Acute Acid Load Research Articles

Acute, but not chronic, metabolic acidosis disturbs 25-hydroxyvitamin D3 metabolism

AbstractAcute, but not chronic, metabolic acidosis disturbs 25-hydroxyvitamin D3 metabolism. Previous studies in vitamin D deficient animals showing that acidosis may impair the production of 1,25(OH)2D3 are in conflict with studies in humans which have not shown convincing disturbances of 25 OHD3 metabolism during acidosis. We have investigated the effect on renal 25 OHD3 1- and 24-hydroxylase of acid loading for periods of 24hr to 21 days. Acid loading resulted in immediate and sustained decrements in arterial pH and bicarbonate and increments in blood-ionized calcium. Acute (24-hr) acid loading decreased 1- and increased 24-hydroxylase activity. After 6 days of acid loading, no effect on 1-hydroxylase activity was observed and that on 24-hydroxylase activity was reduced. By 21 days the effect of acidosis on 24-hydroxylase activity was no longer observed. The results show that acidosis disturbs 25 OHD3 metabolism in the physiological vitamin D and calcium replete state as well as in the vitamin D deficient state, but only acutely. Our data are consistent with studies in humans and suggest that, while short-term disturbances of 25 OHD3 metabolism occur early in acidosis, they are transient and may not be causally related to the development of metabolic bone disease during chronic acidosis.L'acidose métabolique aiguë et non chronique perturbe le métabolisme de la 25-hydroxyvitamine D3. Des études antérieures chez des animaux déficients en vitamine D indiquant que l'acidose pourrait altérer la production de 1,25(OH)2D3 sont en conflit avec des études chez les humains, qui n'ont pas montré d'anomalie convaincante du métabolisme de la 25 OHD3 pendant l'acidose. Nous avons étudié l'effet sur la 25 OHD3 1- et 24-hydroxylase rénale d'une charge acide pendant des périodes de 24 heures à 21 jours. La charge acide a entraîné des diminutions immédiates et prolongées du pH artériel et des bicarbonates, et des augmentations du calcium ionisé sanguin. Une charge aiguë (24 heures) a diminué l'activité de la 1- et augmenté celle de la 24-hydroxylase. Après 6 jours de charge acide, aucun effet sur l'activité 1-hydroxylase n'a été observée, et celui sur l'activité 24-hydroxylase était réduit. Au bout de 21 jours, l'effet de l'acidose sur l'activité 24-hydroxylase n'était plus visible. Ces résultats montrent que l'acidose altère le métabolisme de la 25 OHD3 lors d'un état physiologique en vitamine D et en réplétion calcique, comme dans un état de déficience vitaminique D, mais seulement de façon aiguë. Nos données sont compatibles avec les études chez les humains et suggèrent que bien que des altérations du métabolisme de la 25 OHD3 à court terme puissent se produire tôt en acidose, celles-ci sont transitoires, et ne sont probablement pas reliées au développement d'une ostéopathie métabolique pendant l'acidose chronique.

Steroid modulation of response of plasma bicarbonate concentration to NH4Cl loading: gamma distribution analysis.

Studies have been carried out in awake chronically cannulated adrenalectomized rats subjected to steroid replacement protocols. Our objective was to determine the effects of mineralocorticoid or glucocorticoid or combined replacement on acid-base composition of the extracellular fluid compartment in response to an acute ammonium chloride acid load. We obtained six blood samples from each animal over a 26-h period and, with the help of a specially derived mathematical model, we were able to obtain a continuous dynamic assessment of changes of plasma bicarbonate concentration during induction and recovery of the metabolic acidosis. Assessment of minimum values for plasma bicarbonate concentration, time to reach the minima, and time for 90% recovery showed that dexamethasone did not exert as much protection as aldosterone. The approach we have used, in a sense being a continuous display of plasma bicarbonate concentration, should be useful in further exploring the nature of the interrelations between acid production, net acid excretion, and the resultant plasma acid-base composition.

Na:H exchange and the primary H pump in the proximal tubule.

Cell pH (pHi) transients were monitored at 5-min intervals with the weak acid 5,5-[14C]dimethyloxazolidine-2,4-dione and membrane potentials were estimated from the distribution of [3H]triphenylmethylphosphonium ion in separated proximal tubules (SPT) or rabbit kidney. SPT suspensions were gassed at 37 degrees C first with 5% CO2 and then with 15% CO2. Under normal conditions, pHi rapidly fell during initial 15% CO2 acid loading and then recovered within 20 min. In the presence of 10(-3) M ouabain, which eliminated Na:H exchange as a driving force for H+ secretion, initial cell acidification was still followed by cell pH recovery, which demonstrated a sodium gradient-independent H+ extruding mechanism. In the presence of 10(-3) M ouabain plus 10(-4) M potassium cyanide, there was no pHi recovery following initial cell acidification but, on the contrary, further progressive cell acidification occurred, which is compatible with passive diffusion only of HCO-3 out of the cell. From the cyanide experiments, an apparent permeability coefficient for HCO-3 of the basolateral cell membrane was calculated; this latter result allowed the calculation of rates of passive HCO-3 diffusion and of active H+ extrusion under normal conditions and in the presence of 10(-3) M ouabain. We conclude that in the proximal tubule 1) there is a primary H+ pump additional to Na:H exchange; and 2) this primary H+ pump is responsible for about 25% of active H+ extrusion following acute CO2 cellular acid loading.

Role of PTH, metabolic factors and other hormones in extra-renal acid buffering.

The maintenance of normal acid-base homeostasis is dependent on the renal excretion of acid (as well as the reclamation of bicarbonate) and extra-renal buffering of acid which originates either from normal metabolism or from exogenous sources. The factors influencing renal acid excretion have been well studied. Those events which control the buffering of an acid load are less completely understood (1). Since it is believed that the skeleton is a major source of buffer, events which influence the metabolism of bone may have a profound effect on extra-renal buffering. Fraley and Adler, as well as our own group, have demonstrated that parathyroid hormone (PTH) plays a major role in the buffering of acute acid loads (2,3). More recently we have had the opportunity to study the effects of a number of other hormonal and metabolic factors which influence extra-renal buffering (4). This paper reviews our work; it examines the effect of PTH and other metabolic regulators on extra-renal buffering.

Bone maturation in the vitamin D, phosphate deficient rat and the response to acid loading.

Vitamin D and phosphate deficiency were produced in rats in order (a) to evaluate the degree of bone mineral and matrix maturation using a bromoform/toluene density gradient technique; and (b) to compare the aforementioned bone maturational changes due to vitamin D and phosphate deprivation to those produced with superimposed severe acidosis. Rats were fed a diet deficient in vitamin D and phosphorus (0.2%) from 3 weeks through 7 weeks of age. To examine the additional contribution of dietary calcium, we gave one-half of the animals either a low (0.06%) or high (1.3%) calcium diet. Following the 4 weeks of vitamin D deficiency, one-half of each group was given 1.8% NH4Cl in the drinking water for 4 succeeding days to induce an acute, severe acidosis. The degree of bone maturation was quantitated via bromoformtoulene density gradient fractionation; total mineral and hydroxyproline (collagen) levels were quantitated as well. The vitamin D-deficient rats deprived of adequate dietary phosphate responded by conserving phosphorus, and as a consequence total bone phosphorus levels were maintained within that level for control rats. This conservation was independent of calcium intake but was extremely sensitive to acute acid loading, where a significant reduction in total bone phosphorus was noted. The bone maturational profile obtained from the vitamin D-phosphate deficient rats, however, revealed a significant accumulation of less mature or dense bone collagen and mineral with a corresponding decrease in the most mature or dense moieties. In contrast to the reduction of the total bone phosphorus content by acute acidosis, the skeletal collagen-mineral maturational profile was not significantly affected by the short-term systemic acidosis. The observed retardations in the bone collagen and mineral maturation of the vitamin D-deficient, phosphate-deprived state provide an additional observation which may well relate to the progressive osteopenia documented in states of chronic, mild acidosis.

Lack of effect of amphotericin B on urine-blood pCO2 gradient in spite of urinary acidification defect.

The alkaline urine of normal animals and humans has higher carbon dioxide tension (pCO2) than that of blood. On the other hand, the urine-bloodpCO2 gradient is decreased in some patients and experimental animals having urinary acidification defect. It has been proposed that the urine-bloodpCO2 gradient is dependent on H+ accumulation in the distal nephron to form H2CO3 which is not dehydrated to CO2 until it passes the nephrons. We produced a urinary acidification defect in rats by treating the animals with amphotericin B and studied their ability to excrete acute acid and alkali loads. The amphotericin-treated animals could not decrease their urine pH lower than 5.84, even with a plasma HCO 3 − of 10.0 mEq/l. On the other hand, control animals decreased their urine pH to 5.50 with a plasma HCO 3 − of 18.2 mEq/l. There was no significant differences in urine-bloodpCO2 gradient between amphotericin-treated and control rats. Thus, there was a dissociation between the pH of an acid urine andpCO2 of an alkaline urine in the amphotericin-treated rats. The possible mechanisms of this dissociation along with the mechanisms of urinarypCO2 formation are discussed. It is concluded, that the urine-bloodpCO2 gradient during alkaline infusion is not an accurate index of H+ secretory capability.

An extrarenal role for parathyroid hormone in the disposal of acute acid loads in rats and dogs.

Acid infusion studies were performed in nephrectomized rats and dogs with either intact parathyroid glands (intact) or after thyroparathyroidectomy (thyroparathyroidectomized [TPTX]) to determine the role of parathyroid hormone (PTH) in extrarenal disposal and buffering of acutely administered acid. 29 intact rats given 5 mM/kg HCl and 6 intact dogs given 7 mM/kg HCl developed severe metabolic acidosis but all survived. However, each of 12 TPTX rats and 4 TPTX dogs given the same acid loads died. Intact rats and dogs buffered 39 and 50% of administered acid extracellularly, respectively, whereas extracellular buffering of administered acid was 97 and 78% in TPTX rats and dogs, respectively. 17 TPTX rats and 6 TPTX dogs given synthetic PTH 2 h before acid infusion survived. The blood bicarbonate and extracellular buffering in these animals, measured 2 h after acid infusion, was similar to intact animals. Changes in liver, heart, and skeletal muscle pH determined from [(14)C]5,5-dimethyl-2,4 oxazolidinedione distribution seemed insufficient to account for the increased cell buffering of PTH-replaced animals. Indeed, muscle pH in TPTX dogs given PTH and acid was only 0.06 pH units lower than in control dogs given no acid, suggesting that another tissue, presumably bone, was the target for PTH-mediated increased cell buffering. This conclusion was supported by the observation that PTH did not alter the pH of intact rat diaphragms in vitro. These results indicate that PTH is necessary for the optimal buffering of large, acute acid loads presumably by increasing bone buffering.

Ticrynafen: site of natriuretic activity.

Studies were performed with ticrynafen (tienilic acid) to determine its natriuretic site of action, detailed electrolyte excretion, and its effect on acid excretion and bicarbonate reabsorption. With 500 and 1,000 mg oral doses under conditions of water diuresis, there was a decrease in free water excretion as osmolar clearance increased. During hydropenia, there was no change in free-water reabsorption as osmolar clearance increased. Maximum sodium excretion reached 5.2% of the filtered load. At 500 mg doses, there was no effect on phosphate excretion; with 1,000 mg doses, there was a reduction in phosphate excretion. During bicarbonate infusion, there was no change in the absolute or percent reabsorption of bicarbonate. After chronic administration of ticrynafen, there was no impairment of excretion of al acute acid load. Uric acid excretion increased at least threefold in all studies. The studies indicate that at 500 and 1,000 mg oral doses, ticrynafen has the characteristics of a diuretic which is active in the cortical diluting segment of the distal nephron.

Relation of ammonia excretion adaptation to glutaminase activity in acidotic, subtotalnephrectomized rats

The response of renal ammonia excretion to acidosis was examined in adult rats with reduced renal mass (SNX). Three days after surgical ablation of 70% of renal mass, the activity of renal phosphate-dependent glutaminase (PDG) in SNX rats was 7.7 +/- 1.5 mu moles of ammonia/100 mg of protein min or approximately 50% of the activity in normal rats (14.5 +/- 2.6 mu moles of ammonia/100 mg of protein min), but enhanced ammonia excretion per unit weight was observed in SNX rats (7.2 +/- 0.7 in control vs. 14.6 +/- 3.2 mumoles/g of kidney.hr in SNX rats). The cause (s) of the reduction in the specific activity of PDG (as well as the increase in ammonia excretion) is unknown. The PDG decrease was not due to apparent tissue damage and appeared to be a specific change as the activity of renal succinate dehydrogenase, another mitochondrial inner-membrane enzyme, did not decrease (from the control level) in SNX rats. Ammonia excretion showed no significant response to an acute acid load (ammonium chloride, 5 mmoles/kg of body wt) in SNX rats. Ammonia excretion, however, did adapt to repeated acid-loading (10 mmoles of ammonium chloride per kg of body wt per day for 3 days); ammonia excretion increased more than two-fold by third day of treatment. This adaptive response was associated with a two-fold rise in renal PDG. Administration of actinomycin D, at a dose which produced no gross toxic signs (100 microgram/kg/day i.p.) inhibited virtually all the increase in both ammonia excretion and PDG activity. The correlation of ammonia excretion and PDG adaptations in acidotic SNX rats was similar to that previously observed in infant rats.

The adaptation of hydrogen ion excretion associated with nephron reduction in post-transplant patients.

Extract: The compensatory growth of the single and transplanted kidney is demonstrated by the improvement of glomerular filtration rate (GFR) of the single kidney to achieve 50–90% of the normal value. The compensatory renal tubular acid excretion is also studied. It is clearly demonstrated that in response to a standard ammonium chloride load (75 mEq/m2) systemic acidosis (tCO2 0.95), i.e., each remaining nephron is excreting acid normally (or supernormally) and is supportive of the “intact nephron” hypothesis. The data show a marked difference in urinary pH during acidosis between the two transplant groups: 50% of the cadaveric allografts have an inability to lower urine pH below 5.4 compared with 10% only of the live-related allografts which show such a defect. This would imply that cadaver allografts are more susceptible to an acute acid load. Speculation: The ischemia sustained at the time of transplantation results in acute tubular damage and proportionate lowering of the glomerular filtration rate which account for the impaired acidification capacity in the kidney allografts. It is also possible that circulatory damage to the glomeruli may occur in the transplantation process. It is even possible that intrarenal circulation may be altered by either transplantation or nephrectomy; although, admittedly, this last possibility seems quite unlikely. It is open to speculation what effect the age difference between the donors and the recipients may have on their different responses to acute metabolic acidosis.

48 Urinary Acid Excretion in the Intact Lamb Fetus

The role of the fetal kidney in regulating acid-base elimination in the intact fetus has not been elucidated. This study was designed to investigate the response of the intact fetus to acute acid loading with hydrochloric acid. The lamb fetus is delivered by Caesarean section onto a warm table adjacent to the mother. The umbilical cord was protected and the fetal pulse rate, blood pressure and body temperature were monitored continuously. The fetal external jugular, carotid artery and both ureters were cannulated. The glomerular filtration rates, urine ammonia, titratable acidity (TA), phosphate, chloride and blood pH, pCO2 and chloride were measured during two 30 minute control periods. 0.3 molar hydrochloric acid was then infused into the fetus at a rate to maintain the fetal blood pH between 6.9 and 7.1. The results during the control (Basal) and test periods are shown below for 7 fetal preparations:The mean basal urine pH was 6.91 and the minimum was 5.97 following acid loading. These studies demonstrate that the fetal kidney is able to increase hydrogen ion excretion significantly in response to acid loading. (SPR)