Effect of 24,25(OH)2D3 on PTH levels and bone histology in dogs with chronic uremia

Renal pathophysiology: lessons learned from the canine remnant kidney model

OBJECTIVE

To review the pathophysiology of chronic kidney disease (CKD) in dogs and the contributions of the canine remnant kidney model to our understanding of this disease.

DATA SOURCES

Original studies in the human and veterinary medical fields.

DATA SYNTHESIS

Three of the fundamental principles of modern nephrology-the intact nephron hypothesis, the trade-off hypothesis, and the hyperfiltration theory were developed directly as a result of studies of the remnant kidney model. Most of the pivotal early studies were conducted in dogs. As a result, our understanding of CKD, and of the renal and systemic adaptations to CKD, is largely based on studies of this model.

CONCLUSIONS

Studies of the remnant kidney model have advanced our understanding of the pathophysiology of CKD. Nearly every therapeutic intervention used in CKD, by veterinarians and physicians alike, has its basis in studies of the remnant kidney model or in knowledge that was derived from studies of this model. A great debt is owed to the canine participants in these studies and to a small number of key scientists who conducted this important and insightful research.

Complications of chronic kidney disease: anemia, mineral metabolism, and cardiovascular disease

This article focuses on the importance of three major complications of chronic kidney disease: (1) anemia, (2) calcium-phosphorus regulation and bone disease, and (3) cardiovascular risk profiling and treatment. The arguments for early and effective intervention have been amply made with respect to these three complications. Substantive trial data are sorely need to provide the definitive evidence that effective treatment of these complications results in better outcomes.

Chronic Renal Failure Accelerates Atherogenesis in Apolipoprotein E–Deficient Mice

ABSTRACT. Cardiovascular mortality is 10 to 20 times increased in patients with chronic renal failure (CRF). Risk factors for atherosclerosis are abundant in patients with CRF. However, the pathogenesis of cardiovascular disease in CRF remains to be elucidated. The effect of CRF on the development of atherosclerosis in apolipoprotein E–deficient male mice was examined. Seven-week-old mice underwent 5/6 nephrectomy (CRF, n = 28), unilateral nephrectomy (UNX, n = 24), or no surgery (n = 23). Twenty-two weeks later, CRF mice showed increased aortic plaque area fraction (0.266 ± 0.033 versus 0.045 ± 0.006; P < 0.001), aortic cholesterol content (535 ± 62 versus 100 ± 9 nmol/cm2 intimal surface area; P < 0.001), and aortic root plaque area (205,296 ± 22,098 versus 143,662 ± 13,302 μm2; P < 0.05) as compared with no-surgery mice; UNX mice showed intermediate values. The plaques from uremic mice contained CD11b-positive macrophages and showed strong staining for nitrotyrosine. Systolic BP and plasma homocysteine concentrations were similar in uremic and nonuremic mice. Plasma urea and cholesterol concentrations were elevated 2.6-fold (P < 0.001) and 1.5-fold (P < 0.001) in CRF compared with no-surgery mice. Both variables correlated with aortic plaque area fraction (r2 = 0.5, P < 0.001 and r2 = 0.3, P < 0.001, respectively) and with each other (r2 = 0.5, P < 0.001). On multiple linear regression analysis, only plasma urea was a significant predictor of aortic plaque area fraction. In conclusion, the present findings suggest that uremia markedly accelerates atherogenesis in apolipoprotein E–deficient mice. This effect could not be fully explained by changes in BP, plasma homocysteine levels, or total plasma cholesterol concentrations. Thus, the CRF apolipoprotein E–deficient mouse is a new model for studying the pathogenesis of accelerated atherosclerosis in uremia. E-mail: susannebro@dadlnet.dk

Serum concentrations of 1,25-dihydroxycholecalciferol and 25-hydroxycholecalciferol in clinically normal dogs and dogs with acute and chronic renal failure

OBJECTIVE

To compare serum concentrations of 1,25-dihydroxycholecalciferol (1,25-[OH]2D3) and 25-hydroxycholecalciferol (25-[OH]D3) in healthy control dogs and dogs with naturally occurring acute renal failure (ARF) and chronic renal failure (CRF).

ANIMALS

24 control dogs, 10 dogs with ARF, and 40 dogs with CRF.

PROCEDURE

Serum concentrations of 1,25-(OH)2D3 were measured by use of a quantitative radioimmunoassay, and serum concentrations of 25-(OH)D3 were measured by use of a protein-binding assay.

RESULTS

Mean +/- SD serum concentration of 1,25-(OH)2D3 was 153 +/- 50 pmol/L in control dogs, 75 +/- 25 pmol/L in dogs with ARF, and 93 +/- 67 pmol/L in dogs with CRF. The concentration of 1,25-(OH)2D3 did not differ significantly between dogs with ARF and those with CRF and was in the reference range in most dogs; however, the concentration was significantly lower in dogs with ARF or CRF, compared with the concentration in control dogs. Mean +/- SD concentration of 25-(OH)D3 was 267 +/- 97 nmol/L in control dogs, 130 +/- 82 nmol/L in dogs with ARF, and 84 +/- 60 nmol/L in dogs with CRF. The concentration of 25-(OH)D3 was significantly lower in dogs with ARF or CRF, compared with the concentration in control dogs.

CONCLUSIONS AND CLINICAL RELEVANCE

The concentration of 1,25-(OH)2D3 was within the reference range in most dogs with renal failure. Increased serum concentrations of parathyroid hormone indicated a relative deficiency of 1,25-(OH)2D3. A decrease in the serum concentration of 25-(OH)D3 in dogs with CRF appeared to be attributable to reduced intake and increased urinary loss.

Effect of vitamin D metabolites on calcitriol degradative enzymes in renal failure

We have demonstrated that in renal failure calcitriol degradation is decreased and that administration of vitamin D metabolites increases the degradation. In this study, we measured intestinal 24- and 26-hydroxylase activities and the effects of chronic infusion (7 days) of vitamin D metabolites on these enzymes' activities in rats with experimental renal failure. The enzymatic activity of intestinal 24-hydroxylase, but not 26-hydroxylase, was significantly lower in renal failure rats compared to control sham operated rats. Replacement of calcitriol (3 ng/day) significantly increased 24-hydroxylase activity by 17% in rats with renal failure (P < 0.01), although the activity remained 15% lower than the controls (P < 0.01). Intestinal 26-hydroxylase activity was not lower in renal failure; however, calcitriol treatment increased the activity beyond that of normal controls. In contrast, administration of 25(OH)D3 (600 ng/day) and 24,25(OH)2D3 (1 microgram/day) reduced the conversion of calcitriol to 1,24,25(OH)3D3 by more than 50% and to 1,25,26(OH)3D3 by more than 38%, respectively. We conclude that calcitriol increased its own degradation in renal failure by increasing the enzymatic activities of both 24- and 26-hydroxylase. However, the mechanisms of increased calcitriol degradation by 25(OH)D3 and 24,25(OH)2D3 in renal failure remain unknown.

24,25(OH)2D3 affects the calcemic effects of 1,25(OH)2D3 by mechanisms independent of intestinal calcium absorption

To better define the interaction between 1,25(OH)2D3 and 24,25(OH)2D3 in different states of renal function, the fractional absorption of Ca45 (FCa45) and plasma calcium (PCa) were determined in rats with intact kidneys and in rats with reduced renal mass after 5/6 nephrectomy. The two series of experiments were performed in control animals, and in rats treated with: 1) 1,25(OH)2D3, 54 ng/rat/day, 2) 24,25(OH)2D3 in the same dose, and 3) 1,25 and 24,25(OH)2D3 in the same (and equal) doses. In all animals 1,25(OH)2D3 administration was associated with a significant increase in PCa and in FCa45. In rats with normal renal function, however, 24,25(OH)2D3 enhanced and in rats with reduced renal mass it suppressed the hypercalcemic effect of 1,25(OH)2D3. These variations in PCa were not associated with further alteration in FCa45 which was similar after 1,25(OH)2D3 and after combined 1,25 and 24,25(OH)2D3 in normal rats and in rats with chronic renal failure. FCa45 was found to be of the same magnitude in rats with normal renal function and in rats with reduced renal mass. These results confirm previous findings of normal intestinal absorption of Ca in mild to moderate renal failure. The above data suggest that 24,25(OH)2D3 used in an equivalent dose does not influence the intestinal effects of 1,25(OH)2D3. Therefore, the interaction of 1,25 with 24,25(OH)2D3 in rats with intact kidneys or with reduced renal mass occurs at extra-intestinal sites.

Efficacy of calcium carbonate and low-dose vitamin D/1,25(OH)2D3 in reducing the risk of developing renal osteodystrophy in children on continuous ambulatory peritoneal dialysis

Eight children with terminal renal insufficiency on continuous ambulatory peritoneal dialysis were followed for 12 months to evaluate laboratory parameters of mineral ion and bone metabolism. Calcium carbonate (range 47-295 mg/kg body weight per day) was given in combination with low doses of either vitamin D or 1,25(OH2D3. Blood urea nitrogen and serum phosphate concentrations remained well controlled throughout the observation period. A significant increase in serum calcium levels from 2.35 +/- 0.18 to 2.61 +/- 0.22 mmol/l (mean +/- SD) was observed during the first 6 months. Alkaline phosphatase activity and mid-C-regional parathyroid hormone, both indirect parameters of bone metabolism, revealed no evidence of severe secondary hyperparathyroidism. Our data indicate that calcium carbonate may be sufficient to induce relative hypercalcaemia in uraemic children, and thus reduce the risk of developing renal osteodystrophy. Unwanted side-effects of vitamin D preparations, i.e. increased intestinal phosphate absorption and hypercalcaemia after successful renal transplantation, may thus be avoided.

Effect of vitamin D metabolites on calcitriol metabolism in experimental renal failure

Previous studies from our laboratory have demonstrated that metabolic clearance rate (MCR) of calcitriol is decreased in experimental renal failure. In this experiment, we examined the effects of calcitriol, 25-hydroxyvitamin D3 (25(OH)D3) and 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) on the MCR of calcitriol in renal failure produced in rats by partial nephrectomy. The MCR of calcitriol in these rats with renal failure was significantly lower than in control rats with sham operations. Plasma concentrations of calcitriol did not differ between the rats with moderate renal failure and control rats (sham, 74.7 +/- 3.6 pg/ml, N = 7; renal failure, 67.7 +/- 6.0, N = 6; serum creatinine 0.56 +/- 0.02 mg/dl vs. 0.96 +/- 0.02); however, the levels were significantly lower in rats with severe renal failure (sham, 66.5 +/- 5.1 pg/ml, N = 7, severe renal failure, 49.6 +/- 2.1 pg/ml, N = 8; serum creatinine 0.53 +/- 0.01 mg/dl vs. 1.40 +/- 0.03). Subcutaneous infusion of calcitriol (10 ng/kg/day) in rats with severe renal failure for one week significantly increased the MCR of calcitriol (0.22 +/- .01 vs. 0.17 +/- .01 ml/min/kg, P less than 0.001). Infusion of 25(OH)D3 (600 ng/day) or 24,25(OH)2D3 (1 microgram/day) in rats with renal failure for one week also increased the MCR of calcitriol (25(OH)D3, 0.25 +/- 0.01 ml/min/kg; 24,25(OH)2D3, 0.25 +/- 0.01, both P less than 0.001) when compared to rats with renal failure infused with vehicle (0.21 +/- 0.01). Administration of 24,25(OH)2D3 significantly lowered the plasma levels of calcitriol in rats with renal failure (52.3 +/- 3.1 pg/ml, P less than 0.05) in comparison to the rats with renal failure infused with vehicle (67.7 +/- 6.0).(ABSTRACT TRUNCATED AT 250 WORDS)