Uric acid elimination in the urine. Pathophysiological implications.

Uric acid, a weak organic acid, has very low pH-dependent solubility in aqueous solutions. About 70% of urate elimination occurs in urine, the kidney standing as a major determinant of plasma levels. The complex renal handling results in a fractional clearance of less than 10%. Recently identified urate-specific transporter/channels are involved in tubular handling and extracellular transport. Extracellular fluid, rather than urine output, is the main regulator of urate excretion. A number of interfering agents, including widely used drugs such as aspirin, losartan, diuretics, may decrease or increase urate elimination. Hyperuricemia induced by hypouricosuria often accompanies the metabolic syndrome, and insulin resistance has been hypothesized as the common underlying defect. Hyperuricosuria, associated with dehydration or exercise, results in acute uric acid nephropathy, and causes an obstructive acute renal failure (ARF). This reversible ARF can be prevented by forced hydration with bicarbonate or saline solutions. Renal hypouricemia, due to mutations of urate transporter, is a rare cause of exercise-induced ARF. The existence of chronic urate nephropathy, gouty nephropathy, is still under discussion. Uric acid nephrolithiasis results from supersaturation, strongly influenced by low urine pH, rather than altered urate turnover. Alkali and fluid intake prove successful in managing uric acid stones.