Idiopathic Recurrent Calcium Urolithiasis (IRCU): pathophysiology evaluated in light of oxidative metabolism, without and with variation of several biomarkers in fasting urine and plasma--a comparison of stone-free and -bearing male patients, emphasizing mineral, acid-base, blood pressure and protein status

BACKGROUND

IRCU is traditionally considered as life?style disease (associations with, among others, overweight, obesity, hypertension, type-2 diabetes), arising from excess, in 24 h urine, of calcium (Ca) salts (calcium oxalate (CaOx), calcium phosphate (CaPi)), supersaturation of, and crystallization in, tubular fluid and urine, causing crystal-induced epithelial cell damage, proteinuria, crystal aggregation and uroliths.

METHODS

Another picture emerges from the present uncontrolled study of 154 male adult IRCU patients (75 stone-bearing (SB) and 79 age-matched stone-free (SF)), in whom stone-forming and other parameters in fasting urine and plasma were contrasted with five biomarkers (see footnote) of oxidative metabolism (OM), without and with variation of markers.

RESULTS

1) In SB vs. SF unstratified OM biomarkers were statistically unchanged, but the majority of patients was overweight; despite, in SB vs. SF urine pH, total and non-albumin protein concentration were elevated, fractional urinary uric acid excretion and blood bicarbonate decreased, whereas urine volume, sodium, supersaturation with CaOx and CaPi (as hydroxyapatite) were unchanged; 2) upon variation of OM markers (strata below and above median) numerous stone parameters differed significantly, among others urine volume, total protein, Ca/Pi ratio, pH, sodium, potassium, plasma Ca/Pi ratio and parathyroid hormone, blood pressure, renal excretion of non-albumin protein and other substances; 3) a significant shift from SF to SB patients occurred with increase of urine pH, decrease of blood bicarbonate, and increase of diastolic blood pressure, whereas increase of plasma uric acid impacted only marginally; 4) in both SF and SB patients a strong curvilinear relationship links a rise of urine Ca/Pi to urine Ca/Pi divided by plasma Ca/Pi, but in SB urine Ca/Pi failed to correlate significantly with urine hydroxyapatite supersaturation; 5) also in SB, plasma Ca/Pi and urinary nitrate were negatively correlated, whereas in SF plasma Ca/Pi ratio, PTH and body mass index correlated positively; 6) multivariate regression analysis revealed that PTH, body mass index and nitrate together could explain 22 (p = 0.002) and only 7 (p = 0.06) per cent of variation of plasma Ca/Pi in SF and SB, respectively.

CONCLUSIONS

In IRCU a) numerous constituents of fasting urine, plasma, blood and blood pressure change in response to variation of OM biomarkers, suggesting involvement of OM imbalance as factor in functional deterioration of tissue; b) in the majority of patients a positive exponential relationship links urine Ca/Pi to urine Ca/Pi divided by plasma Ca/Pi, presumably to accumulate Ca outside tubular lumen, thereby minimizing intratubular and urinary Ca salt crystallization; c) alteration of interactions of low urine nitrate, PTH and Ca/Pi in plasma may be of importance in formation of new Ca stone and co-regulation of dynamics of blood vasculature; d) overweight, combined with OM-modified renal interstitial environment appears to facilitate these processes, carrying the risk that CaPi mineral develops within or/and close to blood vessel tissue, and spreads towards urothelium. - For future research focussing on IRCU pathogenesis studies are recommended on the role of affluent lifestyle mediated renal ischemia, mild hypertensive nephropathy, rise of uric acid precursor oxypurines and uricemia, clarifying also why loss of significance of interrelationships of OM biomarkers with traditional Ca stone risk factors is characteristic for SB patients.

Idiopathic recurrent calcium urolithiasis (IRCU): variation of fasting urinary protein is a window to pathophysiology or simple consequence of renal stones in situ? A tripartite study in male patients providing insight into oxidative metabolism as possible driving force towards alteration of urine composition, calcium salt crystallization and stone formation

Background

In IRCU it is uncertain whether variation of urinary protein, especially non-albumin protein (NAlb-P), is due to the presence of stones or reflects alteration of oxidative metabolism.

Aims

To validate in a tripartite cross-sectional study of 187 ambulatory male patients, undergoing a standardized laboratory programme, whether stones impact on N-Alb-P or the state of oxidative metabolism interferes with IRCU pathophysiology.

Methods

In part 1 the strata low and high of fasting urinary excretion rate per 2 h of N-Alb-P, malonedialdehyde, hypoxanthine, xanthine, pH and other urine components were compared, and association with renal stones in situ evaluated; in part 2 the co-variation of oxidatively modulated environment, fasting urinary pH, calcium (Ca) salt crystallization risk and the number of patients with stones in situ was examined; in part 3, the nucleation of Ca oxalate and Ca phosphate was tested in undiluted postprandial urine of patients and related to the state of oxidative metabolism.

Results

In part 1, N-Alb-P excretion > 4.3 mg was associated with increase of blood pressure, excretion of total protein, hypoxanthine (a marker of tissue hypoxia), malonedialdehyde (a marker of lipid peroxidation), sodium, magnesium, citrate, uric acid, volume, pH, and increase of renal fractional excretion of both NAlb-P and uric acid; when stones were present, urinary pH was elevated but other parameters were unaffected. Significant predictors of N-Alb-P excretion were malonedialdehyde, fractional N-Alb-P and hypoxanthine. In part 2, urine pH > 6.14 was associated with unchanged blood pressure and plasma vasopressin, increase of blood pH, urinary volume, malonedialde hyde, fractional excretion of N-Alb-P, uric acid, Ca phosphate, but not Ca oxalate, supersaturation; this spectrum was accompanied by decrease of concentration of urinary total and free magnesium, total and complexed citrate, plasma uric acid (in humans the major circulating antioxidant) and insulin; the number of stone-bearing patients was increased. Significant predictors of urine pH were body mass index, plasma insulin and uric acid (negative), and urinary xanthine (positive). In part 3 low plasma uric acid, not high urinary malonedialdehyde or high ratio malonedialdehyde/uric acid was significantly associated with diminished Ca but not oxalate tolerance, with the first nucleating crystal type being mostly Ca phosphate (hydroxyapatite), in the rest Ca oxalate dihydrate; uricemia correlated marginally positively (p = 0.055) with Ca tolerance of urine, stronger with blood pressure and insulin, and negatively with urinary xanthine, fractional N-Alb-P, volume, sodium.

Conclusions

In IRCU 1) not renal stones in situ, but disturbed oxidative metabolism apparently modulates nephron functionality, ending up in higher renal NAlb-P release, urinary volume, sodium and pH of fasting urine; 2) etiologically unknown decline of uricemia may represent antioxidant deficiency and cause a risk of hydroxyapatite crystallization and stone formation in a weakly acidic or alkaline inhibitor-deficient and NAlb-P-rich milieu; 3) several observations, linking oxidative and systemic metabolism, are compatible with Ca stone initiation beyond tubules.

Idiopathic recurrent calcium urolithiasis (IRCU): an acid meal challenge uncovers inappropriate pH of postprandial, fasting and daily urine: a cross-sectional study of male patients providing insight into post- and pre-load urinary stone substances, crystallization risk, presence of stones, renal transport and systemic metabolic factors

BACKGROUND

In IRCU the possible role of urinary pH (U-pH) as risk factor of calcium (Ca) stones is poorly understood.

OBJECTIVES

To evaluate in IRCU the response to an oral acid load, focussing on post- and pre-load U-pH, other urinary, renal and extra-renal factors, and linkage with Ca stones. -

METHODS

237 male patients, either Ca stone-free (SF) or -bearing (SB), but without overt signs of systemic metabolic acidosis underwent a standardized laboratory programme that included, besides collection of urine and blood, the intake of an oxalate-free acid test meal (proton content 120 mM). Established analytical methods were used.

RESULTS

In 79 patients the post-meal load U-pH was < or = 5.30 (in healthy individuals accepted as the upper limit after the same proton load), but >5.30 in 158; in these two subsets the mean fasting pre-load U-pH was 5.84 and 6.37 (p <0.001), the mean U-pH in 24 h urine 5.70 and 6.03 (p <0.001), the mean score of stone formation activity 32 and 42 (p = 0.12), the SF/SB ratio 35/44 and 76/82 (not significant). However, when in pre-load urine undissociated uric acid concentration was low due to the high pH, the SF/SB ratio was 53/66 (p = 0.038), whereas isolated increase of U-pH with SF/SB ratio 54/65 (p = 0.059), urinary supersaturation with Ca phosphate (hydroxyapatite), Ca oxalate, uric acid, and isolated decrease of concentration of total protein, total uric acid and the crystallization inhibitors magnesium and citrate failed to affect significantly the frequency distribution of SF and SB patients. Pre-load U-pH was positively associated with urinary ratio sodium/proton excretion, renal reclaim of sodium and protein, negatively associated with body mass index, fasting insulinemia and uricemia, urinary protein concentration, renal reclaim of phosphate.

CONCLUSIONS

In IRCU 1) inappropriately high U-pH combined with increase of proteinuria and alteration of renal-tubular transport are frequent; 2) disturbed interactions of renal proton generation with sodium handling, urinary physico-chemical and systemic metabolic factors may initiate the development of Ca-containing concretions, presumably Ca phosphate, at some yet unknown renal anatomic site.

Is there a role for uric acid in an animal model of calcium phosphate nephrocalcinosis and calcium phosphate crystallization in urine of patients with idiopathic calcium urolithiasis? An orientational study

Uric acid (UA), a waste product of purine metabolism, may be involved in calcium phosphate crystallization and deposition. Rats, which develop nephrocalcinosis on high-fat or magnesium-deficient diets, and patients with idiopathic calcium urolithiasis have hyperproteinuria, especially of nonalbumin protein, and a shift toward elevated serum UA. In rats, an increase in UA precursors and renal UA suggests hypoxemia, which stimulates xanthine oxidase. In patients, a primary increase in renal xanthine oxidase would explain the low urine UA in the presence of an elevated serum concentration. For calcium phosphate deposition (rats) or incorporation into stones (humans) to occur, a crucial factor may be xanthine oxidase-mediated overproduction of free radical species and subsequent tissue damage. Another factor may be whether sufficient UA is synthesized to neutralize these free radicals. Allopurinol use, which inhibits xanthine oxidase and has long been favored for the treatment of idiopathic calcium urolithiasis, may not prevent stones, because it also diminishes the availability of UA. An investigation of the factors that control serum UA homeostasis may be rewarding in research into the etiology of idiopathic calcium urolithiasis.

Magnesium, citrate, magnesium citrate and magnesium-alkali citrate as modulators of calcium oxalate crystallization in urine: observations in patients with recurrent idiopathic calcium urolithiasis

The effects of magnesium (Mg) and citrate on the metastable limit of calcium oxalate (CaOx) solubility (synonym: tolerable oxalate TO) were examined in artificial urine and in postprandial urine of male patients with idiopathic calcium urolithiasis (ICU). In artificial urine increasing pH, Mg and citrate elevate TO, decrease CaOx supersaturation only marginally, but elevate considerably free citrate; the effect of Mg alone was small in comparison with citrate alone, and the effects of both substances appeared additive. In ICU patients, matched for sex, age and CaOx supersaturation to non-stone-forming controls, TO was decreased (mean values 0.33 vs. 0.52 mM/l in controls, P < 0.05). Additional significant (P < 0.05) differences were found between ICU and controls: the former exhibited increased CaOx crystal growth, decreased crystal agglomeration time, a more acidic urinary pH, increased concentrations of free calcium and free Mg, and decreased free oxalate and free citrate. After ingestion of a urine-acidifying test meal, or this meal supplemented with either neutral Mg citrate or Mg-alkali citrate, by three groups of male ICU patients, matched for age and CaOx supersaturation, only the last-named preparation evoked an increase in TO and a decrease in crystal diameter, while the normally occurring pH decline from fasting urine was virtually abolished, and the ratios urinary Mg/citrate and calcium/citrate tended towards low values. In contrast, Mg citrate increased crystal agglomeration time, while changes in the other parameters were only insignificant. The crystals formed in urine were CaOx di- and monohydrate (by electron microscopy), and energy dispersive X-ray analysis showed calcium peaks exclusively. However, chemical analysis of crystals verified the presence not only of oxalate and calcium, but also of Mg, phosphate, citrate, and urate; moreover, these crystal constituents seemed to be influenced by Mg citrate and Mg-alkali citrate in different ways. It was concluded that (1) Mg and citrate are effectors of TO in artificial and natural urine; (2) in ICU, low TO and other disturbed CaOx crystallization parameters appear related to the prevailing low urinary pH and low free citrate; (3) Mg-alkali citrate inhibits CaOx crystallization, probably via actions of the citrate, but not the Mg. Because of the eminent role of Mg in human health and ICU, further studies on crystallization after oral intake of Mg in the form of citrate are warranted.

Urinary phosphate excretion in the pathophysiology of idiopathic recurrent calcium urolithiasis: hormonal interactions and lipid metabolism

Previous work in younger males with recurrent idiopathic calcium urolithiasis (RCU) demonstrated inappropriately high postprandial phosphaturia, hyperinsulinemia and insulin resistance, but normal glycemia. To investigate further whether these abnormalities occur also in RCU patients with a mean age corresponding to the life period with peak formation of calcium-containing stones, two trials were carried out in 155 males of comparable age and body mass index. All participants underwent a standardized laboratory examination, including collection of urine and blood before and following a test meal rich in carbohydrate and calcium but low in phosphorus. In trial 1, comprising control subjects (n = 12, mean age 42 years) and RCU patients (n = 24, mean age 41 years), phosphate (Pi) excretion and fractional Pi excretion in postprandial urine of controls did not change compared with the values in fasting urine, but were significantly increased in RCU, despite the fact that there was almost equal suppression of serum parathyroid hormone (PTH) and increase in serum calcitonin. Postprandially, RCU patients were hyperinsulinemic but still normoglycemic versus controls. In trial 2, carried out in unclassified (in terms of calciuria) RCU patients (n = 119, mean age 40 years) only, the post-load Pi-uria was similar in magnitude to Pi-uria of RCU patients in trial 1; increased postprandial Pi-uria was a phenomenon also of normocalciuria but was slightly more pronounced in hypercalciuria, while changes in calcium phosphate (brushite) and calcium oxalate supersaturation of urine were unrelated to calciuria. In RCU patients, but not controls, there was a tendency toward higher urinary glucose in post-load as compared with fasting urine. When urinary Pi and fractional Pi excretion in trial 2 were considered as dependent variables in multivariate regression analysis, they appeared unrelated to age, but positively associated with postprandial glycemia as the best predictor, followed by insulinemia, insulin resistance, to a lesser degree fasting serum PTH and the metabolic activity of stone disease, negatively associated with blood total lipids and very low density lipoprotein (VLDL) cholesterol. It was concluded that RCU males (1) show low Pi-uria during fasting but impaired renal Pi conservation in response to a mixed meal, a situation carrying the risk of Pi deficiency over the long term; (2) represent a population developing hyperPi-uria despite suppressed PTH; (3) exhibit insulin resistance but are still able to maintain normoglycemia at the expense of hyperinsulinemia. It is suggested that calcium-containing renal stones are related to impaired Pi and glucose translocation across cell membranes, and that the role of lipids in this setting deserves further investigation.

Postprandial hyperinsulinaemia, insulin resistance and inappropriately high phosphaturia are features of younger males with idiopathic calcium urolithiasis: attenuation by ascorbic acid supplementation of a test meal

In idiopathic recurrent calcium urolithiasis (RCU) the state of insulin and carbohydrate metabolism, and relationships to minerals such as phosphate, are insufficiently understood. Therefore, in two groups of males with RCU (n = 30) and healthy controls (n = 8) the response to an oral carbohydrate- and calcium-rich test meal was studied with respect to glucose, insulin, and C-peptide in peripheral venous blood (taken before and up to 180 min post-load), and phosphate and glucose in fasting and post-load urine. In one RCU group (n = 16) the meal was supplemented with ascorbic acid (ASC; 5 mg/kg body weight). The mean age (RCU 29, RCU + ASC 30, controls 27 years) and mean body mass index [RCU 24.4, RCU + ASC 25.0, controls 24.0 kg/m2] were similar. Insulin resistance (synonymous sensitivity of peripheral organs to insulin) was calculated from insulin serum concentration, as was also integrated insulin, C-peptide, and glucose. Untreated stone patients (RCU) developed hyperinsulinaemia between 60 and 120 min post-load, increased integrated insulin, and insulin resistance (P < or = 0.05 vs controls), whereas the rise of C-peptide and glycaemia (absolute and integrated values) was only of borderline significance. Fasting phosphaturia was low in both RCU subgroups vs controls; however, phosphaturia in untreated RCU rose in response to the meal, contrasting sharply with a decrease in controls. ASC supplementation of the meal (in the RCU + ASC subgroup) normalized insulin, failed to normalize post-load phosphaturia, but reduced post-load glucosuria and urinary pH significantly (mean pH values 5.55 vs 5.93 in untreated RCU, controls 5.50). Postprandial urinary oxalate, calcium, protein, and supersaturation products were not changed. The postprandial changes in phosphaturia and insulin sensitivity were inversely correlated (n = 38, r = -0.44, P = 0.007). It was concluded that in younger RCU males: (1) postprandial hyperinsulinaemia, the failure to reduce phosphaturia and - within limits - glucosuria, appropriately, as well as poor urine acidification are important features of the metabolism; (2) these phenomena are probably caused by insulin resistance of organs, the kidney included; and (3) the addition of a supraphysiological dose of ASC to a meal, the subsequent abolition of hyperinsulinaemia, and the restoration of normal urine acidification suggest that this antioxidant is capable of counteracting some pre-existing basic abnormality of cell metabolism in RCU.