Role of acidosis-induced increases in calcium on PTH secretion in acute metabolic and respiratory acidosis in the dog

The Role of the Endocrine System in the Regulation of Acid-Base Balance by the Kidney and the Progression of Chronic Kidney Disease

Systemic acid-base status is primarily determined by the interplay of net acid production (NEAP) arising from metabolism of ingested food stuffs, buffering of NEAP in tissues, generation of bicarbonate by the kidney, and capture of any bicarbonate filtered by the kidney. In chronic kidney disease (CKD), acid retention may occur when dietary acid production is not balanced by bicarbonate generation by the diseased kidney. Hormones including aldosterone, angiotensin II, endothelin, PTH, glucocorticoids, insulin, thyroid hormone, and growth hormone can affect acid-base balance in different ways. The levels of some hormones such as aldosterone, angiotensin II and endothelin are increased with acid accumulation and contribute to an adaptive increase in renal acid excretion and bicarbonate generation. However, the persistent elevated levels of these hormones can damage the kidney and accelerate progression of CKD. Measures to slow the progression of CKD have included administration of medications which inhibit the production or action of deleterious hormones. However, since metabolic acidosis accompanying CKD stimulates the secretion of several of these hormones, treatment of CKD should also include administration of base to correct the metabolic acidosis.

Carbon dioxide's direct impact on down-regulating the human species

Much is said about carbon dioxide (CO2) affecting climate; rising global temperatures, rising sea levels, and stagnating ocean currents being often cited results. Less is said about what CO2 does directly to human physiology should anthropogenic trends not abate. Past mass extinctions have been correlated to airborne CO2 levels rapidly rising above 1000 ppm (Ward, 2007); a value that may be seen by the end of this century. This study proposes as humankind confronts climate changes brought on by rising atmospheric CO2 concentration, as levels rise over 1000 ppm humans may also be inundated by the direct effects of CO2. While this has already been proposed by others, this study goes on to introduce a straight forward model for helping quantify the impact of airborne CO2 on human physiology which shows the onset of hypercapnic bloodstream pH levels in humans begin to appear when atmospheric CO2 levels approach 3000 ppm. However, upon examination of data from past submarine studies, a physiological response may occur in humans at much lower atmospheric CO2 levels due to a slow buildup of CO2 in the body over time. A casual link between atmospheric CO2 levels and the calcium balance in the human body is established providing rationale for the possibility of a greater occurrence of vascular calcification and concurrent bone demineralization in the greater general population when atmospheric CO2 levels rise. Noted is the likelihood of neurological effects at CO2 levels around 1000 ppm is suggested by several past studies. Note is also made of other organisms such as fish having much lower arterial pressures than humans making them more vulnerable to environmental CO2 changes as found by other studies. This study concludes CO2's direct impact to the human physiology as well as other life is not as benign as many like to suggest, and beyond climate change, appears as a mechanism for undermining human sustainability deserving closer scrutiny, and greater discussion.

Trauma-induced disturbances in ionized calcium levels correlate parabolically with coagulopathy, transfusion, and mortality: a multicentre cohort analysis from the TraumaRegister DGU®

BACKGROUND

To which extent trauma- induced disturbances in ionized calcium (iCa2+) levels have a linear relationship with adverse outcomes remains controversial. The goal of this study was to determine the association between the distribution and accompanying characteristics of transfusion-independent iCa2+ levels versus outcome in a large cohort of major trauma patients upon arrival at the emergency department.

METHODS

A retrospective observational analysis of the TraumaRegister DGU® (2015-2019) was performed. Adult major trauma patients with direct admission to a European trauma centre were selected as the study cohort. Mortality at 6 h and 24 h, in-hospital mortality, coagulopathy, and need for transfusion were considered as relevant outcome parameters. The distribution of iCa2+ levels upon arrival at the emergency department was calculated in relation to these outcome parameters. Multivariable logistic regression analysis was performed to determine independent associations.

RESULTS

In the TraumaRegister DGU® 30 183 adult major trauma patients were found eligible for inclusion. iCa2+ disturbances affected 16.4% of patients, with hypocalcemia (< 1.10 mmol/l) being more frequent (13.2%) compared to hypercalcemia (≥ 1.30 mmol/l, 3.2%). Patients with hypo- and hypercalcemia were both more likely (P < .001) to have severe injury, shock, acidosis, coagulopathy, transfusion requirement, and haemorrhage as cause of death. Moreover, both groups had significant lower survival rates. All these findings were most distinct in hypercalcemic patients. When adjusting for potential confounders, mortality at 6 h was independently associated with iCa2+ < 0.90 mmol/L (OR 2.69, 95% CI 1.67-4.34; P < .001), iCa2+ 1.30-1.39 mmol/L (OR 1.56, 95% CI 1.04-2.32, P = 0.030), and iCa2+  ≥ 1.40 mmol/L (OR 2.87, 95% CI 1.57-5.26; P < .001). Moreover, an independent relationship was determined for iCa2+ 1.00-1.09 mmol/L with mortality at 24 h (OR 1.25, 95% CI 1.05-1.48; P = .0011), and with in-hospital mortality (OR 1.29, 95% CI 1.13-1.47; P < .001). Both hypocalcemia < 1.10 mmol/L and hypercalcemia ≥ 1.30 mmol/L had an independent association with coagulopathy and transfusion.

CONCLUSIONS

Transfusion-independent iCa2+ levels in major trauma patients upon arrival at the emergency department have a parabolic relationship with coagulopathy, need for transfusion, and mortality. Further research is needed to confirm whether iCa2+ levels change dynamically and are more a reflection of severity of injury and accompanying physiological derangements, rather than an individual parameter that needs to be corrected as such.

Bone metabolism during strict head-down tilt bed rest and exposure to elevated levels of ambient CO2

Astronauts on the International Space Station are exposed to levels of atmospheric carbon dioxide (CO₂) above typical terrestrial levels. We explored the possibility that increased levels of ambient CO₂ further stimulate bone resorption during bed rest. We report here data from 2 ground-based spaceflight analog studies in which 12 male and 7 female subjects were placed in a strict 6° head-down tilt (HDT) position for either 30 days at 0.5% ambient CO₂ or 60 days with nominal environmental exposure to CO₂. Bone mineral density (BMD) and bone mineral content (BMC) were determined using dual-energy X-ray absorptiometry (DXA). Blood and urine were collected before and after HDT for biochemical analysis. No change was detected in either BMD or BMC, as expected given the study duration. Bone resorption markers increased after bed rest as expected; however, elevated CO₂ had no additive effect. Elevated CO₂ did not affect concentrations of minerals in serum and urine. Serum parathyroid hormone and 1,25-dihydroxyvitamin D were both reduced after bed rest, likely secondary to calcium efflux from bone. In summary, exposure to 0.5% CO₂ for 30 days did not exacerbate the typical bone resorption response observed after HDT bed rest. Furthermore, results from these strict HDT studies were similar to data from previous bed rest studies, confirming that strict 30-60 days of HDT can be used to evaluate changes in bone metabolism. This is valuable in the continuing effort to develop and refine efficacious countermeasure protocols to mitigate bone loss during spaceflight in low-Earth orbit and beyond.

Oral Acid Load Down-Regulates Fibroblast Growth Factor 23

Increased dietary acid load has a negative impact on health, particularly when renal function is compromised. Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that is elevated during renal failure. The relationship between metabolic acidosis and FGF23 remains unclear. To investigate the effect of dietary acid load on circulating levels of FGF23, rats with normal renal function and with a graded reduction in renal mass (1/2 Nx and 5/6 Nx) received oral NH4Cl for 1 month. Acid intake resulted in a consistent decrease of plasma FGF23 concentrations in all study groups when compared with their non-acidotic control: 239.3 ± 13.5 vs. 295.0 ± 15.8 pg/mL (intact), 346.4 ± 19.7 vs. 522.6 ± 29.3 pg/mL (1/2 Nx) and 988.0 ± 125.5 vs. 2549.4 ± 469.7 pg/mL (5/6 Nx). Acidosis also decreased plasma PTH in all groups, 96.5 ± 22.3 vs. 107.3 ± 19.1 pg/mL, 113.1 ± 17.3 vs. 185.8 ± 22.2 pg/mL and 504.9 ± 75.7 vs. 1255.4 ± 181.1 pg/mL. FGF23 showed a strong positive correlation with PTH (r = 0.877, p < 0.0001) and further studies demonstrated that acidosis did not influence plasma FGF23 concentrations in parathyroidectomized rats, 190.0 ± 31.6 vs. 215 ± 25.6 pg/mL. In conclusion, plasma concentrations of FGF23 are consistently decreased in rats with metabolic acidosis secondary to increased acid intake, both in animals with intact renal function and with decreased renal function. The in vivo effect of metabolic acidosis on FGF23 appears to be related to the simultaneous decrease in PTH.

Acute catabolic bone metabolism response to exercise in young and older adults: A narrative review

Exercise is recommended for cardiometabolic benefits and to preserve or improve bone health, especially for older adults at increased risk of fracture. However, exercise interventions have modest benefits on areal bone mineral density (aBMD), and exercise can lead to bone loss in young athletes under certain conditions. In this narrative review, we discuss evidence for a disruption in calcium homeostasis during exercise that may diminish the skeletal benefits of exercise. Topics include 1) a general overview of the effects of exercise on aBMD; 2) discussion of the exercise-induced disruption in calcium homeostasis; 3) factors that influence the magnitude of the exercise-induced disruption in calcium homeostasis, including age, sex, and exercise mode, intensity, and duration; 4) oral calcium supplementation to minimize the exercise-induced disruption in calcium homeostasis; and 5) potential for exercise-induced increase in parathyroid hormone to be both catabolic and anabolic to bone.

Environmental Factors That Affect Parathyroid Hormone and Calcitonin Levels

Calciotropic hormones, parathyroid hormone (PTH) and calcitonin are involved in the regulation of bone mineral metabolism and maintenance of calcium and phosphate homeostasis in the body. Therefore, an understanding of environmental and genetic factors influencing PTH and calcitonin levels is crucial. Genetic factors are estimated to account for 60% of variations in PTH levels, while the genetic background of interindividual calcitonin variations has not yet been studied. In this review, we analyzed the literature discussing the influence of environmental factors (lifestyle factors and pollutants) on PTH and calcitonin levels. Among lifestyle factors, smoking, body mass index (BMI), diet, alcohol, and exercise were analyzed; among pollutants, heavy metals and chemicals were analyzed. Lifestyle factors that showed the clearest association with PTH levels were smoking, BMI, exercise, and micronutrients taken from the diet (vitamin D and calcium). Smoking, vitamin D, and calcium intake led to a decrease in PTH levels, while higher BMI and exercise led to an increase in PTH levels. In terms of pollutants, exposure to cadmium led to a decrease in PTH levels, while exposure to lead increased PTH levels. Several studies have investigated the effect of chemicals on PTH levels in humans. Compared to PTH studies, a smaller number of studies analyzed the influence of environmental factors on calcitonin levels, which gives great variability in results. Only a few studies have analyzed the influence of pollutants on calcitonin levels in humans. The lifestyle factor with the clearest relationship with calcitonin was smoking (smokers had increased calcitonin levels). Given the importance of PTH and calcitonin in maintaining calcium and phosphate homeostasis and bone mineral metabolism, additional studies on the influence of environmental factors that could affect PTH and calcitonin levels are crucial.

Maintaining serum ionized calcium during brisk walking attenuates the increase in bone resorption in older adults

BACKGROUND

Endurance exercise can cause a decrease in serum ionized calcium (iCa) and increases in parathyroid hormone (PTH) and bone resorption, reflected by serum carboxy-terminal collagen crosslinks (CTX). We developed a calcium clamp to prevent the decrease in iCa during exercise, which attenuated increases in PTH and CTX during vigorous cycling in young men. The goal was to determine whether this occurs in older adults during brisk walking.

METHODS

Twelve older adults (6 men, 6 women) performed two identical 60-min treadmill walking bouts with Ca gluconate or half-normal saline infusion. Blood sampling for iCa, total calcium (tCa), phosphate (P), PTH, and CTX, occurred before, during, and for 4 h after exercise.

RESULTS

iCa decreased during exercise with the saline infusion (p = 0.04) and this provoked increases in PTH and CTX (both p < 0.01). The Ca clamp prevented the decrease in serum iCa during exercise and attenuated the PTH and CTX responses.

CONCLUSIONS

Preventing the exercise-induced decrease in iCa markedly attenuated the increases in PTH and CTX. The cause of the decrease in iCa during exercise remains unclear, but the increases in PTH and CTX are likely counter-regulatory responses to defend serum iCa. This contention is supported by previous observations that the disruption of Ca homeostasis during exercise occurs regardless of training status. It will be important to establish whether this acute catabolic effect of exercise diminishes the potential chronic anabolic effects of exercise on bone.

Spontaneous alteration of blood pH by a bicarbonate buffer system during experimental hypercalcaemia in cows

Introduction

Maintaining mineral homeostasis as well as the secretion and metabolism of mineralotropic hormones is important for healthy of periparturient dairy cows. To increase the activity of mineralotropic hormones, blood pH can be adjusted. The purpose of this study was to investigate changes in blood pH and the mechanism of action of this change in induced hypercalcaemic cows.

Material and Methods

Six non-lactating Holstein cows were used in a 2 × 2 crossover design. To induce hypercalcaemia, calcium borogluconate was administered subcutaneously to experimental cows and normal saline was administered subcutaneously to control cows. Blood and urine samples were collected serially after administration. Whole blood without any anticoagulant was processed with a portable blood gas analyser. Plasma concentration and urinary excretion of calcium were measured.

Results

In hypercalcaemic cows, both blood and urine calcium levels were significantly increased at 8 h compared to those at 0 h (P < 0.05), and a spontaneous increase in blood pH was also observed. The calcium concentration in plasma was highest at 2 h after administration (3.02 ± 0.27 mmol/L). The change in pH correlated with that in bicarbonate (r = 0.781, P < 0.001) rather than that in partial pressure of CO₂ (r = 0.085, P = 0.424).

Conclusion

Hypercalcaemia induced a spontaneous change in blood pH through the bicarbonate buffer system and this system may be a maintainer of calcium homeostasis.

Dermal Calcium Loss Is Not the Primary Determinant of Parathyroid Hormone Secretion during Exercise

INTRODUCTION

Exercise can cause a decrease in serum ionized calcium (iCa) concentration, which stimulates parathyroid hormone (PTH) secretion and activates bone resorption. We postulated that dermal Ca loss during cycling exercise is the major determinant of the serum iCa, PTH, and bone resorption (C-terminal telopeptide of type 1 collagen [CTX]) responses.

METHODS

To investigate this, women (n = 13) and men (n = 12) age 18 to 45 yr performed the same exercise bout under cool (18°C) and warm (26°C) conditions. Exercise was 60 min of cycling at ~75% of peak aerobic power. Sweat samples were obtained during exercise using a skin patch method, and blood samples were obtained before and during exercise and during 60 min of recovery.

RESULTS

Sweat volume and estimated sweat Ca loss were 50% higher for the warm condition than the cool condition. Despite this, there were no differences between thermal conditions in the changes (mean, 95% confidence interval [95% CI]) in iCa (cool, -0.07 mg·dL; 95% CI, -0.16 to 0.03); warm, -0.07 mg·dL; 95% CI, -0.20 to 0.05), PTH (cool, 34.4 pg·mL; 95% CI, 23.6-45.2; warm: 35.8 pg·mL; 95% CI, 22.4-49.1), or CTX (cool, 0.11 ng·mL; 95% CI, 0.08-0.13; warm, 0.15 ng·mL; 95% CI, 0.11-0.18). Adjusting for exercise-related shifts in plasma volume revealed a marked decline in vascular iCa content in the first 15 min of exercise (cool, -0.85 mg·dL; 95% CI, -1.01 to -0.68; warm, -0.85 mg·dL; 95% CI, -1.05 to -0.66), before substantial sweat Ca loss had occurred.

CONCLUSIONS

This indicates that dermal Ca loss was not the primary trigger for the increases in PTH and CTX during exercise. Further research is necessary to understand the causes and consequences of the disruption in Ca homeostasis during exercise and specifically the extravascular shift in iCa.

Acute Decompensated Heart Failure in Patients with Heart Failure with Preserved Ejection Fraction

There are few treatment options for acute decompensated heart failure patients with preserved ejection fraction, but an increasing number of patients with heart failure with preserved ejection fraction. A deeper understanding of the cause, diagnosis, and prognosis of heart failure with preserved ejection fraction may be informative for clinical practice or clinical decision making and therapeutic investigation in the acute care setting.

Sensing Extracellular Calcium - An Insight into the Structure and Function of the Calcium-Sensing Receptor (CaSR)

The calcium-sensing receptor (CaSR) is a G protein-coupled receptor that plays a key role in calcium homeostasis, by sensing free calcium levels in blood and regulating parathyroid hormone secretion in response. The CaSR is highly expressed in parathyroid gland and kidney where its role is well characterised, but also in other tissues where its function remains to be determined. The CaSR can be activated by a variety of endogenous ligands, as well as by synthetic modulators such as Cinacalcet, used in the clinic to treat secondary hyperparathyroidism in patients with chronic kidney disease. The CaSR couples to multiple G proteins, in a tissue-specific manner, activating several signalling pathways and thus regulating diverse intracellular events. The multifaceted nature of this receptor makes it a valuable therapeutic target for calciotropic and non-calciotropic diseases. It is therefore essential to understand the complexity behind the pharmacology, trafficking, and signalling characteristics of this receptor. This review provides an overview of the latest knowledge about the CaSR and discusses future hot topics in this field.

Effects of pH and the plasma or serum concentrations of total calcium, chloride, magnesium, l-lactate, and albumin on the plasma ionized calcium concentration in calves

BACKGROUND

The plasma ionized calcium concentration (cCa2+ ) represents the biologically active form of calcium and is the preferred method for evaluating calcium status in animals. Different pH-corrective equations have been developed for human plasma, but the validity of the equations for bovine plasma is unknown.

HYPOTHESIS

We hypothesized that pH-corrective equations for bovine plasma would be similar to those used for human plasma; cCa2+ was dependent on the plasma concentrations of total calcium (cTCa), chloride (cCl), L-lactate (cLactate), and albumin (cAlbumin); and the in vitro and in vivo cCa2+ -pH relationships would differ.

ANIMALS

Ten healthy calves (in vitro study), 1426 critically ill calves.

METHODS

The in vitro plasma log10 (cCa2+ )-pH relationship was determined by CO2 tonometry of 465 plasma samples. Plasma cCl was altered by equivolume dilution of plasma with 3 electrolyte solutions of different cCl. The in vivo plasma cCa2+ -pH relationship was investigated and validated using clinicopathologic data extracted from the medical records of 950 (model development) and 476 (model validation) critically ill calves.

RESULTS

pH-corrective equations for bovine plasma were similar to those used for human plasma. Plasma cCa2+ increased in vitro with increases in plasma cCl. Plasma cCa2+ in critically ill calves was associated with plasma cTCa, blood pH, plasma cCl, serum cMg, and cL-lactate (R2  = 0.69) but not plasma cAlbumin.

CONCLUSIONS AND CLINICAL IMPORTANCE

Calculation of cCa2+ from cTCa in calf plasma or serum requires adjustment for at least pH and cCl when 1 or both are outside the reference range.

Maintenance of Serum Ionized Calcium During Exercise Attenuates Parathyroid Hormone and Bone Resorption Responses

Exercise can cause a decrease in serum ionized calcium (iCa) and increases in parathyroid hormone (PTH) and bone resorption. We used a novel intravenous iCa clamp technique to determine whether preventing a decline in serum iCa during exercise prevents increases in PTH and carboxy-terminal collagen crosslinks (CTX). Eleven cycling-trained men (aged 18 to 45 years) underwent two identical 60-min cycling bouts with infusion of Ca gluconate or saline. Blood sampling for iCa, total calcium (tCa), PTH, CTX, and procollagen type 1 amino-terminal propeptide (P1NP) occurred before, during, and for 4 hours after exercise; results are presented as unadjusted and adjusted for plasma volume shifts (denoted with subscript ADJ). iCa decreased during exercise with saline infusion (p = 0.01 at 60 min) and this was prevented by Ca infusion (interaction, p < 0.007); there were abrupt decreases in Ca content (iCa_ADJ and tCa_ADJ ) in the first 15 min of exercise under both conditions. PTH and CTX were increased at the end of exercise (both p < 0.01) on the saline day, and markedly attenuated (-65% and -71%; both p < 0.001) by Ca. CTX remained elevated for 4 hours after exercise on the saline day (p < 0.001), despite the return of PTH to baseline by 1 hour after exercise. P1NP increased in response to exercise (p < 0.001), with no difference between conditions, but the increase in P1NP_ADJ was not significant. Results for PTH_ADJ and CTX_ADJ were similar to unadjusted results. These findings demonstrate that bone resorption is stimulated early in exercise to defend serum iCa. Vascular Ca content decreased early in exercise, but neither the reason why this occurred, nor the fate of Ca, are known. The results suggest that the exercise-induced increase in PTH had an acute catabolic effect on bone. Future research should determine whether the increase in PTH generates an anabolic response that occurs more than 4 hours after exercise. © 2018 American Society for Bone and Mineral Research.

Children and adolescents with cystic fibrosis display moderate bone microarchitecture abnormalities: data from high-resolution peripheral quantitative computed tomography

We investigated whether bone microstructure assessed by high-resolution peripheral quantitative tomography (HR-pQCT) could be altered in children and teenagers with cystic fibrosis (CF). In comparison to their healthy counterparts, bone microstructure was mildly affected at the tibial level only.

INTRODUCTION

Cystic fibrosis-related bone disease (CFBD) may alter bone health, ultimately predisposing patients to bone fractures. Our aim was to assess bone microstructure using high-resolution peripheral quantitative tomography (HR-pQCT) in a cohort of children and teenagers with CF in comparison to age-, puberty-, and gender-matched healthy volunteers (HVs).

METHODS

In this single-center, prospective, cross-sectional study, we evaluated the HR-pQCT bone parameters of CF patients and compared them to those of the healthy volunteers.

RESULTS

At a median age of 15.4 [range, 10.5-17.9] years, 37 CF patients (21 boys) with 91% [range, 46-138%] median forced expiratory volume in 1 s were included. At the ultradistal tibia, CF patients had a smaller bone cross-sectional area (579 [range, 399-1087] mm²) than HVs (655 [range, 445-981] mm²) (p = 0.027), related to a decreased trabecular area, without any significant differences for height. No other differences were found (trabecular number, separation, thickness, or distribution) at the radial or tibial levels. Bone structure was different in patients receiving ursodeoxycholic acid and those bearing two F508del mutations.

CONCLUSION

In our cohort of children and teenagers with good nutritional and lung function status, bone microstructure evaluated with HR-pQCT was not severely affected. Minimal microstructure abnormalities observed at the tibial level may be related to the cystic fibrosis transmembrane conductance regulator defect alone; the long-term consequences of such impairment will require further evaluation.

Calcitonin, the forgotten hormone: does it deserve to be forgotten?

Calcitonin is a 32 amino acid hormone secreted by the C-cells of the thyroid gland. Calcitonin has been preserved during the transition from ocean-based life to land dwellers and is phylogenetically older than parathyroid hormone. Calcitonin secretion is stimulated by increases in the serum calcium concentration and calcitonin protects against the development of hypercalcemia. Calcitonin is also stimulated by gastrointestinal hormones such as gastrin. This has led to the unproven hypothesis that postprandial calcitonin stimulation could play a role in the deposition of calcium and phosphate in bone after feeding. However, no bone or other abnormalities have been described in states of calcitonin deficiency or excess except for diarrhea in a few patients with medullary thyroid carcinoma. Calcitonin is known to stimulate renal 1,25 (OH)2 vitamin D (1,25D) production at a site in the proximal tubule different from parathyroid hormone and hypophosphatemia. During pregnancy and lactation, both calcitonin and 1,25D are increased. The increases in calcitonin and 1,25D may be important in the transfer of maternal calcium to the fetus/infant and in the prevention and recovery of maternal bone loss. Calcitonin has an immediate effect on decreasing osteoclast activity and has been used for treatment of hypercalcemia. Recent studies in the calcitonin gene knockout mouse have shown increases in bone mass and bone formation. This last result together with the presence of calcitonin receptors on the osteocyte suggests that calcitonin could possibly affect osteocyte products which affect bone formation. In summary, a precise role for calcitonin remains elusive more than 50 years after its discovery.

Pathophysiologic Changes in Extracellular pH Modulate Parathyroid Calcium-Sensing Receptor Activity and Secretion via a Histidine-Independent Mechanism

The calcium-sensing receptor (CaR) modulates renal calcium reabsorption and parathyroid hormone (PTH) secretion and is involved in the etiology of secondary hyperparathyroidism in CKD. Supraphysiologic changes in extracellular pH (pHo) modulate CaR responsiveness in HEK-293 (CaR-HEK) cells. Therefore, because acidosis and alkalosis are associated with altered PTH secretion in vivo, we examined whether pathophysiologic changes in pHo can significantly alter CaR responsiveness in both heterologous and endogenous expression systems and whether this affects PTH secretion. In both CaR-HEK and isolated bovine parathyroid cells, decreasing pHo from 7.4 to 7.2 rapidly inhibited CaR-induced intracellular calcium (Ca(2+)i) mobilization, whereas raising pHo to 7.6 potentiated responsiveness to extracellular calcium (Ca(2+)o). Similar pHo effects were observed for Ca(2+)o-induced extracellular signal-regulated kinase phosphorylation and actin polymerization and for L-Phe-induced Ca(2+)i mobilization. Intracellular pH was unaffected by acute 0.4-unit pHo changes, and the presence of physiologic albumin concentrations failed to attenuate the pHo-mediated effects. None of the individual point mutations created at histidine or cysteine residues in the extracellular domain of CaR attenuated pHo sensitivity. Finally, pathophysiologic pHo elevation reversibly suppressed PTH secretion from perifused human parathyroid cells, and acidosis transiently increased PTH secretion. Therefore, pathophysiologic pHo changes can modulate CaR responsiveness in HEK-293 and parathyroid cells independently of extracellular histidine residues. Specifically, pathophysiologic acidification inhibits CaR activity, thus permitting PTH secretion, whereas alkalinization potentiates CaR activity to suppress PTH secretion. These findings suggest that acid-base disturbances may affect the CaR-mediated control of parathyroid function and calcium metabolism in vivo.

Metabolic acidosis-induced hypercalcemia in an azotemic patient with primary hyperparathyroidism

A 58-year-old man with Stage 3b chronic kidney disease and primary hyperparathyroidism treated with cinacalcet was admitted for acute cholecystitis. A cholecystostomy tube was placed, estimated glomerular filtration rate decreased, metabolic acidosis developed and ionized calcium increased from 1.33 to 1.76 mM despite cinacalcet administration. A sodium bicarbonate infusion corrected the metabolic acidosis restoring ionized calcium to normal despite no improvement in renal function. The correlation between the increase in serum bicarbonate and decrease in ionized calcium was r = −0.93, P < 0.001. In summary, severe hypercalcemia was attributable to metabolic acidosis increasing calcium efflux from bone while renal failure decreased the capacity to excrete calcium.

The role of exercise intensity in the bone metabolic response to an acute bout of weight-bearing exercise

We compared the effects of exercise intensity (EI) on bone metabolism during and for 4 days after acute, weight-bearing endurance exercise. Ten males [mean ± SD maximum oxygen uptake (Vo(2max)): 56.2 ± 8.1 ml·min(-1)·kg(-1)] completed three counterbalanced 8-day trials. Following three control days, on day 4, subjects completed 60 min of running at 55%, 65%, and 75% Vo(2max). Markers of bone resorption [COOH-terminal telopeptide region of collagen type 1 (β-CTX)] and formation [NH(2)-terminal propeptides of procollagen type 1 (P1NP), osteocalcin (OC), bone-alkaline phosphatase (ALP)], osteoprotegerin (OPG), parathyroid hormone (PTH), albumin-adjusted calcium (ACa), phosphate (PO(4)), and cortisol were measured during and for 3 h after exercise and on four follow-up days (FU1-FU4). At 75% Vo(2max), β-CTX was not significantly increased from baseline by exercise but was higher compared with 55% (17-19%, P < 0.01) and 65% (11-13%, P < 0.05) Vo(2max) in the first hour postexercise. Concentrations were decreased from baseline in all three groups by 39-42% (P < 0.001) at 3 h postexercise but not thereafter. P1NP increased (P < 0.001) during exercise only, while bone-ALP was increased (P < 0.01) at FU3 and FU4, but neither were affected by EI. PTH and cortisol increased (P < 0.001) with exercise at 75% Vo(2max) only and were higher (P < 0.05) than at 55% and 65% Vo(2max) during and immediately after exercise. The increases (P < 0.001) in OPG, ACa, and PO(4) with exercise were not affected by EI. Increasing EI from 55% to 75% Vo(2max) during 60 min of running resulted in higher β-CTX concentrations in the first hour postexercise but had no effect on bone formation markers. Increased bone-ALP concentrations at 3 and 4 days postexercise suggest a beneficial effect of this type of exercise on bone mineralization. The increase in OPG was not influenced by exercise intensity, whereas PTH was increased at 75% Vo(2max) only, which cannot be fully explained by changes in serum calcium or PO(4) concentrations.

Measuring total blood calcium displays a low sensitivity for the diagnosis of hypercalcemia in incident renal transplant recipients

BACKGROUND AND OBJECTIVES

Hypercalcemia is a common complication in renal transplant recipients and has been associated with nephrocalcinosis and poor graft outcome. The performance of total calcium (tCa) in the diagnosis of blood calcium disturbances in renal transplant recipients is unknown.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We compared the ability of total tCa concentration to identify low, normal, or high ionized calcium (iCa) concentration, i.e., the gold standard, in an unselected cohort of 268 renal transplant recipients. All patients were studied 3 and 12 months after successful engraftment.

RESULTS

Hypercalcemia, defined as a iCa >1.29 mmol/L, was present in 58.6 and 44.8% of the patients at months 3 and 12, respectively. tCa concentrations >10.3 mg/dl, conversely, were observed in only 13.1% of the patients. Measuring tCa had a low sensitivity (20.3 and 24.2% at months 3 and 12, respectively) for the diagnosis of hypercalcemia. The agreement (κ coefficient [95% confidence interval]) between tCa concentrations and iCa was poor (month 3: 0.11 [0.05 to 0.17]; month 12: 0.20 [0.11 to 0.30]). The risk for underestimating iCa was increased by a low total bicarbonate concentration. Metabolic acidosis was observed in 48.1 and 37.3% of the patients at months 3 and 12, respectively.

CONCLUSIONS

Total calcium greatly underestimates the diagnosis of hypercalcemia in incident renal transplant recipients. This is mainly explained by the high prevalence of metabolic acidosis in these patients.

Metabolic acidosis inhibits soft tissue calcification in uremic rats

Metabolic acidosis is common in patients with chronic kidney disease, which is known to affect bone metabolism. We examined the effect of metabolic acidosis on the development of vascular and other soft-tissue calcifications in uremic rats treated with calcitriol. Extraskeletal calcification was measured in vivo, in control rats and rats with a remnant kidney model of uremia with or without ammonium chloride-induced acidosis. Soft-tissue calcification was assessed histologically, by measurement of the expression of the sodium-dependent phosphate cotransporter Pit-1 and by quantification of tissue calcium and phosphorus. Calcitriol administration to uremic rats resulted in significant deposition of material positive for von Kossa stain in the aorta, stomach, and kidney, elevated aortic calcium and phosphorus, increased aortic Pit-1 expression, and high mortality. Calcitriol-treated uremic rats with acidosis did not develop aortic or soft-tissue calcification, did not increase aortic Pit-1 expression, and had significantly lower mortality. Additionally, an acidotic environment prevented calcification of vascular smooth muscle cells in vitro. Our study shows that metabolic acidosis inhibits extraskeletal calcification.

Dynamics of parathyroid hormone secretion in health and secondary hyperparathyroidism

This review examines the dynamics of parathyroid hormone secretion in health and in various causes of secondary hyperparathyroidism. Although most studies of parathyroid hormone and calcium have focused on the modification of parathyroid hormone secretion by serum calcium, the relationship between parathyroid hormone and serum calcium is bifunctional because parathyroid hormone also modifies serum calcium. In normal animals and humans, factors such as phosphorus and vitamin D modify the basal parathyroid hormone level and the maximal parathyroid hormone response to hypocalcemia. Certain medications, such as lithium and estrogen, in normal individuals and sustained changes in the serum calcium concentration in hemodialysis patients change the set point of calcium, which reflects the serum calcium concentration at which parathyroid hormone secretion responds. Hypocalcemia increases the basal/maximal parathyroid hormone ratio, a measure of the relative degree of parathyroid hormone stimulation. The phenomenon of hysteresis, defined as a different parathyroid hormone value for the same serum calcium concentration during the induction of and recovery from hypo- and hypercalcemia, is discussed because it provides important insights into factors that affect parathyroid hormone secretion. In three causes of secondary hyperparathyroidism--chronic kidney disease, vitamin D deficiency, and aging--factors that affect the dynamics of parathyroid hormone secretion are evaluated in detail. During recovery from vitamin D deficiency, the maximal parathyroid hormone remains elevated while the basal parathyroid hormone value rapidly becomes normal because of a shift in the set point of calcium. Much remains to be learned about the dynamics of parathyroid hormone secretion in health and secondary hyperparathyroidism.

Chronic metabolic acidosis stimulated transcellular and solvent drag-induced calcium transport in the duodenum of female rats

Chronic metabolic acidosis results in a negative calcium balance as a result of bone resorption and renal calcium loss. However, reports on the changes in intestinal calcium transport have been controversial. The present investigation therefore aimed to study the effects of chronic metabolic acidosis induced by 1.5% NH(4)Cl administration on the three components of duodenal calcium transport, namely, solvent drag-induced, transcellular active, and passive paracellular components, in rats using an in vitro Ussing chamber technique. The relative mRNA expression of genes related to duodenal calcium transport was also determined. We found that 21-day chronic metabolic acidosis stimulated solvent drag-induced and transcellular active duodenal calcium transport but not passive paracellular calcium transport. Our results further demonstrated that an acute direct exposure to serosal acidic pH, in contrast, decreased solvent drag-induced calcium transport in a pH-dependent fashion but had no effect on transcellular active calcium transport. Neither the transepithelial resistance nor duodenal permeability to Na(+), Cl(-), and Ca(2+) via the passive paracellular pathway were altered by chronic metabolic acidosis, suggesting that widening of the tight junction and changes in the charge-selective property of the tight junction did not occur. Thus the enhanced duodenal calcium transport observed in chronic metabolic acidosis could have resulted from a long-term adaptation, possibly at the molecular level. RT-PCR study revealed that chronic metabolic acidosis significantly increased the relative mRNA expression of duodenal genes associated with solvent drag-induced transport, i.e., the beta(1)-subunit of Na(+)-K(+)-ATPase, zonula occludens-1, occludin, and claudin-3, and with transcellular active transport, i.e., transient receptor potential vanilloid family Ca(2+) channels 5 and 6 and plasma membrane Ca(2+)-ATPase isoform 1b. Total plasma calcium and free ionized calcium and magnesium concentrations were also increased, whereas serum parathyroid hormone and 1alpha,25-dihydroxyvitamin D(3) levels were not changed. The results indicated that 21-day chronic metabolic acidosis affected the calcium metabolism in rats partly through enhancing the mRNA expression of crucial duodenal genes involved in calcium absorption, thereby stimulating solvent drag-induced and transcellular active calcium transport in the duodenum.

Effect of changes in ionized calcium concentration in arterial blood and metabolic acidosis on the arterial partial pressure of oxygen in dogs

OBJECTIVE

To evaluate the effects of metabolic acidosis and changes in ionized calcium (Ca2+) concentration on PaO2 in dogs.

ANIMALS

33 anesthetized dogs receiving assisted ventilation.

PROCEDURE

Normal acid-base status was maintained in 8 dogs (group I), and metabolic acidosis was induced in 25 dogs. For 60 minutes, normocalcemia was maintained in group I and 10 other dogs (group II), and 10 dogs were allowed to become hypercalcemic (group III); hypocalcemia was then induced in groups I and II. Groups II and IV (5 dogs) were treated identically except that, at 90 minutes, the latter underwent parathyroidectomy. At intervals, variables including PaO2, Ca2+ concentration, arterial blood pH (pHa), and systolic blood pressure were assessed.

RESULTS

In group II, PaO2 increased from baseline value (96 +/- 2 mm Hg) within 10 minutes (pHa, 7.33 +/- 0.001); at 60 minutes (pHa, 7.21 +/- 0.02), PaO2 was 108 +/- 2 mm Hg. For the same pHa decrease, the PaO2 increase was less in group III. In group I, hypocalcemia caused PaO2 to progressively increase (from 95 +/- 2 mm Hg to 104 +/- 3 mm Hg), which correlated (r = -0.66) significantly with a decrease in systolic blood pressure (from 156 +/- 9 mm Hg to 118 +/- 10 mm Hg). Parathyroidectomy did not alter PaO2 values.

CONCLUSIONS AND CLINICAL RELEVANCE

Induction of hypocalcemia and metabolic acidosis each increased PaO2 in anesthetized dogs, whereas acidosis-induced hypercalcemia attenuated that increase. In anesthetized dogs, development of metabolic acidosis or hypocalcemia is likely to affect ventilatory control.

Regulation of parathyroid function in chronic renal failure

This review summarizes the factors involved in the development of hyperparathyroidism secondary (2nd-HPTH) to chronic kidney disease (CKD). Calcium and calcitriol act on their respective specific parathyroid cell receptors to inhibit parathyroid function. As well as the well-known effect of calcium and calcitriol on parathyroid cell function, there is experimental work that demonstrates that phosphate, changes in pH, PTHrP, estrogens, and some cytokines also have an effect on PTH secretion. These factors are relevant in patients with chronic kidney disease. However, low calcium, vitamin D deficiency, and an accumulation of phosphate due to the decrease in renal function are the main pathogenic factors involved in the pathogenesis of 2nd-HPTH in CKD patients.