Lithium-induced structural changes in the cortical distal nephron localized by computer-assisted three-dimensional reconstruction

Renal functional deterioration is not affected by the magnitude of sodium consumption in a normotensive model of moderate renal failure

BACKGROUND/AIMS

High sodium consumption has been repeatedly reported to exert deleterious effects on severe chronic renal failure progression, mainly via glomerular mechanisms. However, the role of high sodium intake in renal function deterioration in a model of moderate chronic tubulointerstitial disease has not yet been addressed. We evaluated the effects of exaggerated dietary sodium and the resultant increase in proteinuria on renal function deterioration in experimental tubulointerstitial disease in rats.

METHODS

In 48 Sprague-Dawley rats, moderate renal failure (approximately 50% of normal glomerular filtration rate) was induced by administration of lithium chloride in drinking water. The animals were divided into three groups fed low (<0.2% Na(+)), normal (0.5% Na(+)), or high (8% Na(+)) sodium diets.

RESULTS

Animals in all groups remained normotensive with a similar course of GFR downslope and 100% survival, irrespective of sodium regimen. Rats consuming high sodium diets developed significantly greater proteinuria compared to their counterparts fed normal or low sodium chow.

CONCLUSIONS

(1) Deterioration of renal function in a lithium-induced model of normotensive moderate chronic renal failure was not affected by dietary sodium. (2) Unlike in some other human or experimental renal failure models, the magnitude of proteinuria had no adverse effect on the progression of renal deterioration.

Interstitial capillary changes in lithium nephropathy: effects of antihypertensive treatment

Histopathological changes were investigated in the tubulointerstitium and in the capillaries of male Wistar rats with lithium-induced nephropathy using stereological methods. Two antihypertensive drugs with opposite effects on the renin-angiotensin system, an ACE inhibitor (angiotensin converting enzyme inhibitor) and a thiazide diuretic, modified the nephropathy. Generally, there was a significant positive correlation between the reduction in GFR (glomerular filtration rate) and the reduction in the volume of intact tubular structures and interstitial capillaries. A significant negative correlation was seen between the reduction in GFR and the increase in tubulocapillary distance and the absolute volume of interstitial connective tissue, respectively. Treatment with perindopril, and to some extent hydrochlorothiazide, reversed the rise in systolic blood pressure associated with lithium-induced nephropathy but did not affect the progression to terminal uraemia, the structural renal changes or the mortality. In conclusion, severe tubular and capillary changes are seen in this model of chronic renal failure. Tubular atrophy is associated with a decrease in interstitial capillaries and with an increase in the tubulocapillary distance. Systemic hypertension or activation of the renin-angiotensin system may not be important factors for the progression to terminal renal failure.

Dissociation between water and lithium transport during acute changes in plasma potassium concentration in dog kidney

Lithium clearance is often used as a marker for proximal tubular water transport. Proximal tubular transport may be modulated by changing plasma potassium concentration. The aim of the present study was to examine the effect of acute changes in plasma potassium concentration on proximal tubular fluid and lithium transport. Clearance studies were performed in seven anaesthetised, volume-expanded dogs treated with amiloride (1 mg kg-1 body weight) to block distal tubular potassium secretion, and with bumetanide (30 micrograms kg-1 body weight) to inhibit sodium reabsorption in Henle's loop. When plasma potassium concentration was raised from 2.6 +/- 0.2 to 7.9 +/- 0.2 mmol l-1, water reabsorption decreased from 23.9 +/- 2.9 to 19.8 +/- 2.2 ml min-1, whereas lithium reabsorption increased from 10.5 +/- 2.3 to 18.1 +/- 2.3 mumol min-1, at constant glomerular filtration rate. We conclude that acute elevation of plasma potassium concentration inhibits proximal tubular fluid reabsorption, but stimulates renal lithium reabsorption. Thus, lithium reabsorption cannot be used as a marker for proximal tubular transport during acute changes in plasma potassium concentration.

Effects of perindopril and hydrochlorothiazide on the long-term progression of lithium-induced chronic renal failure in rats

Administration of lithium in the diet to new-born rats induces chronic renal failure associated with hypertension, proteinuria and irreversible tubulo-interstitial morphological changes. In the present study we induced chronic renal failure by administration of lithium for 16 weeks to new-born rats, and examined the spontaneous course of this nephropathy and the effects of antihypertensive treatment with either perindopril (12 mg/kg diet) or hydrochlorothiazide (500-1000 mg/kg diet) during a 24 weeks follow up period without lithium. In the placebo group, progression to terminal uraemia occurred in all rats with severe renal failure (initial Purea > 15 mM) (10 of 18). Rats with mild-moderate renal failure (Purea 9-15 mM) showed no deterioration in renal function despite persistent systolic hypertension and irreversible structural renal changes. Perindopril normalized the blood pressure in all rats but did not prevent the progression to terminal uraemia (8 to 18). Hydrochlorothiazide partially controlled the hypertension and accellerated the progression of uraemia without increasing the mortality (7 of 17). Irrespective of treatments, the predominant quantitative structural changes (e.g. decreased volume of proximal tubular cells) showed significant correlations with the degree of renal dysfunction, but not with systolic blood pressure in the surviving rats. It is concluded that progression of lithium-induced nephropathy to terminal uraemia occurs when the nephrotoxic insult results in a more than 50% reduction of the glomerular filtration rate, judged from Purea levels. The failure of effective antihypertensive treatment with an angiotension-converting enzyme inhibitor to modify the progression suggests that in this model, systemic or glomerular hypertension may not be an important pathophysiological factor. The structural and functional deterioration observed in Li-uraemic rats during treatment with hydrochlorothiazide remains unexplained.

The number and size of glomeruli in long-term lithium-induced nephropathy in rats

Chronic renal failure was induced in 10 Wistar rats using a lithium-containing (40 mmol/kg) diet from time of birth until an age of 55-65 weeks. Nine Wistar rats served as controls. The plasma lithium, the plasma urea, and the inulin clearance were measured, and one kidney was fixed by vascular perfusion with glutaraldehyde. The number of glomeruli was estimated stereologically by the fractionator method. The total number of glomeruli per kidney was 23.9 x 10(3) +/- 3.65 x 10(3) (+/- SD) in controls and 22.0 x 10(3) +/- 1.48 x 10(3) in the lithium-treated group, showing no statistically significant difference. The mean glomerular volume was also estimated using stereological methods. The number-weighted mean volume was reduced by 42% in the lithium-treated group, whereas the volume-weighted mean volume was unchanged. This can be attributed to the occurrence of many small glomeruli and a few very large glomeruli in the lithium-treated group. The many small glomeruli have in a previous study been shown to be atubular. The present study showed that the glomerular population is quite resistant to the deleterious effect of lithium; thus glomerular atrophy was seen, but no loss of glomeruli occurred.

Atubular glomeruli, renal function and hypertrophic response in rats with chronic lithium nephropathy

Experimental lithium nephropathy was induced by administering lithium orally to newborn rats for 8 weeks; thereafter the rats were randomized into four groups which were studied after 8 weeks of further treatment. One group was left untreated, one group was given a high (40%) protein diet, one group was unilaterally nephrectomized and one group was unilaterally nephrectomized and received high protein diet after nephrectomy. Comparable control groups (not lithium-treated) were also studied. Stereological methods were used to estimate the total volume of different parts of the nephron, interstitial fibrosis, and the distribution of the volume of individual glomeruli. The structural integrity between the glomerulus and the proximal tubule was investigated on serial sections. No sclerotic glomeruli were present. The most extensive degree of hypertrophy with almost a doubling of the total volume of proximal and distal tubule cells was seen in the groups that were both nephrectomized and fed a high protein diet. In both controls and lithium-treated animals, high protein and nephrectomy induced enlargement of the glomerular tufts to volumes from 4 to 5 times the normal size. A pronounced heterogeneity of the glomerular population was found in the lithium-treated groups with 36-54% atubular glomeruli with small volumes, and 34-48% enlarged glomeruli connected to qualitatively normal proximal tubules. Only glomeruli connected to proximal tubules had a potential for hypertrophy. In multiple regression analysis the percentage of glomeruli connected to normal proximal tubules was correlated with the reciprocal of plasma creatinine, but the volume of fibrosis also contributed to the decreased renal function.

Loop diuretics reduce lithium reabsorption without affecting bicarbonate and phosphate reabsorption

The effects of the loop diuretics ethacrynic acid and bumetanide on lithium, bicarbonate and phosphate reabsorption were compared in 16 anaesthetized, normovolaemic dogs. In six dogs, ethacrynic acid (3 mg kg-1 body wt) significantly reduced absolute lithium reabsorption from 29.3 +/- 4.1 to 19.0 +/- 3.4 mumol min-1, fractional lithium reabsorption from 0.65 +/- 0.04 to 0.37 +/- 0.04 and fractional chloride reabsorption from 1.00 +/- 0.00 to 0.65 +/- 0.02. Bicarbonate and phosphate reabsorption did not decrease significantly. In six other dogs, bumetanide (30 micrograms kg-1 body wt) gave similar results. Absolute lithium reabsorption significantly decreased from 34.0 +/- 2.2 to 18.1 +/- 2.6 mumol min-1 and fractional lithium reabsorption decreased from 0.50 +/- 0.03 to 0.25 +/- 0.03. Fractional chloride reabsorption decreased from 0.98 +/- 0.00 to 0.61 +/- 0.05, whereas bicarbonate and phosphate reabsorption were not significantly altered. Thus, both loop diuretics greatly reduced lithium reabsorption. We propose that loop diuretics inhibit passive lithium reabsorption in the thick ascending limb of Henle's loop by reducing the lumen-positive electrical potential that drives passive cation transport.