Effect of active acromegaly and its treatment on parathyroid circadian rhythmicity and parathyroid target-organ sensitivity

Vitamin D in pituitary driven osteopathies

The evidence that pituitary hormones may bypass peripheral endocrine glands to exert remarkable effects on the skeleton is gaining ground. Both hormonal excess and deficit may determine impairment in bone structure, and they commonly result in bone loss in patients affected by pituitary and neuroendocrine disorders. Vertebral fractures are the most common skeletal alterations and may occur independently of bone mass. Use of vitamin D (VD) supplementation is still debated in this setting. This review will focus on the interactions between different metabolites of VD and pituitary hormones, and the effects of VD supplementation on bone metabolism in patients with pituitary diseases.

A Systematic Review of the Circadian Rhythm of Bone Markers in Blood

There exists a marked circadian variation for several bone markers (BM), which is influenced by endogenous as well as exogenous factors including hormones, physical activity, and fasting. Consequently, was the aim of this review to provide an overview of the knowledge of the circadian variation of BM and which factors influence this rhythmicity. A systematic search of PubMed was performed for studies evaluating the circadian variation of BM and which factors influence this rhythmicity. The studies were screened for eligibility by a set of predetermined criteria including a list of relevant BM and a minimum study duration of 24 h with at least 3 blood samples of which two should be at least 6 h apart. In total were 29 papers included. There exists a marked circadian variation for most BM including Carboxy-terminal Cross-Linked Telopeptide of Type I Collagen (CTX) and osteocalcin (OC) with nighttime or early morning peak. Pro-collagen Type I N-terminal Propeptide (PINP) and PTH also showed circadian rhythm but with less amplitude. The inter-osteoblast-osteoclast regulatory markers such as OPG, RANKL, FGF23, and sclerostin showed no circadian rhythm. The markers were differently affected by exogenous factors like fasting, which greatly reduced the circadian variation of CTX but did not affect PINP or OC. The marked circadian variation and the factors which influence the rhythmicity, e.g., fasting are of great consequence when measuring BM. To reduce variation and heighten validity should circadian variation and fasting be kept in mind when measuring BM.

Determinants of skeletal fragility in acromegaly: a systematic review and meta-analysis

PURPOSE

Vertebral fractures (VFs) are a potential complication in acromegaly. However, the etiology of this skeletal fragility is unknown. This review aimed to evaluate the effect of acromegaly on VFs, bone turnover, areal bone mineral density (aBMD), and bone quality/microarchitecture. The effect of disease activity and gonadal status in these determinants of skeletal fragility was also evaluated.

METHODS

Articles published in English until September 6, 2020 on PubMed and Embase that reported at least one determinant of skeletal fragility in acromegalic patients, were included. Odds ratio (OR) to evaluate the risk of VFs and the standardized mean difference (SMD) to evaluate bone turnover, aBMD and bone quality/microarchitecture were calculated.

RESULTS

Fifty-eight studies met eligibility criteria, assembling a total of 2412 acromegalic patients. Of these, 49 studies were included in the meta-analysis. Acromegalic patients, when compared to non-acromegalic patients, had higher risk of VFs [OR 7.00; 95% confidence interval (CI) 2.80-17.52; p < 0.0001], higher bone formation (SMD 1.14; 95% CI 0.69-1.59; p < 0.00001), higher bone resorption (SMD 0.60; 95% CI 0.09-1.10; p = 0.02) and higher aBMD at the femoral neck (SMD 0.36; 95% CI 0.15-0.57; p = 0.0009). No significant differences were found regarding aBMD at lumbar spine. Considering the results of the different techniques evaluating bone quality/microarchitecture, the main reported alterations were a decrease in trabecular bone thickness and density, and an increase in trabecular separation. The presence of active disease and/or hypogonadism were associated with worst results.

CONCLUSION

Patients with acromegaly are at increased risk of VFs, mainly because of deterioration in bone microarchitecture.

Characterization of vitamin D metabolism in active acromegaly in the setting of bolus (150,000 IU) cholecalciferol treatment

PURPOSE

To reveal distinctive features of vitamin D metabolism in patients with active acromegaly compared to healthy individuals, particularly in the setting of cholecalciferol treatment.

METHODS

The study group included 34 adults with active acromegaly, and the control group included 30 apparently healthy adults with similar age, sex, and BMI. All participants received a single dose (150,000 IU) of cholecalciferol aqueous solution orally. Laboratory assessments including serum vitamin D metabolites (25(OH)D₃, 25(OH)D₂, 1,25(OH)₂D₃, 3-epi-25(OH)D₃ and 24,25(OH)₂D₃), free 25(OH)D, vitamin D-binding protein (DBP) and parathyroid hormone (PTH) as well as serum and urine biochemical parameters were performed before the intake and on Days 1, 3, and 7 after the administration. All data were analyzed with nonparametric statistics.

RESULTS

Patients with acromegaly had tendency to lower baseline 25(OH)D₃ levels (p = 0.05) and lower 25(OH)D₃ levels (p < 0.05) during the follow-up. They were also characterized by PTH suppression (lower baseline PTH levels and lower prevalence of secondary hyperparathyroidism), altered production of main vitamin D metabolites (higher 1,25(OH)₂D₃ and lower 24,25(OH)₂D₃ levels with corresponding lower 25(ОН)D₃/1,25(ОН)₂D₃ and higher 25(ОН)D₃/24,25(ОН)₂D₃ ratios) as well as concordant biochemical features (higher levels of serum phosphorus and albumin-adjusted calcium levels) throughout the study (p < 0.05). The acromegaly group showed an increase in DBP levels after cholecalciferol intake as opposed to the control group (p < 0.05) and had lower increase in free 25(OH)D levels (p < 0.05). Δ25(OH)D₃ was similar between the groups (p > 0.05), showed a negative correlation with the disease activity markers-both IGF-1 levels (r = -0.44, p < 0.05) and fasting GH levels (r = -0.56, p < 0.05)-and lacked correlation with BMI in the acromegaly group (p > 0.05).

CONCLUSION

Patients with active acromegaly have dysregulated vitamin D metabolism characterized by higher 1,25(ОН)₂D₃, lower 24,25(ОН)₂D₃ and altered DBP production. The response to vitamin D supplementation in acromegaly patients might be influenced by hormonal excess. Obtained results require reproducibility check and further study to develop specific clinical recommendations.

TRIAL REGISTRATION

NCT04844164 (release date: April 9, 2021; retrospectively registered).

Biochemical bone turnover markers in hormonal disorders in adults: a narrative review

BACKGROUND

Hormonal disorders are often associated with abnormal levels of bone turnover markers (BTMs). N-terminal propeptide of type I procollagen (PINP) and serum C-terminal cross-linking telopeptide of type I collagen (CTX-I) are the reference markers of bone formation and bone resorption, respectively.

METHODS

A comprehensive literature search within the MEDLINE and Web of Science databases was performed.

RESULTS

Acromegaly is associated with higher BTM levels, which decrease during the remission after treatment. Adult-onset growth hormone deficiency is often associated with decreased BTM levels. Growth hormone replacement therapy stimulates bone turnover and increases BTM levels. Hypothyroidism is characterized by general slowing of bone metabolism which is reflected by lower BTM levels. The replacement thyroid hormone therapy increases the bone turnover rate and BTM levels increase. Patients with thyroid cancer receive a suppressive dose of thyroid hormones and may have slightly elevated BTM levels. Patients with overt hyperthyroidism had higher BTM levels and anti-thyroid therapy induces a rapid decrease in the BTM levels. Patients with overt primary hyperparathyroidism have higher BTM levels, whereas those with asymptomatic and normocalcemic hyperparathyroidism usually have normal BTM levels. Hypoparathyroidism is characterized by slightly decreased BTM levels. Cushing's syndrome is characterized consistently by markedly decreased osteocalcin concentration, whereas data on other BTMs are discordant.

CONCLUSIONS

BTMs help us to better understand mechanisms of the impact of hormonal disorders and their treatment on bone metabolism. However, it is unknown whether BTMs may be used to monitor the effect of their treatments on bone in the clinical practice.

[Vitamin D metabolism in hypercorticism and acromegaly]

Due to the high prevalence of low vitamin D levels in the overwhelming majority of regions of the world and discovery of extra-skeletal effects of vitamin D, the issue of maintaining adequate levels of vitamin D in the blood remains extremely relevant, especially in people with high risk of severe deficiency. To date, few studies have been performed on the features of vitamin D metabolism in disorders such as hypercorticism and acromegaly. However, vitamin D deficiency in such patients, according to available literature, may be more widespread and more pronounced than in general population. It is now recommended to use standard prophylactic and therapeutic doses of vitamin D for the treatment of these diseases, which may not satisfy the therapeutic goals specific to each disease. This review provides information on normal vitamin D metabolism, as well as literature data on the possible relationship and mutual influence between these endocrinopathies and vitamin D metabolism.

Pituitary Diseases and Bone

Neuroendocrinology of bone is a new area of research based on the evidence that pituitary hormones may directly modulate bone remodeling and metabolism. Skeletal fragility associated with high risk of fractures is a common complication of several pituitary diseases such as hypopituitarism, Cushing disease, acromegaly, and hyperprolactinemia. As in other forms of secondary osteoporosis, pituitary diseases generally affect bone quality more than bone quantity, and fractures may occur even in the presence of normal or low-normal bone mineral density as measured by dual-energy X-ray absorptiometry, making difficult the prediction of fractures in these clinical settings. Treatment of pituitary hormone excess and deficiency generally improves skeletal health, although some patients remain at high risk of fractures, and treatment with bone-active drugs may become mandatory. The aim of this review is to discuss the physiological, pathophysiological, and clinical insights of bone involvement in pituitary diseases.

Vitamin D across growth hormone (GH) disorders: From GH deficiency to GH excess

The interplay between vitamin D and the growth hormone (GH)/insulin-like growth factor (IGF)-I system is very complex and to date it is not fully understood. GH directly regulates renal 1 alpha-hydroxylase activity, although the action of GH in modulating vitamin D metabolism may also be IGF-I mediated. On the other hand, vitamin D increases circulating IGF-I and the vitamin D deficiency should be normalized before measurement of IGF-I concentrations to obtain reliable and unbiased IGF-I values. Indeed, linear growth after treatment of nutritional vitamin D deficiency seems to be mediated through activation of the GH/IGF-I axis and it suggests an important role of vitamin D as a link between the proliferating cartilage cells of the growth plate and GH/IGF-I secretion. Vitamin D levels are commonly lower in patients with GH deficiency (GHD) than in controls, with a variable prevalence of insufficiency or deficiency, and this condition may worsen the already known cardiovascular and metabolic risk of GHD, although this finding is not common to all studies. In addition, data on the impact of GH treatment on vitamin D levels in GHD patients are quite conflicting. Conversely, in active acromegaly, a condition characterized by a chronic GH excess, both increased and decreased vitamin D levels have been highlighted, and the interplay between vitamin D and the GH/IGF-I axis becomes even more complicated when we consider the acromegaly treatment, both medical and surgical. The current review summarizes the available data on vitamin D in the main disorders of the GH/IGF-I axis, providing an overview of the current state of the art.

Hypercalcemia and acromegaly--clarifying the connections. A case report and review of the literature

OBJECTIVE

Hypercalcemia in patients with acromegaly is rare and usually due to co-existent primary hyperparathyroidism. The etiology of hypercalcemia directly related to acromegaly is debated.

METHODS

We present a case report of 1,25(OH)2D3-mediated hypercalcemia in a patient with acromegaly and discuss potential pathophysiological mechanisms contributing to the development of hypercalcemia late in the course of the disease.

RESULTS

A 67-year-old female presented with classical features of acromegaly. A review of her previous photographs suggested a disease duration of approximately 10 years, and her serum calcium (Ca) was normal during this period. A biochemical work up confirmed a combined growth hormone (GH-) and prolactin (PRL-) cosecreting tumor with a GH level of 92.03 ng/mL (normal 0-3.61), an insulin-like growth factor-1 (IGF-1) level of 1,498 ng/mL (59-225), and a PRL level of 223.3 ng/mL (2-17.4). Magnetic resonance imaging (MRI) of the pituitary showed a 1.9-cm macroadenoma. Her preoperative work up revealed new onset hypercalcemia with a corrected serum Ca level of 10.7 mg/dL (8.5-10.5), an ionized Ca level of 1.37 mmol/L (1.08-1.30), a parathyroid hormone (PTH) level of 13.0 pg/mL (10-60), and a high 1,25(OH)2D3 level of 72.6 pg/mL (15-60). She underwent resection of the pituitary adenoma with normalization of GH and PRL levels, and her IGF-1 level decreased to 304 ng/mL. Her serum Ca (9.3 mg/dL), ionized Ca(1.22) and 1,25(OH)2D3 levels (38.6 pg/mL) normalized after surgery.

CONCLUSION

While overt hypercalcemia in acromegaly is rare, it tends to occur late in the disease course. The hypercalcemia is mediated by elevated 1,25(OH)2D3 levels rather than PTH.

Bone turnover, bone mineral density, and fracture risk in acromegaly: a meta-analysis

CONTEXT

GH excess causes an increase in bone turnover, but the consequences in terms of skeletal fragility have long been uncertain due to the heterogeneity of studies dealing with this topic.

OBJECTIVE

We conducted a meta-analysis of studies examining the effects of acromegaly on bone turnover, bone mineral density (BMD), and fractures. Furthermore, we evaluated the effects of sex, gonadal status, and activity of disease on skeletal end-points in acromegaly.

DATA SOURCES

We conducted MEDLINE and EMBASE systematic searches up to December 31, 2013.

STUDY ELIGIBILITY CRITERIA

Studies conducted in patients with acromegaly and reporting at least one determinant of skeletal fragility.

DATA EXTRACTION AND ANALYSIS

Study design, patient characteristics, interventions, and outcomes were independently extracted by two authors. We calculated the standardized mean difference (SMD) of bone turnover and BMD differences, whereas fractures were presented as relative frequencies in acromegaly and odds ratios between patients and controls.

RESULTS

Forty-one studies fulfilled eligibility criteria and were therefore selected for data extraction and analysis. A total of 1935 patients were included (eight to 206 per study). Acromegaly patients had higher bone formation (SMD, 1.49; 95% confidence interval [CI], 0.97-2.01; P < .0001) and bone resorption (SMD, 1.57; 95% CI, 1.03-2.10; P < .0001) as compared to control subjects, without significant differences in lumbar spine BMD. BMD at the femoral neck tended to be higher in acromegaly patients vs control subjects (SMD, 0.67; 95% CI, 0.07-1.27; P = .03). Patients with acromegaly had high frequency of vertebral fractures (odds ratio, 8.26; 95% CI, 2.91-23.39; P < .0001), in close relationship with male gender, hypogonadism, and active acromegaly.

LIMITATIONS

LIMITATIONS included heterogeneous study protocols with possible variability in the assessment of skeletal end-points.

CONCLUSIONS

Skeletal fragility is an emerging complication of acromegaly.

The effect of somatostatin analogs on vitamin D and calcium concentrations in patients with acromegaly

PURPOSE

The goals of this study were to determine: (1) 25OH vitamin D (25OHD) and calcium levels in patients with acromegaly and their association with insulin-like growth factor (IGF-1) and (2) whether somatostatin analog (SSA) therapy effects calcium and 25OHD levels.

METHODS

125 patients with acromegaly were studied. Serum calcium and 25OHD levels were compared prior to and after vitamin D supplementation between patients receiving versus not receiving SSA in whom medical therapy included pegvisomant and/or dopamine agonists. Calcium and 25OHD levels were also evaluated longitudinally prior to and during short-term (mean 3 months, range 1-5) and long-term (mean 49 months, range 7-180) SSA administration. Vitamin D2 50,000 units weekly were given to 3 patients in the cross sectional and 1 in the longitudinal group; 400-4,000 units/day of D3 were given to 11 and 5 in respective groups.

RESULTS

In patients with a comparable mean IGF-1 index and season of testing, mean serum levels of 25OHD prior to vitamin D supplementation did not differ in patients receiving versus not receiving SSA (30 ± 3 vs. 30 ± 1 ng/ml, p = 0.99) and the prevalence of vitamin D sufficiency was similar between SSA and non SSA groups (42 vs. 57%, p = 0.20), prior to vitamin D supplementation. In patients with a comparable mean IGF-1 index and season of testing, mean serum 25OHD levels in patients increased after vitamin D supplementation in both those who were (37 ± 2 ng/ml, N = 23, p = 0.007) and were not receiving SSA (35 ± 1 ng/ml, N = 69, p = 0.005) compared to pre-D supplementation levels but were not different between these groups, p = 0.95) after D supplementation. Calcium and albumin were normal throughout longitudinal follow up. Calcium correlated with IGF-1 index (ρ = 0.29, p = 0.001, N = 125). In the longitudinal subset, serum calcium decreased transiently, in patients receiving short-term SSA (pretreatment 9.9 ± 0.1 mg/dl vs. short-term SSA 9.5 ± 0.1, p = 0.004). After long-term SSA therapy, calcium increased compared to levels on short-term therapy (9.8 ± 0.1 mg/dl vs. 9.5 ± 0.1, p = 0.017) and were unchanged compared to baseline. Mean vitamin D levels were sufficient at baseline prior to SSA therapy (33 ± 5.0 ng/ml), and did not change during short term (29 ± 6 ng/ml, p = 0.85) and long term SSA therapy (35 ± 5 ng/ml, p = 0.43).

CONCLUSIONS

Prior to and after vitamin D supplementation, patients with acromegaly receiving long-term SSA had vitamin D levels similar to those receiving other therapies, suggesting that long-term SSA therapy does not affect serum vitamin D. However, given the limitations of this retrospective study, further prospective studies evaluating the impact of SSA on vitamin D levels are necessary to confirm these findings definitively. Calcium levels are positively associated with IGF-1 index in patients with acromegaly. There is a transient decrease in calcium levels with short-term SSA use. The acute effect of SSA on calcium does not appear to be mediated by albumin, 25OHD or PTH and resolves with long-term SSA treatment. The transient decrease in calcium with short-term SSA use resolved with long-term SSA therapy.

Growth hormone, insulin-like growth factor-1, and the kidney: pathophysiological and clinical implications

Besides their growth-promoting properties, GH and IGF-1 regulate a broad spectrum of biological functions in several organs, including the kidney. This review focuses on the renal actions of GH and IGF-1, taking into account major advances in renal physiology and hormone biology made over the last 20 years, allowing us to move our understanding of GH/IGF-1 regulation of renal functions from a cellular to a molecular level. The main purpose of this review was to analyze how GH and IGF-1 regulate renal development, glomerular functions, and tubular handling of sodium, calcium, phosphate, and glucose. Whenever possible, the relative contributions, the nephronic topology, and the underlying molecular mechanisms of GH and IGF-1 actions were addressed. Beyond the physiological aspects of GH/IGF-1 action on the kidney, the review describes the impact of GH excess and deficiency on renal architecture and functions. It reports in particular new insights into the pathophysiological mechanism of body fluid retention and of changes in phospho-calcium metabolism in acromegaly as well as of the reciprocal changes in sodium, calcium, and phosphate homeostasis observed in GH deficiency. The second aim of this review was to analyze how the GH/IGF-1 axis contributes to major renal diseases such as diabetic nephropathy, renal failure, renal carcinoma, and polycystic renal disease. It summarizes the consequences of chronic renal failure and glucocorticoid therapy after renal transplantation on GH secretion and action and questions the interest of GH therapy in these conditions.

Growth hormone and Klotho

Acromegaly is characterized by excessively high GH and IGF1 levels. Recent data suggest that soluble Klotho (sKlotho) is also elevated in patients with active acromegaly. sKlotho decreases towards normal following removal of the GH-producing pituitary adenoma. The Klotho gene was identified in mice following its accidental disruption by ectopic DNA. It is an ageing suppressor gene of restricted expression (mainly in kidneys, brain, and parathyroid and pituitary glands) encoding a transmembrane protein, mKlotho. mKlotho serves as a co-receptor in fibroblast growth factor 23 (FGF23) signalling. FGF23 promotes urinary phosphate excretion and inhibits the synthesis of calcitriol. The ectodomain of mKlotho is enzymatically released to result in a humoral factor, sKlotho, which exerts systemic effects (on ion channels and signalling pathways), possibly by working as an enzyme that modifies glycans of cell surface glycoproteins. GH enhances renal phosphate reabsorption and calcitriol production, i.e. exerts effects in the proximal tubule opposing those attributed to mKlotho, and attenuates calciuria in the distal tubule similar to sKlotho. sKlotho can be measured in extracellular fluids (serum, urine and cerebrospinal fluid (CSF)) by an ELISA. In line with predominant expression of Klotho in kidneys and choroid plexus, concentrations of sKlotho are particularly high in urine and CSF. Determination of sKlotho in serum and urine (both presumably reflecting GH action on the kidneys) could be used as a supplementary tool in the diagnosis and follow-up of patients with acromegaly. The question arises whether GH exerts selected actions via modifying activities of Klotho.

Diagnosis of acromegaly: state of the art

INTRODUCTION

Biochemical diagnosis of acromegaly relies on measurement of insulin-like growth factor-1 (IGF-1) and growth hormone (GH). An elevated IGF-1 level above the age- and gender-specific normal range and nonsuppression of GH to oral glucose load to a nadir < 0.4 ng/ml in sensitive assays are currently considered diagnostic of acromegaly. Lack of normative data for both IGF-1 and GH across a wide range of populations and ethnicities, interassay and intraassay laboratory variability, pulsatility of GH secretion, and effects of medications and hormones may confound interpretation of these biochemical tests.

AREAS COVERED

Clinical situations in which acromegaly should be suspected and/or investigated. Strengths and limitations of current IGF-1/GH assays are discussed. Clinical scenarios with discordant GH suppression test and IGF-1 levels and, briefly, acromegaly in pregnancy, prolactin-cosecreting tumors, familial acromegaly, and nonpituitary acromegaly are also discussed.

EXPERT OPINION

Serum IGF-1 is the cornerstone and in most cases the stand-alone test in the diagnosis and follow-up in patients with acromegaly. Diagnosis depends on the accurate and reliable measurement of serum IGF-1. GH suppression testing is currently used in limited clinical setting. Standardization of IGF-1 assay and development of normative data across a wide population base are needed. Newer bioassays for IGF-1 hold promise for future.

Vanishing hypercalciuric kidney stones after treating underlying acromegaly

A 53-year-old male presented with recurrent calcium oxalate kidney stones as a first sign of underlying acromegaly, which vanished when his acromegaly was controlled. The exact mechanism behind hypercalciuria and urolithiasis in acromegaly is not yet clear. By discussing this case, a short overview of the pathophysiology of hypercalciuria in acromegaly and practical insights are given.

Acromegaly as a cause of 1,25-dihydroxyvitamin D-dependent hypercalcemia: case reports and review of the literature

Growth hormone excess has been associated with hypercalciuria and nephrolithiasis. Hypercalcemia in acromegaly is rare and usually due to coexistent primary hyperparathyroidism. To report two cases of 1,25-dihydroxyvitamin D (1,25 (OH)(2) D)-dependent hypercalcemia in cromegaly. A 50 year-old female with 2 years history of hypercalcemia presented with features of acromegaly. Serum calcium (Ca) was 10.9 mg/dl (8.6-10.2), parathyroid hormone (PTH) 20 pg/ml (10-65), PTH-related peptide undetectable, and 1,25 (OH)(2) D 119 pg/ml (15-75). Insulin-like growth factor 1 (IGF1) was 911 ng/ml (49-292) and growth hormone (GH) 14.5 ng/ml (0.03-10). MRI showed a 1.7 cm pituitary tumor. Transsphenoidal adenectomy (TSA) resulted in normalization of IGF1, GH, Ca, and 1,25 (OH)(2) D (50 pg/ml) and complete tumor resection. A 52-year-old female was diagnosed with visual field deficits on routine exam. MRI showed a 3 cm invasive pituitary macroadenoma. IGF1 was 416 ng/ml (87-238) and GH 75.8 (0-6.0) ng/ml. Incidentally, she was found with high Ca of 10.8 mg/dl (8.9-10.3) associated with PTH 19 pg/ml and 1,25 (OH)(2) D66 pg/ml. Postoperatively, IGF1 and GH remained abnormal (440 and 12.8 ng/ml, respectively), while MRI showed parasellar tumor residue. Ca remained high (10.1-11.1 mg/dl), along with elevated 1,25 (OH)(2) D level (81.3 pg/ml). In both cases, other causes of hypercalcemia were ruled out. We present 2 cases of 1,25 (OH)(2) D-dependent hypercalcemia associated with growth hormone excess. Complete resection of tumor produced biochemical remission of acromegaly and normalization of calcium and 1,25 (OH)(2) D levels, while incomplete resection was associated with persistent 1,25 (OH)(2) D-dependent hypercalcemia. Acromegaly should be considered a cause of 1,25 (OH)(2) D-dependent hypercalcemia.

Pathophysiology of renal calcium handling in acromegaly: what lies behind hypercalciuria?

BACKGROUND

Hypercalciuria is frequent in patients with acromegaly, but it is unclear how GH/IGF-I regulate renal calcium handling. Elevated fasting plasma calcium levels despite increased glomerular filtration suggest enhanced renal calcium reabsorption.

OBJECTIVE

The aim of this study was to investigate the impact of acromegaly on phosphocalcium metabolism.

DESIGN AND SETTING

We conducted a prospective sequential study at a tertiary referral medical center and clinical investigation center (www.ClinicalTrials.gov Identifier: NCT00531908).

INTERVENTION

Sixteen consecutive patients (five females/11 males) with acromegaly received a single iv infusion of 25 mg of furosemide to induce an acute increase in calcium and magnesium delivery to distal tubular segments during a high-sodium diet with stable dietary calcium, magnesium, and phosphate intake.

MEASUREMENTS

Baseline plasma and urine electrolytes, plasma calciotropic hormones, and furosemide-induced changes in the fractional excretion and tubular reabsorption of Na, Ca, and Mg were measured before and 6 months (range, 1-12) after effective treatment of acromegaly.

RESULTS

Serum IGF-I concentrations normalized in all the patients after acromegaly treatment. Compared with controlled acromegaly, active acromegaly was associated with higher fasting plasma (P = 0.0002) and urinary calcium (P = 0.0003) levels, lower PTH levels (P = 0.0075), higher calcitriol levels (P = 0.0137), higher phosphatemia (P<0.0001) and tubular phosphate reabsorption (P = 0.0002), and a lower calciuric (P = 0.0327) but not magnesiuric response to furosemide related to higher baseline and postfurosemide tubular calcium (P = 0.0034 and P = 0.0081, respectively), but not magnesium reabsorption.

CONCLUSION

The IGF-I-mediated and PTH-independent increase in calcitriol synthesis in acromegaly is responsible for both absorptive hypercalciuria and increased fasting plasma calcium linked to enhanced distal tubular calcium reabsorption, as shown by the selectively diminished calciuric response to furosemide.

Impact of growth hormone hypersecretion on the adult human kidney

Acromegaly is most often secondary to a GH-secreting pituitary adenoma with increased Insulin-like Growth Factor type 1 (IGF-1) level. The consequences of GH/IGF-1 hypersecretion reflect the diversity of action of these hormones. The genes of the GH receptor (GHR), IGF-1, IGF-1 receptor (IGF-1R) and IGF-binding proteins (IGF-BP) are physiologically expressed in the adult kidney, suggesting a potential role of the somatotropic axis on renal structure and functions. The expression of these proteins is highly organized and differs according to the anatomical and functional segments of the nephron suggesting different roles of GH and IGF-1 in these segments. In animals, chronic exposure to high doses of GH induces glomerulosclerosis and increases albuminuria. Studies in patients with GH hypersecretion have identified numerous targets of GH/IGF-1 axis on the kidney: 1) an impact on renal filtration with increased glomerular filtration rate (GFR), 2) a structural impact with an increase in kidney weight and glomerular hypertrophy, and 3) a tubular impact leading to hyperphosphatemia, hypercalciuria and antinatriuretic effects. Despite the increased glomerular filtration rate observed in patients with GH hypersecretion, GH is an inefficient treatment for chronic renal failure. GH and IGF-1 seem to be involved in the physiopathology of diabetic nephropathy; this finding offers the possibility of targeting the GH/IGF-1 axis for the prevention and the treatment of diabetic nephropathy.

Biochemical markers of bone turnover: potential use in the investigation and management of postmenopausal osteoporosis

INTRODUCTION

The aim was to analyse data on the use of biochemical bone turnover markers (BTM) in postmenopausal osteoporosis.

METHODS

We carried out a comparative analysis of the most important papers concerning BTM in postmenopausal osteoporosis that have been published recently.

RESULTS

The BTM levels are influenced by several factors. They are moderately correlated with BMD and subsequent bone loss. Increased levels of bone resorption markers are associated with a higher risk of fracture. Changes in the BTM during the anti-osteoporotic treatment (including combination therapy) reflect the mechanisms of action of the drugs and help to establish their effective doses. Changes in the BTM during the anti-resorptive treatment are correlated with their anti-fracture efficacy.

CONCLUSION

Biological samples should be obtained in a standardised way. BTM cannot be used for prediction of the accelerated bone loss at the level of the individual. BTM help to detect postmenopausal women who are at high risk of fracture; however, adequate practical guidelines are lacking. BTM measurements taken during the anti-resorptive therapy help to identify non-compliers. They may improve adherence to the anti-resorptive therapy and the fall in the BTM levels that exceeds the predefined threshold improves patients' persistence with the treatment. There are no guidelines concerning the use of BTM in monitoring anti-osteoporotic therapy in postmenopausal women.

Growth hormone, insulin-like growth factors, and the skeleton

GH and IGF-I are important regulators of bone homeostasis and are central to the achievement of normal longitudinal bone growth and bone mass. Although GH may act directly on skeletal cells, most of its effects are mediated by IGF-I, which is present in the systemic circulation and is synthesized by peripheral tissues. The availability of IGF-I is regulated by IGF binding proteins. IGF-I enhances the differentiated function of the osteoblast and bone formation. Adult GH deficiency causes low bone turnover osteoporosis with high risk of vertebral and nonvertebral fractures, and the low bone mass can be partially reversed by GH replacement. Acromegaly is characterized by high bone turnover, which can lead to bone loss and vertebral fractures, particularly in patients with coexistent hypogonadism. GH and IGF-I secretion are decreased in aging individuals, and abnormalities in the GH/IGF-I axis play a role in the pathogenesis of the osteoporosis of anorexia nervosa and after glucocorticoid exposure.