Handling of uric acid by the rat kidney. 3. Microperfusion studies on steady state concentration of uric acid in the proximal tubule. Consideration of free flow conditions

Role of obstruction in autosomal dominant polycystic kidney disease in rats

Kidney micropuncture and microdissection studies were carried out on heterozygous 2- to 4-month-old female and male Han:SPRD rats with autosomal dominant polycystic kidney disease (ADPKD) and on normal controls, to determine whether cysts are obstructed. Pressures in proximal tubules and cysts were determined using a servo null device and were recorded before, during, and after intraluminal infusion of an isotonic equilibrium solution at 15 and 50 n1/min. Initial cyst pressures in nine cystic rats averaged 18.5 +/- 5.9 (SD) mm Hg, N = 49, significantly (P < 0.01) higher than in normal proximal tubules in four control rats, 14.3 +/- 1.6 mm Hg, N = 36. Pressures in non-cystic tubules in cystic rats, 16.8 +/- 4.4 mm Hg, N = 25, were not significantly different from pressures in control kidneys or in cysts. When proximal tubules were microinfused at 15 nl/min in control rats, tubule pressure increased by 3.8 +/- 1.2 mm Hg, N = 24. In cysts, the response was highly variable. Twenty out of 33 microinfused cysts (61%) showed responses similar to normal tubules and were considered to be nonobstructed; 13 (39%) showed large pressure increases upon microinfusion, sometimes to values over 100 mm Hg (obstructed cysts). Left kidney inulin clearance (in microliter/min. 100 g body wt) averaged 335 +/- 65 (N = 4) in control rats and 344 +/- 144 (N = 9) in cystic rats; at this early stage of the disease no decline in GFR was seen. Weights of cystic kidneys were twice those of normal animals. Microdissection and scanning electron microscopy revealed the presence of intraluminal casts and debris and constrictions between cysts that would impede fluid flow. We conclude that obstruction is a frequent, early event in PKD and, when present, promotes cyst enlargement. Since many cysts are not obstructed, we suggest that factors other than fixed obstruction initiate cyst formation.

Renal handling of urate in healthy man in hyperuricaemia and renal insufficiency: circadian fluctuation, effect of water diuresis and of uricosuric agents

To differentiate between extrarenal and renal causes of hyperuricaemia and gout, clearances of urate and creatinine were monitored for 3 1/2 days in fifty-two individuals (seven with a history of gout) with no gross impairment of renal function (creatinine clearance 52-137 ml/min). Dietary purine intake was kept constant. Monophasic circadian fluctuations of fractional urate excretion (= urate clearance over creatinine clearance) were observed with peak values in the afternoon, about 50% higher than during the night. Circadian fluctuations of urinary flow rate were almost identical. However, enhancement of urinary flow rate due to water diuresis had no effect on urate clearance. Despite wide variation of plasma urate concentrations among different individuals (+/- 30% SD), daily urate excretion varied little (+/- 4% SD) and did not correlate with plasma urate (r = 0.03). Thus extrarenal factors appear not to account for the occurrence of hyperuricaemia in these patients. In contrast, a clearcut negative correlation was apparent between plasma urate concentration and fractional urate clearance (r = 0.72), which could fully account for the variations of plasma urate concentration. To elucidate further the mechanism responsible for antiuricosuria in hyperuricaemic patients, the effects of the uricosuric agents benzbromarone and probenecid were tested. A clearcut correlation was apparent between control fractional urate excretion and uricosuric effect of both benzbromarone and probenecid (r = 0.83 and 0.88, respectively), suggesting that anti-uricosuria was due to defective secretion. In an additional series, the uricosuric effect of probenecid was tested in ten patients with renal insufficiency. In these patients the uricosuric effect was clearly blunted, indicating that urate reabsorption is reduced in renal insufficiency.

Renal urate transport during variations in urate synthesis in the rat

In order to determine the effect of intrarenal synthesis of urate upon the urinary urate excretion in the rat, we effected large changes in urate synthesis by increasing it with allopurinol. Hypoxanthine infusion increased plasma urate rapidly and also increased the urinary urate excretion and its renal clearance. However, when the plasma urate was maintained constant, hypoxanthine had no effect upon renal urate transport. Conversely, allpurinol infusion rapidly diminished the plasma urate, urinary urate excretion and its renal clearance. Again, the maintenance of a constant plasma urate concentration prevented any change in urate transport during allopurinol. The urinary degradative purine metabolic pattern was altered predictably by hypoxanthine and allopurinol. Assuming than any putative intrarenal component of urate synthesis would be affected predictably and consistently by hypoxanthine and allopurinol, these results suggest that changes in intrarenal urate synthesis are not an important determinant of urate excretion in the rat.

Influence of volume expansion, serum sodium, and fractional excretion of sodium on urate excretion

The relative contributions of volume expansion and increased fractional excretion of sodium to the uricosuria of saline infusion were assessed in 19 subjects by volume expansion with rapid infusion of 21 of hypertonic saline (3%), isotonic saline (0.9%), or hypotonic saline (0.45%). Urate excretion increased 385 mug/min (P less than 0.01) with hypertonic, 145 mug/min (P less than 0.05) with isotonic saline, and 294 mug/min (P less than 0.001) with hyptonic saline. When 150 meq of sodium chloride was administered as appropriate volumes of hypertonic, isotonic of hypotonic saline, the magnitude or uricosuria was correlated with volume load (r = 0.66, P less than 0.002). fractional excretion of sodium correlated with infusion volumes for all studies taken together (r = 0.35, P greater than 0.1). The relationship between fractional excretion of sodium and fractional excretion of urate was entirely attributed to their correlation with infusion volume. Both post-pyrazinamide urate excretion and pyrazinamide suppressible urate excretion increased with volume expansion.