Increased Parathyroid Hormone Secretion Research Articles

Roles of Parathyroid Hormone and Fibroblast Growth Factor 23 in Advanced Chronic Kidney Disease.

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) each play a central role in the pathogenesis of chronic kidney disease (CKD)-mineral and bone disorder. Levels of both hormones increase progressively in advanced CKD and can lead to damage in multiple organs. Secondary hyperparathyroidism (SHPT), characterized by parathyroid hyperplasia with increased PTH secretion, is associated with fractures and mortality. Emerging evidence suggests that these associations may be partially explained by PTH-induced browning of adipose tissue and increased energy expenditure. Observational studies suggest a survival benefit of PTHlowering therapy, and a recent study comparing parathyroidectomy and calcimimetics further suggests the importance of intensive PTH control. The mechanisms underlying the regulation of FGF23 secretion by osteocytes in response to phosphate load have been unclear, but recent experimental studies have identified glycerol-3-phosphate, a byproduct of glycolysis released by the kidney, as a key regulator of FGF23 production. Elevated FGF23 levels have been shown to be associated with mortality, and experimental data suggest off-target adverse effects of FGF23. However, the causal role of FGF23 in adverse outcomes in CKD patients remains to be established. Further studies are needed to determine whether intensive SHPT control improves clinical outcomes and whether treatment targeting FGF23 can improve patient outcomes.

Expect the unexpected: Brown tumor of the mandible as the first manifestation of primary hyperparathyroidism.

Hyperparathyroidism (HPT) is a condition in which one or more parathyroid glands produce increased levels of parathyroid hormone (PTH), causing disturbances in calcium homeostasis. Most commonly HPT presents with asymptomatic hypercalcemia but the clinical spectrum may include disturbances reflecting the combined effects of increased PTH secretion and hypercalcemia. Brown tumors are rare, benign, tumor-like bone lesions, occurring in 1.5% to 4.5% of patients with HPT, as a complication of an uncontrolled disease pathway, and are nowadays rarely seen in clinical practice. The tumor can appear either as a solitary or multifocal lesion and usually presents as an asymptomatic swelling or a painful exophytic mass. Furthermore, it can cause a pathological fracture or skeletal pain and be radiologically described as a lytic bone lesion. The diagnosis of a brown tumor in HPT is typically confirmed by assessing the levels of serum calcium, phosphorus, and PTH. Although when present, brown tumor is quite pathognomonic for HPT, the histologic finding often suggests a giant cell tumor, while clinical presentation might suggest other more frequent pathologies such as metastatic tumors. Treatment of brown tumors frequently focuses on managing the underlying HPT, which can often lead to regression and resolution of the lesion, without the need for surgical intervention. However, in refractory cases or when dealing with large symptomatic lesions, surgical treatment may be necessary.

Hyperparathyroidism and Peripheral Arterial Disease.

Primary hyperparathyroidism (PHPT) is presented in various forms, including classic PHPT, characterised by increased parathyroid hormone (PTH) secretion, normohormonal PHPT, and normocalcaemic PHPT. Secondary hyperparathyroidism is characterised by increased PTH secretion triggered by factors such as vitamin D deficiency and kidney failure. This review aims to discuss the involvement of hyperparathyroidism (HPT) in atherosclerosis, including peripheral arterial disease (PAD). The increased level of PTH is involved in developing subclinical and overt vascular diseases, encompassing endothelial dysfunction, vascular stiffness, hypertension, and coronary and peripheral arterial diseases. It has been consistently associated with an augmented risk of cardiovascular morbidity and mortality, independent of classical risk factors for atherosclerosis. Chronic hypercalcemia associated with increased levels of PTH contributes to the development of calcification of vessel walls and atherosclerotic plaques. Vascular calcification can occur in the intima or media of the arterial wall and is associated with stiffness of peripheral arteries, which the formation of atherosclerotic plaques and narrowing of the vessel lumen can follow. For treating hyperparathyroidism, particularly SHPT, calcimimetics, novel phosphorus binders and novel vitamin D receptor activators are used. However, they are ineffective in severe PHPT. Therefore, parathyroidectomy remains the primary therapeutic option of PHPT.

OR23-01 The Nonredundant Role Of Beta-Amyloid In Mediating Tonic PTH Secretion In Physiological And Pathological States

Abstract Disclosure: C. Tu: None. Z. Cheng: None. K.A. Pena: None. N. Szeto: None. J.A. Sosa: None. J. Vilardaga: None. J. Koh: None. W. Chang: None. Understanding the mechanisms driving parathyroid hormone (PTH) hypersecretion in primary hyperparathyroidism (PHPT) is essential for better management of this common endocrinopathy. Prior studies showed that reduced Ca2+-sensing receptor (CaSR) expression and the subsequent increases in heterodimerization of the CaSR with the type B γ-aminobutyric acid receptor 1 (GABAB1R) are causally linked to PTH hypersecretion in PHPT mouse models (Nat Metab 2:243-255). We further showed that expression of the putative GABAB1R ligands, amyloid precursor protein (APP) and its proteolytic product, β-amyloid (Aβ1-42), is significantly upregulated in adenomas of PHPT patients associated with vitamin D insufficiency/deficiency when compared to normal donor controls. The current study aims to delineate the actions of Aβ1-42 in promoting tonic PTH secretion and its interactions with vitamin D receptor (VDR) signaling in parathyroid glands (PTGs) in basal and PHPT states. We show that Aβ1-42 (200 nM) stimulated tonic PTH secretion in cultures of normal human PTGs without shifting the calcium/PTH secretion setpoint (Ca2+-setpoint). In contrast, conditional knockout (KO) of the App gene in the parathyroid cell (PTC) of PTCAppΔflox/Δflox mice significantly reduced serum PTH levels (KO: 64±13 pg/ml vs Control: 109±12 pg/ml; p<0.05, n=8) despite hypocalcemia, indicating hypoparathyroidism. In PTGs cultured from the PTCAppΔflox/Δflox mice, supplementation of Aβ1-42 dose-dependently (EC50=5.6 nM, p<0.001) increased PTH secretion without altering the Ca2+-setpoint. However, the stimulatory effects of Aβ1-42 on tonic PTH secretion were completely abrogated in the PTGs with concurrent deletions of App, Casr and Gabbr1 genes, supporting a direct action of Aβ1-42 on CaSR and/or GABAB1R. The latter notion is further supported by the ability of Aβ1-42 to stimulate cAMP production in cells co-expressing CaSR and GABAB1R. PTC-specific deletion of the Vdr gene in the PTCVdrΔflox/Δflox mice led to elevated serum PTH levels in vivo and increased tonic PTH secretion with unaffected Ca2+-setpoint in PTGs in vitro. Concurrent ablation of the App gene completely normalized serum PTH levels in the PTCVdrΔflox/Δflox mice and prevented PTH hypersecretion in their PTGs in culture. These findings support a critical role of the APP-derived Aβ1-42 in mediating tonic PTH secretion in the normal physiological state and suggest a new mechanism driving PTH hypersecretion in PHPT due to vitamin D deficiency. Presentation: Saturday, June 17, 2023

THU395 Phosphate Inhibits Calcium-sensing Receptor Expressed Endogenously In TT Cells

Abstract Disclosure: K.A. Alghamdi: None. D. Ward: None. Calcium-sensing receptor (CaR) is the key controller of parathyroid hormone (PTH) secretion and extracellular calcium homeostasis. Hyperphosphataemia increases PTH secretion and we reported recently that pathophysiologic phosphate (Pi) concentrations can attenuate CaR activity directly in transfected HEK-293 cells and can increase PTH secretion from human and murine parathyroid cells (1). This was investigated further using thyroidal C cell-derived TT cells (ATCC), which express CaR endogenously. Intracellular Ca2+ (Ca2+i) mobilisation was assayed by epifluorescence microscopy, calcitonin secretion by ELISA, protein expression by immunoblotting and CaR mRNA expression by qRT-PCR. Co-stimulation of TT cells with the (CaR-activating) calcimimetic R568 (1µM) and spermine (1mM) elicited robust Ca2+i mobilisation, that was inhibited (33±4%; P<0.001; N=4) by 2mM (pathophysiologic) Pi-containing buffer vs 0.8mM Pi control. In contrast, raising Pi concentration was without effect on carbachol-induced Ca2+i mobilisation (acting via muscarinic receptors) (P=0.89; N=3). Also, 1.2mM (high) sulphate elicited a similar CaR inhibition as for Pi vs 0.3mM control (-28±16%; P<0.05; N=4). Next, it was found that CaR-mediated stimulation of the TT cells with 1µM R568 & 1mM spermine (in 2mM Ca2+ and 0.8mM Pi) increased calcitonin secretion, as expected, whereas the same stimulation in 2mM Pi (in the presence of 2.2mM Ca2+ to correct for any reduction in free Ca2+) significantly blunted this response (-59.7±15%; P<0.05;N=4). Interestingly, CaR mRNA expression was significantly downregulated 45±13%; P<0.05;N=6) after 48 hours treatment with media containing 2mM Pi (vs 0.8mM control). However, while CaR protein expression in the TT cells was confirmed by immunoblotting, its abundance was unaffected by the same 2mM Pi cotreatment (P=0.79; N=8). Therefore, pathophysiologic Pi treatment inhibits endogenous CaR-induced signalling, calcitonin secretion and CaR mRNA expression in thyroidal TT cells but without affecting CaR abundance. These results further support the idea that the CaR represents a mineral sensor, at which Pi acts directly as a non-competitive antagonist to limit CaR-induced suppression of PTH secretion. Reference: (1) Centeno et al. Nat Commun. 2019;10:4693. Presentation: Thursday, June 15, 2023

Metabolic syndrome in parathyroid diseases

Metabolic syndrome is a cluster of metabolic abnormalities, including central obesity, insulin resistance, dyslipidemia, and hypertension. Parathyroid diseases are a group of disorders that affect the parathyroid glands’ function, which regulates the body’s calcium and phosphorus levels. There is growing evidence that parathyroid diseases may be associated with metabolic syndrome. There is a growing body of evidence suggesting a possible association between parathyroid diseases and metabolic syndrome. Hyperparathyroidism and secondary hyperparathyroidism are associated with insulin resistance and dyslipidemia, while hypoparathyroidism is associated with hypocalcemia-induced metabolic abnormalities. Further studies are needed better to understand the relationship between parathyroid diseases and metabolic syndrome and to develop effective treatment strategies. Hyperparathyroidism is a common parathyroid disease characterized by excessive secretion of parathyroid hormone (PTH). Parathormone plays a crucial role in calcium homeostasis and bone metabolism. However, excessive PTH secretion can lead to hypercalcemia, which can cause metabolic abnormalities such as insulin resistance and dyslipidemia. Studies have shown that patients with primary hyperparathyroidism have a higher metabolic syndrome prevalence than the general population. Secondary hyperparathyroidism is another parathyroid disease in patients with chronic renal failure. Chronic kidney disease leads to impaired renal function, which results in decreased vitamin D synthesis and increased PTH secretion. Secondary hyperparathyroidism is associated with bone mineral disorders and cardiovascular disease. Studies have also shown that secondary hyperparathyroidism is associated with metabolic abnormalities such as insulin resistance and dyslipidemia.

High sodium promotes the secretion and synthesis of PTH through PiT-1-IKKβ pathway in parathyroid gland in vitro.

Parathyroid hormone (PTH) is secreted by the parathyroid glands (PTGs) and is an important hormone regulating mineral metabolism. Previous studies reported that high sodium diet will cause the increase in serum PTH, but the specific mechanism is unknown. Consequently, the present study aims to investigate the effects and mechanisms of high sodium on PTH synthesis and secretion from PTGs. We developed a tissue culture model using normal rat PTGs, discovered that sodium elicited and promoted concentration-dependent and time-dependent PTH secretion. Changes in sodium-associated transporters from PTGs incubated with high sodium were thoroughly examined. Increased expression of sodium-phosphate cotransporter Slc20a1 (also known as PiT-1) was observed. Further tests revealed that PiT-1 activated the NF-κB signaling pathway, resulting in increased IKKβ phosphorylation, IKBα degradation, and increased p65 phosphorylation followed by nuclear entry, which led to increased PTH transcription. Meanwhile, IKKβ phosphorylated SNAP23, promoting exocytosis and eventually led to increased PTH secretion. In conclusion, our findings indicate that PiT-1 plays an important role in the increased secretion and synthesis of PTH directly induced by high sodium under physiological conditions, and may provide a potential therapeutic target for secondary hyperparathyroidism (SHPT).

The Eucalcemic Patient With Elevated Parathyroid Hormone Levels.

Primary hyperparathyroidism (PHPT) is classically characterized by hypercalcemia with elevated or inappropriately normal parathyroid hormone (PTH) levels. Elevated PTH levels in the presence of normal calcium levels are not infrequently found during the evaluation of metabolic bone disorders or kidney stone disease. This can be caused by secondary hyperparathyroidism (SHPT) or normocalcemic primary hyperparathyroidism (NPHPT). NPHPT is due to autonomous parathyroid function whereas SHPT is caused by a physiologic stimulation to PTH secretion. Many medical conditions and medications can contribute to SHPT, and differentiation between SHPT and NPHPT may be difficult. Cases are presented to illustrate examples. In this paper, we review the distinction between SHPT and NPHPT as well as end organ effects of NPHPT and outcomes of surgery in NPHPT. We suggest that the diagnosis of NPHPT be made only after careful exclusion of causes of SHPT and consideration of medications that can increase PTH secretion. Further, we advise a conservative approach to surgery in NPHPT.

Identification of microRNAs targeting vitamin D receptor and their effect on parathyroid hormone secretion in secondary hyperparathyroidism

To identify the miRNAs targeting vitamin D receptor (VDR) gene and their effect on parathyroid hormone (PTH) secretion in secondary hyperparathyroidism. Primary parathyroid cells with secondary hyperparathyroidism were isolated by collagenase digestion and cultured. The miRNAs targeting VDR were screened by bioinformatics methods and full transcriptome sequencing, and dual-luciferase reporter assay was used to verify the targeting relationship between VDR and the screened miRNA. The effects of overexpression or inhibition of the candidate miRNA on VDR mRNA and protein expressions and PTH secretion were evaluated using qRT-PCR and Western blotting. The expression levels of the candidate miRNAs and VDR mRNA in clinical specimens of parathyroid tissues were verified by qRT-PCR, and the expression of VDR protein was detected by immunohistochemistry. We successfully isolated primary parathyroid cells. Dual-luciferase reporter assay verified the targeting relationship of hsa-miR-149-5p, hsa-miR-221-5p, hsa-miR-222-3p, hsa-miR-29a-5p, hsa-miR-301a-5p, hsa-miR-873-5p, hsa-miR-93-3p with VDR, and among them, the overexpression of hsa-miR-149-5p and hsa-miR-301a-5p significantly increased PTH secretion in the parathyroid cells. In patients with secondary hyperparathyroidism, hsa-miR-149-5p was highly expressed in the parathyroid tissues (P=0.046), where the expressions of VDR mRNA (P=0.0267) and protein were both decreased. The two miRNAs, hsa-miR-149-5p and hsa-miR-301a-5p, may promote the secretion of PTH in patients with secondary hyperparathyroidism by down-regulating the expression of VDR gene.

Prevalence and Determinants of a Blunted Parathyroid Hormone Response in Young Saudi Women with Vitamin D Deficiency: A Cross-Sectional Study

Vitamin D deficiency is highly prevalent among the Saudi population. Increased parathyroid hormone (PTH) secretion is an appropriate homeostatic response to correct the resultant hypocalcemia. However, not all vitamin D deficiency patients have increased PTH levels. This study determined the prevalence of a blunted PTH response to vitamin D deficiency among apparently healthy young Saudi women and assessed anthropometric and biochemical factors associated with this response by performing a secondary analysis of data obtained from a cross-sectional study conducted at the “Center of Excellence for Osteoporosis research.” Overall, 315 women (aged 20–45 years) with vitamin D deficiency (serum 25-hydroxyvitamin D (25(OH)D) levels <30 nmol/L) were included. They were divided into two groups according to the laboratory cutoff value of PTH (<7 or ≥7 pmol/L), and anthropometric and biochemical characteristics of both groups were compared. Women with a blunted PTH response (n = 62, 19.7%) had a significantly lower body mass index (BMI) (P < 0.001) and smaller waist circumference (P=0.001). They also had significantly higher serum 25(OH)D (P=0.001), corrected serum calcium (P < 0.001), and phosphate (P=0.003) levels than those with an elevated PTH response (n = 253, 80.3%). Multiple logistic regression analysis showed that lower BMI (OR = 0.925; 95% CI: 0.949–0.987) and higher 25(OH)D (OR = 1.068; 95% CI: 1.014–1.124) and serum calcium (OR = 8.600; 95% CI: 1.614–45.809) levels were significantly associated with a blunted PTH response (R2 = 0.178). A blunted PTH response to vitamin D deficiency is mainly observed among women with lower BMI. Higher serum calcium and 25(OH)D levels and lower BMI were significant predictors of a blunted PTH response, which may indicate that these subjects are adapting to lower 25(OH)D levels and maintaining normal calcium levels without the need to increase PTH secretion. The mechanisms underlying this adaptation are unclear, and future studies to explore these mechanisms are warranted.

Tertiary hyperparathyroidism: a review.

Tertiary hyperparathyroidism (HPT III) occurs when an excess of parathyroid hormone (PTH) is secreted by parathyroid glands, usually after longstanding secondary hyperparathyroidism. Some authorities reserve the term for secondary hyperparathyroidism that persists after successful renal transplantation. Long-standing chronic kidney disease (CKD) is associated with several metabolic disturbances that lead to increased secretion of PTH, including hyperphosphatemia, calcit-riol deficiency, and hypocalcaemia. Hyperphosphatemia has a direct stimulatory effect on the parathyroid gland cell resulting in nodular hyperplasia and increased PTH secretion. Prolonged hypocalcaemia also causes parathyroid chief cell hyperplasia and excess PTH. Af-ter correction of the primary disorder CKD by renal transplant, the hypertrophied parathyroid tissue fails to resolute, enlarge over and continues to oversecrete PTH, despite serum calcium levels that are within the reference range or even elevated. They also may become resistant to calcimimetic treatment. The main indication for treatment is persistent hypercalcemia and/or an increased PTH, and the primary treatment is surgery. Three procedures are commonly performed: total parathyroidectomy with or without autotransplantation, subtotal parathyroidectomy, and limited parathyroidectomy. It is important to remove superior parts of thymus as well. The most appropriate surgical procedure, whether it be total, subtotal, or anything less than subtotal including "limited" or "focused" parathyroidectomies, continues to be unclear and controversial. Surgical complications are rare, and para-thyroidectomy appears to be a safe and feasible treatment option for HPT III.

Calcium loss in sweat does not stimulate PTH release: A study of Bikram hot yoga

It has been hypothesized that sweat loss during exercise causes a disruption in calcium homeostasis that activates bone resorption and over time leads to low bone mineral density. The purpose of this small pilot study was to determine whether dermal calcium loss from a bout of excessive sweating during light intensity physical activity triggers an increase in biomarkers of bone resorption. Biochemical markers related to bone homeostasis were measured before and after a 90 min Bikram hot yoga practice performed in a room heated to 105 °F with 40 % humidity. Participants were five females with a mean age of 47.4 ± 4.7 years. Nude body weight, serum total calcium (Ca2+), free ionized calcium, albumin, parathyroid hormone (PTH) and CTX-I were measured before and after a Bikram hot yoga practice. Mean estimated sweat loss was 1.54 ± 0.65 L, which elicited a 1.9 ± 0.9 % decrease in participant’s body weight. Mean Ca2+ concentration in sweat was 2.9 ± 1.7 mg/dl and the estimated mean total calcium lost was 41.3 ± 16.4 mg. Serum ionized Ca2+ increased from 4.76 ± 0.29 mg/dl to 5.35 ± 0.36 mg/dl after the Bikram hot yoga practice (p = 0.0118). Serum PTH decreased from pre- 33.9 ± 3.3 pg/ml to post- 29.9 ± 2.1 pg/ml yoga practice (p = 0.0015) when adjusted for hemoconcentration (PTHADJ), implying a decrease in PTH secretion. We conclude that calcium loss in sweat during 90 min of Bikram hot yoga did not trigger an increase in PTH secretion and did not initiate bone resorption.

Hormonal and Mineral Imbalance Effect on Bone Resorption in Predialysis Iraqi Patients with Chronic Kidney Disease

Introduction: Chronic kidney disease mineral bone disorder is a metabolic bone disease present in almost all uremic patients. The aim of this research to indicate the stage of chronic kidney disease (CKD) that affect the bone metabolism that leading to the mineral and hormonal imbalance by studying the relationship among osteocalcin , glomerular filtration rate (GFR), parathyroid hormone, calcium and phosphorus levels in the blood. Method: The study included 52 patients with predialysis chronic kidney disease stage 3-5 and 40 apparently healthy relatives accompanying the patients. Glomerular filtration rate (GFR) was calculated for each patient. Renal function tests, including serum levels of urea, creatinine, a biochemical marker of bone metabolism: osteocalcin (OSN), calcium, phosphorus, and parathyroid hormone (PTH), were measured for each participant. Results: Serum urea and creatinine levels were significantly higher in CKD patients Than that of apparently healthy control. There is significantly higher serum parathyroid hormone, serum phosphorus, serum osteocalcin (P&lt;0.01, P&lt;0.01, P&lt;0.01 respectively) in CKD patients than that of the healthy control group. While low serum calcium level in CKD patients as compared to the corresponding group (P&lt;0.01). Conclusion: Hyperphosphatemia and hypocalcemia in the end stage of predialysis CKD patients lead to increase parathyroid hormone secretion, which causes high bone turnover characterized by significantly high serum osteocalcin in these patients. Parathyroid hormone and osteocalcin were used as a biomarker for the development of bone and mineral disorders in predialysis CKD patients.

HYPERPARATHYROIDISM AND PARATHYROID LESIONS IN THE ChNPP ACCIDENT SURVIVORS

Study of the status of parathyroid glands in individuals exposed to ionizing radiation as a result of the ChNPP accident and comparison with the general population of Ukraine. Subjects exposed as a result of the ChNPP accident (n = 1,348) and people from the general population of Ukraine (n=655) were examined. Diagnostic ultrasound scan of thyroid and parathyroid glands (PTG) was conducted in all study subjects. The technique of parathyroid ultrasound screening was developed, which led to an increase in the efficiency of their imaging. Additionally, the 25-hydroxyvitamin D (25(OH)D), parathyroid hor- mone, ionized calcium and some other parameters were selectively assayed in serum. High incidence of parathyroid hyperplasia was detected 27-32 years after the irradiation in persons ex- posed as a result of the ChNPP accident, especially in evacuees from the 30-km exclusion zone (71.4%; χ2Yates' = 24.1; р = 0) and residents of radilogically contaminated territories (41.7%; χ2Yates' = 6.45; p < 0.01) having no primary hyperparathyroidism. High prevalence of vitamin D insufficiency and deficiency was revealed in all study subgroups, namely in 83.1 % of the general population of Ukraine along with a bit better vitamin D status in the ChNPP acci- dent survivors i.e. the vitamin insufficiency and deficiency was found in 78.7 % of them. Incidence of hyperparathy- roidism, predominantly of the secondary (normocalcemic) one, was 33.8 % among persons exposed as a result of the ChNPP accident (p > 0.3) being somewhat higher than in the general population of Ukraine (26.1%), despite above- mentioned better supply of vitamin D. There is a widespread insufficiency or deficiency of vitamin D (over 78.7%) in the population of Ukraine in general providing an unfavorable background for the higher prevalence of health disorders associated with calcium and phosphorus metabolism. The latter features parathyroid hyperplasia and musculosceletal, immune, cardiovascular, and endocrine system comorbidities. Such disorders should exacerbate with a secondary increase in parathyroid hormone secretion (26.1 %). Higher incidence of normocalcemic hyperparathyroidism (33.8% versus 26.1%) against a background of better vitamin D status among irradiated individuals indicates the existence of other factors, where the past combined effects of Chornobyl radioactive fallout and external parathyroid exposure are most likely to be involved. This could explain the greater number of cases of parathyroid hyperplasia and hyper- parathyroidism among the exposed subjects. However the additional precise studies are required here with clarifi- cation of the personal data in population groups of the ChNPP accident survivors. Participants of the ChNPP acci- dent clean-up work in the «iodine period» of 1986 are of especial concern here. Besides that, the study population should be expanded with inclusion of subjects exposed in prenatal period.

Prevalência de hipovitaminose D e hiperparatireoidismo secundário em mulheres na pós-menopausa

Introdução: A vitamina D e o paratormônio (PTH) são os principais moduladores do metabolismo mineral, contribuindo para homeostase do cálcio e fósforo. Considera-se como insuficientes níveis séricos abaixo de 30 ng/mL de 25-hidroxi-vitamina D [25(OH)D] e deficientes quando abaixo de 20 ng/mL. Tal deficiência também ocasiona aumento da secreção de PTH e, assim, há aumento da desmineralização óssea. Este estudo visa avaliar os níveis séricos de 25(OH)D em mulheres na pós-menopausa e correlacioná-los com as concentrações de PTH e a densidade mineral óssea (DMO). Materiais e métodos: Foram coletados dados clínicos, laboratoriais e densitométricos de 114 mulheres na pós-menopausa que participaram de estudo clínico prévio. As pacientes incluídas apresentavam diagnóstico de osteopenia ou osteoporose sem tratamento prévio. A análise estatística foi realizada pelo software Statistica - Statsoft®. Resultados: As pacientes foram classificadas em osteoporóticas 82,9% (n=92) e osteopênicas 17,1% (n = 19). A média dos valores de 25(OH)D encontrada foi 26,21 ± 9,69 ng/mL. Identificou-se deficiência (16,05 ± 2,81 ng/mL) em 32,4% das pacientes (n = 37) e insuficiência (24,71 ± 3,18 ng/mL) em 33,3% (n = 38). Os valores de PTH apresentaram média de 53,7 ± 20,1 pg/mL. Destes, 25% (n = 29) apresentaram-se acima dos valores de referência. Os valores séricos de PTH correlacionaram-se negativamente com os valores séricos de 25(OH)D (p &lt; 0,01; rho = -0.2971). Houve uma correlação negativa do T-score de colo de fêmur em duas situações: histórico de fraturas e no diagnóstico de fraturas de coluna (p &lt; 0,01). Conclusão: Neste grupo de mulheres pós-menopausadas houve uma alta prevalência de hipovitaminose D, a qual se correlacionou negativamente com o PTH, não sendo observado relação com as estações do ano. Houve uma correlação entre a história de fraturas e a DMO, evidenciando que a DMO de fêmur pode predizer risco de fraturas em outros sítios.

Functions of vitamin D in bone.

Vitamin D, synthesized in the skin or absorbed from the diet, undergoes multi-step enzymatic conversion to its active form, 1,25-dihydroxy vitamin D [1,25(OH)2D], followed by interaction with the vitamin D receptor (VDR), to modulate target gene expression. Loss-of function mutations in the genes encoding the enzymes regulating these processes, or in the VDR, result in human diseases, which have demonstrated the paramount role of 1,25(OH)2D in mineral and skeletal homeostasis. Mouse genetics has been used to create disease phenocopies which have produced considerable insight into the mechanisms of 1,25(OH)2D regulation of mineral and skeletal metabolism. Hypophosphatemia resulting from 1,25(OH)2D deficiency or resistance can inhibit apoptosis in hypertrophic chondrocytes leading to abnormal development of the cartilaginous growth plate in rickets. Decreased 1,25(OH)2D may also cause decreased vascular invasion associated with reduced chondroclast and osteoclast activity and thereby contribute to growth plate abnormalities. Reduced 1,25(OH)2D-mediated intestinal and renal calcium transport can reduce calcium availability, increase parathyroid hormone secretion and phosphaturia, and impair mineral availability for normal matrix mineralization, resulting in reduced growth plate mineralization and osteomalacia. 1,25(OH)2D may exert an anabolic effect in bone, apparently via the VDR in mature osteoblasts, by increasing osteoblast activity and reducing osteoclast activity. High ambient levels of exogenous 1,25(OH)2D, or of elevated endogenous 1,25(OH)2D in the presence of reduced calcium balance, can enhance bone resorption, and apparently prevent mineral deposition in bone. These actions demonstrate the critical role of vitamin D in regulating skeletal homeostasis both indirectly and directly via the 1,25(OH)2D/VDR system.

Fibroblast Growth Factor (FGF) 23 Regulates the Plasma Levels of Parathyroid Hormone In Vivo Through the FGF Receptor in Normocalcemia, But Not in Hypocalcemia

The calcium and phosphate homeostasis is regulated by a complex interplay between parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and calcitriol. Experimental studies have demonstrated an inhibitory effect of FG23 on PTH production and secretion; the physiological role of this regulation is however not well understood. Surprisingly, in uremia, concomitantly elevated FGF23 and PTH levels are observed. The parathyroid gland rapidly loses its responsiveness to extracellular calcium in vitro and a functional parathyroid cell line has currently not been established. Therefore, the aim of the present investigation was to study the impact of FGF23 on the Ca2+/PTH relationship in vivo under conditions of normocalcemia and hypocalcemia. Wistar rats were allocated to treatment with intravenous recombinant FGF23 and inhibition of the FGF receptor in the setting of normocalcemia and acute hypocalcemia. We demonstrated that FGF23 rapidly inhibited PTH secretion and that this effect was completely blocked by inhibition of the FGF receptor. Furthermore, inhibition of the FGF receptor by itself significantly increased PTH levels, indicating that FGF23 has a suppressive tonus on the parathyroid gland’s PTH secretion. In acute hypocalcemia, there was no effect of either recombinant FGF23 or FGF receptor inhibition on the physiological response to the low ionized calcium levels. In conclusion, FGF23 has an inhibitory tonus on PTH secretion in normocalcemia and signals through the FGF receptor. In acute hypocalcemia, when increased PTH secretion is needed to restore the calcium homeostasis, this inhibitory effect of FGF23 is abolished.

Normocalcemic primary hyperparathyroidism: a survey in a small village of Southern Italy

We investigated the prevalence of normocalcemic primary hyperparathyroidism (NPHPT) in the adult population living in a village in Southern Italy. All residents in 2010 (n=2045) were invited by calls and 1046 individuals accepted to participate. Medical history, calcium intake, calcium, albumin, creatinine, parathyroid hormone (PTH) and 25OHD were evaluated. NPHPT was defined by normal albumin-adjusted serum calcium, elevated plasma PTH, and exclusion of common causes of secondary hyperparathyroidism (SHPT) (serum 25OHD <30 ng/ml, estimated glomerular filtration rate (eGFR) <60 ml/min per 1.73 m2 and thiazide diuretics use), overt gastrointestinal and metabolic bone diseases. Complete data were available for 685 of 1046 subjects. Twenty subjects did not meet the inclusion criteria and 341 could not be evaluated because of thawing of plasma samples. Classical PHPT was diagnosed in four women (0.58%). For diagnosing NPHPT the upper normal limit of PTH was established in the sample of the population (n=100) who had 25OHD ≥30 ng/ml and eGFR ≥60 ml/min per 1.73 m2 and was set at the mean+3s.d. Three males (0.44%) met the diagnostic criteria of NPHPT. These subjects were younger and with lower BMI than those with classical PHPT. Our data suggest, in line with previous studies, that NPHPT might be a distinct clinical entity, being either an early phenotype of asymptomatic PHPT or a distinct variant of it. However, we cannot exclude that NPHPT might also represent an early phase of non-classical SHPT, since other variables, in addition to those currently taken into account for the diagnosis of NPHPT, might cumulate in a normocalcemic subject to increase PTH secretion.

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.

Effects of high phosphorus diet on bone metabolism-related gene expression in young and aged mice.

In this study, the effects of high phosphorus (P) diet on bone metabolism-related gene expression were investigated in young and aged mice. Twelve- and 80-week-old ddY male mice were divided into two groups, respectively, and fed a control diet containing 0.3% P or a high P diet containing 1.2% P. After 4 weeks of treatment, serum parathyroid hormone (PTH) concentration was significantly higher in the high P groups than in the control groups in both young and aged mice and was significantly higher in aged mice than in young mice fed the high P diet. High P diet significantly increased receptor activator of NF-κB ligand (RANKL) mRNA in the femur of both young and aged mice and significantly increased the RANKL/osteoprotegerin (OPG) mRNA ratio only in aged mice. High P diet significantly increased mRNA expression of transient receptor potential vanilloid type 6, calbindin-D9k, and plasma membrane Ca2+-ATPase 1b in the duodenum of both young and aged mice. These results suggest that high P diet increased RANKL mRNA expression in the femur and calcium absorption-related gene expression in the duodenum regardless of age. Furthermore, the high P diet-induced increase in PTH secretion might increase the RANKL/OPG mRNA ratio in aged mice.