PRODUCTION, EXCRETION, AND NET BALANCE OF FIXED ACID IN PATIENTS WITH RENAL ACIDOSIS

Metabolic, Electrolytes, and Nutritional Concerns in Critical Illness

This article discusses metabolic, electrolyte, and nutritional concerns in critical illness.

Acid loading during treatment with sevelamer hydrochloride: mechanisms and clinical implications

Short-term and long-term studies indicate that patients treated with sevelamer hydrochloride have lower serum bicarbonate levels than patients treated with calcium-containing phosphate binders. This observation has previously been attributed to withdrawal of a source of base with discontinuation of calcium carbonate or calcium acetate. However, understanding of the chemistry of sevelamer hydrochloride suggests at least three potential mechanisms whereby it might induce a dietary acid load. Moreover, preliminary results from an animal model demonstrate that treatment with sevelamer hydrochloride results in a fall in urine pH, as well as an increase in urinary ammonium and calcium excretion consistent with an increase in net acid excretion. Chronic metabolic acidosis in maintenance dialysis patients is associated with major systemic effects. It is independently associated with an increased risk of death in dialysis patients. Metabolic acidosis has both catabolic and antianabolic effects that may lead to a net negative nitrogen balance and total body protein balance. Metabolic acidosis also leads to physiochemical dissolution of bone and promotes cell-mediated bone resorption due to enhanced osteoclast activity and reduced osteoblast activity. It may also exacerbate secondary hyperparathyroidism and renal osteodystrophy. Given the long-term risks of chronic metabolic acidosis in maintenance dialysis patients, Kidney/Dialysis Outcome Quality Initiative (K/DOQI) guidelines have recently recommended maintaining predialysis serum levels of CO2 above 22 mmol/L in order to improve bone histology, and to ameliorate excess protein catabolism.

Mechanism of acid-induced bone resorption

PURPOSE OF REVIEW

This review presents our current understanding of the way metabolic acidosis induces calcium efflux from bone, and in the process, buffers additional systemic hydrogen ions associated with acidosis.

RECENT FINDINGS

Acid-induced changes in bone mineral are consistent with a role for bone as a proton buffer. In response to metabolic acidosis in an in-vitro bone organ culture system, we observed a fall in mineral sodium, potassium, carbonate and phosphate, which each buffer protons and in vivo should increase systemic pH towards the physiologic normal. Initially, metabolic acidosis stimulates physicochemical mineral dissolution and subsequently cell-mediated bone resorption. Acidosis suppresses the activity of bone-resorbing cells, osteoblasts, decreasing gene expression of specific matrix proteins and alkaline phosphatase activity. There is concomitant acid stimulation of prostaglandin production by osteoblasts, which acting in a paracrine manner increases synthesis of the osteoblastic receptor activator of nuclear factor kappa B ligand (RANKL). The acid induction of RANKL then stimulates osteoclastic activity and recruitment of new osteoclasts to promote bone resorption and buffering of the proton load. Both the regulation of RANKL and acid-induced calcium efflux from bone are mediated by prostaglandins.

SUMMARY

Metabolic acidosis, which occurs during renal failure, renal insufficiency or renal tubular acidosis, results in decreased systemic pH and is associated with an increase in urine calcium excretion. The apparent protective function of bone to help maintain systemic pH, which has a clear survival advantage for mammals, will come partly at the expense of its mineral stores.

Metabolic acidosis in maintenance dialysis patients: clinical considerations

Metabolic acidosis in maintenance dialysis patients: Clinical considerations. Metabolic acidosis is a common consequence of advanced chronic renal failure (CRF) and maintenance dialysis (MD) therapies are not infrequently unable to completely correct the base deficit. In MD patients, severe metabolic acidosis is associated with an increased relative risk for death. The chronic metabolic acidosis of the severity commonly encountered in patients with advanced CRF has two well-recognized major systemic consequences. First, metabolic acidosis induces net negative nitrogen and total body protein balance, which improves upon bicarbonate supplementation. The data suggest that metabolic acidosis is both catabolic and antianabolic. Emerging data also indicate that metabolic acidosis may be one of the triggers for chronic inflammation, which may in turn promote protein catabolism among MD patients. In contrast to these findings, metabolic acidosis may be associated with a decrease in hyperleptinemia associated with CRF. Several studies have shown that correction of metabolic acidosis among MD patients is associated with modest improvements in the nutritional status. Second, metabolic acidosis has several effects on bone, causing physicochemical dissolution of bone and cell-mediated bone resorption (inhibition of osteoblast and stimulation of osteoclast function). Metabolic acidosis is probably also associated with worsening of secondary hyperparathyroidism. Data on the effect of correction of metabolic acidosis on renal osteodystrophy, however, are limited. Preliminary evidence suggest that metabolic acidosis may play a role in beta2-microglobulin accumulation, as well as the hypertriglyceridemia seen in renal failure. Given the body of evidence pointing to the several systemic consequences of metabolic acidosis, a more aggressive approach to the correction of metabolic acidosis is proposed.

Acid–base balance in acute renal failure and renal replacement therapy

The approach to acid-base balance based on the concept of strong ions, initially proposed by Stewart, is briefly overviewed. The anion gap and the strong anion gap are both discussed. Comments are made on the strong ion difference of fluids administered to patients and their impact on acid-base status will be commented. Renal failure patients have an altered acid-base balance; most commonly, a mixed type of metabolic acidosis (hyperchloraemic, and of a high anion gap) is observed. The consequences of renal metabolic acidosis are described. Finally, the impact of renal replacement therapy on acid-base balance is exposed; different modalities of renal replacement are considered in regard to their alkalinizing performance.

Toxicity and carcinogenicity of acidogenic or alkalogenic diets in rats; effects of feeding NH 4 Cl, KHCO 3 or KCl

The effects of diet-induced acid-base disturbances were examined in 4-week, 13-week and 18-month toxicity studies, and in a 30-month carcinogenicity study. Rats were fed a natural ingredient diet (controls), supplemented with 2% or 4% KHCO(3) (base-forming diets), or with 1% or 2.1% NH(4)Cl (acid-forming diets). Additional controls were fed 3% KCl (neutral diet providing K(+) and Cl(-) in amounts equimolar to those in the 4% KHCO(3) diet and the 2.1% NH(4)Cl diet, respectively). NH(4)Cl induced the expected metabolic acidosis, as shown by decreased base excess in blood, decreased urinary pH and increased urinary net acid excretion. KHCO(3) induced the opposite effects. KCl did not affect the acid-base balance. Clinical condition and death rate were not affected. The feeding of high levels of each salt resulted in growth retardation and increased water intake and urinary volume. Plasma potassium and urinary potassium excretion were increased with KHCO(3) and KCl. Plasma chloride was increased with NH(4)Cl, but not with KCl. Urinary calcium and phosphate excretion were increased with NH(4)Cl, but there were no indications that bone minerals were involved (weight, calcium content and fat free solid of the femur were not affected). Standard haematological and clinical chemistry parameters were not affected. Kidney weights were increased with 2.1% NH(4)Cl. Hypertrophy of the adrenal zona glomerulosa occurred with KHCO(3), KCl and NH(4)Cl, due to chronic stimulation of the adrenal cortex by either K(+) or by NH(4)Cl-induced acidosis. An early onset (from week 13) of oncocytic tubules was noted in the kidneys of rats fed KHCO(3) and, after 30 months, the incidence of this lesion was much higher than the background incidence in ageing controls. No progression to oncocytomas was noted. KCl showed only slight effects on the early onset of oncocytic tubules (from 18 months). In contrast, the severity of nephrosis and the incidence of oncocytic tubules were decreased with 2.1% NH(4)Cl, suggesting a protective effect of acidosis. The feeding of KHCO(3) resulted in hyperplasia, papillomas and carcinomas of the urinary bladder. With KCl only a slight increase in proliferative urothelial lesions was noted. Apart from these (pre-)neoplastic lesions in the urinary bladder there were no treatment-related differences in tumour response among the groups. We concluded that most of the observed changes represent physiological adaptations to the feeding of acid- or base-forming salts. Remarkable effects noted with KHCO(3), and to a far lesser extent with KCl, consisted of renal oncocytic tubules and (pre-)neoplastic lesions of the urinary bladder epithelium. NH(4)Cl-induced chronic metabolic acidosis was not associated with dissolution of alkaline bone salts in rats. Finally, a protective effect of chronic acidosis on tumour development was not found.

Bone buffering of acid and base in humans

The sources and rates of metabolic acid production in relation to renal net acid excretion and thus acid balance in humans have remained controversial. The techniques and possible errors in these measurements are reviewed, as is the relationship of charge balance to acid balance. The results demonstrate that when acid production is experimentally increased among healthy subjects, renal net acid excretion does not increase as much as acid production so that acid balances become positive. These positive imbalances are accompanied by equivalently negative charge balances that are the result of bone buffering of retained H+ and loss of bone Ca2+ into the urine. The data also demonstrate that when acid production is experimentally reduced during the administration of KHCO3, renal net acid excretion does not decrease as much as the decrease in acid production so that acid balances become negative, or, in opposite terms, there are equivalently positive HCO3- balances. Equivalently positive K+ and Ca2+ balances, and thus positive charge balances, accompany these negative acid imbalances. Similarly, positive Na+ balances, and thus positive charge balances, accompany these negative acid balances during the administration of NaHCO3. These charge balances are likely the result of the adsorption of HCO3- onto the crystal surfaces of bone mineral. There do not appear to be significant errors in the measurements.

Endogenous endothelins mediate increased acidification in remnant kidneys

Because endothelins (ET) mediate increased renal acidification induced by dietary acid and animals with reduced renal mass exhibit increased urinary ET-1 excretion, the hypothesis that ET mediate increased renal acidification in remnant kidneys was tested. Four weeks before the study, rats underwent a 5/6 nephrectomy (Nx) and a microdialysis apparatus was inserted into the remnant left kidney and the left kidney of sham-treated control animals, for measurements of renal ET-1 contents. Nx animals exhibited greater ET-1 addition to the renal dialysate than did control animals (681 +/- 91 versus 290 +/- 39 fmol/g kidney wt per min, P < 0.002) and greater urinary ET-1 excretion (346 +/- 79 versus 125 +/- 24 fmol/d, P < 0.02). Urinary net acid excretion rates were similar for Nx and control animals (732 +/- 106 versus 1005 +/- 293 microEq/d, P = 0.4), but Nx animals exhibited greater in situ HCO(3)(-) reabsorption in proximal (972.3 +/- 77 versus 482.6 +/- 42.4 pmol/min, P < 0.001) and distal (62.7 +/- 6.7 versus 24.3 +/- 2.5 pmol/min, P < 0.001) tubules. Orally administered bosentan, an ET(A/B) receptor antagonist, decreased urinary net acid excretion in Nx animals (to 394 +/- 99 microEq/d, P < 0.04 versus without bosentan); the decrease was mediated by decreased HCO(3)(-) reabsorption in both the proximal and distal tubules. Furthermore, bosentan decreased blood base excess in Nx animals (0.1 +/- 0.3 to -0.12 +/- 0.03 microM/ml blood, P < 0.002), consistent with acid retention. The data demonstrate that endogenous ET mediate increased urinary acid excretion in the remnant kidneys of Nx animals.

Acid and mineral balances and bone in familial proximal renal tubular acidosis

BACKGROUND

Metabolic acidosis caused by increased rates of fixed acid production is associated with increased urinary excretion of Ca and negative Ca balances. Metabolic acidosis caused by a reduced capacity of the kidneys to excrete acid contributes to the development of bone disease in the course of chronic renal failure and may be associated with bone disease among some patients with renal tubular acidosis.

METHODS

To assess the effects of life-long metabolic acidosis alone in the absence of other physiological disturbances, we measured the net balances of fixed acid and minerals in two brothers in a Costa Rican family with hereditary proximal renal tubular acidosis. Bone radiographs were assessed, and radial bone densities were measured. On a subsequent occasion, transiliac bone biopsies, following double-tetracycline labeling, were obtained from these two patients and an unaffected brother.

RESULTS

During the balance studies, serum [HCO3-] concentrations of the two affected patients were stable at 12.5 +/- 0.9 and 19.2 +/- 0.7 mmol/L, respectively. Their rates of net fixed acid production were normal and appropriate for their body weights, averaging 0.90 and 1.02 mEq/kg/day. Because their distal renal tubular function was normal, they were capable of acidifying their urine maximally, allowing sufficient urinary excretion of titratable acid and ammonium to maintain net acid excretion at a level that matched acid production. Thus, their acid balances were near zero, as observed among healthy subjects, at -1.9 +/- 2.3 and -2.2 +/- 2.2 mEq/day, respectively. Their rates of urinary Ca excretion were normal at 1.6 +/- 0.3 and 2.7 +/- 2.4 mmol/day, and the their balances of Ca and other minerals were close to zero so that ongoing bone loss was not occurring despite the acidosis. Nevertheless, their heights, relative to their ages, were shorter than the heights of their unaffected relatives. Their radial bone densities were lower than normal for their age and sex, and their iliac cortices were thinner than that of their unaffected brother. However, they had no histomorphometric evidence of osteomalacia or osteitis fibrosa, and their rates of bone mineralization were normal.

CONCLUSIONS

The results indicate that this chronic metabolic acidosis reduces growth, including that of bone. We speculate, without direct supporting evidence, that bone stores of HCO3-/CO3= are reduced, as has been observed in patients with the metabolic acidosis of chronic renal failure and in experimental metabolic acidosis in animals.

Chronic metabolic acidosis in azotemic rats on a high-phosphate diet halts the progression of renal disease

BACKGROUND

Hyperphosphatemia and metabolic acidosis are general features of advanced chronic renal failure (RF), and each may affect mineral metabolism. The goal of the present study was to evaluate the effect of chronic metabolic acidosis on the development of hyperparathyroidism and bone disease in normal and azotemic rats on a high-phosphate diet. Our assumption that the two groups of azotemic rats (acid-loaded vs. non-acid-loaded) would have the same degree of renal failure at the end of the study proved to be incorrect.

METHODS

Four groups of rats receiving a high-phosphate (1.2%), normal-calcium (0.6%) diet for 30 days were studied: (1) normal (N); (2) normal + acid (N + Ac) in which 1.5% ammonium chloride (NH4Cl) was added to the drinking water to induce acidosis; (3) RF, 5/6 nephrectomized rats; and (4) RF + acid (RF + Ac) in which 0.75% NH4Cl was added to the drinking water of 5/6 nephrectomized rats to induce acidosis.

RESULTS

At sacrifice, the arterial pH and serum bicarbonate were lowest in the RF + Ac group and were intermediate in the N + Ac group. Serum creatinine (0.76 +/- 0.08 vs. 1.15 +/- 0.08 mg/dL), blood urea nitrogen (52 +/- 8 vs. 86 +/- 13 mg/dL), parathyroid hormone (PTH; 180 +/- 50 vs. 484 +/- 51 pg/mL), and serum phosphate (7.46 +/- 0.60 vs. 12.87 +/- 1.4 mg/dL) values were less (P < 0.05), and serum calcium (9.00 +/- 0.28 vs. 7.75 +/- 0.28 mg/dL) values were greater (P < 0.05) in the RF + Ac group than in the RF group. The fractional excretion of phosphate (FEP) was greater (P < 0.05) in the two azotemic groups than in the two nonazotemic groups. In the azotemic groups, the FEP was similar even though PTH and serum phosphate values were less in the RF + Ac than in the RF group. NH4Cl-induced acidosis produced hypercalciuria in the N + Ac and RF + Ac groups. When acid-loaded (N + Ac and RF + Ac) and non-acid-loaded (N and RF) rats were combined as separate groups, serum phosphate and PTH values were less for a similarly elevated serum creatinine value in acid-loaded than in non-acid-loaded rats. Finally, the osteoblast surface was less in the N + Ac group than in the other groups. However, in the acid-loaded azotemic group (RF + Ac), the osteoblast surface was not reduced.

CONCLUSIONS

The presence of chronic metabolic acidosis in 5/6 nephrectomized rats on a high-phosphate diet (1) protected against the progression of RF, (2) enhanced the renal clearance of phosphate, (3) resulted in a lesser degree of hyperparathyroidism, and (4) did not reduce the osteoblast surface. The combination of metabolic acidosis and phosphate loading may protect against the progression of RF and possibly bone disease because the harmful effects of acidosis and phosphate loading may be counterbalanced.

The effects of acid on bone

Metabolic acidosis induces calcium efflux from bone and in the process buffers the additional hydrogen ions. Initially metabolic acidosis stimulates physicochemical mineral dissolution and then cell-mediated bone resorption. Acidosis increases activity of the bone resorbing cells, the osteoclasts, and decreases activity of the bone forming cells, the osteoblasts. Osteoblastic immediate early response genes are inhibited as are genes controlling matrix formation.

New perspectives on acid-base balance

This review will cover two main areas of acid-base balance, both of which are attended with much misconception and misunderstanding. One is the external balance of acids and alkali; the other is the contribution of bone buffering in acute and chronic metabolic acidosis.

Acidosis in chronic renal insufficiency

In this article we deal with three aspects of acidosis in patients with chronic renal insufficiency: clinical characteristics, pathophysiology, and therapeutic approach.

Disturbances of acid-base balance and bone disease in end-stage renal disease

Bone disease in patients with chronic renal failure (CRF) is thought to be the consequence primarily of the interplay of several factors, including the serum levels of parathyroid hormone (PTH), vitamin D, calcium, and phosphorus, and exposure to bone toxins such as aluminum or amyloid. Recently the metabolic acidosis noted with CRF has been implicated as an additional factor contributing to the genesis of bone disease. Although metabolic acidosis might be the dominant factor in the cause of bone disease in some instances, more commonly this acid-base disturbance interacts with other factors contributing to the development of bone disease. The following article summarizes the data in support of an important role for metabolic acidosis in the genesis of bone disease in patients with CRF and presents our recommendations for treatment of uremic acidosis to prevent or treat the bone disease.

Renal osteodystrophy

Renal osteodystrophy is a general complication of chronic renal failure and end-stage renal disease. The nature of renal osteodystrophy has changed since osteomalacia due to aluminum intoxication has become less prevalent. Osteomalacia has been replaced by the adynamic bone disorder. Suppression of osteitis fibrosa, calcitrol and control of secondary hyperparathyroidism has been shown to produce the adynamic bone disorder. Thus, many other factors besides secondary hyperparathyroidism and calcitrol deficiency contribute to the pathogenesis of renal osteodystrophy. Some of these factors, according to our current state of knowledge, are discussed in this chapter along with the presentation and treatment of renal osteodystrophy.

A re-evaluation of the urinary parameters of acid production and excretion in patients with chronic renal acidosis

We have studied acid-base balance in 32 patients attending the renal clinic of Mount Sinai Hospital. The parameters of acid-base balance measured included acid production measured as urinary sulfate and organic anions, net acid excretion measured as urinary ammonia plus titratable acid minus bicarbonate, and net GI absorption of alkali measured by a new method utilizing urinary electrolytes. Net GI absorption of alkali by urinary electrolytes measures alkali addition to the body from the GI tract as well as from any other sources, including bone. All patients had a creatinine clearance less than 80 ml/min and they were divided into two groups: those with normal serum bicarbonate (Group 1; N = 12) and those with subnormal serum bicarbonate (Group 2; N = 20). Hydrogen ion balance was -0.6 +/- 9 mEq/day in the first group, while those in the second group had a hydrogen ion balance of +16 +/- 5 mEq/day. A group of 8 normal controls had a hydrogen ion balance of -0.3 +/- 5 mEq/day. When the sum of all cations was compared with the sum of all anions in the urine, a cation gap of exactly the same magnitude as the positive hydrogen ion balance was found in patients with low serum bicarbonate. In conclusion, our data show that patients with decreased GFR and low serum bicarbonate appear to have a significantly positive hydrogen ion balance. However, we believe that the positive hydrogen ion balance is only apparent, but not real for the following reasons.(ABSTRACT TRUNCATED AT 250 WORDS)

Diseases of renal adenosine triphosphatase

Most renal transport is a primary or secondary result of the action of one of three membrane bound ion translocating ATPase pumps. The proximal tubule mechanisms for the reabsorption of salt, volume, organic compounds, phosphate, and most bicarbonate reabsorption depend upon the generation and maintenance of a low intracellular sodium concentration by the basolateral membrane Na-K-ATPase pump. The reabsorption of fluid and salt in the loop of Henle is similarly dependent on the energy provided by Na-K-ATPase activity. Some proximal tubule bicarbonate reabsorption and all distal nephron proton excretion is a product of one of two proton translocating ATPase pumps, either an electrogenic H-ATPase or an electroneutral H-K-ATPase. In this article, the authors review the biochemistry and physiology of pump activity and consider the pathophysiology of proximal and distal renal tubular acidosis, the Fanconi syndrome, and Bartter's syndrome as disorders of ATPase pump function.

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.

Calcium balance, growth and skeletal mineralisation in patients with cystoplasties

Twelve adult female patients and 16 children who had undergone augmentation cystoplasty at least 2 years previously were studied to assess calcium balance and skeletal mineralisation. The serum and 24-h urinary calcium levels were measured and arterial blood gas analysis was performed in all patients. In children, skeletal mineralisation was assessed by serial study of their growth charts, comparing their growth centiles before and after cystoplasty. In adults, skeletal mineralisation was assessed by dual photon absorptiometry (DPA). As previously reported, all patients had a metabolic acidosis, usually with respiratory compensation. Serum and 24-h urinary calcium levels were all within the normal range. Growth charts of the 6 children with colocystoplasties showed an average of 20% reduction in growth potential in 3 of them. Growth charts in the 10 children with ileocystoplasties did not show any change in growth pattern. DPA bone scans in adults were all normal. These results suggest that if calcium is mobilised from bone in patients with a cystoplasty as a result of the metabolic acidosis, then it is reabsorbed from the bladder by the gut segment after an ileocystoplasty; colonic segments are less efficient than ileal segments, however, so that after a colocystoplasty skeletal demineralisation or a reduction in growth potential is more likely.

H(+)-stimulated release of prostaglandin E2 and cyclic adenosine 3',5'-monophosphoric acid and their relationship to bone resorption in neonatal mouse calvaria cultures

The addition of protons to the medium of neonatal mouse calvaria cultures stimulated bone resorption and release of calcium into the medium. In addition, added protons significantly increased the release of prostaglandin E2 (PGE2) and cyclic adenosine 3',5'-monophosphoric acid (cAMP) from the bones. Indomethacin significantly inhibited the release of calcium, PGE2 and cAMP from proton-treated cultures. The positive control, parathyroid hormone (PTH)-treated cultures, also gave rise to bone resorption and calcium release into the medium. However, unlike the addition of protons, the addition of PTH did not stimulate PGE2 release nor did indomethacin inhibit calcium release from PTH-treated cultures. In addition, indomethacin only slightly inhibited cAMP release from PTH-treated cultures, as compared to the marked inhibition by indomethacin of cAMP release from proton-treated cultures. These findings indicate that bone resorption due to added protons is dependent on both PGE2 and cAMP production, whereas bone resorption due to PTH only involves cAMP production.

Trabecular bone remodeling and bone mineral density in the adult cat during chronic dietary acidification with ammonium chloride

Ammonium chloride (NH4Cl) is used as a urinary acidifier in the treatment and prevention of feline urologic syndrome. It is reported to cause alterations in calcium and bone metabolism in humans, dogs, and rats. Adult cats with normal renal function were fed 1.5% NH4Cl for 6 months to study the effects of chronic dietary acidification on trabecular bone remodeling of the iliac crest and bone mineral density (BMD) of lumbar vertebral trabecular bone and femoral cortex. Histomorphometric analyses of iliac crest biopsies were performed before and after treatment. Static and dynamic parameters of bone resorption and formation were determined. Single-energy quantitative computed tomography (SEQCT) was used to measure lumbar trabecular and femoral cortical BMD. There were no significant treatment effects in iliac crest trabecular bone remodeling or BMD of the vertebrae and femora. Bone remodeling activity decreased with time in both acidotic and control cats. Vertebral BMD increased with time in both groups of cats, whereas no change was seen in the femora. Thus, chronic dietary acidification for 6 months with therapeutic levels of NH4Cl produced no significant changes in trabecular bone remodeling or bone mineral density in adult cats.

Bisphosphonates and extrarenal acid buffering capacity

Both the intracellular compartment and bone mineral are supposed to play a role in acid-base balance by contributing to the extrarenal acid buffering capacity. Bisphosphonates could affect extrarenal acid buffering capacity by interfering with the formation and/or dissolution of bone mineral. In the present study, rats were pretreated with either 1-hydroxyethylidene-1, 1-bisphosphonate (HEBP, 10 mg/kg.day sc), with prevailing inhibitory action on bone mineral formation, or dichloromethylene bisphosphonate (Cl2MBP, 10 mg p/kg.day sc) with prevailing action on bone resorption, or NaCl injections (controls) for 7 days. In intact rats, blood acid-base variables were influenced by neither HEBP, nor Cl2MBP. Two hours after nephrectomy and before acute acid loading, HEBP-but not Cl2MBP-pretreated rats displayed a significant increase in both blood HCO3- and PCO2. After HCl infusion (2.5 mEq/kg), the relative decrement in blood HCO3- (difference in blood HCO3- before and after acid loading) was transiently more important in the two bisphosphonate pretreated groups than in controls. After a 24 hour fasting period, nephrectomized animals pretreated with Cl2MBP displayed significantly lower blood HCO3- and pH values than controls or HEBP-pretreated rats. These results suggest that bisphosphonates influence extrarenal buffering capacity according to their prevailing inhibitory action on either bone mineral formation and/or dissolution. These compounds could interfere with the release rate of bone proton buffers. However, in the presence of normal renal function, this effect does not disturb the blood acid-base equilibrium.

H+ stimulation of cell-mediated bone resorption in tissue culture

The addition of protons in the form of hydrochloric acid (10.5, 17.2, or 26.6 meq/l resulting in an initial media pH of 7.28, 7.15, and 6.94, respectively) to neonatal mouse calvaria maintained in a chemically defined medium in tissue culture for 1 wk increased calcium release in a dose-response fashion. The same amounts of protons added to the media of devitalized calvaria caused no increase in calcium release into the medium. The net cell-mediated calcium release resulting from the addition of 26.6 meq/l of protons amounted to approximately 50% of the initial calvarial calcium content. Hydroxyproline determinations revealed that active resorption was taking place, wherein both mineral and organic matrix are removed simultaneously. Histological examination of the extensively resorbed calvaria demonstrated the presence of numerous osteoclasts in different stages of bone destruction. The addition of indomethacin (100 ng/ml) strongly inhibited the increase in calcium release by added protons, suggesting that prostaglandin synthesis is involved in the phenomenon. The addition of thyrocalcitonin also inhibited proton-induced calcium release, providing additional evidence that the calcium release from cultures exposed to added protons involved osteoclastic activity.

The urine anion gap: a clinically useful index of ammonium excretion

In patients with a normal plasma anion gap type of metabolic acidosis, knowledge of the rate of ammonium excretion can provide valuable information to determine if there is a renal cause for the disorder. Unfortunately, few hospital biochemistry laboratories offer routine determination of the urine ammonium concentration. Data are presented that demonstrate a direct linear relationship between the urine anion gap (Na+ + K+ - Cl-) and the urine ammonium concentration. In a 24-hour urine collection, the relationship is urine ammonium equals -0.8 (urine anion gap) +82 (r = 0.97 p less than 0.01). The applications of this index of ammonium excretion are discussed.

Serum electrolyte patterns in end-stage renal disease

The charts of 70 successive patients presenting for dialysis therapy for end-stage renal disease (ESRD) were evaluated for their serum electrolyte values. The "classical" pattern of low total CO2 (tCO2), elevated anion gap ("delta"), and normal chloride was found in a minority of patients (14 of 70, or 20%). Hyperchloremia was noted in 46%; in 21 patients (30%), this was associated with a normal delta and in 11 (16%), hyperchloremia was accompanied by an elevated delta. Fourteen patients (20%), most with diabetic nephropathy, had normal serum electrolytes. Patients with chronic glomerulonephritis had a hyperchloremic pattern as often as not, and two of four patients with interstitial nephritis demonstrated hyperchloremia without an elevated delta. We conclude that the previously held thesis that hyperchloremia is a rare or absent finding by the time renal failure progresses to ESRD is no longer tenable. Furthermore, a significant minority of ESRD patients may require the initiation of dialysis at a time when their serum electrolytes are still normal.

The renal response to chronic mineral acid feeding: a re-examination of the role of systemic pH

It has been widely held that systemic acidemia represents the proximate event signaling the kidney to elicit its acidification response to chronic metabolic acidosis. However, a previous study from this laboratory has cast serious doubt on the validity of this conventional viewpoint. When a large acid load (7 mEq/kg/day) was fed chronically to dogs as HCl, H2SO4 or HNO3, net acid excretion increased similarly in all three groups of animals despite wide variability in the prevailing systemic acid-base composition. Marked or moderate hypobicarbonatemia and acidemia were observed in the HCl- or H2SO4-fed animals respectively, but strikingly, plasma [HCO3-] and pH did not change significantly from the control in the HNO3-fed animals. That study concluded that the renal response to chronic mineral acid feeding appears to be triggered, not by acidemia, but by the interplay of sodium delivery to and sodium avidity of the distal nephron as modulated by the reabsorbability of the "acid" anion. We have re-examined the above provocative conclusion in the light of the observation that the only evidence for a dissociation of the renal response from systemic acidemia in that study was derived from preprandial (8:00 a.m.) blood samples obtained some 23 hr after the ingestion of the daily acid load (administered at 9:00 a.m.). We investigated the diurnal variation of plasma acid-base composition in two groups of dogs fed chronically a large acid load (7 mEq/kg/day) as either HCl or HNO3. Both groups exhibited significant diurnal oscillations of plasma acid-base composition.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbon dioxide removal in acetate hemodialysis: effects on acid base balance

Studies were performed in patients on maintenance acetate hemodialysis to assess the quantity and processes involved in the removal of carbon dioxide (CO2) during the treatment. For this purpose the CO2 losses from whole blood were evaluated in vivo using a mass balance technique. The data demonstrated that the CO2 recovered in the dialysate exceeded the amount calculated to have left the blood in the same period. This observation suggested that CO2 may be generated by the blood cells as they go through the dialyzer. In vitro studies confirmed this observation and established that the uremic blood cells can generate CO2 when exposed to a low PCO2 and/or HCO3. The net effect of this CO2 generation may be hydrogen ion gain by the patient. The contribution of ultrafiltration to the losses of CO2 depends on the volume of ultrafiltrate and the plasma HCO3 concentration. The dialysance of total CO2 was found to be equal to that of urea nitrogen. Treatments with high urea dialysance may interfere with the acute and chronic correction of the acid base balance in these patients. The data presented suggest that multiple factors related to the removal of CO2 during acetate dialysis may be responsible in part for the low plasma bicarbonate observed in patients on chronic maintenance hemodialysis.

Effect of diet on plasma acid-base composition in normal humans

Steady-state plasma and urine acid-base composition was assessed in 19 studies of 16 normal subjects who ingested constant amounts of one of three diets that resulted in different rates of endogenous noncarbonic acid production (EAP) within the normal range. Renal net acid excretion (NAE) was used to quantify EAP since the two variables are positively correlated in normal subjects. A significant positive correlation was observed between plasma [H+] and plasma PCO2, and between plasma [HCO3-] and plasma PCO2, among the subjects. Multiple correlation analysis revealed a significant interrelationship among plasma [H+], plasma PCO2, and NAE (r = 0.71, P less than 0.001), and among plasma [HCO3-], plasma PCO2, and NAE (r = 0.77, P less than 0.001). The partial correlation coefficients indicated a significant positive correlation between plasma [H+] and NAE, and a significant negative correlation between plasma [HCO3-] and NAE, when plasma PCO2 was held constant. These findings indicate that two factors influence the level at which plasma [H+] is maintained in normal subjects: (1) the steady-state rate of endogenous noncarbonic acid production, and (2) the setpoint at which plasma PCO2 is regulated by the respiratory system. Plasma [HCO3-] is also co-determined by these two factors. In disease states, therefore, both factors must be known before a disturbance in acid-base homeostasis can be excluded.

Acid-base physiology in uremia

Chronic renal failure represents the most common disorder responsible for chronic stable metabolic acidosis. This type of metabolic acidosis has been characterized as "hyperchloremic" in pre-end-stage disease and the "anion-gap" form as the GFR falls below 20 ml/min. The early hyperchloremic, hyperkalemic variety may result from disease of the juxtaglomerular apparatus, a distal acidification defect, or volume depletion. The anion-gap acidosis of advanced renal disease occurs as a result of the inability of the diminished nephron mass to keep pace with the metabolic acid load which depletes extracellular fluid bicarbonate. Total ammonium excretion diminishes despite an adaptive increase in ammonia production per nephron. The observation that the serum bicarbonate rarely falls below 15 mEq/L and the anion gap stays below 20 mEq/L despite positive hydrogen ion balance attests to the important role of extrarenal buffers. Bone buffers, primarily calcium carbonate, titrate a portion of the excess hydrogen ions at the expense of progressive loss of bone salts. Parathyroid hormone (PTH) appears to be involved in the control of bone buffering capacity. Both PTH-dependent and PTH-independent mechanisms must therefore be considered. PTH mediates bone buffering capacity by activating intracellular shifts of calcium, phosphorus, and carbonate or by stimulation of bone carbonic anhydrase. A direct effect of pH on bone mineral mobilization has been demonstrated. Adequate alkali therapy to maintain serum bicarbonate levels of 20-22 mEq/L may prevent bone dissolution and minimize risk of volume overload.

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.

Familial absorptive hypercalciuria and renal tubular acidosis

Hypercalciuria was considered as a secondary condition when associated with familial renal tubular acidosis. Later studies suggested that hypercalciuria could lead to renal tubular acidosis and nephrocalcinosis. Selected members of a family spanning five generations were studied. Renal tubular acidosis was present in eight subjects in three consecutive generations. Increased 24-hour urinary calcium excretion was present in nine subjects in three consecutive generations, alone in the younger generation, and in combination with renal tubular acidosis and nephrocalcinosis in the older generation. Calcium loading tests showed the absorptive nature of hypercalciuria in nine of 18 subjects studied. This report suggests that in this family the absorptive hypercalciuria is an autosomal dominant genetic defect with complete penetrance and variable expressivity which leads to renal tubular acidosis and nephrocalcinosis.