Renal excretion of hydrogen in primarygout

Uric acid nephrolithiasis: An update

Uric acid nephrolithiasis appears to increase in prevalence. While a relationship between uric acid stones and low urinary pH has been for long known, additional association with various metabolic conditions and pathophysiological basis has recently been elucidated. Some conditions such as diabetes and metabolic syndrome disease, excessive dietary intake, and increased endogenous uric acid production and/or defect in ammoniagenesis are associated with low urinary pH. In addition, the phenomenon of global warming could result in an increase in areas with greater climate risk for uric acid stone formation. There are three therapeutic steps to be taken for management of uric acid stones: identification of urinary pH profiles, assessment of urinary volume status, and identification of disorders leading to excessive uric acid production. However, the most important factor for uric acid stone formation is acid urinary pH, which is a prerequisite for uric acid precipitation. This article reviews recent insights into the pathophysiology of uric acid stones and their management.

Pathophysiology of uric acid nephrolithiasis

Humans although a predominantly ureotylic organism, has preserved the ability to excrete nitrogen as uric acid and ammonia. An imbalance between these two secondary modes of nitrogen excretion has resulted in uric acid precipitation in human urine. Uric acid nephrolithiasis can arise from diverse etiologies all with distinct underlying defects converging to one or more of three defects of hyperuricosuria, acidic urine pH, and low urinary volume, originating from secondary, genetic or heretofore undefined (idiopathic) causes. A subset of idiopathic uric acid nephrolithiasis (gouty diathesis) may be the "tip of the icebergp" of a broader systemic illness characterized by insulin resistance. A novel renal manifestation of insulin resistance is a mild defect in ammonium excretion, which is not severe enough to disturb acid-base homeostasis, but is sufficient to set up the chemical milieu for uric acid nephrolithiasis.

Uric acid nephrolithiasis: current concepts and controversies

PURPOSE

Uric acid calculi with or without a calcium component comprise a significant proportion of urinary stones. Knowledge of the pathophysiology of stone formation is important to direct medical treatment. The aim of this review is to provide an update on the epidemiology, pathophysiology and management of uric acid renal stones.

MATERIALS AND METHODS

A MEDLINE search was performed on the topic of uric acid stones. Current literature was reviewed with regard to the epidemiology, pathophysiology, associated medical conditions and management of uric acid stones.

RESULTS

The incidence of uric acid stones varies between countries and accounts for 5% to 40% of all urinary calculi. Hyperuricuria, low urinary output and acidic urine are well known contributing factors. However, the most important factor for uric acid stone formation is persistently acidic urine. Gout and myeloproliferative disorders are associated with uric acid stones. Why most patients with gout present with acidic urine yet only 20% have uric acid stone formation remains unclear. The pathophysiological basis for persistent urine acidity also remains unclear although various mechanisms have been proposed. Urinary alkalization with potassium citrate or sodium bicarbonate is a highly effective treatment, resulting in dissolution of existing stones and prevention of recurrence.

CONCLUSIONS

Acidic urine is a prerequisite for uric acid stone formation and growth. Medical management with urinary alkalization for stone dissolution and prevention of recurrence is effective and should be the cornerstone of treatment.

Uric acid nephrolithiasis: current concepts and controversies

PURPOSE

Uric acid calculi with or without a calcium component comprise a significant proportion of urinary stones. Knowledge of the pathophysiology of stone formation is important to direct medical treatment. The aim of this review is to provide an update on the epidemiology, pathophysiology and management of uric acid renal stones.

MATERIALS AND METHODS

A MEDLINE search was performed on the topic of uric acid stones. Current literature was reviewed with regard to the epidemiology, pathophysiology, associated medical conditions and management of uric acid stones.

RESULTS

The incidence of uric acid stones varies between countries and accounts for 5% to 40% of all urinary calculi. Hyperuricuria, low urinary output and acidic urine are well known contributing factors. However, the most important factor for uric acid stone formation is persistently acidic urine. Gout and myeloproliferative disorders are associated with uric acid stones. Why most patients with gout present with acidic urine yet only 20% have uric acid stone formation remains unclear. The pathophysiological basis for persistent urine acidity also remains unclear although various mechanisms have been proposed. Urinary alkalization with potassium citrate or sodium bicarbonate is a highly effective treatment, resulting in dissolution of existing stones and prevention of recurrence.

CONCLUSIONS

Acidic urine is a prerequisite for uric acid stone formation and growth. Medical management with urinary alkalization for stone dissolution and prevention of recurrence is effective and should be the cornerstone of treatment.

Pathophysiologic basis for normouricosuric uric acid nephrolithiasis

BACKGROUND

Low urinary pH is the commonest and by far the most important factor in uric acid nephrolithiasis but the reason(s) for this defect is (are) unknown. Patients with uric acid nephrolithaisis have normal acid-base parameters according conventional clinical tests.

METHODS

We studied steady-state plasma and urinary parameters of acid-base balance in subjects with normouricosuric pure uric acid stones. We also tested the ability of these subjects to excrete ammonium in response to an acute acid load. We compared these parameters in patients with pure uric acid stones to patients with mixed uric acid/calcium oxalate stones, pure calcium stones, and normal volunteers.

RESULTS

Pure uric acid stone formers have a much higher incidence of either diabetes or glucose intolerance. After equilibration to a control diet, patients with uric acid stones have lower urinary pH and they excrete less of their acid as ammonium. This is compensated by higher titratable acidity and hypocitraturia. Despite their low baseline urinary pH, uric acid stone formers further acidify their urine after an acid load because of a severely impaired ammonia excretory response. Their characteristics are significantly different from normal volunteers and pure calcium stone formers. Patients with mixed uric acid/calcium stones exhibit intermediate characteristics.

CONCLUSION

We propose that certain patients with normouricosuric uric acid nephrolithiasis have a renal acidification disease. The primary defect lies in renal ammonium excretion, which may be linked to the insulin-resistant state. Although net acid excretion is maintained at the expense of increased titratable acidity and to some degree hypocitraturia, the compromise is acid urine pH and may result in uric acid nephrolithiasis.

Renal ammonia in autosomal dominant polycystic kidney disease

Recent studies have suggested that defective medullary trapping of ammonia underlies the acidosis associated with renal failure and sets in motion maladaptive compensatory mechanisms that contribute to the progression of renal disease. Since a renal concentrating defect is an early functional abnormality in autosomal dominant polycystic kidney disease (ADPKD), defective medullary trapping and urinary excretion of ammonia may also occur early and have important pathophysiological consequences. The urinary pH and excretions of ammonia, titratable acid, and bicarbonate, were measured during a 24-hour baseline period and following the administration of ammonium chloride (100 mg/kg body wt) in ADPKD patients with normal glomerular filtration rate and in age- and gender-matched healthy control subjects. The distal nephron hydrogen ion secretory capacity was assessed during a bicarbonate infusion. Ammonia, sodium, pH, C3dg, and C5b-9 were measured in cyst fluid samples. The excretion rates of ammonia during the 24-hour baseline period and following the administration of ammonium chloride were significantly lower, and the relationship of ammonia excretion to urinary pH was significantly shifted downward in ADPKD. No difference in the increment of urinary pCO2 (delta pCO2) or the peripheral blood-urine pCO2 gradient (U-B pCO2) between ADPKD patients and control subjects was detected during a sodium bicarbonate infusion. Calculated concentrations of free-base ammonia in cyst fluid samples exceeded those calculated from reported concentrations of ammonia in renal venous blood of normal subjects. C3dg and C5b-9 were detected in some cyst fluids. The urinary excretion of ammonia is reduced in ADPKD patients with normal glomerular filtration rate.(ABSTRACT TRUNCATED AT 250 WORDS)

The association of nephrolithiasis and autosomal dominant polycystic kidney disease

Despite the frequency and morbidity of nephrolithiasis in autosomal dominant polycystic kidney disease (ADPKD), this association has not been subject to a detailed study. One hundred fifty-one of 751 ADPKD patients seen at the Mayo Clinic between 1976 and 1986 had nephrolithiasis. Seventy-four had passed calculi or had stones surgically removed. Stone analysis was available in 30 patients: uric acid, calcium oxalate, calcium phosphate, and struvite were present in 56.6%, 46.6%, 20%, and 10%, respectively. Calculi were observed in 71 of 79 patients with excretory urograms available for review. Faintly opaque and bull's eye stones, probably containing uric acid, were present in 12.7% and 14.1% of these patients, respectively. Precaliceal tubular ectasia was observed in 15.5%. Ninety-seven patients had preserved renal function (serum creatinine less than 1.5 mg/dL) at the initial evaluation. Six were excluded because they had other known causes of stone disease. The most common metabolic abnormality in the remaining 91 patients was hypocitric aciduria (ten of 15 patients with measurements). The urine pH in the first voided morning specimens (5.66 +/- 0.05) was significantly lower than that of an unselected control population (5.92 +/- 0.03, P less than 0.001). Hyperuricosuria, hyperoxaluria, and hypercalciuria were observed in six of 32 (18.8%), six of 31 (19.4%), and three of 39 (9.7%) patients with preserved renal function. The composition of the stones, the frequency of hypocitric aciduria, and the low urine pH (possibly related to the defect in excretion of ammonia described in ADPKD), suggest that metabolic, along with mechanical, factors are responsible for the frequent occurrence of nephrolithiasis in this disease.

Allopurinol treatment and its effect on renal function in gout: a controlled study

Fifty-nine patients with primary gout were treated with either a combination of colchicine and allopurinol or colchicine alone. Assessments of renal function over 2 years revealed a statistically significant fall of glomerular filtration rate an urine concentrating ability in those receiving only colchicine. The renal function of patients given allopurinol did not change. Treatment with allopurinol resulted ina significant reduction of ammonium excretion, a phenomenon which could not be readily explained. Urate clearance also declined during allopurinol treatment, and the impaired urate clearance associated with gout became more evident. The most important observation was that allopurinol retarded an apparent decline of renal function. Presumably this was achieved through its hypouricaemic effect and implies that the hyperuricaemia of gouty patients is deleterious to the kidneys.