Evidence for increased postprandial distal nephron calcium delivery in hypercalciuric stone-forming patients

Evidence for altered renal tubule function in idiopathic calcium stone formers

Patients who form calcium kidney stones often have metabolic disorders such as idiopathic hypercalciuria (IH) that reflect abnormalities in mineral handling in the kidney. Renal handling of calcium is altered by ingestion of nutrients such as carbohydrates, protein, and sodium, and patients with IH appear to be more sensitive to these stimuli. Studies using probes such as diuretics or lithium clearance have the ability to clarify which nephron segments are involved in the altered renal calcium transport with nutrient seen in IH. Studies in the genetic hypercalciuric rat demonstrate alterations in both proximal tubule and thick ascending limb calcium reabsorption. Similar studies in humans have begun to provide evidence about the corresponding abnormalities in stone formers with IH. A pattern of altered renal tubule transport in calcium stone formers is suggested by the frequency of such findings as decreased tubular maximal reabsorption of phosphate and abnormal urine acidification as well as hypercalciuria in such patients, not explained by monogenic transport abnormalities.

Plaque and deposits in nine human stone diseases

Data concerning nine forms of human stone disease, along with observations on normal people give new insights into formation of interstitial apatite plaque and intra-tubular crystal deposits. In general, across multiple disease states, one can reproduce the same relationships between plaque abundance as is seen among patients within individual disease states, so that the link between plaque and high urine calcium excretion, and low urine volume and pH seems increasingly secure. From this, one can propose a specific model of plaque formation, susceptible to experimental test. In many diseases, formation of inner medullary collecting duct and Bellini duct deposits is compatible with simple crystallization driven by urine supersaturations; this is expected in that these segments contain tubule fluid quite close in composition to final urine. But in ileostomy, small bowel disease and obesity bypass patients, crystals found in deposits are not those expected: apatite and urates in deposits, despite formation of highly acidic urine. Also, this discrepancy suggests the possibility of divergence between bulk urine pH and pH of focal collecting ducts, a new kind of possibility that is susceptible to experimental test.

Three pathways for human kidney stone formation

No single theory of pathogenesis can properly account for human kidney stones, they are too various and their formation is too complex for simple understanding. Using human tissue biopsies, intraoperative imaging and such physiology data from ten different stone forming groups, we have identified at least three pathways that lead to stones. The first pathway is overgrowth on interstitial apatite plaque as seen in idiopathic calcium oxalate stone formers, as well as stone formers with primary hyperparathyroidism, ileostomy, and small bowel resection, and in brushite stone formers. In the second pathway, there are crystal deposits in renal tubules that were seen in all stone forming groups except the idiopathic calcium oxalate stone formers. The third pathway is free solution crystallization. Clear examples of this pathway are those patient groups with cystinuria or hyperoxaluria associated with bypass surgery for obesity. Although the final products may be very similar, the ways of creation are so different that in attempting to create animal and cell models of the processes one needs to be careful that the details of the human condition are included.

Familial hyperkalemia and hypertension: pathogenetic insights based on lithium clearance

CONTEXT

Familial hyperkalemia and hypertension (FHHt) is caused by mutations in WNK kinases. Its pathogenesis is not completely understood.

OBJECTIVE

Our objective was to investigate the mechanism of hypercalciuria in FHHt.

DESIGN AND SETTING

We conducted a study of a large family with FHHt and WNK4 Q565E mutation and of control subjects at a referral medical center.

SUBJECTS

Forty-six members of a family with FHHt and WNK4 Q565E mutation, 23 of them affected, and 12 control subjects participated.

MAIN OUTCOME MEASURES

Urinary calcium and sodium concentrations, endogenous lithium clearance, age of hypertension appearance were assessed.

RESULTS

In 40 urine samples of 20 affected subjects, urinary calcium was correlated to urinary sodium (r = 0.567; P = 0.0001). In 28 urinary samples of 22 unaffected members, no correlation was found (r = 0.285; P = 0.14). Mean ratio of urinary calcium to urinary sodium was 2.7-fold higher in affected compared with unaffected members (58.7 +/- 25.9 vs. 22.1 +/- 14.0 micromol/mmol, P < 0.0001). Endogenous lithium clearance in eight affected members was about 50% lower than in 12 controls (16.2 +/- 7.7 vs. 28.8 +/- 9.8 ml/min, P = 0.0073). Hypertension was detected in males 12 yr earlier than in females (26.0 +/- 7.5 vs. 37.9 +/- 11.3 yr; P = 0.031).

CONCLUSIONS

Hypercalciuria in FHHt seems to be dependent on urinary sodium. According to molecular studies, FHHt patients are presumed to have increased distal nephron sodium reabsorption and therefore decreased proximal reabsorption of sodium, lithium, and calcium. The observed decreased lithium clearance reflects probable abnormal renal handling of lithium, i.e. distal nephron lithium reabsorption. Therefore, hypercalciuria may result from proximal nephron aberration. Finally, earlier appearance of hypertension in males may be the result of sex-hormone activity.

A test of the hypothesis that the collecting duct calcium-sensing receptor limits rise of urine calcium molarity in hypercalciuric calcium kidney stone formers

The process of kidney stone formation depends on an imbalance between excretion of water and insoluble stone-forming salts, leading to high concentrations that supersaturate urine and inner medullary collecting duct (IMCD) fluid. For common calcium-containing stones, a critical mechanism that has been proposed for integrating water and calcium salt excretions is activation of the cell surface calcium-sensing receptor (CaSR) on the apical membranes of IMCD cells. High deliveries of calcium into the IMCD would be predicted to activate CaSR, leading to reduced membrane abundance of aquaporin-2, thereby limiting water conservation and protecting against stone formation. We have tested this hypothesis in 16 idiopathic hypercalciuric calcium stone formers and 14 matched normal men and women in the General Clinical Research Center. Subjects were fed identical diets; we collected 14 urine samples at 1-h intervals during a single study day, and one sample overnight. Hypercalciuria did not increase urine volume, so urine calcium molarity and supersaturation with respect to calcium oxalate and calcium phosphate rose proportionately to calcium excretion. Thus CaSR modulation of urine volume via IMCD CaSR activation does not appear to be an important mechanism of protection against stone formation. The overnight period, one of maximal water conservation, was a time of maximal stone risk and perhaps a target of specific clinical intervention.