Renal tubular effects of chronic phosphate depletion.

The effects of chronic phosphate depletion on renal tubular function were evaluated by micropuncture and free water clearance studies in the dog. Proximal tubular punctures demonstrated that chronic hypophosphatemia led to a reduction in ratio of tubular fluid to plasma inulin in late superficial tubular from 1.59+/-0.08 in control animals to 1.29+/-0.06 in phosphate-depleted dogs, with proportional inhibition of calcium and sodium reabsorption. The chronic decrease in proximal tubular fluid reabsorption was confirmed by the analysis of sustained water diuresis in conscious, phosphate-depleted dogs, before and after repletion of body PO4 stores, and in control animals. Urine flow rate/100 ml glomerular filtration rate (V/GFR) was significantly higher in PO4 DEPLETION THAN CONTROL (15.8+/-1.1 VS. 10.7+/-0.82). In addition, acetazolamide infusion did not increase V/GFR in phosphate-depleted dogs (15.8+/-1.1 vs. 17.16+/-0.9), supporting the conclusion that inhibition of proximal tubular fluid reabsorption was responsible for the elevated urine flow rate. PO4 repletion over 5 days reduced V/GFR to 9.2+/-0.7 despite no change in urine osmolality and no change in GFR, further suggesting a specific reversible alteration in proximal tubular reabsorption in phosphate depletion. Although hypercalciuria was a constant finding in phosphate depletion (fractional excretion of calcium of 2.04+/-0.4% vs. 0.47+/-0.13% in controls), the enhanced distal delivery of calcium was not a crucial factor; acute phosphate infusion reduced urinary calcium excretion to control values without affecting the reduced proximal tubular reabsorption in either intact or thyroparathyroidectomized phosphate-depleted dogs the change in distal nephron calcium reabsorption was independent of parathyroid hormone (PTH) levels since infusion of PTH failed to alter urinary calcium excretion. We conclude that chronic phosphate depletion leads to a reversible, sustained inhibition in proximal tubular reabsorptive fuction as well as a specific decrease in distal nephron calcium reabsorption. This latter reabsorptive defect is sensitive to phosplate infusion but not corrected by PTH.     

The Hypercalciurias CAUSES, PARATHYROID FUNCTIONS, AND DIAGNOSTIC CRITERIA

The causes for the hypercalciuria and diagnostic criteria for the various forms of hypercalciuria were sought in 56 patients with hypercalcemia or nephrolithiasis (Ca stones), by a careful assessment of parathyroid function and calcium metabolism. A study protocol for the evaluation of hypercalciuria, based on a constant liquid synthetic diet, was developed. In 26 cases of primary hyperparathyroidism, characteristic features were: hypercalcemia, high urinary cyclic AMP (cAMP, 8.58+/-3.63 SD mumol/g creatinine; normal, 4.02+/-0.70 mumol/g creatinine), high immunoreactive serum parathyroid hormone (PTH), hypercalciuria, the urinary Ca exceeding absorbed Ca from intestinal tract (Ca(A)), high fasting urinary Ca (0.2 mg/mg creatinine or greater), and low bone density by (125)I photon absorption. The results suggest that hypercalciuria is partly secondary to an excessive skeletal resorption (resorptive hypercalciuria). The 22 cases with renal stones had normocalcemia, hypercalciuria, intestinal hyperabsorption of calcium, normal or low serum PTH and urinary cAMP, normal fasting urinary Ca, and normal bone density. Since their Ca(A) exceeded urinary Ca, the hypercalciuria probably resulted from an intestinal hyperabsorption of Ca (absorptive hypercalciuria). The primacy of intestinal Ca hyperabsorption was confirmed by responses to Ca load and deprivation under a metabolic dietary regimen. During a Ca load of 1,700 mg/day, there was an exaggerated increase in the renal excretion of Ca and a suppression of cAMP excretion. The urinary Ca of 453+/-154 SD mg/day was significantly higher than the control group's 211+/-42 mg/day. The urinary cAMP of 2.26+/-0.56 mumol/g creatinine was significantly lower than in the control group. In contrast, when the intestinal absorption of calcium was limited by cellulose phosphate, the hypercalciuria was corrected and the suppressed renal excretion of cAMP returned towards normal. Two cases with renal stones had normocalcemia, hypercalciuria, and high urinary cAMP or serum PTH. Since Ca(A) was less than urinary Ca, the hypercalciuria may have been secondary to an impaired renal tubular reabsorption of Ca (renal hypercalciuria). Six cases with renal stones had normal values of serum Ca, urinary Ca, urinary cAMP, and serum PTH (normocalciuric nephrolithiasis). Their Ca(A) exceeded urinary Ca, and fasting urinary Ca and bone density were normal. The results support the proposed mechanisms for the hypercalciuria and provide reliable diagnostic criteria for the various forms of hypercalciuria.     

Familial benign hypercalcaemia. Study of a large family

Twenty-seven hypercalcaemic subjects were identified in three generations of a family. There were no clinical complications of chronic hypercalcaemia, but five had had parathyroid surgery which was unsuccessful in four. Twenty of the twenty-seven subjects were compared with twenty-four normocalcaemic controls from the same family and the findings were also compared with those from forty patients with surgically proven primary hyperparathyroidism. The relation between the serum and urinary calcium levels was studied by means of an oral calcium loading test. The ratio of calcium clearance to creatinine clearance was normal in this family (but elevated in the patients with primary hyperparathyroidism) and the concentration of parathyroid hormone was normal, as was the total urinary excretion of cyclic AMP. Thus, there was no evidence of either suppressed or increased parathyroid activity in this familial condition. Basal urinary calcium excretion was normal under steady-state conditions indicating that the hypercalcaemia could not be attributed to either increased bone resorption or increased calcium absorption from the gut. In accordance with this, the serum levels of 1,25-dihydroxycholecalciferol were normal. The hypercalcaemia in this condition can be accounted for in full by an increase in renal tubular reabsorption of calcium, and thus differs from that of primary hyperparathyroidism in which there is increased production of calcium from gut and/or bone as well as an increase in renal tubular reabsorption of calcium. Although the serum phosphate and renal tubular reabsorption of phosphate were both low in patients with familial benign hypercalcaemia, they were not as low as in patients with the same degree of hypercalcaemia due to primary hyperparathyroidism. The changes in phosphate transport in familial benign hypercalcaemia could be explained as a secondary effect of the increased filtered load of calcium in the kidney. The tendency towards hypermagnesaemia in our patients, which contrasts with a tendency towards hypomagnesaemia in primary hyperparathyroidism, could also be explained as a secondary effect of the abnormality of renal tubular reabsorption of calcium. Increased renal tubular calcium reabsorption and persistent normal functioning of the parathyroid glands in the face of hypercalcaemia remain the sole definite abnormalities of the syndrome.     

Plasma oxalate and creatinine and oxalate/creatinine clearance ratios in normal subjects and in primary hyperoxaluria. Evidence for renal hyperoxaluria

Plasma oxalate and creatinine were measured repeatedly in healthy individuals and in 12 patients with type 1 primary hyperoxaluria unresponsive to pyridoxine. The mean ratios were 0.025 (SD 0.006) and 0.120 (SD 0.048), respectively. One patient repeatedly had normal plasma oxalate despite markedly raised urinary oxalate and it seems unlikely that this excess oxalate could have come from the liver. Oxalate/creatinine clearance ratios in the normal group had an overall mean of 0.59 (SD 0.27) in 24 h urine collections and 0.741 (SD 0.297) in repeated short clearance periods. Both renal tubular absorption and secretion of oxalate apparently occurred on different days, but this did not depend upon urinary flow rate. Oxalate/creatinine clearance ratios in type 1 primary hyperoxaluria had a mean of 2.88 (SD 3.11). The raised oxalate/creatinine clearance ratios in the patients were not correlated with either plasma oxalate or creatinine. A few patients showed much higher clearance ratios and in some were sufficiently high to indicate that oxalate was generated and secreted in the kidneys.     

Familial Benign Hypercalcaemia: STUDY OF A LARGE FAMILY

Twenty-seven hypercalcaemic subjects were identified in threegenerations of a family. There were no clinical complicationsof chronic hypercalcaemia, but five had had parathyroid surgerywhich was unsuccessful in four. Twenty of the twenty-seven subjectswere compared with twenty-four normocalcaemic controls fromthe same family and the findings were also compared with thosefrom forty patients with surgically proven primary hyperparathyroidism.The relation between the serum and urinary calcium levels wasstudied by means of an oral calcium loading test. The ratio of calcium clearance to creatinine clearance was normalin this family (but elevated in the patients with primary hyperparathyroidism)and the concentration of parathyroid hormone was normal, aswas the total urinary excretion of cyclic AMP. Thus, there wasno evidence of either supressed or increased parathyroid activityin this familial condition. Basal urinary calcium excretionwas normal under steady-state conditions indicating that thehypercalcaemia could not be attributed to either increased boneresorption or increased calcium absorption from the gut. Inaccordance with this, the serum levels of 1, 25-dihydroxycholecalciferolwere normal. The hypercalcaemia in this condition can be accountedfor in full by an increase in renal tubular reabsorption ofcalcium, and thus differs from that of primary hyperparathyroidismin which there is increased production of calcium from gut and/orbone as well as an increase in renal tubular reabsorption ofcalcium. Although the serum phosphate and renal tubular reabsorptionof phosphate were both low in patients with familial benignhypercalcaemia, they were not as low as in patients with thesame degree of hypercalcaemia due to primary hyperparathyroidism.The changes in phosphate transport in familial benign hypercalcaemiacould be explained as a secondary effect of the increased filteredload of calcium in the kidney. The tendency towards hypermagnesamiain our patients, which contrasts with a tendency towards hypomagnesaemiain primary hyperparathyroidism, could also be explained as asecondary effect of the abnormality of renal tubular reabsorptionof calcium. Increased renal tubular calcium reabsorption and persistentnormal functioning of the parathyroid glands in the face ofhypercalcaemia remain the sole definite abnormalities of thesyndrome.     

Changes in serum and urinary calcium during phosphate depletion: studies on mechanisms

The changes in serum calcium and the renal handling of this ion were evaluated during phosphate depletion. 96 renal clearance studies were carried out in 10 dogs before and after prolonged phosphate depletion (30-160 days) and after repletion. Depletion was produced by reducing phosphate intake and administering aluminum hydroxide gel while intakes of sodium, calcium, and magnesium were constant. With phosphate depletion, serum phosphorus fell to less than 1.0 mg/100 ml and diffusible serum calcium either remained unchanged or rose transiently. Glomerular filtration rate (GFR) fell by 15 to 53%. Despite the reduced filtered load of calcium, its fractional excretion increased in most experiments. This hypercalciuria was not dependent upon changes in sodium or magnesium excretion, or the urinary concentration of complexing anions, and persisted after sodium restriction. Phosphate repletion reversed the effects on GFR and calcium excretion. The intravenous infusion of small quantities of phosphate (0.04 mmole/min) into either intact or thyroparathyroidectomized (T-PTX), phosphate-depleted animals caused a significant reduction in fractional excretion of calcium, but the intrarenal infusion of 0.02 mmole/min of phosphate into one kidney failed to produce an ipsilateral effect. The administration of parathyroid extract reduced fractional calcium excretion, but the latter remained significantly elevated. After T-PTX, fractional calcium excretion did not increase in the phosphate-depleted animals. Furthermore, serum calcium was normal after T-PTX until serum phosphorus increased slightly, and only then did hypocalcemia develop. These observations indicate that (a) phosphate depletion produces hypercalciuria through a reduction in tubular reabsorption of calcium which is not due to changes in the tubular reabsorption of other ions; this effect is not reversed by the direct intrarenal infusion of phosphate; (b) a state of functional hypoparathyroidsm occurs during phosphate depletion which may, in part, cause reduced tubular reabsorption of calcium; (c) other extra renal mechanism(s), possibly related to events occurring in bone as a result of phosphate depletion, may have an effect on urinary calcium excretion; and (d) in the phosphatedepleted state, parathyroid hormone is not required for the maintenance of a normal level of serum calcium.     

PTH-independent regulation of blood calcium concentration by the calcium-sensing receptor

Tight regulation of calcium levels is required for many critical biological functions. The Ca2+-sensing receptor (CaSR) expressed by parathyroid cells controls blood calcium concentration by regulating parathyroid hormone (PTH) secretion. However, CaSR is also expressed in other organs, such as the kidney, but the importance of extraparathyroid CaSR in calcium metabolism remains unknown. Here, we investigated the role of extraparathyroid CaSR using thyroparathyroidectomized, PTH-supplemented rats. Chronic inhibition of CaSR selectively increased renal tubular calcium absorption and blood calcium concentration independent of PTH secretion change and without altering intestinal calcium absorption. CaSR inhibition increased blood calcium concentration in animals pretreated with a bisphosphonate, indicating that the increase did not result from release of bone calcium. Kidney CaSR was expressed primarily in the thick ascending limb of the loop of Henle (TAL). As measured by in vitro microperfusion of cortical TAL, CaSR inhibitors increased calcium reabsorption and paracellular pathway permeability but did not change NaCl reabsorption. We conclude that CaSR is a direct determinant of blood calcium concentration, independent of PTH, and modulates renal tubular calcium transport in the TAL via the permeability of the paracellular pathway. These findings suggest that CaSR inhibitors may provide a new specific treatment for disorders related to impaired PTH secretion, such as primary hypoparathyroidism.     

Is there a renal phosphorus leak in recurrent renal stone formers with absorptive hypercalciuria?

Abstract. In ten male hypophosphataemic hypercal-ciuric recurrent renal stone formers with absorptive hypercalciuria and ten male normophosphataemic normocalciuric control persons, fasting plasma and urine chemistry was studied throughout the day under basal conditions and following an oral phosphorus load. After overnight fasting, plasma phosphorus and TMP/GFR were lower and urinary calcium higher in patients than in controls. Both in patients and controls, plasma phosphorus rose throughout the morning hours. In the afternoon, plasma phosphorus was almost equal in patients and controls. The circadian rise of plasma phosphorus despite no increase of urinary phosphorus argues against the presence of a fixed renal tubular phosphorus leak in absorptive hypercalciuria, at least in the fasting state.
Patients differed from controls not only with respect to urinary calcium, but also with respect to fasting absolute and fractional urinary excretion of sodium and chloride. Increased fractional urinary sodium was found both in normotensive and hypertensive patients. Since tubular reabsorption of phosphorus and the setting of fasting plasma phosphorus depend, among other factors, on tubular handling of sodium, the finding may be relevant for the genesis of transient fasting hypophosphataemia in absorptive hypercalciuria.     

Evidence for an Intrinsic Renal Tubular Defect in Mice with Genetic Hypophosphatemic Rickets

To investigate the role of parathyroid hormone (PTH) and(or) an intrinsic renal tubular reabsorptive defect for phosphate in mice with hereditary hypophosphatemic rickets, we performed clearance and micropuncture studies in hypophosphatemic mutants and nonaffected littermate controls. Increased fractional excretion of phosphate in mutants (47.2+/-4 vs. 30.8+/-2% in controls) was associated with reduced fractional and absolute reabsorption in the proximal convoluted tubule and more distal sites. Acute thyropara-thyroidectomy (TPTX) increased phosphate reabsorption in both mutants and controls with a fall in fractional phosphate excretion to congruent with7.5% in both groups indicating that PTH modified the degree of phosphaturia in the intact mutants. Absolute reabsorption in the proximal tubule and beyond remained reduced in the mutants, however, possibly because of the reduced filtered load. Serum PTH levels were the same in intact mutants and normals as was renal cortical adenylate cyclase activity both before and after PTH stimulation.To evaluate the possibility that the phosphate wasting was caused by an intrinsic tubular defect that was masked by TPTX, glomerular fluid phosphate concentration was raised by phosphate infusion in TPTX mutants to levels approaching those of control mice. Phosphate excretion rose markedly and fractional reabsorption fell, but there was no change in absolute phosphate reabsorption in either the proximal tubule or beyond, indicating a persistent reabsorptive defect in the absence of PTH.We conclude that hereditary hypophosphatemia in the mouse is associated with a renal tubular defect in phosphate reabsorption, which is independent of PTH and therefore represents a specific intrinsic abnormality of phosphate transport.     

Oxalate measurement in the picomol range by ion chromatography: values in fasting plasma and urine of controls and patients with idiopathic calcium urolithiasis

Oxalate was measured by ion chromatography in the ultrafiltrate of heparinized plasma from peripheral venous blood, using a membrane with a cut-off molecular weight (Mr). The following criteria were established: sensitivity 0.7 mumol.l-1; intra- and inter-assay coefficients of variation 4% and 12%, respectively; precision of duplicate determinations (expressed as standard deviation) 0.08 mumol.l-1; overall recovery (oxalate added and diluted, respectively) 100.7%. These qualified the method for assessment of plasma oxalate in healthy human controls (males: n = 12) as well as patients with idiopathic renal calcium urolithiasis (males: n = 22; females: n = 16). Renal calcium urolithiasis patients were subclassified into those with normocalciuria and idiopathic hypercalciuria. In male and female controls the mean values (and range) of plasma oxalate were 1.98 (1.4-2.5) and 1.78 (0.7-2.9) mumol.l-1, respectively. In male controls ultrafiltration (membrane cut off Mr 10,000) revealed that 11-16% plasma oxalate was bound to constituents having an apparent Mr above 10,000, and that with use of membranes with smaller pore size, the ultrafilterability of oxalate decreases further. In renal calcium urolithiasis the following values were elicited (mumol.l-1): male normocalciuria 1.78 (0.8-4.0), idiopathic hypercalciuria 1.58 (1.2-2.2); female normocalciuria 1.69 (0.8-3.6), idiopathic hypercalciuria 1.21 (0.8-2.1). The difference from controls is significant in idiopathic hypercalciuria (males and females). In contrast, in fasting urine of renal calcium urolithiasis the oxalate excretion rate (5-45 mumol per 120 min) and oxalate clearance (21-328 ml per min) resemble those in controls, whereas in renal calcium urolithiasis the fractional oxalate clearance (30-357% of creatinine clearance) tended to higher values (p less than 0.01, in male idiopathic hypercalciuria versus controls). It is suggested that 1) ion chromatography allows the reliable assessment of ultrafiltrable plasma oxalate in health and disease states, 2) in renal calcium urolithiasis this technique may help to elucidate oxalate pathophysiology, especially the mode of renal handling of oxalate.     

Evidence for Secondary Hyperparathyroidism in Idiopathic Hypercalciuria

Circulating levels of immunoreactive parathyroid hormone (PTH) were measured in 40 patients with idiopathic hypercalciuria (IH) before and during reversal of hypercalciuria with thiazide, and in four normal subjects before and during induction of hypercalciuria with furosemide. 26 patients with IH had elevated serum PTH levels. The remaining patients had normal levels. Although the correlation was not complete, high PTH levels were generally found in patients who had more severe average urinary calcium losses. When initially elevated. PTH levels fell to normal or nearly normal values during periods of thiazide administration lasting up to 22 months. When initially normal, PTH levels were not altered by thiazide. Reversal of hyperparathyroidism by thiazide could not be ascribed to the induction of hypercalcemia, since serum calcium concentration failed to rise in a majority of patients. Renal hypercalciuria produced by furosemide administration elevated serum PTH to levels equivalent to those observed in patients with IH.The findings in this study help to distinguish between several current alternative views of IH and its relationship to hyperparathyroidism. Alimentary calcium hyperabsorption cannot be the major cause of IH with high PTH levels, because this mechanism could not elevate PTH. Idiopathic hypercalciuria cannot be a variety of primary hyperparathyroidism, as this disease is usually defined, because PTH levels are not elevated in all patients and, when high, are lowered by reversal of hypercalciuria. Primary renal loss of calcium could explain the variable occurrence of reversible hyperparathyroidism in IH, since renal hypercalciuria from furosemide elevates serum PTH in normal subjects. Consequently, a reasonable working hypothesis is that IH is often due to a primary renal defect of calcium handling that leads, by unknown pathways, to secondary hyperparathyroidism.     

Metabolic studies in kidney stone disease.

Patients with kidney stones (n = 59) and healthy controls (n = 31) collected a 24-hour urine sample and later underwent a 6-hour 'fast and load' test in which an oral calcium load was taken after 2 hours. In the 24-hour urine sample, mean calcium excretion was higher in patients than controls, while mean urate, oxalate and citrate levels were similar. The patients had higher levels of fasting plasma calcium, serum calcitriol and fasting urinary calcium, and lower levels of plasma phosphate than did the controls. Following the calcium load, plasma and urinary calcium increased similarly in both groups. Serum parathyroid hormone (PTH) levels were similar in both groups and decreased similarly following the calcium load. Multiple linear regression, relating the presence or absence of stone formation to all variables, found the only variables significantly related to stone formation to be plasma levels of calcium (p less than 0.001) and phosphate (p = 0.001) and fasting urinary urea (p less than 0.001), and 24-hour urinary calcium excretion (p less than 0.05). Urinary oxalate and citrate were not related to stone formation. The data do not support the hypothesis that primary stimulation by calcitriol produces a normal fasting plasma calcium level, with an exaggerated increase after an oral calcium load. The findings instead suggest an abnormality of parathyroid cell 'set point', such that PTH secretion continues until the plasma calcium level is a little higher and the phosphate a little lower than in controls.     

Absorptive hyperoxaluria: a new clinical entity--successful treatment with hydrochlorothiazide

This report describes studies performed over an 11 year period in a 13 year old girl with hyperoxaluria and calcium oxalate nephrolithiasis who did not have primary hyperoxaluria or any of the recognized causes of secondary hyperoxaluria. The patient also had increased urinary excretion of calcium and magnesium and hyperabsorption of dietary calcium and magnesium. It is suggested that the hyperoxaluria resulted from hyperabsorption of dietary oxalate secondary to hyperabsorption of dietary calcium. Hyperabsorption of dietary magnesium and increased urinary magnesium excretion have not previously been reported in this context. Stone formation ceased and urinary oxalate excretion gradually fell to normal during long term thiazide therapy but hyperoxaluria recurred when orthophosphate therapy was substituted for the hydrochlorothiazide. This is the first report of normalization of urine oxalate excretion during thiazide therapy in a patient with frank hyperoxaluria.     

Prevention of recurrent nephrolithiasis

The first episode of nephrolithiasis provides an opportunity to advise patients about measures for preventing future stones. Low fluid intake and excessive intake of protein, salt and oxalate are important modifiable risk factors for kidney stones. Calcium restriction is not useful and may potentiate osteoporosis. Diseases such as hyperparathyroidism, sarcoidosis and renal tubular acidosis should be considered in patients with nephrolithiasis. A 24-hour urine collection with measurement of the important analytes is usually reserved for use in patients with recurrent stone formation. In these patients, the major urinary risk factors include hypercalciuria, hyperoxaluria, hypocitraturia and hyperuricosuria. Effective preventive and treatment measures include thiazide therapy to lower the urinary calcium level, citrate supplementation to increase the urinary citrate level and, sometimes, allopurinol therapy to lower uric acid excretion. Uric acid stones are most often treated with citrate supplementation. Data now support the cost-effectiveness of evaluation and treatment of patients with recurrent stones.     

Mechanism of hypercalciuria in genetic hypercalciuric rats. Inherited defect in intestinal calcium transport.

Excessive urine calcium excretion in patients with idiopathic hypercalciuria may involve a primary increase in intestinal calcium absorption, overproduction of 1,25-dihydroxyvitamin D3 or a defect in renal tubular calcium reabsorption. To determine the mechanism of hypercalciuria in an animal model, hypercalciuria was selected for in rats and the most hypercalciuric animals inbred. Animals from the fourth generation were utilized to study mineral balance and intestinal transport in relation to levels of serum 1,25(OH)2D3. Both urine calcium excretion and net intestinal calcium absorption were greater in hypercalciuric males (HM) than in normocalciuric males (NM) and in hypercalciuric females (HF) than in normocalciuric females (NF). However, serum 1,25(OH)2D3 was lower in HM than in NM and not different in HF than in NF. Net calcium balance was more positive in HM than in NM and in HF than in NF. In vitro duodenal calcium net flux was correlated with serum 1,25(OH)2D3 in HM and HF and in NM and NF. However, with increasing serum 1,25(OH)2D3 there was greater calcium net flux in hypercalciuric rats than in normocalciuric controls. Hypercalciuria in this colony of hypercalciuric rats is due to a primary intestinal overabsorption of dietary calcium and not an overproduction of 1,25(OH)2D3 or a defect in the renal tubular reabsorption of calcium.     

The effect of pyridoxine on oxalate dynamics in three cases of primary hyperoxaluria (with glycollic aciduria)

We have measured glomerular filtration rate (GFR), extracellular fluid volume (ECF), oxalate distribution volume (OxDV), plasma oxalate concentration (POx.), plasma total clearance of oxalate (PCOx.), oxalate metabolic pool size [(OxDV) X (POx.)], renal clearance of oxalate (RCOx.), oxalate excretion, tissue clearance of oxalate (TCOx.) and tissue oxalate accumulation rate [(TOx.A) = (TCOx.) X (POx.)] in three patients with type I primary hyperoxaluria (hyperoxaluria with hyperglycollic aciduria) when they were taking pyridoxine and after discontinuation of the vitamin. Seven days after stopping pyridoxine the plasma oxalate concentration, oxalate metabolic pool size and the urinary excretion of oxalate had all increased between seven- and eight-fold in two of the patients. The third patient showed no changes on stopping pyridoxine. These results support the view that pyridoxine acts by reducing oxalate biosynthesis in some patients with type I primary hyperoxaluria. The possible biochemical basis for this effect is discussed.     

Effect of indapamide on urinary calcium excretion in patients with and without urinary stone disease

BACKGROUND: Indapamide is an antihypertensive agent similar to thiazides, but with some different effects. Thiazide and thiazide-like diuretics are useful in preventing recurrent urinary stone formation due to their hypocalciuric effects. OBJECTIVE: To determine the hypocalciuric and other effects on certain laboratory parameters of indapamide 1.5 mg in different patient groups. METHODS: Four groups of patients recruited from urology and nephrology outpatient departments were experiencing non-hypercalciuric urinary stone disease (group 1), idiopathic hypercalciuria (group 2), urinary stone disease with hypercalciuria (group 3), and essential hypertension (group 4). In all patients, fasting serum uric acid, calcium, sodium, potassium, cholesterol, triglyceride, parathyroid hormone (PTH) values, and morning second-spot urine calcium and creatinine levels were assessed before and 8 weeks after treatment with indapamide. RESULTS: Urinary calcium excretion was reduced significantly in all groups: group 1 from 0.10 +/- 0.02 to 0.07 +/- 0.03 (mean +/-SD; 30% reduction; p < 0.001), group 2 from 0.30 +/- 0.15 to 0.15 +/- 0.10 (50% reduction; p < 0.001), group 3 from 0.35 +/- 0.15 to 0.20 +/- 0.10 (43% reduction; p < 0.001), and group 4 from 0.10 +/- 0.03 to 0.08 +/- 0.02 (20% reduction; p < 0.0010). These results should be interpreted with caution since no control group was included in this study. Mean serum uric acid and triglyceride levels were significantly increased, and mean PTH and potassium levels and diastolic and systolic blood pressure were significantly decreased in all groups. Few temporary adverse effects, such as dizziness and fatigue, were noticed and none of them caused discontinuation of treatment. CONCLUSIONS: Indapamide 1.5 mg/day is effective in decreasing calciuria in patients with non-hypercalciuric urinary stone disease, idiopathic hypercalciuria, urinary stone disease with hypercalciuria, and essential hypertension. This could be achieved with few adverse effects similar to those of thiazides and indapamide 2.5 mg. Indapamide decreased the PTH levels in all groups. Long-term clinical benefits of these effects should be evaluated prospectively with further randomized studies.     

Effect of citrate on the urinary excretion of calcium and oxalate: relevance to calcium oxalate nephrolithiasis

Studies in 24 recurrent oxalate stone-formers have shown that values for urinary calcium excretion for this group on at-home diets vary significantly (P less than 0.001) more than values for creatinine excretions. By placing stone-formers on controlled in-hospital diets and measuring their calcium excretions, we were able to predict probable outpatient hypercalciuria (greater than 7.5 mmol/day) with a sensitivity of 95% and a specificity of 95%. In this study, the renal loss of calcium during low-calcium diets was proportional to the absorptive hypercalciuria during high-calcium diets. Calcium loading experiments in fasted stone-formers and normal subjects indicated that citrate, at citrate:calcium molar ratios ranging from 0.12 to 1, stimulated urinary calcium excretion more than did calcium carbonate loading alone. In addition, citrate also significantly (P less than 0.05) increased the excretion of urinary oxalate by two normal subjects for a given load of calcium oxalate. Malabsorption of citrate and possibly other hydroxycarboxylic acids may thus predispose to oxalate nephrolithiasis by promoting calcium and oxalate absorption.     

Renal handling of fleroxacin in rabbits, dogs, and humans.

The renal handling of fleroxacin was studied by renal clearance and stop-flow techniques in rabbits and dogs and by analyzing the pharmacokinetics with and without probenecid in humans. In rabbits the excretion ratios (fleroxacin intrinsic renal clearance/glomerular filtration rate) were greater than unity (2.01) without probenecid and were decreased to a value below unity (0.680) with probenecid. In dogs, on the other hand, the excretion ratios were less than unity (0.608 and 0.456) both without and with probenecid, and so were not affected by probenecid. This fact suggested that fleroxacin was excreted into urine by both glomerular filtration and renal tubular secretion in rabbits, but only by glomerular filtration in dogs, accompanied by partial renal tubular reabsorption in both species; these mechanisms were also supported by stop-flow experiments. In humans probenecid treatment induced increases in the elimination half-life and area under the serum concentration-time curve and decreases in apparent serum clearance, renal clearance, and urinary recovery of fleroxacin. The excretion ratio without probenecid was 1.13, which was significantly decreased to 0.750 with probenecid. These results indicated that both renal tubular secretion and reabsorption contributed to renal excretion of fleroxacin in humans. The contribution of tubular secretion was species dependent and was extensive in rabbits, minimal in dogs, and moderate in humans. Renal tubular reabsorption was commonly found in every species. The long elimination half-life of fleroxacin in humans might be explained by its small total serum clearance and small renal clearance, which are attributed to less tubular secretion and more tubular reabsorption.     

Studies on some possible biochemical treatments of primary hyperoxaluria.

The effects of some putative inhibitors of oxalate production or urinary oxalate excretion have been investigated in the Cynamolgus monkey and in patients with Type I primary hyperoxaluria (hyperoxaluria with glycollic aciduria). Sodium-1-hydroxybutan-sulphonate, D,L-phenyllactate, succinimide and isocarboxazide did not reduce the urinary oxalate excretion in the monkeys. Pyridoxine reduced the excretion of oxalate and glycollate in some patients, and its therapeutic use has been documented over a five-year period. Succinimide, which has been used by other workers for the treatment of non-hyperoxaluric stone formers, did not decrease the excretion of either oxalate or glycollate in three patients in whom it was tried. It did not change the inhibitory activity of the urine with respect to the growth and aggregation of calcium oxalate crystals in any of the three patients, and it did not have any consistent effect on the excretion of calcium oxalate crystals in the one patient who had detectable crystaluria before treatment. We have identified several metabolites of succinimide in the urine of patients taking the drug. These include 2,3-dehydrosuccinamic, 2-hydroxysuccinamic and 3-hydroxysuccinamic acids. Isocarboxazide, cholestyramine and thiamine did not affect the urinary oxalate excretion in the patients. The significance of these observations from the viewpoint of the treatment of primary hyperoxaluria is discussed.