Prophylaxis of infection-induced kidney stone formation

Quantitative phase analysis and microstructural characterization of urinary tract calculi with X-ray diffraction Rietveld analysis on a Caribbean island

In the twin-island state of Trinidad and Tobago, urinary stone analysis is not routinely performed. This study investigates, via powder X-ray diffraction, 52 urinary tract calculi collected from hospitals in Trinidad. Of these, 46 stones were analysed with Rietveld refinement for quantitative analysis and materials characterization. Refined unit-cell, microstructural and weight fraction parameters were obtained, with the last being used for stone classification. The results revealed seven distinct mineralogical phases of varying frequency: calcium oxalate monohydrate (COM, 58%), calcium oxalate dihydrate (COD, 23%), carbonated apatite (APA, 48%), brushite (BRU, 6%), struvite (STR, 42%), uric acid (UA, 23%) and ammonium acid urate (AAU, 19%). The average refined crystallite sizes were 1352 ± 90 Å (COM), 1921 ± 285 Å (COD), 83 ± 5 Å (APA), 1172 ± 9 Å (BRU), 1843 ± 138 Å (STR), 981 ± 87 Å (UA) and 292 ± 83 Å (AAU). Subsequently, 36.5% of stones were categorized as phosphates, 34.6% as oxalates, 13.5% as uric acid/urates and 15.4% as mixed compositions. The study findings highlight the importance of stone analysis as a necessary step towards disease management of local patients, and endorse the application of Rietveld refinement as a natural extension to diffraction-based kidney stone investigations.

Tamm-Horsfall protein is a critical renal defense factor protecting against calcium oxalate crystal formation

BACKGROUND

The tubular fluid of the mammalian kidney is often supersaturated with mineral salts, but crystallization rarely occurs under normal conditions. The unique ability of the kidney to avoid harmful crystal formation has long been attributed to the inhibitory activity of the urinary macromolecules, although few in vivo studies have been carried out to examine this hypothesis. Here we examined the role of Tamm-Horsfall protein (THP), the principal urinary protein, in urinary defense against renal calcium crystal formation, using a THP knockout model that we recently developed.

METHODS

Wild-type and THP knockout mice were examined for the spontaneous formation of renal calcium crystals using von Kossa staining. The susceptibility of these mice to experimentally induced renal crystal formation was evaluated by administering mice with ethylene glycol, a precursor of oxalate, and vitamin D(3), which increases calcium absorption. Renal calcium crystals were visualized by von Kossa stain, dark field microscopy with polarized light and scanning electron microscopy.

RESULTS

Inactivating the THP gene in mouse embryonic stem cells results in spontaneous formation of calcium crystals in adult kidneys. Excessive intake of calcium and oxalate, precursors of the most common type of human renal stones, dramatically increases both the frequency and the severity of renal calcium crystal formation in THP-deficient, but not in wild-type mice. Under high calcium/oxalate conditions, the absence of THP triggers a marked, adaptive induction in renal epithelial cells of osteopontin (OPN), a potent inhibitor of bone mineralization and vascular calcification. Thus, OPN may serve as an inducible inhibitor of calcium crystallization, whereas THP can serve as a constitutive and apparently more effective inhibitor.

CONCLUSION

These results provide the first in vivo evidence that THP is a critical urinary defense factor and suggest that its deficiency could be an important contributing factor in human nephrolithiasis, a condition afflicting tens of millions of people in the world annually.

Gastropyeloplasty: a swine model

OBJECTIVES

Struvite calculus formation requires an alkaline urinary pH. An acidic urinary pH will dissolve struvite calculi and prevent recurrent struvite stone formation. Long-term urinary acidification has been unsuccessful. We sought to determine whether a gastric patch (with viable parietal cells) anastomosed to the renal pelvis could create an acidic urinary milieu.

METHODS

A vascularized stomach patch (from the greater curvature) was anastomosed to the left renal pelvis in 15 female pigs. The right kidney was used as a control. The first 6 pigs were used to refine the surgical technique. The remaining 9 pigs were subjected to a formal gastropyeloplasty and followed up for 4 weeks. Urine was collected before and after stimulation with pentagastrin. Urine pH was measured from both kidneys in response to gastrin stimulation and oral intake. The kidneys, ureters, and bladders were examined for gross changes and histologic review.

RESULTS

The 9 test animals had more acidic urine in the control kidney than in the gastropyeloplasty kidney. Pentagastrin had no significant impact on urinary pH. Hydroureteronephrosis and a concentrating defect were noted in the treated kidney. Histologic review revealed smooth muscle hyperplasia of the left ureter and viable parietal cells in the stomach patch.

CONCLUSIONS

An animal model was developed to transfer a gastric patch to the renal pelvis. Hydronephrosis and ureteral dilation were associated with this gastric patch. We were unable to acidify the urine despite viable parietal cells in the transposed stomach segment. Further refinements of this concept may be successful in acidifying urine in the hope of preventing recurrent struvite nephrolithiasis.

Prospective therapeutic studies in nephrolithiasis

Prospective randomized trials with respect to stone prophylaxis deal mainly with idiopathic calcium stone disease. There is a lot of evidence that alkali citrate, thiazides and allopurinol are effective in many patients. However, appropriate patient selection seems to be crucial. Alkali citrate and allopurinol have proven clinical efficacy in patients with hypocitraturia or hyperuricosuria, respectively. The correct method of patient selection for treatment with thiazides remains unclear. Despite the lack of prospective randomized trials, it is generally accepted that the first steps in the prophylaxis of idiopathic calcium stone disease are high fluid intake and a sensible diet.

In vitro inhibition of struvite crystal growth by acetohydroxamic acid

Struvite (MgNH4PO46H2O) crystals were produced by Proteus mirabilis growth in artificial urine, in the presence and absence of the urease inhibitor, acetohydroxamic acid (AHA). In the absence of AHA, struvite crystals assumed an "X-shaped" or dendritic crystal habit due to rapid growth along their 100 axis. When AHA was present, crystal growth, as monitored by phase contrast light microscopy, was greatly slowed, and the crystals assumed an octahedral crystal habit. Scanning electron microscopy revealed that crystals grown in the presence of AHA were pitted on their surface. This pitting was absent in control samples. While most of this inhibition by AHA was due to lowered urease activity, some crystal growth inhibition occurred in struvite produced in the absence of urease activity through NH4OH titration of artificial urine. We conclude that while AHA is primarily a urease inhibitor, it may also disrupt struvite growth and formation directly through interference with the molecular growth processes on crystal surfaces.