Effect of bolus and divided feeding on urine ions and supersaturation in genetic hypercalciuric stone-forming rats
2007; Elsevier BV; Volume: 73; Issue: 4 Linguagem: Inglês
10.1038/sj.ki.5002699
ISSN1523-1755
AutoresDavid A. Bushinsky, Anne Michalenka, Kelly L. Strutz, S. Donahue, John R. Asplin,
Tópico(s)Paleopathology and ancient diseases
ResumoBecause urine ion excretion varies throughout the day, clinicians monitor 24 h urine samples to measure ion excretion and supersaturation in kidney stone patients. However, these results are averages and may not reflect maximal supersaturation which drives stone formation. We measured ion excretion and saturation in genetic hypercalciuric stone-forming rats on both a normal or low calcium diet over 0–3, 3–6 and 6–24 h using two feeding protocols, where the daily food allotment was fed either as a bolus or divided into three portions. With a normal calcium diet, urine calcium, oxalate, volume, and calcium oxalate supersaturation were significantly greater on the bolus compared to the divided feeds in the prandial and postprandial periods. Bolus eaters also excreted more calcium and oxalate and had increased volume over 24 h. Maximal calcium oxalate supersaturation was greater during the initial time periods than during the entire 24 h, regardless of the feeding schedule. With the low calcium diet, the effect of bolus feeding was reduced. Thus, urine ion excretion and supersaturation vary with the type of feeding. If these results are confirmed in man, it suggests that eating as a bolus may result in greater prandial and postprandial calcium oxalate supersaturation. This may increase growth on Randall's plaques and promote stone disease. Because urine ion excretion varies throughout the day, clinicians monitor 24 h urine samples to measure ion excretion and supersaturation in kidney stone patients. However, these results are averages and may not reflect maximal supersaturation which drives stone formation. We measured ion excretion and saturation in genetic hypercalciuric stone-forming rats on both a normal or low calcium diet over 0–3, 3–6 and 6–24 h using two feeding protocols, where the daily food allotment was fed either as a bolus or divided into three portions. With a normal calcium diet, urine calcium, oxalate, volume, and calcium oxalate supersaturation were significantly greater on the bolus compared to the divided feeds in the prandial and postprandial periods. Bolus eaters also excreted more calcium and oxalate and had increased volume over 24 h. Maximal calcium oxalate supersaturation was greater during the initial time periods than during the entire 24 h, regardless of the feeding schedule. With the low calcium diet, the effect of bolus feeding was reduced. Thus, urine ion excretion and supersaturation vary with the type of feeding. If these results are confirmed in man, it suggests that eating as a bolus may result in greater prandial and postprandial calcium oxalate supersaturation. This may increase growth on Randall's plaques and promote stone disease. Hypercalciuria is the most common metabolic abnormality found in humans with nephrolithiasis.1.Monk R.D. Bushinsky D.A. Kidney stones.in: Larsen P.R. Kronenberg H.M. Melmed S. Polonsky K.S. Williams Textbook of Endocrinology. 10th edn. W.B. Saunders, Philadelphia2003: 1411-1425Google Scholar, 2.Monk R.D. Bushinsky D.A. Nephrolithiasis and nephrocalcinosis.in: Johnson R. Frehally J. Comprehensive Clinical Nephrology. 2nd edn. Mosby, London2003: 731-744Google Scholar, 3.Bushinsky D.A. Nephrolithiasis.J Am Soc Nephrol. 1998; 9: 917-924PubMed Google Scholar, 4.Coe F.L. Favus M.J. Asplin J.R. Nephrolithiasis.in: Brenner B.M. The Kidney. 7th edn. W.B. Saunders Company, Philadelphia2004: 1819-1866Google Scholar, 5.Bushinsky D.A. Renal lithiasis.in: Humes H.D. Kelly's Textbook of Medicine. Lippincott Williams & Wilkens, New York2000: 1243-1248Google Scholar Hypercalciuria raises urine saturation with respect to the solid phases of calcium hydrogen phosphate (CaHPO4, brushite) and calcium oxalate (CaOx) enhancing the probability of nucleation and growth of crystals into clinically significant kidney stones.1.Monk R.D. Bushinsky D.A. Kidney stones.in: Larsen P.R. Kronenberg H.M. Melmed S. Polonsky K.S. Williams Textbook of Endocrinology. 10th edn. W.B. Saunders, Philadelphia2003: 1411-1425Google Scholar, 2.Monk R.D. Bushinsky D.A. Nephrolithiasis and nephrocalcinosis.in: Johnson R. Frehally J. Comprehensive Clinical Nephrology. 2nd edn. Mosby, London2003: 731-744Google Scholar, 4.Coe F.L. Favus M.J. Asplin J.R. Nephrolithiasis.in: Brenner B.M. The Kidney. 7th edn. W.B. Saunders Company, Philadelphia2004: 1819-1866Google Scholar We have established a model of hypercalciuria and nephrolithiasis by successively inbreeding 71 generations of the most hypercalciuric progeny of the most hypercalciuric Sprague–Dawley rats found on an initial screen. Each rat now excretes 8–10 times as much urinary (U) calcium (Ca) as similarly fed controls.6.Bushinsky D.A. Favus M.J. Mechanism of hypercalciuria in genetic hypercalciuric rats: inherited defect in intestinal calcium transport.J Clin Invest. 1988; 82: 1585-1591Crossref PubMed Scopus (86) Google Scholar, 7.Kim M. Sessler N.E. Tembe V. et al.Response of genetic hypercalciuric rats to a low calcium diet.Kidney Int. 1993; 43: 189-196Abstract Full Text PDF PubMed Scopus (61) Google Scholar, 8.Li X.Q. Tembe V. Horwitz G.M. et al.Increased intestinal vitamin D receptor in genetic hypercalciuric rats: a cause of intestinal calcium hyperabsorption.J Clin Invest. 1993; 91: 661-667Crossref PubMed Scopus (142) Google Scholar, 9.Bushinsky D.A. Kim M. Sessler N.E. et al.Increased urinary saturation and kidney calcium content in genetic hypercalciuric rats.Kidney Int. 1994; 45: 58-65Abstract Full Text PDF PubMed Scopus (51) Google Scholar, 10.Bushinsky D.A. Grynpas M.D. Nilsson E.L. et al.Stone formation in genetic hypercalciuric rats.Kidney Int. 1995; 48: 1705-1713Abstract Full Text PDF PubMed Scopus (77) Google Scholar, 11.Krieger N.S. Stathopoulos V.M. Bushinsky D.A. Increased sensitivity to 1,25(OH)2D3 in bone from genetic hypercalciuric rats.Am J Physiol (Cell Physiol). 1996; 271: C130-C135PubMed Google Scholar, 12.Bushinsky D.A. Bashir M.A. Riordon D.R. et al.Increased dietary oxalate does not increase urinary calcium oxalate saturation in hypercalciuric rats.Kidney Int. 1999; 55: 602-612Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar, 13.Bushinsky D.A. Genetic hypercalciuric stone forming rats.Semin Nephrol. 1996; 16: 448-457PubMed Google Scholar, 14.Asplin J.R. Bushinsky D.A. Singharetnam W. et al.Relationship between supersaturation and crystal inhibition in hypercalciuric rats.Kidney Int. 1997; 51: 640-645Abstract Full Text PDF PubMed Scopus (54) Google Scholar, 15.Tsuruoka S. Bushinsky D.A. Schwartz G.J. Defective renal calcium reabsorption in genetic hypercalciuric rats.Kidney Int. 1997; 51: 1540-1547Abstract Full Text PDF PubMed Scopus (77) Google Scholar, 16.Bushinsky D.A. Neumann K.J. Asplin J. Krieger N.S. Alendronate decreases urine calcium and supersaturation in genetic hypercalciuric rats.Kidney Int. 1999; 55: 234-243Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar, 17.Yao J. Kathpalia P. Bushinsky D.A. Favus M.J. Hyperresponsiveness of vitamin D receptor gene expression to 1,25-dihydroxyvitamin D3: A new characteristic of genetic hypercalciuric stone-forming rats.J Clin Invest. 1998; 101: 2223-2232Crossref PubMed Scopus (89) Google Scholar, 18.Evan A.P. Bledsoe S.B. Smith S.B. Bushinsky D.A. Calcium oxalate crystal localization and osteopontin immunostaining in genetic hypercalciuric stone-forming rats.Kidney Int. 2004; 65: 154-161Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar, 19.Bushinsky D.A. Genetic hypercalciuric stone-forming rats.Curr Opin Nephrol Hypertens. 1999; 8: 479-488Crossref PubMed Scopus (40) Google Scholar, 20.Bushinsky D.A. Parker W.R. Asplin J.R. Calcium phosphate supersaturation regulates stone formation in genetic hypercalciuric stone-forming rats.Kidney Int. 2000; 57: 550-560Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar, 21.Bushinsky D.A. Grynpas M.D. Asplin J.R. Effect of acidosis on urine supersaturation and stone formation in genetic hypercalciuric stone forming rats.Kidney Int. 2001; 59: 1415-1423Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar, 22.Bushinsky D.A. Asplin J.R. Grynpas M.D. et al.Calcium oxalate stone formation in genetic hypercalciuric stone-forming rats.Kidney Int. 2002; 61: 975-987Abstract Full Text Full Text PDF PubMed Scopus (85) Google Scholar, 23.Yao J. Karnauskas A.J. Bushinsky D.A. Favus M.J. Regulation of renal calcium receptor gene expression in response to 1,25(OH)2D3 in genetic hypercalciuric stone-forming rats.J Am Soc Nephrol. 2005; 16: 1300-1308Crossref PubMed Scopus (58) Google Scholar, 24.Bushinsky D.A. Asplin J.R. Thiazides reduce brushite, but not calcium oxalate, supersaturation and stone formation in genetic hypercalciuric stone-forming rats.J Am Soc Nephrol. 2005; 16: 417-424Crossref PubMed Scopus (30) Google Scholar, 25.Karnauskas A.J. van Leeuwen J.P. van den Bemd G.J. et al.Mechanism and function of high vitamin D receptor levels in genetic hypercalciuric stone-forming rats.J Bone Miner Res. 2005; 20: 447-454Crossref PubMed Scopus (54) Google Scholar, 26.Bushinsky D.A. LaPlante K. Asplin J.R. Effect of cinacalcet on urine calcium excretion and supersaturation in genetic hypercalciuric stone-forming rats.Kidney Int. 2006; 69: 1586-1592Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar, 27.Bushinsky D.A. Frick K.K. Nehrke K. Genetic hypercalciuric stone-forming rats.Curr Opin Nephrol Hypertens. 2006; 15: 403-418Crossref PubMed Scopus (43) Google Scholar, 28.Hoopes Jr, R.R. Middleton F.A. Sen S. et al.Isolation and confirmation of a calcium excretion quantitative trait locus on chromosome 1 in genetic hypercalciuric stone-forming congenic rats.J Am Soc Nephrol. 2006; 17: 1292-1304Crossref PubMed Scopus (28) Google Scholar The hypercalciuria is due to increased intestinal Ca absorption6.Bushinsky D.A. Favus M.J. Mechanism of hypercalciuria in genetic hypercalciuric rats: inherited defect in intestinal calcium transport.J Clin Invest. 1988; 82: 1585-1591Crossref PubMed Scopus (86) Google Scholar,7.Kim M. Sessler N.E. Tembe V. et al.Response of genetic hypercalciuric rats to a low calcium diet.Kidney Int. 1993; 43: 189-196Abstract Full Text PDF PubMed Scopus (61) Google Scholar coupled to a defect in renal tubular Ca reabsorption7.Kim M. Sessler N.E. Tembe V. et al.Response of genetic hypercalciuric rats to a low calcium diet.Kidney Int. 1993; 43: 189-196Abstract Full Text PDF PubMed Scopus (61) Google Scholar,15.Tsuruoka S. Bushinsky D.A. Schwartz G.J. Defective renal calcium reabsorption in genetic hypercalciuric rats.Kidney Int. 1997; 51: 1540-1547Abstract Full Text PDF PubMed Scopus (77) Google Scholar and enhanced bone resorption,11.Krieger N.S. Stathopoulos V.M. Bushinsky D.A. Increased sensitivity to 1,25(OH)2D3 in bone from genetic hypercalciuric rats.Am J Physiol (Cell Physiol). 1996; 271: C130-C135PubMed Google Scholar suggesting a systemic dysregulation of Ca homeostasis.8.Li X.Q. Tembe V. Horwitz G.M. et al.Increased intestinal vitamin D receptor in genetic hypercalciuric rats: a cause of intestinal calcium hyperabsorption.J Clin Invest. 1993; 91: 661-667Crossref PubMed Scopus (142) Google Scholar Stone formation in both humans and rats shares many metabolic features; stone formation in humans also has increased intestinal Ca absorption, increased bone resorption, and decreased renal tubule Ca reabsorption.1.Monk R.D. Bushinsky D.A. Kidney stones.in: Larsen P.R. Kronenberg H.M. Melmed S. Polonsky K.S. Williams Textbook of Endocrinology. 10th edn. W.B. Saunders, Philadelphia2003: 1411-1425Google Scholar,3.Bushinsky D.A. Nephrolithiasis.J Am Soc Nephrol. 1998; 9: 917-924PubMed Google Scholar Virtually, all of these hypercalciuric rats form kidney stones while there is no evidence of stone formation in the control rats.10.Bushinsky D.A. Grynpas M.D. Nilsson E.L. et al.Stone formation in genetic hypercalciuric rats.Kidney Int. 1995; 48: 1705-1713Abstract Full Text PDF PubMed Scopus (77) Google Scholar We have termed the rats genetic hypercalciuric stone-forming (GHS) rats.10.Bushinsky D.A. Grynpas M.D. Nilsson E.L. et al.Stone formation in genetic hypercalciuric rats.Kidney Int. 1995; 48: 1705-1713Abstract Full Text PDF PubMed Scopus (77) Google Scholar, 12.Bushinsky D.A. Bashir M.A. Riordon D.R. et al.Increased dietary oxalate does not increase urinary calcium oxalate saturation in hypercalciuric rats.Kidney Int. 1999; 55: 602-612Abstract Full Text Full Text PDF PubMed Scopus (38) Google Scholar, 14.Asplin J.R. Bushinsky D.A. Singharetnam W. et al.Relationship between supersaturation and crystal inhibition in hypercalciuric rats.Kidney Int. 1997; 51: 640-645Abstract Full Text PDF PubMed Scopus (54) Google Scholar, 16.Bushinsky D.A. Neumann K.J. Asplin J. Krieger N.S. Alendronate decreases urine calcium and supersaturation in genetic hypercalciuric rats.Kidney Int. 1999; 55: 234-243Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar, 18.Evan A.P. Bledsoe S.B. Smith S.B. Bushinsky D.A. Calcium oxalate crystal localization and osteopontin immunostaining in genetic hypercalciuric stone-forming rats.Kidney Int. 2004; 65: 154-161Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar, 20.Bushinsky D.A. Parker W.R. Asplin J.R. Calcium phosphate supersaturation regulates stone formation in genetic hypercalciuric stone-forming rats.Kidney Int. 2000; 57: 550-560Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar, 21.Bushinsky D.A. Grynpas M.D. Asplin J.R. Effect of acidosis on urine supersaturation and stone formation in genetic hypercalciuric stone forming rats.Kidney Int. 2001; 59: 1415-1423Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar, 22.Bushinsky D.A. Asplin J.R. Grynpas M.D. et al.Calcium oxalate stone formation in genetic hypercalciuric stone-forming rats.Kidney Int. 2002; 61: 975-987Abstract Full Text Full Text PDF PubMed Scopus (85) Google Scholar The stones formed by the GHS rats fed normal rat chow contain only Ca and P.10.Bushinsky D.A. Grynpas M.D. Nilsson E.L. et al.Stone formation in genetic hypercalciuric rats.Kidney Int. 1995; 48: 1705-1713Abstract Full Text PDF PubMed Scopus (77) Google Scholar, 14.Asplin J.R. Bushinsky D.A. Singharetnam W. et al.Relationship between supersaturation and crystal inhibition in hypercalciuric rats.Kidney Int. 1997; 51: 640-645Abstract Full Text PDF PubMed Scopus (54) Google Scholar, 20.Bushinsky D.A. Parker W.R. Asplin J.R. Calcium phosphate supersaturation regulates stone formation in genetic hypercalciuric stone-forming rats.Kidney Int. 2000; 57: 550-560Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar, 21.Bushinsky D.A. Grynpas M.D. Asplin J.R. Effect of acidosis on urine supersaturation and stone formation in genetic hypercalciuric stone forming rats.Kidney Int. 2001; 59: 1415-1423Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar The dietary addition of hydroxyproline, a common amino acid and an oxalate precursor,29.Hagler L. Herman R.H. Oxalate metabolism. I.Am J Clin Nutr. 1973; 26: 758-765PubMed Google Scholar results in formation of CaOx kidney stones.18.Evan A.P. Bledsoe S.B. Smith S.B. Bushinsky D.A. Calcium oxalate crystal localization and osteopontin immunostaining in genetic hypercalciuric stone-forming rats.Kidney Int. 2004; 65: 154-161Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar,22.Bushinsky D.A. Asplin J.R. Grynpas M.D. et al.Calcium oxalate stone formation in genetic hypercalciuric stone-forming rats.Kidney Int. 2002; 61: 975-987Abstract Full Text Full Text PDF PubMed Scopus (85) Google Scholar In our clinical treatment strategies for human nephrolithiasis, we assume that a 24 h U collection with calculation of supersaturation (SS) is the best predictor of whether an individual will form a stone and, if a stone is formed, of its solid phase.1.Monk R.D. Bushinsky D.A. Kidney stones.in: Larsen P.R. Kronenberg H.M. Melmed S. Polonsky K.S. Williams Textbook of Endocrinology. 10th edn. W.B. Saunders, Philadelphia2003: 1411-1425Google Scholar, 2.Monk R.D. Bushinsky D.A. Nephrolithiasis and nephrocalcinosis.in: Johnson R. Frehally J. Comprehensive Clinical Nephrology. 2nd edn. Mosby, London2003: 731-744Google Scholar, 3.Bushinsky D.A. Nephrolithiasis.J Am Soc Nephrol. 1998; 9: 917-924PubMed Google Scholar, 5.Bushinsky D.A. Renal lithiasis.in: Humes H.D. Kelly's Textbook of Medicine. Lippincott Williams & Wilkens, New York2000: 1243-1248Google Scholar, 30.Bushinsky D.A. Recurrent hypercalciuric nephrolithiasis—does diet help?.N Engl J Med. 2002; 346: 124-125Crossref PubMed Scopus (24) Google Scholar, 31.Bushinsky D.A. Nephrolithiasis: site of the initial solid phase.J Clin Invest. 2003; 111: 602-605Crossref PubMed Scopus (61) Google Scholar, 32.Monk R.D. Bushinsky D.A. Pathogenesis of idiopathic hypercalciuria.in: Coe F. Favus M. Pak C. Parks J. Preminger P. Kidney Stones: Medical and Surgical Management. Lippincott-Raven, Philadelphia1996: 759-772Google Scholar, 33.Coe F.L. Bushinsky D.A. Pathophysiology of hypercalciuria.Am J Physiol (Renal Fluid Electrolyte Physiol). 1984; 247: F1-F13PubMed Google Scholar However, recent human studies have demonstrated CaOx stone formation in patients whose 24 h U is normal with respect to this solid phase.34.Evan A.P. Lingeman J.E. Coe F.L. et al.Randall's plaque of patients with nephrolithiasis begins in basement membranes of thin loops of Henle.J Clin Invest. 2003; 111: 607-616Crossref PubMed Scopus (468) Google Scholar The lack of demonstrable increase in SS may be a function of the averaging nature of a 24 h urine collection. Serum Ca fluctuates throughout the day.35.Bushinsky D.A. Calcium, magnesium, and phosphorus: renal handling and urinary excretion.in: Favus M.J. Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism. 5th edn. American Society for Bone and Mineral Research, Washington, DC2003: 97-105Google Scholar, 36.Monk R.D. Bushinsky D.A. Treatment of calcium, phosphorus, and magnesium disorders.in: Halperin M. Therapy in Nephrology and Hypertension: A Companion to Brenner and Rector's The Kidney. W.B. Saunders Company, Philadelphia1999: 303-315Google Scholar, 37.Bushinsky D.A. Monk R.D. Electrolyte quintet: Calcium.Lancet. 1998; 352: 306-311Abstract Full Text Full Text PDF PubMed Scopus (276) Google Scholar, 38.Bushinsky D.A. Disorders of calcium and phosphorus homeostasis.in: Greenberg A. Primer on Kidney Diseases. 4th edn. Academic Press, San Diego2005: 120-130Google Scholar If SS also varies throughout the day, the maximal supersaturation and thus the maximal driving force for crystallization will not be measured.31.Bushinsky D.A. Nephrolithiasis: site of the initial solid phase.J Clin Invest. 2003; 111: 602-605Crossref PubMed Scopus (61) Google Scholar After eating, Ca is rapidly absorbed leading to small, but measurable, increases in serum levels.35.Bushinsky D.A. Calcium, magnesium, and phosphorus: renal handling and urinary excretion.in: Favus M.J. Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism. 5th edn. American Society for Bone and Mineral Research, Washington, DC2003: 97-105Google Scholar,38.Bushinsky D.A. Disorders of calcium and phosphorus homeostasis.in: Greenberg A. Primer on Kidney Diseases. 4th edn. Academic Press, San Diego2005: 120-130Google Scholar An increased serum level, with no change in kidney function, leads to an increase in the filtered load of Ca. An increased filtered load results in increased UCa and, may increase the SS, unless U volume (V) also increases after eating, for the solid phases CaOx and CaHPO4. Urine Ca while eating, and in the immediate postprandial period, exceeds UCa during periods of fasting, such as prior to the morning meal;39.Worcester E.M. Gillen D.L. Evan A.P. et al.Evidence that postprandial reduction of renal calcium reabsorption mediates hypercalciuria of patients with calcium nephrolithiasis.Am J Physiol Renal Physiol. 2007; 292: F66-F75Crossref PubMed Scopus (64) Google Scholar however, the potential benefit of multiple small feedings vs large bolus feedings in reducing peak UCa and USS has never been tested. The goal of this study is to test the hypothesis that USS with respect to CaOx and CaHPO4 is maximal soon after eating and is greater when the daily food allotment is eaten as a bolus rather than in a divided fashion. While being fed the normal Ca (1.2% Ca) diet, the bolus-fed GHS rats excreted significantly more UCa at 0–3 h, 3–6 h, the prandial and postprandial periods, respectively, and 0–24 h when compared to the rats fed in a divided manner (Figure 1). When being fed the low Ca diet (0.02% Ca), the bolus-fed GHS rats excreted significantly more UCa at 3–6 h, 6–24 h, and 0–24 h when compared to the rats fed in a divided manner. While being fed the normal Ca diet, the bolus-fed GHS rats excreted significantly more UOx at 0–3 h, 3–6 h, and 0–24 h when compared to the rats fed in a divided manner (Figure 2). When being fed the low Ca diet, the bolus-fed GHS rats excreted significantly more UOx at 0–3 h, but not 0–24 h, when compared to the rats fed in a divided manner. While being fed the normal Ca diet, the bolus-fed GHS rats excreted significantly more UP at 0–3 h and significantly less UP at 6–24 h when compared to the rats fed in a divided manner (Figure 3). When being fed the low Ca diet, the bolus-fed GHS rats had no differences in UP when compared to the rats fed in a divided manner during any time period. While being fed the normal Ca diet, the bolus-fed GHS rats had a significantly higher UpH at 3–6 h and a significantly lower UpH at 6–24 h when compared to the rats fed in a divided manner (Figure 4). When being fed the low Ca diet, the bolus-fed GHS rats had no differences in UpH when compared to the rats fed in a divided manner during any time period. While being fed the normal Ca diet, the bolus-fed GHS rats excreted significantly more UV at 0–3 h, 3–6 h, 6–24 h, and 0–24 h when compared to the rats fed in a divided manner (Figure 5). When being fed the low Ca diet, the bolus-fed GHS rats excreted significantly more UV only at 6–24 h when compared to the rats fed in a divided manner. While being fed the normal Ca diet, the bolus-fed GHS rats had a higher CaOx SS at 0–3 h and 3–6 h when compared to the rats fed in a divided manner (Figure 6). While being fed the normal Ca diet, both the bolus-fed and the GHS rats fed in a divided manner had greater SS with respect to CaOx at both 0–3 h and 3–6 h compared to both 6–24 h and 0–24 h and with the rats fed in a divided manner, the SS at 3–6 h was greater than at 0–3 h. While being fed the low Ca diet, the bolus-fed GHS rats had a higher CaOx SS at 3–6 h when compared to the rats fed in a divided manner. While being fed the low Ca diet, there were no differences with respect to CaOx in the bolus-fed GHS rats at any time period. While being fed the low Ca diet, the GHS rats fed in a divided manner had greater SS with respect to CaOx at both 6–24 h and 0–24 h compared to 3–6 h. While being fed the normal Ca diet, the bolus-fed GHS rats had a significantly lower CaHPO4 SS at 3–6 h, 6–24 h, and 0–24 h when compared to the rats fed in a divided manner (Figure 7). While being fed the normal Ca diet, both the bolus-fed and GHS rats fed in a divided manner had greater SS with respect to CaHPO4 at both 0–3 h and 3–6 h compared to both 6–24 h and 0–24 h, and in the rats fed in a divided manner, the SS at 3–6 h was greater than at 0–3 h. While being fed the low Ca diet, the bolus-fed GHS rats did not have any differences in CaHPO4 SS at any time period when compared to the rats fed in a divided manner. While being fed the low Ca diet, both the bolus-fed and the GHS rats fed in a divided manner had greater SS with respect to CaHPO4 at 3–6 h and a lower SS at 6–24 h compared to 0–3 h. The GHS rats were bred for hypercalciuria, the most common metabolic abnormality in patients with nephrolithiasis. Studies of the pathophysiology of the hypercalciuria in the GHS rats reveal that, similar to many patients with idiopathic hypercalciuria, they have increased intestinal Ca absorption,6.Bushinsky D.A. Favus M.J. Mechanism of hypercalciuria in genetic hypercalciuric rats: inherited defect in intestinal calcium transport.J Clin Invest. 1988; 82: 1585-1591Crossref PubMed Scopus (86) Google Scholar,7.Kim M. Sessler N.E. Tembe V. et al.Response of genetic hypercalciuric rats to a low calcium diet.Kidney Int. 1993; 43: 189-196Abstract Full Text PDF PubMed Scopus (61) Google Scholar reduced renal Ca reabsorption,7.Kim M. Sessler N.E. Tembe V. et al.Response of genetic hypercalciuric rats to a low calcium diet.Kidney Int. 1993; 43: 189-196Abstract Full Text PDF PubMed Scopus (61) Google Scholar,15.Tsuruoka S. Bushinsky D.A. Schwartz G.J. Defective renal calcium reabsorption in genetic hypercalciuric rats.Kidney Int. 1997; 51: 1540-1547Abstract Full Text PDF PubMed Scopus (77) Google Scholar and excessive bone resorption.11.Krieger N.S. Stathopoulos V.M. Bushinsky D.A. Increased sensitivity to 1,25(OH)2D3 in bone from genetic hypercalciuric rats.Am J Physiol (Cell Physiol). 1996; 271: C130-C135PubMed Google Scholar The dysregulation of Ca transport at these sites suggests a systemic abnormality in Ca homeostasis.8.Li X.Q. Tembe V. Horwitz G.M. et al.Increased intestinal vitamin D receptor in genetic hypercalciuric rats: a cause of intestinal calcium hyperabsorption.J Clin Invest. 1993; 91: 661-667Crossref PubMed Scopus (142) Google Scholar Indeed, we have shown that there is an increase in the number of vitamin D receptors in intestine, bone, and kidney8.Li X.Q. Tembe V. Horwitz G.M. et al.Increased intestinal vitamin D receptor in genetic hypercalciuric rats: a cause of intestinal calcium hyperabsorption.J Clin Invest. 1993; 91: 661-667Crossref PubMed Scopus (142) Google Scholar, 11.Krieger N.S. Stathopoulos V.M. Bushinsky D.A. Increased sensitivity to 1,25(OH)2D3 in bone from genetic hypercalciuric rats.Am J Physiol (Cell Physiol). 1996; 271: C130-C135PubMed Google Scholar, 17.Yao J. Kathpalia P. Bushinsky D.A. Favus M.J. Hyperresponsiveness of vitamin D receptor gene expression to 1,25-dihydroxyvitamin D3: A new characteristic of genetic hypercalciuric stone-forming rats.J Clin Invest. 1998; 101: 2223-2232Crossref PubMed Scopus (89) Google Scholar, 25.Karnauskas A.J. van Leeuwen J.P. van den Bemd G.J. et al.Mechanism and function of high vitamin D receptor levels in genetic hypercalciuric stone-forming rats.J Bone Miner Res. 2005; 20: 447-454Crossref PubMed Scopus (54) Google Scholar and calcium receptors in kidney23.Yao J. Karnauskas A.J. Bushinsky D.A. Favus M.J. Regulation of renal calcium receptor gene expression in response to 1,25(OH)2D3 in genetic hypercalciuric stone-forming rats.J Am Soc Nephrol. 2005; 16: 1300-1308Crossref PubMed Scopus (58) Google Scholar of the GHS compared to non-hypercalciuric Sprague–Dawley rats, the parental strain of the GHS rats. When eating a normal Ca diet, virtually all of the GHS rats form kidney stones composed solely of CaHPO4.10.Bushinsky D.A. Grynpas M.D. Nilsson E.L. et al.Stone formation in genetic hypercalciuric rats.Kidney Int. 1995; 48: 1705-1713Abstract Full Text PDF PubMed Scopus (77) Google Scholar, 14.Asplin J.R. Bushinsky D.A. Singharetnam W. et al.Relationship between supersaturation and crystal inhibition in hypercalciuric rats.Kidney Int. 1997; 51: 640-645Abstract Full Text PDF PubMed Scopus (54) Google Scholar, 20.Bushinsky D.A. Parker W.R. Asplin J.R. Calcium phosphate supersaturation regulates stone formation in genetic hypercalciuric stone-forming rats.Kidney Int. 2000; 57: 550-560Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar, 21.Bushinsky D.A. Grynpas M.D. Asplin J.R. Effect of acidosis on urine supersaturation and stone formation in genetic hypercalciuric stone forming rats.Kidney Int. 2001; 59: 1415-1423Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar The addition of hydroxyproline, an oxalate precursor, to the diet induces the GHS rats to form CaOx stones.18.Evan A.P. Bledsoe S.B. Smith S.B. Bushinsky D.A. Calcium oxalate crystal localization and osteopontin immunostaining in genetic hypercalciuric stone-forming rats.Kidney Int. 2004; 65: 154-161Abstract Full Text Full Text PDF PubMed Scopus (48) Google Scholar,22.Bushinsky D.A. Asplin J.R. Grynpas M.D. et al.Calcium oxalate stone formation in genetic hypercalciuric stone-forming rats.Kidney Int. 2002; 61: 975-987Abstract Full Text Full Text PDF PubMed Scopus (85) Google Scholar As the diet and environment of these rats can be rigorously controlled, they are an ideal animal model to test hypotheses that are difficult to test in man. In this study, we used the GHS rats to test the hypotheses that urine supersaturation with respect to CaOx and CaHPO4 is maximal soon after eating and is greater when the daily food allotment is eaten as a bolus rather than in a divided fashion. On an ample Ca diet, UCa excretion was greater in the prandial (0–3 h) and postprandial (3–6 h) periods, when the GHS rats were offered their daily food allotment as a bolus compared to when the rats were given the first of three equal portions of a food at time 0. Total 24-h UCa excretion was greater in the GHS rats fed as a bolus compared to when the rats received a similar amount of food given in divided doses. That more Ca is absorbed in this ample Ca diet in the prandial and postprandial period is not surprising, as a portion of the increased amount of dietary Ca would be absorbed resulting in a greater filtered load of Ca and greater UCa excretion. However, what is surprising is that the bolus-fed rats actually had greater UCa excretion over the entire 24-h perio
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