Équilibre phosphocalcique : régulation et explorations

Phosphocalcic metabolism and the role of vitamin D, vitamin K2, and nattokinase supplementation

Calcium is involved in bone metabolism, regulation of nerve signaling, and release of neurotransmitters. Phosphorus is a structural component of ATP, participates in metabolic energy regulation, and ensures stability to biological membranes and cells. Vitamin D and vitamin K are important for intestinal absorption and renal excretion of calcium and phosphorus. Vitamin D plays a regulatory role in bone formation, carbohydrate metabolism, immune responses, and cardiovascular regulation. Research has linked vitamin D deficiency to the development of diabetes mellitus, hypertension, cancer, and osteoporosis. Vitamin K has been associated with a reduced risk of osteoporosis, cancer, and cardiovascular diseases (due to improved vascular elasticity). This review highlights the importance of vitamins D and K in the metabolism of calcium and phosphorus and explores various molecular mechanisms that help maintain the system's mineral homeostasis. Moreover, the paper reviews the enzyme nattokinase's role in thrombotic prevention due to its fibrinolytic activity.

[Nephrolithiasis: From mechanisms to preventive medical treatment]

Renal lithiasis is a frequent pathology (prevalence ranging from 10 to 12% in France) and a recurrent condition. It is associated with chronic kidney disease and is responsible for 2 to 3% of cases of end-stage renal disease, especially if it is associated with nephrocalcinosis and/or is part of a monogenic disease (1.6% of lithiasis in adults, including 1% of cystinuria). In order to understand the pathophysiology of the nephrolithiasis, the analysis of stones (morphological and by infrared spectrophotometry) as well as a minimal biological evaluation including crystalluria must be carried out. Calcium nephrolithiasis is the most common form (more than 80%). Its preventive medical treatment relies on simple hygienic dietetics: non-alkaline hyperdiuresis greater than 2liters/day, normalization of calcium intakes (1g/day to be distributed over the three meals), restriction of sodium intakes (6g/day) and of protein intakes (0.8-1g/kg of theoretical weight/day), and avoidance of foods rich in oxalate. If there is a hypercalciuria (greater than 0.1mmol/kg of theoretical weight/day with normal calcium intakes), its mechanism should be explored with an oral calcium load test. In the absence of primary hyperparathyroidism, thiazide diuretics can be prescribed, taking care to prevent hypokalemia and iatrogenic hypocitraturia. The treatment of uric acid lithiasis includes alkaline hyperdiuresis (urinary pH 6.2 to 6.8). Allopurinol is only justified if the urinary excretion of uric acid exceeds 4mmol/day. With a well-managed medical treatment, more than 80% of recurrent lithiasis can be stopped, making nephrolithiasis one of the kidney diseases the more accessible to the preventive medical treatment.

[Severe hypercalcemia of unusual cause, looking for the culprit: Case report and review of the literature]

INTRODUCTION

Hypercalcemia is not a rare event and can lead to severe consequences. Its main etiologies are primary hyperparathyroidism and neoplasic conditions. The iatrogenic etiology by vitamin D intoxication is more rarely found.

CASE PRESENTATION

A 76-year-old finish woman comes to the emergency room for chest pain. Her medical history is impossible to specify due to the language barrier and initial confusion. She has severe hypercalcaemia (4.14mmol/L), renal insufficiency, cardiac arrhythmia later complicated by an ischemic cardiac episode. Clinic and biologic examinations initially guided the research towards a hematological and neoplasic pathology. The iatrogenic etiology will be permitted by the contribution of details on its medical history and treatment learnt secondly. She was treated for post-surgical hypoparathyroidism by dihydrotachysterol, a vitamin D derivative. The cessation of substitution, treatment with hydration and biphosphonates allowed the rapid correction of hypercalcemia.

DISCUSSION

Dihydrotachysterol intoxication is a rare etiology of hypercalcemia. Because of the longer half-life of this molecule, the risk of hypercalcemia seems to be greater than with other vitamin D derivatives. This molecule, withdrawn from the French market in 1982, is not detected by the dosage of 25 and 1.25 OH vitamin D.

CONCLUSION

We report an original case of intoxication by dihydrotachysterol. The risk of hypercalcemia encountered with this molecule must be known. The close medical follow-up recommended in case of hypoparathyroidism seems to be particularly necessary in case of supplementation by this molecule.

Trajectories of CKD-MBD biochemical parameters over a 2-year period following diagnosis of secondary hyperparathyroidism: a pharmacoepidemiological study

OBJECTIVES

To define groups of patients according to the changes of biochemical parameters, that is, serum calcium, phosphate and parathyroid hormone (PTH), over a 2-year follow-up period using group-based multi-trajectory modeling (GBMM) among a cohort of dialysis patients with newly diagnosed secondary hyperparathyroidism (SHPT) (ie, PTH≥500 ng/L for the first time) and to compare their patient characteristics and treatments.

DESIGN

Pharmacoepidemiological study.

SETTING

In the 12 dialysis units located in the French region of Lorraine.

PARTICIPANTS

A total of 269 dialysis patients with newly diagnosed SHPT were prospectively included from December 2009 to May 2012 and followed-up for 2 years.

RESULTS

We identified four distinct trajectory groups: 'rapid PTH drop' experiencing a rapid and sharp decrease (over weeks) in PTH level associated with decreasing phosphate level within normal range (n=34; 12.7%), 'gradual PTH decrease' experiencing a gradual and continuous decrease (over months) in PTH level and maintaining phosphate at a middle level throughout the study (n=98; 36.4%), 'slow PTH decrease with high phosphate' experiencing a slow decrease in PTH level associated with a relatively high phosphate level (n=105; 39.0%) and 'uncontrolled SHPT' with high levels of PTH and phosphate throughout the study (n=32; 11.9%). Patients in the 'uncontrolled SHPT' group were significantly (p<0.00001) younger than patients in other groups. Kidney Disease Improving Global Outcomes (KDIGO) targets for PTH, phosphate and calcium were reached simultaneously for 14.9% of patients at baseline and 16.7% at the end of the study. Patients were given cinacalcet more frequently at months 3 and 6 in the 'rapid PTH drop' and at month 24 in the 'uncontrolled SHPT' groups.

CONCLUSIONS

Over 2 years following a new SHPT diagnosis, a younger age and a higher rate of alkaline phosphatase were associated to a continuous uncontrolled SHPT. Patients with the lowest PTH at the end of the follow-up tended to receive more often cinacalcet.

TRIAL REGISTRATION NUMBER

ClinicalTrials.gov number, NCT02888639, post results.

A model of calcium homeostasis in the rat

We developed a model of calcium homeostasis in the rat to better understand the impact of dysfunctions such as primary hyperparathyroidism and vitamin D deficiency on calcium balance. The model accounts for the regulation of calcium intestinal uptake, bone resorption, and renal reabsorption by parathyroid hormone (PTH), vitamin D₃, and Ca²⁺ itself. It is the first such model to incorporate recent findings regarding the role of the calcium-sensing receptor (CaSR) in the kidney, the presence of a rapidly exchangeable pool in bone, and the delayed response of vitamin D₃ synthesis. Accounting for two (fast and slow) calcium storage compartments in bone allows the model to properly predict the effects of bisphophonates on the plasma levels of Ca²⁺ ([Ca²⁺]_p), PTH, and vitamin D₃ Our model also suggests that Ca²⁺ exchange rates between plasma and the fast pool vary with both sex and age, allowing [Ca²⁺]_p to remain constant in spite of sex- and age-based hormonal and other differences. Our results suggest that the inconstant hypercalciuria that is observed in primary hyperparathyroidism can be attributed in part to counterbalancing effects of PTH and CaSR in the kidney. Our model also correctly predicts that calcimimetic agents such as cinacalcet bring down [Ca²⁺]_p to within its normal range in primary hyperparathyroidism. In addition, the model provides a simulation of CYP24A1 inactivation that leads to a situation reminiscent of infantile hypercalcemia. In summary, our model of calcium handling can be used to decipher the complex regulation of calcium homeostasis.

[Nephrolithiasis of adult: From mechanisms to preventive medical treatment]

Nephrolithiasis is a very common (prevalence around 10 to 12% in France) and recurrent disorder. Nephrolithiasis is associated to chronic kidney disease and is responsible for 2 to 3% of cases of end-stage renal disease, mainly if it is associated to nephrocalcinosis or to a monogenic disorder (1.6% of nephrolithiasis in adults, among them 1% of cystinuria). To understand the underlying pathophysiological processes, stone analysis (morphology and using infrared spectrophotometry) as well as minimal biological assessment including urine crystal research are required. The calcic nephrolithiasis is the more frequent subtype (>80%). Its medical treatment relies on simple dietary rules: non-alkaline hyperdiuresis>2 liters/day, calcium intake normalization (1 gram per day divided between the three principal meals), normalization of sodium (6 to 7 grams per day) and protein intake (1g/kg of theoretical body weight/day), and eviction of foods rich in oxalate. In case of persistent hypercalciuria (>0.1mmol/kg of theoretical body weight/day on free diet), a thiazide diuretic can be started while being aware to correct iatrogenic decrease in plasma potassium and urine citrate excretion. Measurement of bone mineral density must systematically be performed in patients with high 24 h-urinary calcium excretion. The medical treatment of uric acid nephrolithiasis relies on alkaline hyperdiuresis (goal of urine pH: 6.2 to 6.8). The use of allopurinol is justified only if urine uric acid is over 4mmol/day. Thanks to a well-managed preventive medical treatment, one can expect to stop the activity of nephrolithiasis in more than 80% of cases, making it one of the most accessible renal pathologies to preventive medical treatment.

[Changes in mineral and bone disorder management in a French cohort of hemodialysis patients between 2008 and 2012: The National Bone and Mineral Metabolism observatory (Photo-Graphe 2 and 3)]

INTRODUCTION

Chronic kidney disease progressively induces a disorder of mineral and bone metabolism (CKD-MBD) which also leads to cardiovascular abnormalities. Previous studies showed that only few hemodialysis patients had serum calcium, phosphate and parathyroid hormone levels within the K/DOQI (Kidney-Disease Outcomes Quality Initiative) targets of 2003. Our aim was to identify the impact of different therapeutic strategies and that of the KDIGO (Kidney-Disease: Improving Global Outcomes) targets of 2009 on the control of CKD-MBD.

PATIENTS AND METHODS

The French calcium and phosphate observatory monitors the mineral metabolism of patients with CKD at the local, regional and national level every six months. We compared the data recorded in June 2008 (n=1914 patients) with those collected in October 2012 (n=2481) for patients aged 18 years or more, who started hemodialysis therapy within the last 12 months.

RESULTS

As compared with 2008, in 2012 fewer patients had hyperphosphatemia (55.1 % versus 64.7 %), hypocalcemia (35.5 % versus 40.3 %) and hyperparathyroidism (9.8 % versus 10.1 %) according to the KDIGO guideline, and more had hypophosphatemia (9.6 % versus 6.5 %), hypercalcemia (3.9 % versus 2.2 %) and hypoparathyroidism (31.5 % versus 25.8 %) (P<0.001, P<0.001 and P=0.002 respectively for differences in serum phosphate, calcium and PTH levels). Mean (± standard deviation [SD]) serum 25 OH vitamin D levels increased by 1.6-fold, from 48.3±42.6 nmol/L in 2008 to 76.6±45.8 nmol/L in 2012. Between 2008 and 2012, the prescription of native vitamin D derivatives and sevelamer (HCl or carbonate) increased whereas that of cinacalcet, lanthanum carbonate, calcium-chelating agents and active vitamin D derivatives decreased.

CONCLUSION

Despite a slight improvement of biochemical CKD-MBD parameters in the observation period only few patients reached the three KDIGO targets (11.5 % in 2012 versus 11.1 % in 2008).

Vitamin D deficiency and insufficiency in HIV-infected children and young adults

BACKGROUND

Vitamin D insufficiency and HIV infection are both risk factors for chronic disorders, so it is important to consider vitamin D status in HIV-infected patients.

METHODS

We prospectively investigated serum 25-hydroxyvitamin D (25(OH)D) concentrations, determined by radioimmunoassay, in 113 HIV-infected children (age≤24 years) and 54 healthy controls matched for age and phototype. We assessed the prevalence of vitamin D deficiency and insufficiency (VDD and VDI) defined as 25(OH)D titers of <10 ng/mL and between 10 and 30 ng/mL, respectively, and their predictive factors.

RESULTS

The overall prevalence of VDD and VDI was 38.9% and 58.7%, respectively. Mean serum 25(OH)D concentrations were significantly higher in the HIV group than the control group (14.2±6.9 ng/mL vs. 10.4±5 ng/mL, P<0.001). Variables significantly associated with low serum 25(OH)D concentrations in HIV-infected children were dark phototype (P<0.001) and age (r=-0.19, P=0.03). Patients receiving efavirenz had a trend toward lower serum 25(OH)D concentrations (11.1±4.6 ng/mL vs. 14.6±7 ng/mL, P=0.1). Dark phototype was the only independent risk factor for VDD in HIV-infected children (odds ratio=14.6; 95% confidence interval: 2.4-89.9, P=0.004).

CONCLUSIONS

VDD and VDI were common in both HIV-infected and control groups, and serum 25(OH)D concentrations were significantly lower in controls than in HIV-infected children.

Secondary hyperparathyroidism in HIV patients: is there any responsibility of highly active antiretroviral therapy?

Secondary hyperparathyroidism may develop in the presence of hypovitaminosis D in order to maintain calcium homeostasis. We conducted a cross-sectional analysis in a cohort of 371 patients, identifying secondary hyperparathyroidism in 65 patients. This high prevalence (17.5%) was in part justified by the high prevalence of hypovitaminosis D (77.4%) in the whole sample, but we also identified an independent association with the use of tenofovir.