Effect of flow rate and the extracellular fluid volume on proximal urate and water absorption

Fractional excretion of uric acid as a predictor for saline responsiveness in long-term kidney transplant patients

BACKGROUND/AIMS

Subclinical hypovolemia may contribute to allograft dysfunction in long-term kidney transplant (KT) patients. In order to predict responsiveness to saline hydration, indices for tubular transport were investigated.

METHODS

Fifty-four clinically euvolemic long-term KT patients with recently aggravated azotemia were given intravenous hydration as follows: 0.9% saline 5 ml/kg over 1 h, followed by 0.9% saline 1 ml/kg/h over 12 h and 1 liter of 0.45% saline over the next 24 h. Serum and urine data were collected and analyzed to assess responses.

RESULTS

In all patients, saline hydration relieved azotemia, as shown by blood urea nitrogen (46.9 ± 17.2 vs. 39.3 ± 15.4 mg/dl; p < 0.01) and serum creatinine levels (2.9 ± 1.1 vs. 2.5 ± 1.1 mg/dl; p < 0.01) on day 0 versus day 2. In 38 patients, serum creatinine did not increase in the following month (70% responders). Compared with the nonresponders, the responders had a higher urine-to-plasma creatinine ratio and lower fractional excretion of sodium, uric acid and urea at admission. Multivariate logistic regression analysis revealed that responsiveness to saline hydration was independently associated with lower fractional excretion of uric acid.

CONCLUSION

Subclinical hypovolemia should be considered in long-term KT patients with azotemia of unexplainable causes. Fractional excretion of uric acid may predict responsiveness to saline hydration.

Uric acid metabolism in children

The renal excretion of uric acid in children differs quantitatively, and perhaps qualitatively, from that in adult humans. The younger the child, the greater the renal clearance of uric acid and the greater the excretion of uric acid expressed as mg per kg body weight. During infancy, the reduced ability to maximally concentrate the urine may protect against precipitation of uric acid crystals within the kidney. Conversely, the extremely high urinary uric concentrations places the very small infant at jeopardy during sudden increases in the filtered load of uric acid. Understanding the pharmacologic and physiologic modulators of renal uric acid clearance will allow the pediatrician to minimize the risk of uric acid nephropathy, and to understand the implications of uric acid in the serum or urine in children with fluid and electrolyte disorders. Certainly evaluation of serum and urinary uric acid concentrations is essential in any child with acute renal failure.

Acute natriuretic effect of nifedipine in hypertensive patients and normotensive controls--a proximal tubular effect?

The acute effects of buccal nifedipine 20 mg on blood pressure, renal haemodynamics and electrolyte excretion were compared in 16 untreated patients (HT) with uncomplicated arterial hypertension (WHO I-II), 11 normotensives (NT) and 6 normotensives given a placebo. Nifedipine caused a significant fall in the systolic and diastolic blood pressures (BP) of 25.7 +/- 12/26.5 +/- 10 mmHg in the hypertensives, and a minor but significant fall in diastolic BP in the normotensives. Renal vascular resistance fell significantly and renal plasma flow was increased non-significantly in the hypertensives. No changes in these parameters were seen in NT. Glomerular filtration rate remained constant in all groups, also in HT despite the marked haemodynamic changes. Natriuresis was significantly increased to the same degree in the HT and NT groups, in spite of their different haemodynamic responses. Uric acid excretion showed a parallel acute increase in both groups. The significant and close relationship between the acute changes in the excretion of sodium and uric acid provides evidence for a proximal tubular natriuretic effect of nifedipine.

Axial heterogeneity of bicarbonate, chloride, and water transport in the rat proximal convoluted tubule. Effects of change in luminal flow rate and of alkalemia

These studies examined regulation of superficial proximal convoluted tubule (PCT) transport as a function of length. When single nephron glomerular filtration rate (SNGFR) increased from 28.7 +/- 0.7 nl/min in hydropenia to 41.5 +/- 0.4 nl/min in euvolemia, bicarbonate, chloride, and water reabsorption in the early (1st mm) PCT increased proportionally: from 354 +/- 21 peq/mm X min, 206 +/- 55 peq/mm X min, and 5.9 +/- 0.4 nl/mm X min to 520 +/- 12 peq/mm X min, 585 +/- 21 peq/mm X min, and 10.1 +/- 0.4 nl/mm X min, respectively. These high transport rates did not increase further, however, when SNGFR went to 51.2 +/- 0.7 or 50.7 +/- 0.6 nl/min after atrial natriuretic factor or glucagon administration. Anion and water transport rates in the late PCT were lower and exhibited less flow dependence. During chronic metabolic alkalosis, acidification was inhibited in the late but not early PCT. In conclusion, the early PCT is distinguished from the late PCT by having high-capacity, flow-responsive but saturable, anion- and water-reabsorptive processes relatively unaffected by alkalemia.

Effects of bendroflumethiazide on urate metabolism during treatment of patients with renal stones

Treatment with bendroflumethiazide (2.5 mg, twice daily with potassium supplements) in 63 patients with calcium-containing renal stones for a minimum period of 1 year (average duration of treatment 2.6 years) increased the serum urate values in all patients with an average of 90 mumol./l. (conversion factor: 1 mmol. urate = 183 mg.). Despite this the mean urinary urate excretion was unchanged. In the individual patient a systematic effect of therapy was evident since in patients with low pretreatment values the thiazide increased the urate output while in those with the highest levels before therapy it caused a reduction. In most patients there was a reduction of the urate clearance during therapy, which was most evident in those with the highest pretreatment clearance values. In patients with incomplete types of renal acidification defects the same effects were seen on urate metabolism during thiazide treatment as in the other, idiopathic, stone formers. Although the possible role of urate in calcium stone formation has not been definitely settled this study shows that thiazides do not cause hyperuricosuria and hence their beneficial effects on calcium excretion are not counteracted.

Ionic requirements of proximal tubular fluid reabsorption flow dependence of fluid transport

The effects of changes in luminal flow rate on fluid absorption in rat renal proximal convoluted tubules were studied by continuous luminal and peritubular microperfusion methods. Luminal flow rate was varied over a range from 5 to 45 nl . min-1, and the effects of transepithelial chloride and bicarbonate gradients were tested. Fluid absorption across the proximal convoluted tubule increased with luminal flow rate in the absence of luminal bicarbonate and organic solutes but in the presence of transepithelial chloride and bicarbonate gradients and active sodium transport. Augmenting perfusion rate from 5 to 45 nl . min-1 resulted in an increase of volume absorption from 0.49 to 3.37 nl . min-1 per millimeter length of tubule-1. The chloride concentration change in the collected perfusate decreased from 5.9 to 2.6 mEq . liter-1 . mm length tubule-1 over the same perfusion range. Thus, tubular chloride concentration rises with perfusion rate such that the steepest transepithelial chloride gradients are maintained at the highest flow rates. Flow dependence continued, albeit at reduced rate, in the absence of active sodium transport (cyanide perfusion) but in the presence of chloride and bicarbonate gradients. Flow dependence disappeared in the absence of both active sodium transport and transepithelial anion gradients. Luminal and peritubular perfusion experiments with symmetrical bicarbonate-free solutions that contained only phosphate buffer showed that even under those conditions fluid movement driven by cyanide-sensitive active transport increased with flow rate.