β1-Adrenergic Blockade Augments Pulsatile PTH Secretion in Humans

Antihypertensive Drugs and Risk of Bone Fractures

Antihypertensive drugs are among the most documented regimens worldwide with an overall survival and cardioprotective benefit. However, there is evidence that they cause symptoms of orthostatic hypotension (i.e., dizziness and syncope) placing patients at risk for falls and fall-related injuries such as bone fractures. Moreover, it seems that they might impact bone metabolism and architecture impairing bone health. The aim of this review was to summarize the accumulative literature exploring any potential association between several antihypertensive medications including diuretics, renin-angiotensin-aldosterone system inhibitors, beta-blockers and calcium channel blockers and the risk of fractures.

Efficacy and safety of weekly vitamin D3 in patients with fibromyalgia: 12-week, double-blind, randomized, controlled placebo trial

INTRODUCTION

FM is a chronic musculoskeletal disorder characterized by the presence of generalized pain. There are contradictory results regarding the prevalence and supplementation effect of vitamin D deficiency on FM patients. We aim to determine the safety and efficacy of a 12-week vitamin D supplementation on FM patients.

METHODS

We conducted a randomized, placebo-controlled, double-blind clinical trial. We included female participants of 18 years old or older, who met 1990 or 2010 ACR criteria for fibromyalgia. The Spanish validated FIQ and the VAS of pain were applied at baseline. The participants were then randomized to receive placebo or 50,000 IU of Vitamin D₃ PO, weekly for 12 weeks.

RESULTS

We included 80 patients. There was no statistical difference in the initial and final FIQ between both groups. The FIQ delta also did not prove to be different at the end of the study. The increase in vitamin D levels in the intervention group was corroborated. Regarding serious adverse effects, none was reported in both groups. There was no statistical difference in minor adverse events.

CONCLUSION

In this double-blind placebo-controlled randomized study conducted to measure the efficacy and safety of vitamin D exclusively in patients with FM, we found that there is no evidence of a trend in favor of vitamin D treatment, since we did not observe improvement in the VAS of pain or FIQ.

TRIAL REGISTRY

Clinical Trials.gov number: NCT03369379 Key Points • There are conflicting results in vitamin D to treat fibromyalgia. • In this double-blind, randomized controlled trial, we did not find a difference in the VAS nor FIQ with vitamin D supplementation. • The increase in vitamin D levels in the intervention group was corroborated.

The role of nervous system in adaptive response of bone to mechanical loading

Bone tissue is remodeled through the catabolic function of the osteoclasts and the anabolic function of the osteoblasts. The process of bone homeostasis and metabolism has been identified to be co-ordinated with several local and systemic factors, of which mechanical stimulation acts as an important regulator. Very recent studies have shown a mutual effect between bone and other organs, which means bone influences the activity of other organs and is also influenced by other organs and systems of the body, especially the nervous system. With the discovery of neuropeptide (calcitonin gene-related peptide, vasoactive intestinal peptide, substance P, and neuropeptide Y) and neurotransmitter in bone and the adrenergic receptor observed in osteoclasts and osteoblasts, the function of peripheral nervous system including sympathetic and sensor nerves in bone resorption and its reaction to on osteoclasts and osteoblasts under mechanical stimulus cannot be ignored. Taken together, bone tissue is not only the mechanical transmitter, but as well the receptor of neural system under mechanical loading. This review aims to summarize the relationship among bone, nervous system, and mechanotransduction.

Association of parathyroid hormone and vitamin D with untreated hypertension: Is it different in white-coat or sustained hypertension?

BACKGROUND

Previous reports about the relationship between a high parathyroid hormone (PTH) and low vitamin D levels with blood pressure in different hypertension groups are conflicting.

OBJECTIVE

We studied serum PTH and vitamin D levels in white-coat (WCHT) and sustained hypertension (SHT) patients who had not been on antihypertensive treatment. We also investigated the association between serum PTH and vitamin D levels with respect to blood pressure in SHT and WCHT patients.

METHODS

We included 52 SHT patients (54.06 ± 9.2 years, 32 newly diagnosed and 20 previously diagnosed with SHT who had not been treated with antihypertensive medication for 3 months or more), 48 WCHT patients (53.64 ± 9.5 years), and 50 normotensive (NT) healthy controls (53.44 ± 8.4 years) in our study. In addition to routine tests, PTH and vitamin D levels were measured.

RESULTS

Serum PTH levels were significantly higher in SHT patients not taking antihypertensive medications than in WCHT patients and NT controls (p = 0.004). Although PTH levels were higher in WCHT than in NT groups, the difference was not statistically significant. In SHT patients, PTH levels showed a positive correlation with office systolic (r = 0.363, p = 0.008), office diastolic (r = 0.282, p = 0.038), home systolic (r = 0.390, p = 0.004), and home diastolic blood pressures (r = 0.397, p = 0.003). Serum vitamin D levels were similar in SHT, WCHT and NT groups. Vitamin D levels were not associated with blood pressures in the entire study group. Furthermore, no significant relation was found between vitamin D and PTH levels in SHT and WCHT groups.

CONCLUSION

PTH levels are significantly higher in untreated SHT patients than WCHT patients and NT subjects. However, vitamin D levels are similar in SHT, WCHT and NT groups. There is a significant association between PTH levels and blood pressures suggesting PTH has a role in increase of blood pressure in SHT.

Effect of general anesthesia and intubation on parathyroid levels in normal patients and those with hyperparathyroidism

BACKGROUND

Induction of general anesthesia and endotracheal intubation may precipitate parathyroid hormone (PTH) elevation in patients with primary hyperparathyroidism (HPT). The purposes of this study were to revisit this observation and to study its impact in healthy patients.

METHODS

Patients with primary HPT who underwent parathyroidectomy were retrospectively studied. The PTH was sampled and compared: before, immediately after general anesthesia and endotracheal intubation, and 15 minutes after parathyroidectomy. Healthy adults who underwent elective operations were prospectively studied. The PTH was sampled before general anesthesia and endotracheal intubation, immediately after, and 15 minutes later.

RESULTS

Thirty-one patients, aged 28-89 years (mean 60.1 ± 13 years), were retrospectively studied. The PTH was significantly elevated after general anesthesia and endotracheal intubation (P = .014). Fifty patients, aged 21-86 years (mean 54 ± 15 years), were prospectively studied. The PTH elevation after general anesthesia and endotracheal intubation was not significant.

CONCLUSION

General anesthesia and endotracheal intubation causes an immediate, steep, and significant PTH elevation in patients with primary HPT but only a minor change in healthy adults. The difference may be attributed to an impaired adrenergic response in patients with primary HPT.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Negative association between serum parathyroid hormone levels and urinary perchlorate, nitrate, and thiocyanate concentrations in U.S. adults: the National Health and Nutrition Examination Survey 2005-2006

OBJECTIVES

Perchlorate, nitrate, and thiocyanate are well-known inhibitors of the sodium-iodide symporter and may disrupt thyroid function. This exploratory study investigated the association among urinary perchlorate, nitrate, and thiocyanate concentrations and parathyroid hormone (PTH) levels in the general U.S. population.

METHODS

We analyzed data on 4265 adults (aged 20 years and older) from the National Health and Nutrition Examination Survey in 2005 through 2006 to evaluate the relationship among urinary perchlorate, nitrate, and thiocyanate concentration and PTH levels and the presence of hyperparathyroidism cross-sectionally.

RESULTS

The geometric means and 95% confidence interval (95% CI) concentrations of urinary perchlorate, nitrate, and thiocyanate were 3.38 (3.15-3.62), 40363 (37512-43431), and 1129 (1029-1239) ng/mL, respectively. After adjusting for confounding variables and sample weights, creatinine-corrected urinary perchlorate was negatively associated with serum PTH levels in women (P = 0.001), and creatinine-corrected urinary nitrate and thiocyanate were negatively associated with serum PTH levels in both sex groups (P = 0.001 and P<0.001 for men, P = 0.018 and P<0.001 for women, respectively). Similar results were obtained from sensitivity analyses performed for exposure variables unadjusted for creatinine with urinary creatinine added as a separate covariate. There was a negative relationship between hyperparathyroidism and urinary nitrate and thiocyanate [odds ratio (95% CI) = 0.77 (0.60-0.98) and 0.69 (0.61-0.79), respectively].

CONCLUSIONS

A higher urinary concentration of perchlorate, nitrate, and thiocyanate is associated with lower serum PTH levels. Future studies are needed to determine the pathophysiological background of the observation.

The role of vitamin D in pathophysiology and treatment of fibromyalgia

Recent studies showed that most cells have receptors and enzymes responsible for metabolism of vitamin D. Several diseases have been linked to vitamin D deficiency, such as hypertension, diabetes, depression, Alzheimer's disease, Parkinson's disease, multiple sclerosis, and chronic pain syndromes such as fibromyalgia. The association between fibromyalgia and vitamin D deficiency is very controversial in the literature with conflicting studies and methodological problems, which leads to more questions than answers. The purpose of this article is to raise questions about the association of hypovitaminosis D with fibromyalgia considering causal relationships, treatment, and pathophysiological explanations.

β-Adrenergic receptor antagonists and fracture risk: a meta-analysis of selectivity, gender, and site-specific effects

By meta-analysis, the risk of fracture was 15% lower in patients treated with β-adrenergic blockers compared to controls independent of gender, fracture site, and dose. This might be attributable to β1-selective blockers.

INTRODUCTION

The aim of this study is to determine by meta-analysis whether β-adrenergic blockers (BBs) reduce fracture risk and whether the effect, if demonstrable, is dependent upon selectivity, dose, gender, or fracture site.

METHODS

A literature search was performed in electronic databases MEDLINE, EMBASE, and reference sections of relevant articles to identify eligible studies. Adjusted estimates of fracture risk effect size (ES) were pooled across studies using fixed or random-effects (RE) meta-analysis as appropriate. Dose-related effects were evaluated using meta-regression. To explore the relative efficacy of β1-selective blockers in comparison to nonselective BBs, adjusted indirect comparison was performed.

RESULTS

A total of 16 studies (7 cohort and 9 case-control studies), involving 1,644,570 subjects, were identified. The risk of any fracture was found to be significantly reduced in subjects receiving BBs as compared to control subjects (16 studies, RE pooled ES = 0.86, 95% CI 0.78-0.93; I(2) = 87 %). In a sensitivity analysis limited to those studies deemed to be most robust, the BB effect to reduce fracture risk was sustained (four studies, pooled ES = 0.79, 95% CI 0.67-0.94; I(2) = 96%). The risk of a hip fracture was lower in both women and men receiving BBs (women: pooled ES = 0.86, 95% CI 0.80-0.91; I(2) = 1% and men: pooled ES = 0.80, 95% CI 0.71-0.90; I(2) = 0%). Similar risk reductions were found for clinical vertebral and forearm fractures, although statistical significance was not reached. The reduction in risk did not appear to be dose-related (test for a linear trend p value 0.150). Using adjusted indirect comparisons, it was estimated that β1-selective agents were significantly more effective than nonselective BBs in reducing the risk of any fracture (six studies, β1-selective blockers vs. nonselective BBs: RE pooled ES = 0.82, 95% CI = 0.69-0.97).

CONCLUSIONS

The findings suggest that the risk of fracture is approximately 15% lower in patients treated with BBs compared to controls independent of gender, fracture site, and dose. This risk reduction might be associated with the effects of β1-selective blockers.

Vitamin D in chronic kidney disease

Vitamin D deficiency is widespread in both the pediatric and adult chronic kidney disease (CKD) population. CKD is characterized by dysregulation of vitamin D and mineral metabolism. Secondary hyperparathyroidism and its management puts patients with CKD at increased cardiovascular risk. Emergence of experimental and some clinical data suggesting beneficial effects of vitamin D on proteinuria, blood pressure, inflammation and cardiovascular outcomes has pushed it to the center stage of CKD research. Pediatric data on vitamin D dysregulation and its consequences are still in its infancy. Ongoing prospective studies such as Chronic Kidney disease in Children (CKiD) and the Cardiovascular Comorbidity in Children with CKD (4 C) should help to delineate the evolution of disturbances in mineral metabolism and its adverse effects on growth, CKD progression and cardiovascular outcomes.

Aldosterone and parathyroid hormone: a precarious couple for cardiovascular disease

Animal and human studies support a clinically relevant interaction between aldosterone and parathyroid hormone (PTH) levels and suggest an impact of the interaction on cardiovascular (CV) health. This review focuses on mechanisms behind the bidirectional interactions between aldosterone and PTH and their potential impact on the CV system. There is evidence that PTH increases the secretion of aldosterone from the adrenals directly as well as indirectly by activating the renin-angiotensin system. Upregulation of aldosterone synthesis might contribute to the higher risk of arterial hypertension and of CV damage in patients with primary hyperparathyroidism. Furthermore, parathyroidectomy is followed by decreased blood pressure levels and reduced CV morbidity as well as lower renin and aldosterone levels. In chronic heart failure, the aldosterone activity is inappropriately elevated, causing salt retention; it has been argued that the resulting calcium wasting causes secondary hyperparathyroidism. The ensuing intracellular calcium overload and oxidative stress, caused by PTH and amplified by the relative aldosterone excess, may increase the risk of CV events. In the setting of primary aldosteronism, renal and faecal calcium loss triggers increased PTH secretion which in turn aggravates aldosterone secretion and CV damage. This sequence explains why adrenalectomy and blockade of the mineralocorticoid receptor tend to decrease PTH levels in patients with primary aldosteronism. In view of the reciprocal interaction between aldosterone and PTH and the potentially ensuing CV damage, studies are urgently needed to evaluate diagnostic and therapeutic strategies addressing the interaction between the two hormones.

A 7-day continuous infusion of PTH or PTHrP suppresses bone formation and uncouples bone turnover

Human in vivo models of primary hyperparathyroidism (HPT), humoral hypercalcemia of malignancy (HHM), or lactational bone mobilization for more than 48 hours have not been described previously. We therefore developed 7-day continuous-infusion models using human parathyroid hormone(1-34) [hPTH(1-34)] and human parathyroid hormone-related protein(1-36) [hPTHrP(1-36)] in healthy human adult volunteers. Study subjects developed sustained mild increases in serum calcium (10.0 mg/dL), with marked suppression of endogenous PTH(1-84). The maximal tolerated infused doses over a 7-day period (2 and 4 pmol/kg/h for PTH and PTHrP, respectively) were far lower than in prior, briefer human studies (8 to 28 pmol/kg/h). In contrast to prior reports using higher PTH and PTHrP doses, both 1,25-dihydroxyvitamin D(3) [1,25(OH)(2) D(3) ] and tubular maximum for phosphorus (TmP/GFR) remained unaltered with these low doses despite achievement of hypercalcemia and hypercalciuria. As expected, bone resorption increased rapidly and reversed promptly with cessation of the infusion. However, in contrast to events in primary HPT, bone formation was suppressed by 30% to 40% for the 7 days of the infusions. With cessation of PTH and PTHrP infusion, bone-formation markers abruptly rebounded upward, confirming that bone formation is suppressed by continuous PTH or PTHrP infusion. These studies demonstrate that continuous exposure of the human skeleton to PTH or PTHrP in vivo recruits and activates the bone-resorption program but causes sustained arrest in the osteoblast maturation program. These events would most closely mimic and model events in HHM. Although not a perfect model for lactation, the increase in resorption and the rebound increase in formation with cessation of the infusions are reminiscent of the maternal skeletal calcium mobilization and reversal that occur following lactation. The findings also highlight similarities and differences between the model and HPT.

Association between beta-blocker use and fracture risk: the Dubbo Osteoporosis Epidemiology Study

INTRODUCTION

In animal model, mice treated with beta-blockers (BB) had increased bone mass. In humans, high bone mass is associated with reduce fracture risk. The present study sought to test the hypothesis that BB use is associated with reduced fracture risk.

MATERIALS AND METHODS

Data from 3488 participants (1285 men) aged 50 years and above in the Dubbo Osteoporosis Epidemiology Study (DOES) were analyzed. Baseline characteristics of participants were obtained at the initial visit which had taken place between 1989 and 1993. Bone mineral density (BMD) at the lumbar spine and femoral neck was measured by dual energy X-ray absorptiometry (GE-LUNAR Corp, Madison, WI). Two hundred and sixty two (20%) men and 411 (19%) women had been on BB, as ascertained by direct interview and verification with medication history. The incidence of fragility fractures was ascertained during the follow-up period (1989-2008).

RESULTS

In men, BB use was associated with higher BMD at the femoral neck (0.96 versus 0.92 g/cm², P < 0.01), higher lumbar spine (1.32 versus 1.25 g/cm², P < 0.01), and lower fracture risk than those not on BB (odds ratio [OR]: 0.49; 95% CI: 0.32-0.75). In women, BB users also had higher femoral neck BMD (0.83 versus 0.81 g/cm², P < 0.01), higher lumbar spine BMD (1.11 versus 1.06 g/cm², P < 0.01), and lower risk of fracture than non-users (OR 0.68, 95% CI: 0.53-0.87). The associations between BB use and fracture risk were independent of age, BMD, and clinical risk factors. Subgroup analysis suggested that the association was mainly found in selective BB, not in non-selective BB.

CONCLUSION

Beta-blockers use, particularly selective BB, was associated with reduced fracture risk in both men and women, and the association was independent of BMD.

Investigational anabolic therapies for osteoporosis

IMPORTANCE OF THE FIELD

Anabolic therapy, or stimulating the function of bone-forming osteoblasts, is the preferred pharmacological intervention for osteoporosis.

AREAS COVERED IN THIS REVIEW

We reviewed bone anabolic agents currently under active investigation. The bone anabolic potential of IGF-I and parathyroid hormone-related protein is discussed in the light of animal data and human studies. We also discuss the use of antagonists of the calcium-sensing receptor (calcilytics) as orally administered small molecules capable of transiently elevating serum parathyroid hormone (PTH). Further, we reviewed novel anabolic agents targeting members of the wingless tail (Wnt) signaling family that regulate bone formation including DKK-1, sclerostin, Thp1, and glycogen synthase kinase 3beta. We have also followed up on the promise shown by beta-blockers in modulating the activity of sympathetic nervous system, thus affecting bone anabolism. We give critical consideration to neutralizing the activity of activin A, a negative regulator of bone mass by soluble activin receptor IIA, as a strategy to promote bone formation.

WHAT THE READER WILL GAIN

Update on various strategies to promote osteoblast function currently under evaluation.

TAKE HOME MESSAGE

In spite of favorable results in experimental models, none of these strategies has yet achieved the ultimate goal of providing an alternative to injectable PTH, the sole anabolic therapy in clinical use.

Effects of a beta-blocker on bone turnover in normal postmenopausal women: a randomized controlled trial

INTRODUCTION

The central nervous system has been demonstrated to regulate bone mass in mice, possibly via the beta2-adrenoreceptors on osteoblasts. beta-blockers increase bone mass in mice, and some observational studies have suggested a beneficial effect of these drugs on bone in humans

EXPERIMENTAL SUBJECTS

We studied 41 normal postmenopausal women.

MATERIALS AND METHODS

We conducted a randomized, placebo- controlled trial, comparing the effects on bone markers of propranolol 160 mg/d and placebo over 3 months.

RESULTS

Serum osteocalcin declined by almost 20% in the first 2 wk of propranolol treatment, and this effect increased over time (P < 0.0001). Other osteoblast markers, procollagen type-I N-terminal propeptide and total alkaline phosphatase activity, were not significantly changed by propranolol. Urine free deoxypyridinoline declined by approximately 10% between 0 and 6 wk (P = 0.019) in the beta-blocker group and was stable thereafter. Serum C-terminal telopeptide of type I collagen also showed a small decrease, but this was not significantly different between groups. Serum albumin concentrations decreased by more than 2 g/liter in the first 2 wk of propranolol treatment, remaining stable subsequently (P = 0.007). Serum creatinine tended to increase in the propranolol group (P = 0.06), as did weight. Bone densities in the lumbar spine and total proximal femur did not change significantly in either group.

CONCLUSIONS

The present study provides no evidence that beta-blocker drugs stimulate bone formation; if anything, propranolol reduces osteoblast activity. It also influences renal function and fluid balance, effects that might indirectly affect bone metabolism. Current evidence does not justify the use of beta-blockers for treatment of osteoporosis.

beta-Blocker use, BMD, and fractures in the study of osteoporotic fractures

A role for osteoblastic beta-adrenoreceptors in bone regulation is suggested by the finding that beta-blockers increase bone mass in mice. We studied the association of beta-blocker use with BMD and fractures in the Study of Osteoporotic Fractures. beta-blocker use and BMD are unrelated in this cohort, and associations with fracture risk are inconsistent.

INTRODUCTION

The central nervous system has been shown to regulate bone mass in mice, possibly by way of the beta(2)-adrenoreceptors on osteoblasts. beta-blockers have been shown to increase bone mass in mice. Because these agents are widely used therapeutically, it is possible that they may influence fracture epidemiology in humans, and they are a potential therapy for osteoporosis.

MATERIALS AND METHODS

We have studied the association of beta-blocker use with BMD and fracture rates in the Study of Osteoporotic Fractures. beta-blocker use was recorded at the fourth visit, in 8412 women, of whom 1099 were users, and these women were followed for 7 years.

RESULTS

Users had significantly higher weight, more thiazide use, more estrogen use, less glucocorticoid use, more statin use, and more hypertension than nonusers, and they smoked less. Total hip BMD at the fourth visit was higher in the beta-blocker users (0.746 versus 0.735 g/cm(2), p = 0.02), but adjustment for weight alone, or together with these other variables, eliminated this difference (p = 0.62). There was no effect of beta-blocker use on loss of hip BMD over a mean follow-up of 4 years (p = 0.48). Os calcis BMD at visit 4 was also higher in those taking beta-blockers (0.385 versus 0.375 g/cm(2), p = 0.005), but weight adjustment eliminated this difference (p = 0.14). The frequencies of hip or any fracture (since age 50) were similar in users and nonusers (p = 0.80 and p = 0.51, respectively). Over a mean follow-up of 7 years, there were 2167 total fractures, including 431 at the wrist and 585 at the hip. Among beta-blocker users, hazards ratios were 0.92 (0.81, 1.05) for any fracture, 0.74 (0.54, 1.01) for wrist fracture, and 0.76 (0.58, 0.99) for hip fracture. Adjustment for weight and other factors previously shown to influence hip fracture incidence in this cohort made little difference to the outcome. When fracture data were analyzed for nonselective and beta(1)-selective agents separately, trends toward fewer fractures were confined to the users of selective beta(1)-blockers.

CONCLUSIONS

beta-Blocker use and BMD are unrelated in this cohort, and associations with fracture risk are inconsistent. Therefore, a history of use of these drugs is not useful in assessing fracture risk, nor do they have a role in osteoporosis management at this time. The relationship between beta-blocker use and hip fracture deserves further study.

Structural organization and biological relevance of oscillatory parathyroid hormone secretion

Parathyroid gland secretory activity exhibits seasonal and circadian fluctuations, which are in synchrony with changes in serum calcium, phosphate, and bone turnover. In addition, an ultradian rhythm exists, which comprises seven pulses per hour, accounts for 30% of basal parathyroid hormone (PTH) release, and is highly sensitive to changes in ionized calcium. Acute hypocalcemia induces a selective, severalfold increase in pulse frequency and amplitude, whereas hypercalcemia suppresses the pulsatile secretion component, as does prolonged calcitriol therapy. Chronic renal failure is associated with a GFR dependent decrease in metabolic PTH clearance accounting for a two- to threefold increase in plasma PTH concentrations, a consistent increase of PTH burst mass and frequency, and a markedly reduced capacity to counteract changes in ionized calcium by modulation of pulsatile PTH release. Continuous PTH excess destroys bone, whereas intermittent administration of pharmacological doses of PTH improves bone morphology and strength in experimental and clinical settings. The molecular mechanisms of the exposure pattern dependent, contrasting biological effects of PTH may involve differential regulation of osteoblastic G protein signaling feedback circuits. In this context, calcimimetic and calcilytic agents are promising new therapeutic tools allowing for tight control of plasma PTH and restoration of circadian PTH rhythmicity.