The effects in the rat of varying intakes of dietary calcium, phosphorus, and hydrogen ion on hyperparathyroidism due to chronic renal failure

Recent Advances in the Role of Diet in Bone and Mineral Disorders in Chronic Kidney Disease

PURPOSE OF REVIEW

Chronic kidney disease mineral and bone disease (CKD-MBD) is a common complication of kidney disease and is strongly influenced by diet. The purpose of this manuscript is to review recent advances in the role of diet in CKD-MBD over the last 5 years.

RECENT FINDINGS

Many of the recent studies examining the role of diet in CKD-MBD have focused on the adverse effects of high phosphorus consumption on bone health and metabolism. In general, the studies have shown that high phosphorus consumption worsens markers of bone and mineral metabolism but that eating a diet with a calcium to phosphorus ratio closer to 1:1 can attenuate some of these effects. Recent studies also showed that dietary counseling is efficacious for improving markers of CKD-MBD. High consumption of phosphorus aggravates CKD-MBD. Dietary counseling may ameliorate these effects, for example, by consuming diets with higher calcium to phosphorus ratios.

Dietary phosphorus restriction in advanced chronic kidney disease: merits, challenges, and emerging strategies

Hyperphosphatemia is an independent risk factor for mortality in patients on maintenance dialysis. Since phosphorus clearance by standard three times-weekly dialysis is insufficient to balance ongoing dietary phosphorus intake, strategies to prevent absorption of dietary phosphorus are essential for attenuating increased serum levels. Dietary phosphorus binders are used widely for this purpose but dietary phosphorus restriction is relatively underutilized, most likely because of the logistical complexity of instituting and monitoring a low phosphorus diet, and for fear of worsening protein-energy wasting, which itself is a potent risk factor for mortality. In this review, we propose sustainable strategies for reducing phosphorus intake while avoiding exacerbation of protein-energy wasting. The approach is based on recognition of the dissociation between protein and phosphorus content in phosphorus-rich processed foods and the varying phosphorus bioavailability in different dietary sources. Controlling serum phosphate levels is among the most challenging aspects of day-to-day dialysis care but integration of sensible dietary interventions will likely improve phosphorus control.

Metabolic acidosis up-regulates PTH/PTHrP receptors in UMR 106-01 osteoblast-like cells

BACKGROUND

Metabolic acidosis results in skeletal demineralization by multiple mechanisms. One of these involves the inorganic phase of bone by which hydrogen ion is buffered by bone carbonate. In addition, the cellular components of bone participate by the induction and repression of several skeletal genes. Previous studies have suggested that the action of parathyroid hormone (PTH), a major regulator of bone turnover, might be altered by acidosis. The present studies were designed to test directly, in vitro, whether acidosis altered the effects of PTH in UMR 106-01 osteoblast-like cells.

METHODS

Studies were conducted in confluent cultures of UMR 106-01 cells in modified Eagle's medium (MEM) with 5% fetal bovine serum (FBS) at pH values varying from 7.4 to 7.1 by addition of HCl. After time periods of 4 to 48 hours, cells were tested for cyclic AMP generation in response to PTH. PTH binding and PTH/PTHrP receptor mRNA levels were determined by radioligand binding assay and Northern analysis respectively.

RESULTS

After 48 hours, decreases in pH from 7.4 to 7.1 resulted in a progressive increase in PTH-stimulated cyclic-AMP generation from 1978 +/- 294 to 4968 +/- 929 pmol/culture/5 min (P < 0.05). Basal cyclic AMP concentrations were unchanged. PTH binding increased 1.5- to twofold. Competitive inhibition binding revealed an increase in receptor number supported by up-regulation of PTH/PTHrP receptor mRNA up to twofold from control levels.

CONCLUSIONS

These findings demonstrate that metabolic acidosis stimulates the response to PTH in UMR 106-01 osteoblast-like cells by a mechanism that involves an increase in the levels of PTH/PTHrP receptor mRNA. Thus, the skeletal response to acidosis that includes an increase in bone resorption may result, at least in part, from an increase in PTH/PTHrP receptors leading to an enhanced effect of PTH on bone.

Genetic lineage, bone mass, and physical activity in mice

Five strains of inbred mice were found to have variations in bone mass although they were similar in body weight. Two of these strains, C57BL/6J and A/J, were studied in greater detail and the former had more bone in both femur and tibia. The increased bone mass was associated with larger quadriceps muscles in the C57BL/6J animals when measured at 18 weeks of age. Activities of animals from these two strains were studied over 24 h periods using a cage with an ultrasonic movement detector and automatic counter. The C57BL/6J animals were more active than the A/J mice. The male C57BL/6J mice tended to have larger testicles and higher testosterone levels than the A/J animals, whereas the female A/J animals had larger ovaries and higher oestradiol levels. As both male and female C57BL/6J animals were more active, it was concluded that the sex hormone differences between the two strains was not responsible for either the changes in bone mass or physical activity. These findings indicate that in the mouse, activity is in part genetically determined. We have hypothesized that this, in turn, could affect muscle mass and secondarily, bone size and strength. If these results can be applied to humans, it would suggest that differences at birth between individuals are important for bone mass in later life and that muscle mass and activity are in part genetically influenced. If this was the case, then muscle mass and strength could be a factor in bone mass and one of the goals in prevention and treatment of osteoporosis should be directed toward preservation and/or augmentation of muscle strength.

The role of metabolic acidosis in the pathogenesis of renal osteodystrophy

Renal osteodystrophy is thought to be the result of abnormalities in the serum levels of parathyroid hormone, vitamin D, calcium, and phosphorus, and excess exposure to certain substances such as aluminum and iron. However, a significant amount of data suggest that the metabolic acidosis that develops in the course of chronic renal failure may also play a contributory role. Metabolic acidosis may effect changes in bone by directly inducing dissolution of bone, stimulating osteoclast-mediated bone resorption, inhibiting osteoblast-mediated bone formation, and altering the serum concentrations or the biological actions of parathyroid hormone and vitamin D. As a consequence, in some patients with normal renal function, osteoporosis and osteomalacia have been reported that are linked in part to metabolic acidosis. Also, in patients with chronic renal failure before and after initiation of dialysis, the severity of the metabolic acidosis appears to have a bearing on the presence and degree of hyperparathyroidism, osteitis fibrosa, and osteomalacia. Taken as a whole, these data suggest that correction of the metabolic acidosis of chronic renal failure may have a beneficial effect on the bone disease observed in these patients. This article reviews (1) the data indicating the mechanisms by which metabolic acidosis causes alterations in bone; (2) the types of bone lesions observed in animals and humans with metabolic acidosis in the presence of normal and abnormal renal function; (3) the impact of correction of the acidosis on the bone lesions; and (4) specific recommendations for treatment in patients with chronic renal failure both before and after initiation of maintenance dialysis.

Calcemic response to parathyroid hormone in renal failure: role of phosphorus and its effect on calcitriol

The calcemic response to parathyroid hormone (PTH) is decreased in renal failure. The reduction of hyperphosphatemia improves the calcemic response to PTH in animals with advanced renal failure. However, since low calcitriol levels in renal failure may also contribute to the decreased calcemic response to PTH, the improved calcemic response observed during the reduction of serum phosphorus may be partially mediated by an increase in serum calcitriol levels. The present study evaluated the calcemic response to PTH in rats with moderate and advanced renal failure and how this response was modified by a high and a low phosphorus diet. In addition, the effect of a change in dietary phosphorus on calcitriol levels was also evaluated. A 48-hour continuous infusion of 1-34 rat PTH increased the serum calcium level to 18.2 +/- 0.4 mg/dl in normal rats, versus 13.7 +/- 0.9 and 12.1 +/- 0.2 mg/dl in rats with moderate and advanced renal failure, respectively. During the PTH infusion, a high phosphorus diet increased the serum phosphorus and resulted in a reduced calcemic response to PTH at each level of renal function; respective serum calcium levels were 13.8 +/- 0.6 mg/dl in normals, 11.2 +/- 0.2 mg/dl in moderate renal failure and 9.6 +/- 0.5 mg/dl in advanced renal failure. In normal rats and in rats with moderate renal failure, dietary phosphorus restriction during the PTH infusion increased serum calcitriol levels. In rats with advanced renal failure, serum calcitriol levels were lower than in the other two groups and were not affected by changes in dietary phosphorus.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcemic response to parathyroid hormone in renal failure: role of calcitriol and the effect of parathyroidectomy

Hyperparathyroidism due to renal failure begins in the early stages of renal insufficiency and is in part secondary to skeletal resistance to the calcemic action of parathyroid hormone (PTH). Factors which have been reported to reduce the calcemic response to PTH include: decreased calcitriol levels, hyperphosphatemia and down regulation of PTH receptors in bone. While hyperphosphatemia may directly decrease the calcemic response to PTH, it may also act indirectly by a suppression of calcitriol synthesis. In this study, the effect of calcitriol on the calcemic response to PTH was evaluated in normal rats and in rats with moderate and advanced renal failure. To determine the combined effect of calcitriol and phosphorus on the calcemic response to PTH, rats receiving calcitriol were fed either a high (1.0%) or low (0.2%) phosphorus diet during a 48-hour PTH infusion. In advanced renal failure, calcitriol administration increased the calcemic response to PTH independent of the dietary phosphorus intake. During ingestion of a low phosphorus diet, a 48 hour PTH infusion resulted in a serum calcium level of 13.7 +/- 0.5 and 12.1 +/- 0.2 mg/dl (P less than 0.02) with and without calcitriol administration, respectively. In normal rats and in rats with moderate renal failure, calcitriol administration improved the calcemic response only during a high phosphorus intake. After a 48-hour PTH infusion in normal rats, the serum calcium levels with and without calcitriol were 16.1 +/- 0.9 and 14.8 +/- 0.6 mg/dl, P less than 0.01 respectively; in rats with moderate renal failure, calcitriol administration increased serum calcium, 13.2 +/- 0.5 versus 11.2 +/- 0.4 mg/dl, P less than 0.01.(ABSTRACT TRUNCATED AT 250 WORDS)

Protection from progressive renal failure and hyperparathyroid bone remodeling by WR-2721

In chronic renal failure (CRF), secondary hyperparathyroidism (sHPT) plays a major role in skeletal lesions and also probably in the deterioration of renal functions consecutive to nephrocalcinosis. In this study we tested whether WR-2721 [S-,2-(3-aminopropylamino)-ethylphosphorothioic acid], an inhibitor of parathyroid hormone (PTH) secretion, could prevent the deleterious effects of sHPT at both the bone and kidney level in an animal model of CRF. Rats were either subtotally nephrectomized (NX) or sham-operated (SHAM). They were then pairfed a high phosphorus (1.4%), middle Ca (0.6%) diet. This regimen accelerated the deterioration of renal function in NX rats which displayed signs of severe sHPT in bone (three- to fourfold increase in osteoclast number and resorption surfaces) and kidney (sixfold increase in Ca content) after four weeks. Chronic administration of WR-2721 (20 mg = 0.093 mmol/kg s.c. twice daily) during four weeks completely prevented the progressive elevation of both plasma urea and inorganic phosphate, and the fall of plasma Ca. It also prevented kidney Ca accumulation to the same extent as parathyroidectomy. However, WR-2721 given at this dose only partially prevented the histomorphometric indices of increased bone resorption and formation. This discrepant response suggests that WR-2721 could exert an additional protective effect at the kidney level that might be related to its property of acting as a free radical scavenger. In conclusion, this study suggests that WR-2721 might be a useful compound in CRF, not only because it inhibits PTH secretion, but also because it could protect against the deleterious effect of secondary hyperparathyroidism on kidney functions.

Myocardial hypertrophy in rats with renal insufficiency

Increased defatted dry wt of the heart and increased heart calcium content were observed in subtotally nephrectomized male Sprague-Dawley rats compared with sham-operated pairfed controls. Increased heart wt contrasted with no change of the weight of viscera (liver, spleen) and markedly decreased weight of striated muscle. Heart wt was unchanged after 5 days of renal insufficiency, but significantly increased after 14 or 21 days. Increased heart wt persisted despite effective beta adrenoreceptor blockade (2 X 10 mg metroprolol/kg/day i.p.) or effective alpha-1-adrenoreceptor blockade (2 X 2 mg prazosin/kg/day i.p.). Increased heart wt was also demonstrable despite normalization of basal blood pressure (intraarterial blood pressure measurement in conscious animals): blood pressure was lowered in one series with hydralazine/nadolol in drinking water (calculated to deliver 20 and 2 mg/kg/day, respectively) and in another series with furosemide in drinking water (15 mg/kg/day) combined with metoprolol (2 X 10 mg/kg/day i.p.). Increased heart wt was also noted despite correction of anemia by blood transfusion (Hct greater than 40%) and after parathyroidectomy in animals kept eucalcemic with high dietary calcium. Micromorphometry of left ventricular myocardium in perfusion-fixed tissue showed no significant change of the relative proportion of connective tissue and myocardial fibers. Myocardial isomyosin pattern was changed with an increase of fast-migrating V1 isomyosin in animals with renal insufficiency compared to sham-operated pairfed controls.

The effect of metabolic acidosis on vitamin D metabolites and bone histology in uremic rats

Biochemical data and skeletal histomorphometric measurements are presented for normal rats and for two groups of rats rendered uremic by partial nephrectomy. In one of these groups chronic acidosis was induced by the oral administration of hydrochloric acid. Uremic animals had higher urine calcium excretion rates and lower serum concentrations of vitamin D metabolites than normal rats. Chronic acid loading of uremic rats resulted in hypercalcemia, elevated serum parathyroid hormone concentrations, and a significant loss of trabecular bone in addition to the above changes. greater osteoclast densities and higher resorption surfaces wee seen in the uremic acidotic animals than in the other two groups. The acidotic uremic state induced more potent changes in calcium metabolism and bone structure than uremia alone.

Hydrogen ion balance in dialysis therapy

A model to describe hydrogen ion balance (H+B) in acetate and bicarbonate dialysis therapy was developed based on measurement of metabolic addition of hydrogen ion (H+) to the body between and during dialyses and measurement of net buffer repletion during dialysis. Metabolic H+ generation was shown to be equal to 0.77 times the protein catabolic rate plus the total net removal of lactate and beta-hydroxybutyrate ions during dialysis. Buffer repletion was calculated from total net flux of acetate and bicarbonate during dialysis. The model was used for eight paired studies of H+B on one week each of acetate and bicarbonate dialysis and showed that cumulative H+B with acetate was -7 +/- 28 (M +/- SEM) mmol/week compared to -175 +/- 45 mmol/week with bicarbonate (P less than 0.001). It is concluded that there is an initial, strongly negative H+B when patients on acetate dialysis are converted to bicarbonate. The possible physiologic significance of this is discussed.

Action of phosphorus on calcium release in isolated perfused rat tails

A bone perfusion system using isolated rat tails was developed to study the action of inorganic phosphorus (Pi) on calcium (Ca) release in bone. Rats were either pre-labelled with 45Ca to study calcium removal from bone or 45Ca was added to the perfusate to study Ca deposition into bone. When perfusate Pi was increased from 3.5 to 5.25 mg/dl, a marked inhibition of 45Ca release was demonstrated. 85Sr microspheres, used to demonstrate differences in the flow of perfusate between bone and soft tissue, showed that the Pi effect was not due to alterations in perfusate flow to bone. Increasing the perfusate Pi concentration from 3.5 to 7 mg/dl did not increase 45Ca deposition into bone, nor did it affect baseline or PTH-stimulated cyclic AMP production. The Pi effect was inhibited by reducing the perfusate temperature to 4 degrees C, by using an anerobic perfusate, adding iodoacetate to the perfusate, and finally, by pre-treating the rats with actinomycin D prior to the experiment. Thus, in this model an increase in the Pi concentration of the perfusate inhibits Ca release from bone but does not increase Ca uptake by bone. The inhibitory effect appears to be dependent on normal cell metabolism.

Evidence that resistance to the calcemic action of parathyroid hormone in rats with acute uremia is caused by phosphate retention

An animal model was developed to examine the cause of resistance to the calcemic action of PTH in renal failure. Thyroparathyroidectomized (TPTX) rats were repeatedly reinfused with their excreted urine, over a 5-hour period, to produce an acute uremic animal with normal kidneys. Nonuremic controls were infused with equivalent volumes of a simple electrolyte solution. Using this model, we have demonstrated impaired calcemic response to PTH (Lilly Parathyroid Extract, 80 U/100 g) in urine-infused rats compared with electrolyte-infused rats. The final plasma calcium concentrations were 12.0 +/- 0.3 and 14.9 +/- 0.3 mg/dl, respectively (P less than 0.001). The cause of this impaired calcemic response was investigated by reinfusing rats with their own urine that had been pretreated with either activated charcoal or zirconium oxide in two different anionic forms, or urine that had been ultrafiltrated through an Amicon membrane of which the stated molecular-weight cut-off of the smallest pore-size membrane was 500 daltons. It was found that charcoal and ultrafiltration techniques had no effect, whereas the zirconium oxide treatment completely corrected the impaired calcemic response and returned the plasma phosphorus to a concentration similar to that in nonuremic controls. This can be explained by the fact that the principal effect of zirconium oxide on urine is to remove inorganic phosphate. Other groups of TPTX rats given PTH extract were infused with an electrolyte solution containing varying amounts of phosphate up to a maximum similar to the amount that a urine-infused rat would receive. A highly significant inverse relationship was found between the dose of phosphate infused with the electrolyte solution and the measured calcemic response to PTH. This relationship is represented by the following equation: calcium (mg/dl) = 14.84 - 0.139 inorganic phosphate; r = 0.915, P less than 0.001. From these series of studies, we conclude that phosphate retention is the cause of resistance to the calcemic action of PTH extract in this acute uremic model.

Resistance to parathyroid hormone in renal failure: role of vitamin D metabolites

Resistance to the calcemic action of parathyroid extract (PTE) was shown in thyroparathyroidectomized rats after 5 hr of renal failure that was induced by either bilateral nephrectomy (NPX) or ureter ligation (UL). Studies were carried out to investigate the relationship of parathyroid resistance to the vitamin D status of the animal. Concentrations of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) and 24,25-dihydroxycholecalciferol (24,25(OH)2D3) were similar in pooled sera samples from rats either UL or sham-operated and treated with PTE. Pretreatment with oral 25-hydroxycholecalciferol or with a combination of i.v. 24,25(OH)2D3 and 1,25(OH)2D3 prior to UL failed to alter the resistance. Resistance was also present in a group in vitamin-D-deficient rats. A similar group given 1 microgram of vitamin D2 showed more parathyroid resistance than did the group not given vitamin D2. In chronic renal failure of 28 day's duration, parathyroid resistance was again demonstrated, but, in contrast to the acute renal failure models, this was partly corrected by prior 1,25(OH)2D3 administration. These studies show that parathyroid resistance is not caused by an abnormality of vitamin D metabolism in the acute renal failure model, and we suggest that the phenomenon is due to the accumulation of one or more uremic factors.

A micropuncture study of renal phosphate transport in rats with chronic renal failure and secondary hyperparathyroidism

Micropuncture studies were carried out in rats to determine changes in tubular transport of phosphate which occur in chronic renal failure and secondary hyperparathyroidism. Rats underwent subtotal nephrectomy (NX) and were fed a low calcium, high phosphorus diet for 3--4 wk. Other groups consisted of normal control animals, normal rats infused with sodium phosphate to raise filtered load of phosphate, subtotal NX rats parathyroidectomized (PTX) on the day of experiment, and normal PTX rats infused with sodium phosphate. It was found that filtered phosphate/nephron is markedly increased in subtotal NX rats due to high single nephron filtration rates, proximal tubular fluid plasma phosphate ratios are less than 1.0, and fractional reabsorption of phosphate is decreased in the proximal tubule. More phosphate was present in the final urine than in surface distal convoluted tubules. Acute PTX in subtotal NX rats resulted in a striking increase in proximal phosphate reabsorption, and urinary phosphate became approximately equal to that remaining in surface distal tubules. Phosphate loading in normal rats reduced fractional reabsorption in the proximal tubule, but urinary phosphate was not greater than that at the end of surface distal tubules. Acute PTX in normal phosphate-loaded animals had no significant effect on proximal tubular phosphate reabsorption. These observations suggest that phosphate homeostasis in chronic renal failure is acheived by inhibition of proximal phosphate reabsorption, counteracting a greatly enhanced intrinsic capacity for reabsorption. In addition, the large amount of urinary phosphate is consistent either with secretion by the collecting ducts or with a disproportionately high contribution by deep nephrons. The changes in phosphate transport are mediated by parathyroid hormone and are completely abolished by acute removal of the hormone.

A micropuncture study of HCO3 reabsorption by the hypertrophied proximal tubule

In rats with renal failure produced by excision of one kidney and infarction of large portions of the other kidney, given a low calcium, high phosphorus diet for 2-3 weeks, GFR was reduced by 80 percent, the fractional excretion of sodium increased from 7 to 23 percent, that of bicarbonate from 16 to 23 percent and that of water from 4 to 13 percent. Single nephron GFR in the remaining nephrons was nearly doubled and end-proximal TF/P(In) was depressed from 2.3 to 1.8, and proximal TF/P(HCO3) from 0.52 to 0.35, the latter figure corresponding to an increase of absolute proximal HCO(3) reabsorption from 1.7 to 3.5 nEq/min or from 2.8 to 3.2 Eq/L of single nephron glomerular filtrate. Acute parathyroidectomy had no influence on the fall of GFR or the rise of SNGFR in the remaining nephrons and failed to cause any significant changes in proximal tubular bicarbonate reabsorption. Parathyroidectomy, on the other hand, practically prevented the rise of the fractional excretion of sodium and of water and inverted the rise of the fractional excretion of bicarbonate to a fall. The data are interpreted to indicate that secondary hyperparathyroidism in renal failure impairs distal nephron bicarbonate and sodium reabsorption and, thus, contributes to the maintenance of sodium balance, but could possibly aggravate acidosis.

Changes in red cell space of bone in hypocalcemic states: studies in rat femur

With the use of technetium-99m-labeled red cells, the red cell space of the femur was found to be increased in rats fed a 0.01 percent Ca diet for 4 wk, at which time they were also hypocalcemic. Rats fed the same diet for 2 wk did not demonstrate hypocalcemia nor hypervascularity of the femur. Thyroparathyroidectomized (TPTX) rats fed the calcium-deficient diet produced hypocalcemia nor hypervascularity as did TPTX rats fed a normal diet for 4 wk. However, bone vascularity was normal at 2 wk post-TPTX (normal diet) when the degree of hypocalcemia was less marked. Inhibition of prostaglandin synthetase with oral indomethacin 0.2 mg/100 g per 12 h for 60 h did not alter the red cell space estimates but did increase bone blood flow in both normal and calcium-deficient animals. Acute restoration of normocalcemia with CaCl2 orally in the calcium-deficient rats did not revert the bone hypervascularity. The evidence suggests that in these models of hypocalcemic states, the rise in the vascular space of bone does not result from the influence of PTH, calcitonin, diet, or prostaglandins.