and protein was significantly decreased in rats with HgCl(2)-induced ARF relative to control rats. These observations suggest that the production of the vasodilator NO derived from bNOS is decreased at the glomerulus level in the HgCl(2)-induced ARF setting. Thus, the reduction in bNOS expression may in part contribute to the progression of HgCl(2)-induced ARF through the deterioration of glomerular hemodynamics. In addition, the decrease in bNOS expression may be primarily the result of cell injury caused by the cytotoxic effect of HgCl(2).

Role of angiotensin II, endothelin-1, and nitric oxide in HgCl2-induced acute renal failure

To elucidate the mechanisms underlying the development of HgCl2-induced acute renal failure (ARF), we examined the expression of endothelin (ET)-1, endothelial (e) nitric oxide synthase (NOS) and inducible (i) NOS, and a role of angiotensin II (ANG II) and tumor necrosis factor (TNF) in glomeruli and cortices from rats at 20 h after exposure of HgCl2. Prepro-ET-1 and iNOS mRNA were significantly increased in glomeruli and cortices from rats with HgCl2-induced ARF. However, eNOS mRNA was markedly decreased in glomeruli of rats with HgCl2-induced ARF. Blockade of the action of endogenous ANG II with TCV-116, an ANG II receptor type 1 antagonist, or prior administration of TNF antibody (Ab) neutralizing TNF bioactivity or aminoguanidine, an iNOS inhibitor, substantially suppressed the increase in the expression of prepro-ET-1 or iNOS mRNA seen in rats with HgCl2-induced ARF. Both TCV-116 and TNF Ab had no effects on the expression of eNOS mRNA. The abundance of ET-1, iNOS, and eNOS proteins was paralleled by the magnitude of each mRNA expression. Additionally, the aggravation of blood urea nitrogen and serum Cr observed in rats with HgCl2-induced ARF were significantly ameliorated together with the alleviation of proximal tubule epithelial cell injury when the expression of prepro-ET-1 or iNOS mRNA was blunted by prior administration of TCV-116 or prior injection of TNF Ab or aminoguanidine. These observations indicate that ANG II, ET-1, and NO may play an important role in the progression of HgCl2-induced ARF through the acceleration of proximal tubule epithelial cell injury and the deterioration of glomerular hemodynamics. In HgCl2-induced ARF, the gene expression of ET-1 or iNOS is at least in part up-regulated at the transcription level by endogenous ANG II or TNF.

Verapamil protection against mercuric chloride-induced renal glomerular injury in rats

We have examined the effects of the calcium channel blocker verapamil on the renal glomerular structural damage produced by mercuric chloride in rats. Verapamil (75 micrograms/kg body wt iv) was administered 30 min prior to mercuric chloride injection (HgCl2, 5 mg/kg body wt sc). Verapamil prevented the glomerular proteinuria observed in HgCl2-treated rats. Isolated glomeruli from mercury-treated rats 1 h after injection presented a diminished cross-sectional area as compared with control glomeruli (control [micron2], 26,310 +/- 2545; HgCl2 [micron2], 18,474 +/- 1828) and increased glomerular calcium content (control, 23 +/- 6 nmol/mg protein; HgCl2, 43 +/- 7 nmol/mg protein). Verapamil pretreatment prevented glomerular cross-sectional area (GCSA) diminution and glomerular calcium content rise (GCSA [micron2] Vp + Hg, 28,281 +/- 4654, Ca2+ [nmol/mg protein] Vp + Hg, 18 +/- 5). Renal sections prepared for immunohistochemical detection and histochemical analysis showed increased deposits of fibronectin and lipids and enhanced cellularity in glomerular structures from HgCl2-treated rats. Renal sections from animals pretreated with verapamil showed fibronectin and lipid contents not different from control sections and their histological studies did not show any changes when compared with control. Verapamil pretreatment also protected glomeruli from enhanced leukocyte content (myeloperoxidase activity/mg protein): control, 59 +/- 7; HgCl2, 134 +/- 10; Vp + Hg, 79 +/- 11). HgCl2 also contracts GCSA in vitro; Vp prevented this GCSA diminution. The results described in this study indicate that mercuric chloride nephrotoxicity may be associated not only with changes in renal glomerular haemodynamics, but also with a direct effect on glomerular cells.

Altered expression of endothelin-1 and endothelial nitric oxide synthase in the juxtaglomerular apparatus of rats with HgCl2-induced acute renal failure

A progressive fall in glomerular capillary plasma flow (QA) is observed in mercury chloride (HgCl2)-induced acute renal failure (ARF) although the site of the main lesion of this ARF is the proximal tubule. To elucidate this mechanism, we examined the expression of endothelin (ET)-1 and endothelial nitric oxide synthase (eNOS) protein at the glomerulus level in the kidneys of control rats and rats with HgCl2-induced ARF. Both ET-1 and eNOS protein were detected in the juxtaglomerular cells of afferent arterioles. The expression of ET-1 was significantly increased in ARF rats when compared to control rats. Inversely, the expression of eNOS protein was markedly reduced in ARF rats as opposed to control rats. These observations suggest the participation of the vasoconstrictor, ET-1 and the vasodilator, NO in a reduction in QA observed in HgCl2-induced ARF.

Evidence for renal ischaemia as a cause of mercuric chloride nephrotoxicity

The present study was undertaken to investigate if the source of oxidative stress and the renal injury produced by mercuric chloride could be renal ischaemia. Verapamil Vp was used because it was described that calcium channel blockers protect cells from nephrotoxicants and from ischaemia. Vp (75 micrograms/kg, i.v.; 30 min before HgCl2 injection) prevented mercuric chloride renal injury observed 1 h post-HgCl2 injection as measured by clearance techniques. Vp also prevented the diminution of non-protein-sulfhydryls (NPSH) and the increased lipid peroxidation (LPO) induced by HgCl2 in renal tissue. Hg2+ toxicokinetic alterations were not observed in Vp plus HgCl2 treated rats, nor was Vp ability found as a free radical scavenger in renal tissue homogenates. The results described in this study give some evidence for the role of renal ischaemia in the production of oxidative stress, generating LPO and functional and morphological renal injury described in mercuric chloride treated rats.

Metal accumulation and nephron heterogeneity in mercuric chloride-induced acute renal failure

The present study was designed to assess the effects of mercury on glomerular integrity during the early phase of acute renal failure. The silver amplification method showed distribution of mercury in midcortical and juxtamedullary glomeruli and on the brush border of the S2 segment of the proximal tubule 15 min after treatment. At 30 min, there was a decrease in glomerular staining and increased mercury in the proximal tubule. After 3 hr, mercury was no longer detectable in glomeruli but was widespread in the lumen of the proximal tubule. By 24 hr, mercury was prominent in all proximal tubular segments throughout the cortex. The presence of mercury in glomeruli was not related to hemodynamic changes, as there was no evidence for blood redistribution toward juxtamedullary glomeruli as assessed by the filling of the microvascular system with Monastral Blue B. The reduced activity of horseradish peroxidase (administered i.v. 90 sec and 10 min before sacrifice) in juxtamedullary glomeruli 30 min after mercury administration suggests a decreased uptake of horseradish peroxidase or an increased glomerular protein filtration. These data support glomerular filtration as the predominant excretory route for mercury, highlight the marked nephron heterogeneity in the distribution of this metal, and show that impairment of glomerular integrity occurs before necrosis of the proximal tubules and acute renal failure.

Effect of different renal glutathione levels on renal mercury disposition and excretion in the rat

Mercury renal disposition has been studied following HgCl2 injection (5.0 mg/kg body wt., s.c.) in controls, diethylmaleate and N-acetylcysteine-treated rats. The different treatments were used to generate statistically different degrees of non-protein sulfhydryls concentration in kidneys. Diethylmaleate (4 mmol/kg body wt., i.p.) diminished kidney glutathione levels to 25% and N-acetylcysteine (2 mmol/kg body wt., i.p.) increased kidney non-protein sulfhydryls levels up to 75% compared with new controls. The amount of mercury in the kidneys, the mercury excretion rate in urine and the mercury plasma disappearance curves were calculated during 3 h post HgCl2 injection. BUN was measured in plasma at the same time period to determine the onset of kidney damage. The results indicate a higher HgCl2 renal clearance in N-acetylcysteine-treated rats compared to controls and less renal mercury accumulation. The data agree with diminished renal toxicity. On the other hand, renal mercury accumulation was higher and mercury renal clearance lower in diethylmaleate-treated animals, associated with higher renal toxicity. The results suggest that non-protein sulfhydryl levels (principally glutathione) might determine renal accumulation of mercury as well as its elimination rate and hence might enhance or mitigate the nephrotoxicity induced by the metal.

Renal epithelial amino acid concentrations in mercury-induced and postischemic acute renal failure

The concentration of 18 alpha-amino acids (AAs) in plasma and renal cortical cell water were measured 3 or 24 hr after 1 hr of unilateral renal artery clamping or 24 or 48 hr after 15 mg/kg body weight HgCl2 injection sc as a test of epithelial integrity. Cellular glycine (Gly), hydroxyproline (Hpr), ornithine (Orn), phenylalanine (Phe), serine (Ser), and tryptophan (Trp) concentrations were depressed 24 hr after HgCl2 (p less than 0.05), but the remaining 12 AAs were not distinguishable from control despite the presence of severe renal failure. ARginine (Arg), glutamic acid (Glu), and valine (Val) also were decreased (P less than 0.05) 24 hr later, but concentrations of half of all measured AAs were still normal. Cellular alanine (Ala), Arg, Glu, Gly, Phe, and Ser concentrations were decreased 3 hr after ischemia, p less than 0.05, but 12 AAs were unchanged and only Arg, Phe, Ser, and threonine (Thr) were reduced 24 hr after ischemia was reversed. Concentrations of even the most affected AAs remained notably higher than in plasma in both forms of acute renal failure (ARF). Total loss of AAs from a small proportion of tubular cells would be hidden by essentially normal concentrations in the rest, and such losses may well have occurred. Unless cellular AAs in ARF are almost completely bound, however, the well-maintained cell:plasma AA concentration ratios indicate that cellular energetics were adequate for AA uptake and that epithelial permeability to AAs in the vast majority of cells was not greatly disturbed. Such findings suggest that most of the epithelium, although seriously damaged, had remained viable.

Glomerular hemodynamics in established glycerol-induced acute renal failure in the rat

The glomerular dynamic correlates of failed filtration were studied in volume replete rats with established glycerol-induced acute renal failure (ARF). Over one-half of all nephrons formed virtually no filtrate, while the single nephron glomerular filtration rate (SNGFR) of fluid-filled nephrons, measured at the glomerulotubular junction to preclude the possibility of covert tubular leakage, averaged one-sixth of control (P less than 0.001). Even that low mean value was elevated by a few nephrons with a near normal SNGFR. Renal failure thus reflected both total filtration failure in the majority of nephrons and massively reduced filtration in most of the remainder. Glomerular capillary pressure (Pg) averaged some 14 mmHg below control (P less than 0.001), whereas the arterial colloid osmotic and Bowman's space pressures were not significantly altered. Renocortical and whole kidney blood flow were also unchanged. Marked internephron functional heterogeneity precluded estimates of the ultrafiltration coefficient. However, the fall in SNGFR correlated well with the markedly depressed Pg and afferent net filtration pressure (delta PnetA, P less than 0.001), which in turn were caused by increased preglomerular resistance and a reciprocal fall in efferent arteriolar resistance. This complex change in intrarenal resistances was largely, if not entirely, responsible for failed filtration in this ARF model.

The implication of renal glutathione levels in mercuric chloride nephrotoxicity

The effects of renal glutathione (GSH) depletion on renal injury following a single injection of mercuric chloride (HgCl2) were evaluated in the rat. Animals were injected with different doses of HgCl2 and the renal function were studied 1 h later. Diethylmaleate (DEM) (4 mmol/kg body wt, i.p.) induced a significant depletion of GSH by reducing renal GSH levels to 25% of control values. This effect lasted for 6 h. HgCl2-induced nephrotoxicity, as measured by fractional excretion of glucose, lithium, sodium, potassium and water was increased in rats treated with DEM. The time course of HgCl2 nephrotoxicity was also investigated by determining the renal function at different times after HgCl2 and HgCl2 plus DEM injection. Renal impairment was significantly more marked in rat depleted of GSH.

Effects of pentoxifylline in experimental acute renal failure

The beneficial effects of post-insult administration of pentoxifylline, a novel hemorheologic agent experimentally studied in various ischemic diseases, were evaluated in two models of acute renal failure (ARF): direct nephrotoxicity (mercuric chloride 4 mg/kg via femoral vein) and hemoglobinuria (glycerol 10 ml/kg i.m.). Glomerular filtration rate (GFR) was estimated at baseline and following drug administration by creatinine clearances; tubular function was assessed by renal fractional and absolute electrolyte excretions. The incidence of mortality was decreased with a single dose of pentoxifylline 45 mg/kg (21.4%) compared to control rats (71.4%) 48 hours following induction of ARF with mercuric chloride. Although GFR and renal electrolyte excretion were significantly greater in rats given pentoxifylline compared to saline, the magnitude of difference was minimal. A return to baseline GFR was observed in the glycerol group administered a single i.p. dose of pentoxifylline 45 mg/kg (100.8 +/- 54.8%) compared to saline controls (45.6 +/- 22.7%; P less than 0.05). No differences in renal electrolyte excretion or mortality were observed in this model. Taken together, these data suggest that pentoxifylline, administered shortly after the initiation of ARF, exerts an ameliorative effect on the course and mortality of experimental ARF. The mechanism of amelioration most likely involves the stimulation of renal vasodilator prostaglandins as well as prevention of vascular congestion.