Increases in Urinary Inhibitor Activity and Excretion of an Inhibitor of Crystalluria in Pregnancy: A Defense Against the Hypercalciuria of Normal Gestation

Defective urinary crystallization inhibition and urinary stone formation

INTRODUCTION

Nephrocalcin (NC) is a glycoprotein produced in the kidney and inhibits calcium oxalate crystal formation. It has been separated into 4 isoforms (A, B, C, and D) and found that (A + B) are more abundant than (C + D) in urine of healthy subjects, but the reverse is seen in human urine of kidney stone patients. To further examine the role of this protein in inhibition of urinary crystallization, nephrocalcin isoforms were purified from 2 genetically pure dog species.

MATERIALS AND METHODS

We studied healthy Beagles, known to be non-stone forming dogs, and Mini-Schnauzers, known to be calcium oxalate stone formers. NC was isolated and purified from each group. Urinary biochemistry and calcium oxalate crystal growth inhibition were measured.

RESULTS

Specific crystal growth inhibition activity was significantly higher in non-stone forming dogs (9.79 +/- 2.25 in Beagles vs. 2.75 +/- 1.34 of Mini-Schnauzers, p < 0.005). Dissociation constants toward calcium oxalate monohydrate were 10-fold different, with Beagles' isoforms being 10 times stronger inhibitors compare to those of Mini-Schnauzers'. Isoforms C + D of NC were the main isoforms isolated in stone-forming dogs.

CONCLUSION

NC of these two species of dogs differently affects calcium oxalate crystallization and might have a role in determining ulterior urinary stone formation.

Racial variation in gender frequency of calcium urolithiasis

Calcium oxalate stones are known to occur most frequently in the fifth and sixth decades of life and in a male-to-female ratio of approximately 2:1. However, previous data are from predominantly white populations or from multiracial populations that have included patients with infection stones. We reviewed a population of 444 consecutive patients (189 white, 123 black and 132 Hispanic) from 1 urban hospital treated by shock wave lithotripsy for symptomatic renal and ureteral calculi, and added a population of 260 patients treated at a university hospital in Japan. Patients with infection stones and those with uric acid or cystine stones were excluded. We found men to be more commonly afflicted in the white (62%) and Asian (64%) populations but noted a reversal of gender frequency in the black and Hispanic populations, of which women comprised 68% and 60% of the stone population, respectively. We conclude that calcium urolithiasis is more common in women than previously reported, and discuss the possible etiologies.

Nephrocalcin in patients with renal cell carcinoma

Nephrocalcin, an acidic glycoprotein that inhibits calcium oxalate crystal growth, has been previously localized in proximal tubules of kidneys by an immunohistochemical staining method and purified from tissue culture media of 2 renal carcinoma cell lines. A polyclonal antibody specific to nephrocalcin was raised in rabbits and the level of nephrocalcin was quantitatively determined in urine of 19 renal cell carcinoma patients (0.241 +/- 0.341 microgram nephrocalcin per mg. creatinine) and compared to healthy controls (0.022 +/- 0.012 micrograms nephrocalcin per mg. creatinine). Nephrocalcin levels after tumor nephrectomy decreased dramatically in 5 patients and to a lesser degree in 7. A specific nephrocalcin fraction that was eluted from an anion exchange column with low ionic strength was detected in urine of the renal cell carcinoma patients, and this fraction decreased or disappeared after tumor nephrectomy in 6 of 9 patients studied. Amino acid composition, phosphate content and dissociation constants toward calcium oxalate monohydrate crystals were investigated in the nephrocalcin from tumor patients and compared to that from healthy controls. Our studies demonstrate that nephrocalcin in patients with renal cell carcinoma is atypical and usually in much higher quantity. Further studies are needed to determine the clinical significance of these observations.

Nephrolithiasis in pregnancy

Although the anatomical and physiological changes of normal pregnancy may predispose to kidney stone formation, it remains an uncommon occurrence. Correct diagnosis is often difficult. Ultrasonography has become the primary radiological diagnostic tool, with a limited excretory urogram only necessary in complicated cases. Nephrolithiasis during pregnancy occurs more frequently during the later stages of gestation, in multiparas, and without a difference in laterality. Conservative management with bed rest, hydration and analgesia can result in spontaneous passage of most stones in gravidas. Past experience of several groups suggests that cystoscopy and/or surgery can usually be done safely when absolutely necessary. Pre-existing stone disease can increase the incidence of maternal urinary tract infections by 10-20%. The most common obstetric complications of stones during gestation is premature labour induced by renal colic. Most drugs normally used to treat stone disease are contraindicated in gestation. Known inhibitors of stone formation are present in gestation and may partially explain why the incidence of stones is not increased in this hypercalciuric state.

Renal haemodynamics and tubular function in human pregnancy

In human pregnancy, effective renal plasma flow and glomerular filtration rate increase to levels 50-80% above non-pregnant values. The increments occur shortly after conception, persist throughout the second trimester and reduce slightly in late pregnancy. The hyperfiltration of pregnancy does not seem to be a potentially damaging process, as intraglomerular pressure remains unchanged. The increased excretion of glucose and other nutrients, as well as uric acid and protein, is related in part to altered tubular function. Renal physiology is altered so much in pregnancy that non-pregnant norms cannot be used in antenatal care.

Hypertension in pregnancy

High blood pressure, which complicates approximately 10% of all pregnancies, remains a major cause of morbidity and mortality for both mother and fetus. A relative paucity of investigative data, as well as the frequent difficulty in making an etiological diagnosis by clinical criteria alone, may be among the reasons why there are many conflicts about the management of hypertension during pregnancy. This clinical conference summarizes current concepts regarding the hypertensive disorders of gestation, focusing on the most dangerous cause, preeclampsia-eclampsia. It further highlights a recent report of the Working Group on High Blood Pressure in Pregnancy convened by the National High Blood Pressure Education Program at the National Heart, Lung, and Blood Institute (the Consensus Report). Among the Working Group's most interesting recommendations in controversial areas were a return to the classification schema suggested by the American College of Obstetricians and Gynecologists in 1972, use of the fifth Korotkoff sound to determine diastolic blood pressure levels, and institution of treatment with antihypertensive drugs for sudden elevations of blood pressure near term to diastolic levels greater than or equal to 105 mm Hg or for levels of 100 mm Hg or higher in pregnant women with chronic hypertension. The Consensus Report further recommended parenteral hydralazine and methyldopa as the drugs of choice for the acute hypertensive crisis and management of chronic hypertension, respectively, based on the long histories of safe use of these agents in gravidas. Parenteral magnesium sulfate remained the preferred therapeutic approach for avoiding or treating the convulsive complication, eclampsia, but the Working Group underscored the need for controlled trials of magnesium's efficacy. Finally, they noted that diuretics should be avoided in preeclampsia, but that these drugs can be continued during gestation if taken before conception, and may be prescribed to pregnant women with chronic hypertension who appear overly salt sensitive.