Factors in the development of secondary hyperparathyroidism during graded renal failure in the rat

Silver jubilee: 25 years of the first demonstration of the direct effect of phosphate on the parathyroid cell

Although phosphorus is an essential element for life, it is not found in nature in its native state but rather combined in the form of inorganic phosphates (PO₄^3-), with tightly regulated plasma levels that are associated with deleterious effects and mortality when these are out of bounds. The growing interest in the accumulation of PO₄^3- in human pathophysiology originated in its attributed role in the pathogenesis of secondary hyperparathyroidism (SHPT) in chronic kidney disease. In this article, we review the mechanisms by which this effect was justified and we commemorate the important contribution of a Spanish group led by Dr. M. Rodríguez, just 25 years ago, when they first demonstrated the direct effect of PO₄^3- on the regulation of the synthesis and secretion of parathyroid hormone by maintaining the structural integrity of the parathyroid glands in their original experimental model. In addition to demonstrating the importance of arachidonic acid (AA) and the phospholipase A2-AA pathway as a mediator of parathyroid gland response, these findings were predecessors of the recent description of the important role of PO₄^3- on the activity of the calcium sensor-receptor, and also fueled various lines of research on the importance of PO₄^3- overload not only for the pathophysiology of SHPT but also in its systemic pathogenic role.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

Hyporesponsiveness or resistance to the action of parathyroid hormone in chronic kidney disease

Secondary hyperparathyroidism (SHPT) is an integral component of the chronic kidney disease-mineral and bone disorder (CKD-MBD). Many factors have been associated with the development and progression of SHPT but the presence of skeletal or calcemic resistance to the action of PTH in CKD has often gone unnoticed. The term hyporesponsiveness to PTH is currently preferred and, in this chapter, we will not only review the scientific timeline but also some of the molecular mechanisms behind. Moreover, the presence of resistance to the biological action of PTH is not unique in CKD since resistance to other hormones has also been described ("uremia as a receptor disease"). This hyporesponsiveness carries out important clinical implications since it explains, at least partially, not only the progressive nature of the pathogenesis of CKD-related PTH hypersecretion and parathyroid hyperplasia but also the increasing prevalence of adynamic bone disease in the CKD population. Therefore, we underline the importance of PTH control in all CKD stages, but not aiming to completely normalize PTH levels since a certain degree of SHPT may represent an adaptive clinical response. Future studies at the molecular level, i.e. on uremia, or the recent description of the calcium-sensing receptor as a phosphate sensor, may become of great value beyond their significance to explain just the hyporesponsiveness to PTH in CKD.

Alkaline Phosphatases in the Complex Chronic Kidney Disease-Mineral and Bone Disorders

Alkaline phosphatases (APs) remove the phosphate (dephosphorylation) needed in multiple metabolic processes (from many molecules such as proteins, nucleotides, or pyrophosphate). Therefore, APs are important for bone mineralization but paradoxically they can also be deleterious for other processes, such as vascular calcification and the increasingly known cross-talk between bone and vessels. A proper balance between beneficial and harmful activities is further complicated in the context of chronic kidney disease (CKD). In this narrative review, we will briefly update the complexity of the enzyme, including its different isoforms such as the bone-specific alkaline phosphatase or the most recently discovered B1x. We will also analyze the correlations and potential discrepancies with parathyroid hormone and bone turnover and, most importantly, the valuable recent associations of AP's with cardiovascular disease and/or vascular calcification, and survival. Finally, a basic knowledge of the synthetic and degradation pathways of APs promises to open new therapeutic strategies for the treatment of the CKD-Mineral and Bone Disorder (CKD-MBD) in the near future, as well as for other processes such as sepsis, acute kidney injury, inflammation, endothelial dysfunction, metabolic syndrome or, in diabetes, cardiovascular complications. However, no studies have been done using APs as a primary therapeutic target for clinical outcomes, and therefore, AP's levels cannot yet be used alone as an isolated primary target in the treatment of CKD-MBD. Nonetheless, its diagnostic and prognostic potential should be underlined.

Phosphate Additive Avoidance in Chronic Kidney Disease

IN BRIEF Dietary guidelines for patients with diabetes extend beyond glycemic management to include recommendations for mitigating chronic disease risk. This review summarizes the literature suggesting that excess dietary phosphorus intake may increase the risk of skeletal and cardiovascular disease in patients who are in the early stages of chronic kidney disease (CKD) despite having normal serum phosphorus concentrations. It explores strategies for limiting dietary phosphorus, emphasizing that food additives, as a major source of highly bioavailable dietary phosphorus, may be a suitable target. Although the evidence for restricting phosphorus-based food additives in early CKD is limited, diabetes clinicians should monitor ongoing research aimed at assessing its efficacy.

Tradeoff-in-the-Nephron: A Theory to Explain the Primacy of Phosphate in the Pathogenesis of Secondary Hyperparathyroidism

Chronic kidney disease (CKD) causes secondary hyperparathyroidism (SHPT). The cardinal features of SHPT are persistence of normocalcemia as CKD progresses and dependence of the parathyroid hormone concentration ([PTH]) on phosphate influx (IP). The tradeoff-in-the-nephron hypothesis integrates these features. It states that as the glomerular filtration rate (GFR) falls, the phosphate concentration ([P]CDN) rises in the cortical distal nephron, the calcium concentration ([Ca]CDN) in that segment falls, and [PTH] rises to maintain normal calcium reabsorption per volume of filtrate (TRCa/GFR). In a clinical study, we set GFR equal to creatinine clearance (Ccr) and IP equal to the urinary excretion rate of phosphorus (EP). We employed EP/Ccr as a surrogate for [P]CDN. We showed that TRCa/Ccr was high in patients with primary hyperparathyroidism (PHPT) and normal in those with SHPT despite comparably increased [PTH] in each group. In subjects with SHPT, we examined regressions of [PTH] on EP/Ccr before and after treatment with sevelamer carbonate or a placebo. All regressions were significant, and ∆[PTH] correlated with ∆EP/Ccr in each treatment cohort. We concluded that [P]CDN determines [PTH] in CKD. This inference explains the cardinal features of SHPT, much of the evidence on which other pathogenic theories are based, and many ancillary observations.

Adynamic bone disease: from bone to vessels in chronic kidney disease

Adynamic bone disease (ABD) is a well-recognized clinical entity in the complex chronic kidney disease (CKD)-mineral and bone disorder. Although the combination of low intact parathyroid hormone (PTH) and low bone alkaline phosphatase levels may be suggestive of ABD, the gold standard for precise diagnosis is histomorphometric analysis of tetracycline double-labeled bone biopsies. ABD essentially is characterized by low bone turnover, low bone volume, normal mineralization, and markedly decreased cellularity with minimal or no fibrosis. ABD is increasing in prevalence relative to other forms of renal osteodystrophy, and is becoming the most frequent type of bone lesion in some series. ABD develops in situations with reduced osteoanabolic stimulation caused by oversuppression of PTH, multifactorial skeletal resistance to PTH actions in uremia, and/or dysregulation of Wnt signaling. All may contribute not only to bone disease but also to the early vascular calcification processes observed in CKD. Various risk factors have been linked to ABD, including calcium loading, ageing, diabetes, hypogonadism, parathyroidectomy, peritoneal dialysis, and antiresorptive therapies, among others. The relationship between low PTH level, ABD, increased risk fracture, and vascular calcifications may at least partially explain the association of ABD with increased mortality rates. To achieve optimal bone and cardiovascular health, attention should be focused not only on classic control of secondary hyperparathyroidism but also on prevention of ABD, especially in the steadily growing proportions of diabetic, white, and elderly patients. Overcoming the insufficient osteoanabolic stimulation in ABD is the ultimate treatment goal.

Lanthanum carbonate for the control of hyperphosphatemia in chronic renal failure patients: a new oral powder formulation - safety, efficacy, and patient adherence

Chronic kidney disease (CKD) is associated with very high mortality rates, mainly of cardiovascular origin. The retention of phosphate (P) and increased fibroblast growth factor-23 levels are common, even at early stages of CKD, due to disturbances in normal P homeostasis. Later, hyperphosphatemia appears, which has also been strongly associated with high mortality rates linked to P-mediated cardiovascular and procalcifying effects. Treatment guidelines for these patients continue to be poorly implemented, at least partially due to the lack of adherence to a P-restricted diet and P-binder therapy. Calcium-free P binders, such as lanthanum carbonate, have been associated with a decreased progression of vascular calcification, rendering them an important therapeutic alternative for these high cardiovascular risk CKD patients. Lanthanum carbonate has typically been available as chewable tablets, and the new presentation as an oral powder may provide a useful alternative in the therapeutic armamentarium. This powder is a tasteless, odorless, and colorless semisolid compound miscible with food. In a recent study in healthy individuals, the safety and efficacy of this novel form were evaluated, and it was concluded that it is well tolerated and pharmacodynamically equivalent to the chewable form. In the long run, individualization of preferences and treatments seems an achievable goal prior to final demonstration of improvements in hard outcomes in wide clinical trials in CKD patients.

Total parathyroidectomy with forearm autotransplantation as the treatment of choice for secondary hyperparathyroidism

Chronic kidney disease-mineral and bone disease (CKD-MBD) is associated with uraemic bone disease, vascular calcification, reduced quality of life and reduced survival. This study evaluated the efficacy of parathyroidectomy (PTX) with autotransplantation in improving short-term and long-term outcomes. Dialysis patients who underwent PTX showed significantly more favourable biochemical parameters after PTX. These changes were accompanied by a lower coronary artery calcification score, reduced thickness of the intimae media and comparable bone mineral density measures compared with control dialysis patients who did not undergo PTX. Despite the risk of a substantially lower intact parathyroid hormone level postoperatively that might lead to adynamic bone disease, none of the patients reported clinical signs of this disease, such as bone pain or fractures. In conclusion, PTX with autotransplantation led to improvement of CKD-MBD so may be considered in patients with secondary hyperparathyroidism that is resistant to treatment with vitamin D analogues and calcimimetics.

Mineral and bone disorders in chronic kidney disease and end-stage renal disease patients: new insights into vitamin D receptor activation

Progressive loss of kidney function leads to reduced production of calcitriol (1,25-dihydroxyvitamin D; active vitamin D) and an imbalance in serum calcium (Ca) and phosphorus (P) levels, which are associated with progression of renal failure as well as increased rates of cardiovascular (CV) events and mortality. In addition, multifactorial hypocalcemia and resistance to parathyroid hormone (PTH) can lead to prolonged and excessive synthesis and secretion of PTH, eventually leading to development of secondary hyperparathyroidism and renal osteodystrophy. These changes associated with chronic kidney disease (CKD), extending beyond bone and related biochemical abnormalities, have prompted the development of the term CKD–mineral and bone disorder to describe its systemic nature. Excessive P loading, among other factors, will promote vascular calcification (VC), and PTH production will affect bone remodeling. Although administration of calcitriol increases serum Ca levels and decreases PTH, it is also associated with elevated Ca × P product. Therefore, compounds that selectively activate vitamin D receptors (VDR activators), potentially reducing Ca–P toxicity and distinctly affecting pathogenic mechanisms of VC, might enhance CV and renal protection, increase the vitamin D therapeutic window, and thus provide a significant clinical benefit. Moreover, selective VDR activators have been associated with improvement in survival, at least among dialysis patients. Thus, selective VDR activators should be considered a novel and interesting approach to enhance the standard of care in CKD patients.

Long-term treatment with lanthanum carbonate reduces mineral and bone abnormalities in rats with chronic renal failure

Background. Lanthanum carbonate (FOSRENOL^®, Shire Pharmaceuticals) is an effective non-calcium, non-resin phosphate binder for the treatment of hyperphosphataemia in patients with chronic kidney disease (CKD). In this study, we used a rat model of chronic renal failure (CRF) to examine the long-term effects of controlling serum phosphorus with lanthanum carbonate treatment on the biochemical and bone abnormalities associated with CKD–mineral and bone disorder (CKD–MBD).

Methods. Rats were fed a normal diet (normal renal function, NRF), or a diet containing 0.75% adenine for 3 weeks to induce CRF. NRF rats continued to receive normal diet plus vehicle or normal diet supplemented with 2% (w/w) lanthanum carbonate for 22 weeks. CRF rats received a diet containing 0.1% adenine, with or without 2% (w/w) lanthanum carbonate. Blood and urine biochemistry were assessed, and bone histomorphometry was performed at study completion.

Results. Treatment with 0.75% adenine induced severe CRF, as demonstrated by elevated serum creatinine. Hyperphosphataemia, hypocalcaemia, elevated calcium × phosphorus product and secondary hyperparathyroidism were evident in CRF + vehicle animals. Treatment with lanthanum carbonate reduced hyperphosphataemia and secondary hyperparathyroidism in CRF animals (P < 0.05), and had little effect in NRF animals. Bone histomorphometry revealed a severe form of bone disease with fibrosis in CRF + vehicle animals; lanthanum carbonate treatment reduced the severity of the bone abnormalities observed, particularly woven bone formation and fibrosis.

Conclusions. Long-term treatment with lanthanum carbonate reduced the biochemical and bone abnormalities of CKD–MBD in a rat model of CRF.

Update on the treatment of chronic kidney disease-mineral and bone disorder

Chronic kidney disease (CKD) is associated with increased morbidity and mortality. Mineral metabolism disturbances appear to contribute to the high mortality rate. A CKD-mineral bone disorder (CKD-MBD) has recently been defined as a systemic disorder manifested by one or a combination of abnormalities in bone biopsy, laboratory parameters and/or vascular or other soft-tissue calcifications. New available treatments have contributed to move from the former treatment paradigm of renal osteodystrophy to CKD-MBD management, beyond mere control of parathyroid hormone (PTH) and trying to improve cardiovascular or survival outcomes. Thus, the recommended multidisciplinary approach among nurses, dieticians and clinicians, helping not only through dietary assessment but also through education, behaviour control and by increasing the patient's personal motivation, may have additional important benefits. This article will review the current therapeutic approach with phosphate binders including the latest developments, vitamin D derivatives and selective vitamin D receptor activators as well as the new calcimimetics, all used in the treatment of this systemic disease.

Role of dietary phosphorus and degree of uremia in the development of renal bone disease in rats

The remnant kidney rat model has been extensively used for the evaluation of bone changes due to uremia. The present study aimed to assess the effect of the dietary phosphorus availability and of the severity of renal failure on bone histomorphometric changes and various biochemical markers over time in this model. Chronic renal failure (CRF) was induced in male Wistar rats by 5/6th nephrectomy. Half of the number of animals received a standard rat diet (STD) (0.67% P, containing low bioavailable phosphorus of plant origin); the other animals were fed a high phosphorus diet (HPD) (0.93% P, containing inorganic phosphorus with high bioavailability). Every two weeks, blood and urine samples were collected. At sacrifice after 6 or 12 weeks, bone samples were taken for the measurement of histological and histodynamic parameters. Serum creatinine measurements indicated the development of mild to moderate renal failure in both diet groups. Phosphaturia was unexpectedly low in all animals that received the STD, indicating relative phosphorus depletion despite the normal dietary phosphorus content. In the HPD CRF group, a decrease in calcemia and a rise in phosphatemia were seen after 12 weeks of CRF, which were more pronounced in animals with higher serum creatinine. Serum iPTH levels were distinctly increased in CRF rats fed a HPD, especially those with more pronounced renal failure. Serum osteocalcin and to a lesser extend tartrate-resistant acid phosphatase and urinary pyridinoline and deoxypyridinoline crosslinks were higher in the CRF animals compared to the shams, particularly in the animals of the HPD group with more pronounced CRF. In both diet groups, the CRF animals had significantly higher amounts of osteoid compared to shams. Only the animals that received a HPD developed distinct histological signs of secondary hyperparathyroidism (sHPTH), that is, an increased bone formation rate, mineral apposition rate, osteoblast perimeter, and eroded perimeter. Again, this effect was most prominent in rats with more severe CRF. In conclusion, data of the present study indicate that in experimental studies using the remnant kidney rat model, both the dietary phosphorus bioavailability and the degree of renal failure in the development of hyperparathyroidism should be considered.

Dynamics of parathyroid hormone secretion in health and secondary hyperparathyroidism

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

Effect of endothelin receptor antagonist on parathyroid gland growth, PTH values and cell proliferation in azotemic rats

BACKGROUND

A variety of stimuli are involved in the pathogenesis of parathyroid gland hyperplasia in renal failure. Recently, it was shown that blocking the signal from the endothelin-1 (ET-1) receptor (ET(A)R/ET(B)R) by a non-selective receptor antagonist, bosentan, reduced parathyroid cell proliferation, parathyroid gland hyperplasia and parathyroid hormone (PTH) levels in normal rats on a calcium deficient diet. Our goal was to determine whether in 5/6 nephrectomized (NPX) rats with developing or established hyperparathyroidism, the endothelin receptor blocker, bosentan, reduced the increase in parathyroid cell proliferation, parathyroid gland hyperplasia and PTH values.

METHODS

High (HPD, 1.2%) or normal phosphorus diets (PD) (NPD, 0.6%) were given to 5/6 NPX rats for 15 days (NPX(15)). In each dietary group, one-half the rats were given bosentan (B) i.p. 100 mg/kg/day. The four groups of rats were: (1) NPX(15)-1.2% P; (2) NPX(15)-1.2% P+B; (3) NPX(15)-0.6% P; and (4) NPX(15)-0.6% P+B. In a second study in which hyperparathyroidism was already established in 5/6 NPX rats fed a HPD for 15 days, rats were divided into two groups in which one group was maintained on a HPD and the other group was changed to very low PD (VLPD, <0.05%) for an additional 15 days. In each dietary group, one-half the rats were given bosentan i.p. 100 mg/kg-day. The four groups of rats were: (1) NPX(30)-1.2% P; (2) NPX(30)-1.2% P+B; (3) NPX(30)-0.05% P and (4) NPX(30)-0.05% P+B. Parathyroid cell proliferation was measured by proliferating cell nuclear antigen (PCNA) staining and ET-1 expression by immunohistochemical techniques.

RESULTS

In the study of developing hyperparathyroidism, bosentan reduced ET-1 expression in the parathyroid glands of rats on the NPD and HPD (P<0.05). But only in rats on the NPD did bosentan result in a reduced increase in parathyroid gland weight (P<0.05). In the study of established hyperparathyroidism, in which 5/6 NPX rats were given a HPD for 15 days, bosentan started on day 15 reduced (P<0.05) ET-1 expression in rats maintained for 15 additional days on the HPD or the VLPD. On the VLPD, parathyroid gland weight was less (P<0.05) than that in rats on the HPD sacrificed at 15 or 30 days. Bosentan did not reduce parathyroid cell proliferation or parathyroid gland weight in rats maintained on the HPD or further reduce these parameters beyond that obtained with dietary phosphorus restriction. PTH values were lowest in the VLPD group, intermediate in the NPD group, and highest in the HPD group, but in none of the three groups did bosentan decrease PTH values.

CONCLUSIONS

In azotemic rats with developing hyperparathyroidism, bosentan resulted in a reduced increase in parathyroid gland weight when dietary phosphorus content was normal. Despite a reduction in ET-1 expression in rats on a HPD with developing or established hyperparathyroidism, bosentan did not reduce the increase in parathyroid cell proliferation, parathyroid gland growth or PTH values. Thus, ET-1 blockade with bosentan did not prevent parathyroid gland growth in the azotemic rat.

Effect of ammonium chloride and dietary phosphorus in the azotaemic rat. I. Renal function and biochemical changes

BACKGROUND

Both dietary phosphorus restriction and the ingestion of ammonium chloride (NH(4)Cl) given to rats on a high-phosphorus diet have been shown to preserve renal function in the azotaemic rat. Parathyroidectomy also has been reported to preserve renal function and, in addition, to prevent kidney hypertrophy in the remnant kidney model. Our goals were (i) to evaluate in azotaemic rats the effect of dietary phosphorus on renal function in a shorter time frame than previously studied and (ii) to determine whether NH(4)Cl administration (a) enhances the renoprotective effect of dietary phosphorus restriction and (b) improves renal function in the absence of parathyroid hormone (PTH).

METHODS

High (H; 1.2%), normal (N; 0.6%) and low (L; <0.05%) phosphorus diets (PD) were given for 30 days to 5/6 nephrectomized rats. In each dietary group, one-half of the rats were given NH(4)Cl in the drinking water. The six groups were HPD + NH(4)Cl, HPD, NPD + NH(4)Cl, NPD, LPD + NH(4)Cl and LPD. The effect of NH(4)Cl administration was also evaluated in 5/6 nephrectomized, parathyroidectomized (PTX) rats on NPD.

RESULTS

In each of the three dietary phosphorus groups, creatinine and urea clearances were greater (P<0.01) in rats receiving NH(4)Cl. Neither creatinine nor urea clearance was reduced by high dietary phosphorus. Urine calcium excretion was greatest in the LPD group and was increased (P < or = 0.001) in all three groups by NH(4)Cl ingestion. An inverse correlation was present between plasma calcium and phosphorus in the parathyroid intact (r = -0.79, P<0.001) and PTX groups (r = -0.46, P = 0.02). In PTX rats, NH(4)Cl ingestion increased (P < or = 0.01) creatinine and urea clearances and both an increasing plasma calcium concentration (r = 0.67, P<0.001) and urine calcium excretion (r = 0.73, P<0.001) increased urine phosphorus excretion.

CONCLUSIONS

At 30 days of renal failure (i) NH(4)Cl ingestion increased creatinine and urea clearances, irrespective of dietary phosphorus; (ii) high urine calcium excretion, induced by dietary phosphorus restriction and NH(4)Cl ingestion, did not adversely affect renal function; (iii) high dietary phosphorus did not decrease renal function; (iv) the absence of PTH did not preserve renal function or prevent NH(4)Cl from improving renal function; and (v) both an increasing plasma calcium concentration and urine calcium excretion resulted in an increase in urine phosphorus excretion in PTX rats.

Differential regulation of PHEX expression in bone and parathyroid gland by chronic renal insufficiency and 1,25-dihydroxyvitamin D3

Mutations in the PHEX gene are responsible for X-linked hypophosphatemia, a renal phosphate-wasting disorder associated with defective skeletal mineralization. PHEX is predominantly expressed in bones and teeth and in the parathyroid gland of patients with chronic renal failure and tertiary hyperparathyroidism. The purpose of the present study was to examine the effects of renal insufficiency and 1,25-dihydroxyvitamin D3[1,25(OH)2D3] on the regulation of PHEX expression in rat tibia and parathyroid gland. In rats fed a high-phosphate (Pi) diet, ⅚ nephrectomy elicited a significant increase in the serum parathyroid hormone (PTH) concentration that was associated with a significant increase in the abundance of PHEX mRNA and protein in the tibia and a significant increase in PHEX mRNA in the parathyroid gland. In contrast, 1,25(OH)2D3administration to intact rats fed a control diet elicited a significant decrease in the serum PTH concentration that was accompanied by a significant decrease in PHEX mRNA and protein abundance in the tibia and a significant decrease in PHEX mRNA in the parathyroid gland. In addition, the increases in serum PTH levels and PHEX mRNA in the tibia and parathyroid gland in ⅚ nephrectomized rats fed a high-Pidiet were blunted by 1,25(OH)2D3. Serum PTH concentration was positively and significantly correlated with tibial PHEX mRNA and protein abundance. In summary, we demonstrate that PHEX expression in the tibia and parathyroid gland is increased by chronic renal insufficiency and decreased by 1,25(OH)2D3administration and suggest that PTH status may play an important role in mediating these changes in PHEX expression.

Dose-dependent effects of strontium on bone of chronic renal failure rats

BACKGROUND

We previously reported on increased bone strontium (Sr) levels in dialysis patients with osteomalacia versus those presenting other types of renal osteodystrophy. A causal role of strontium in the development of osteomalacia was established in a chronic renal failure (CRF) rat model.

METHODS

In the present study we investigated whether the effect of Sr on bone was related to dosage. Four groups of CRF rats were studied: a control group (control-CFR; N=6) not receiving strontium and three groups of animals loaded orally with Sr during 18 weeks by adding the element as the SrCl2. H20 compound to the drinking water at concentrations of 0.03 g/100mL (Sr-30; N=6), 0.075 g/100mL (Sr-75; N=6), or 0.15 g/100mL (Sr-150; N=6) respectively. A fifth group consisting of seven animals with intact renal function (control-NRF), not receiving Sr served as controls for the effect of CRF on bone histology.

RESULTS

As compared to the control-NRF and control-CRF groups, Sr administration resulted in a dose-dependent increase in bone and serum Sr levels. No difference in body weight and biochemical serum and urinary parameters [i.e., calcium (Ca), phosphorus (P), and creatinine] was noted between the various CRF groups. At sacrifice, intact parathyroid hormone (iPTH) levels of CRF groups were significantly (P < 0.05) higher than the values measured in the control-NRF group indicating the development of hyperparathyroidism secondary to the installation of the CRF. This is further supported by the differences in bone histomorphometry between the control-CRF and control-NRF animals, which, respectively, showed an increased amount of osteoid (mean +/- SEM 3.4 +/- 1.2% vs. 0.37 +/- 0.14%, P < 0.05) in combination with a distinct osteoblastic activity (35 +/- 11% vs. <2%, P < 0.05) and an increased bone formation rate [(BFR), 677 +/- 177 microm 2/mm2/day vs. 130 +/- 50 microm 2/mm2/day, P < 0.05]. Bone surface area and erodic perimeter did not differ between the various study groups. In the Sr-30 group, Sr loading went along with a dramatic reduction of the BFR as indicated by the total absence of double tetracyclin labels and osteoblastic activity, which in the presence of a low to normal amount of osteoid (2.7 +/- 1.9%) points to the development of the adynamic type of renal osteodystrophy. Interestingly, compared to the control-CRF group, histodynamic and histologic parameters of the Sr-75 group did not differ significantly and a substantial osteoblastic activity (7.6 +/- 4.0%) was seen also. In the Sr-150 group, the various osteoid parameters were significantly (P < 0.05) increased vs. all other groups and were accompanied by a reduced BFR and mineral apposition rate (MAR) and an increased mineralization lag time (MLT), indicating a mineralization defect and the development of osteomalacia.

CONCLUSIONS

Our findings indicate that the role of Sr in the development of bone lesions in renal failure is complex and that, depending on the dose, the element may act via multiple pathways.

Failure of high doses of calcitriol and hypercalcaemia to induce apoptosis in hyperplastic parathyroid glands of azotaemic rats

BACKGROUND

Whether calcitriol administration, which is used to treat secondary hyperparathyroidism in dialysis patients, induces regression of parathyroid-gland hyperplasia remains a subject of interest and debate. If regression of the parathyroid gland were to occur, the presumed mechanism would be apoptosis. However, information on whether high doses of calcitriol can induce apoptosis of parathyroid cells in hyperplastic parathyroid glands is lacking. Consequently, high doses of calcitriol were given to azotaemic rats and the parathyroid glands were evaluated for apoptosis.

METHODS

Rats were either sham-operated (two groups) or underwent a two-stage 5/6 nephrectomy (three groups). For the first 4 weeks, all rats were given a high (1.2%) phosphorus (P) diet to stimulate parathyroid gland growth and then were changed to a normal (0.6%) P diet for 2 weeks. At week 7, three of the five groups were given high doses of calcitriol (500 pmol/100 g body weight) intraperitoneally every 24 h during 72 h before sacrifice. The five groups during week 7 were: (i) normal renal function (NRF)+0.6% P diet; (ii) NRF+0.6% P+calcitriol; (iii) renal failure (RF)+0.6% P; (iv) RF+1.2% P+calcitriol; and (v) RF+0.6% P+calcitriol. Parathyroid glands were removed at sacrifice and the TUNEL stain was performed to detect apoptosis.

RESULTS

At sacrifice, the respective serum calcium values in calcitriol-treated groups (groups 2, 4, and 5) were 15.52+/-0.26, 13.41+/-0.39 and 15.12+/-0.32 mg/dl. In group 3, PTH was 178+/-42 pg/ml, but in calcitriol-treated groups, PTH values were suppressed, 8+/-1 (group 2), 12+/-2 (group 4), and 7+/-1 pg/ml (group 5). Despite, the severe hypercalcaemia and marked PTH suppression in calcitriol-treated groups, the percentage of apoptotic cells in the parathyroid glands was very low (range 0.08+/-0.04 to 0.25+/-0.20%) and not different among the five groups.

CONCLUSIONS

We found no evidence in hyperplastic parathyroid glands that apoptosis could be induced in azotaemic rats by the combination of high doses of calcitriol and severe hypercalcaemia despite the marked reduction in PTH levels that was observed.

Ultrasonography of the parathyroid glands as an aid in differentiation of acute and chronic renal failure in dogs

OBJECTIVE

To evaluate whether determination of parathyroid gland size by use of ultrasonography is helpful in differentiating acute renal failure (ARF) from chronic renal failure (CRF) in dogs.

DESIGN

Prospective study.

ANIMALS

20 dogs with renal failure in which serum creatinine concentration was at least 5 times the upper reference limit. Seven dogs had ARF, and 13 dogs had CRF. Twenty-three healthy dogs were used as controls.

PROCEDURE

Dogs were positioned in dorsal recumbency for ultrasonographic examination of the ventral portion of the neck, A 10-MHz linear-array high-resolution transducer was used. The size of the parathyroid gland was determined by measuring the maximal length of the gland on the screen when it was imaged in longitudinal section. For comparison among groups, the longest linear dimension of any of the parathyroid glands of each dog was used.

RESULTS

Size of the parathyroid glands in the control dogs varied from 2.0 to 4.6 mm (median, 3.3 mm). In the dogs with ARF, gland size ranged from 2.4 to 4.0 mm (median, 2.7), which was not significantly different from controls. In dogs with CRF, the glands were more distinctly demarcated from the surrounding thyroid tissue, than those of controls and dogs with ARF. Sizes ranged from 3.9 to 8.1 mm (median, 5.7 mm), which was significantly larger, compared with controls and dogs with ARF.

CONCLUSIONS AND CLINICAL RELEVANCE

In dogs with severe azotemia, ultrasonographic examination of the parathyroid glands was helpful in differentiating ARF from CRF Size of the parathyroid glands appeared to be related to body weight.

Chronic metabolic acidosis in azotemic rats on a high-phosphate diet halts the progression of renal disease

BACKGROUND

Hyperphosphatemia and metabolic acidosis are general features of advanced chronic renal failure (RF), and each may affect mineral metabolism. The goal of the present study was to evaluate the effect of chronic metabolic acidosis on the development of hyperparathyroidism and bone disease in normal and azotemic rats on a high-phosphate diet. Our assumption that the two groups of azotemic rats (acid-loaded vs. non-acid-loaded) would have the same degree of renal failure at the end of the study proved to be incorrect.

METHODS

Four groups of rats receiving a high-phosphate (1.2%), normal-calcium (0.6%) diet for 30 days were studied: (1) normal (N); (2) normal + acid (N + Ac) in which 1.5% ammonium chloride (NH4Cl) was added to the drinking water to induce acidosis; (3) RF, 5/6 nephrectomized rats; and (4) RF + acid (RF + Ac) in which 0.75% NH4Cl was added to the drinking water of 5/6 nephrectomized rats to induce acidosis.

RESULTS

At sacrifice, the arterial pH and serum bicarbonate were lowest in the RF + Ac group and were intermediate in the N + Ac group. Serum creatinine (0.76 +/- 0.08 vs. 1.15 +/- 0.08 mg/dL), blood urea nitrogen (52 +/- 8 vs. 86 +/- 13 mg/dL), parathyroid hormone (PTH; 180 +/- 50 vs. 484 +/- 51 pg/mL), and serum phosphate (7.46 +/- 0.60 vs. 12.87 +/- 1.4 mg/dL) values were less (P < 0.05), and serum calcium (9.00 +/- 0.28 vs. 7.75 +/- 0.28 mg/dL) values were greater (P < 0.05) in the RF + Ac group than in the RF group. The fractional excretion of phosphate (FEP) was greater (P < 0.05) in the two azotemic groups than in the two nonazotemic groups. In the azotemic groups, the FEP was similar even though PTH and serum phosphate values were less in the RF + Ac than in the RF group. NH4Cl-induced acidosis produced hypercalciuria in the N + Ac and RF + Ac groups. When acid-loaded (N + Ac and RF + Ac) and non-acid-loaded (N and RF) rats were combined as separate groups, serum phosphate and PTH values were less for a similarly elevated serum creatinine value in acid-loaded than in non-acid-loaded rats. Finally, the osteoblast surface was less in the N + Ac group than in the other groups. However, in the acid-loaded azotemic group (RF + Ac), the osteoblast surface was not reduced.

CONCLUSIONS

The presence of chronic metabolic acidosis in 5/6 nephrectomized rats on a high-phosphate diet (1) protected against the progression of RF, (2) enhanced the renal clearance of phosphate, (3) resulted in a lesser degree of hyperparathyroidism, and (4) did not reduce the osteoblast surface. The combination of metabolic acidosis and phosphate loading may protect against the progression of RF and possibly bone disease because the harmful effects of acidosis and phosphate loading may be counterbalanced.

Effect of high extracellular phosphate concentration on arachidonic acid production by parathyroid tissue in vitro

Recent in vivo and in vitro studies show that high phosphate directly stimulates parathyroid hormone (PTH) secretion. However, little is known about the intracellular signaling system involved in the regulation of PTH secretion by extracellular phosphate. High extracellular calcium is coupled to the activation of phospholipase A(2) (PLA(2)) and the formation of arachidonic acid (AA), a potent inhibitor of PTH release. The present study was designed to evaluate whether a high phosphate concentration has an effect on the PLA(2)-AA pathway in parathyroid cells. In vitro experiments were performed in parathyroid tissue obtained from normal rats and dogs. AA production was measured in parathyroid tissue in response to 1- and 4-mM phosphate concentration and after addition of PLA(2) to the medium. To determine whether the effect of phosphate on AA production in parathyroid cells was tissue specific, separate experiments were performed to test the effect of phosphate in rat adrenal glomerulosa cells, which are known to increase AA production in response to angiotensin II. The effect of sulfate, an ion with chemical characteristics similar to phosphate, on PTH secretion was also evaluated. In parathyroid tissue, a high phosphate concentration decreased the high calcium-induced AA production. This effect of phosphate was associated with an increase in PTH secretion. The addition of AA reversed the stimulatory effect of phosphate on PTH secretion. In another type of AA-responsive tissue, the adrenal glomerulosa, a high phosphate concentration did not affect the production of AA when stimulated by angiotensin II. In a normal phosphate concentration, the addition of PLA(2) stimulated AA production and decreased the PTH secretion. However, in a 4-mM phosphate concentration, the addition of PLA(2) did not reduce PTH secretion and did not stimulate AA production. Finally, sulfate did not affect PTH secretion. In conclusion, a high phosphate concentration affects the production of AA by parathyroid tissue. This effect of phosphate may be the mechanism by which a high phosphate concentration stimulates PTH secretion.

Effect of phosphate on parathyroid hormone secretion in vivo

Alterations in phosphate homeostasis play an important role in the development of secondary hyperparathyroidism in renal failure. Until recently, it was accepted that phosphate retention only increased parathyroid hormone (PTH) secretion through indirect mechanisms affecting calcium regulation and calcitriol synthesis. However, recent in vitro studies have suggested that phosphate may directly affect PTH secretion. Our goal was to determine whether in vivo an intravenous phosphate infusion stimulated PTH secretion in the absence of changes in serum calcium. Three different doses of phosphate were infused intravenously during 120 minutes to increase the serum phosphate concentration in dogs. Sulfate was also infused intravenously as a separate experimental control. A simultaneous calcium clamp was performed to maintain a normal ionized calcium concentration throughout all studies. At the lowest dose of infused phosphate (1.2 mmol/kg), serum phosphate values increased to approximately 3 mM, but PTH values did not increase. At higher doses of infused phosphate (1.6 mmol/kg and 2.4 mmol/kg), the increase in serum phosphate to values of approximately 4 mM and 5 mM, respectively, was associated with increases in PTH, even though the ionized calcium concentration did not change. Increases in PTH were not observed until 30-60 minutes into the study. These increases were not sustained, since by 120 minutes PTH values were not different from baseline or controls despite the maintenance of marked hyperphosphatemia. During the sulfate infusion, serum sulfate values increased by approximately 3-fold, but no change in PTH values were observed. In conclusion, an acute elevation in serum phosphate stimulated PTH secretion in the intact animal, but the magnitude of hyperphosphatemia exceeded the physiologic range. Future studies are needed to determine whether PTH stimulation is more sensitive to phosphate loading in states of chronic phosphate retention. Moreover, the mechanisms responsible for the delay in PTH stimulation and the failure to sustain the increased PTH secretion need further evaluation.

Dynamics of skeletal resistance to parathyroid hormone in the rat: effect of renal failure and dietary phosphorus

Secondary hyperparathyroidism develops in renal failure and is generally ascribed to factors directly affecting parathyroid hormone (PTH) production and/or secretion. These include hypocalcemia, phosphorus retention, and a calcitriol deficiency. However, not often emphasized is that skeletal resistance to PTH is an important factor. Our study evaluated: (1) the relative effects of uremia and dietary phosphorus on the skeletal resistance to PTH; and (2) how, during a PTH infusion, the dynamics of skeletal resistance to PTH were affected by renal failure. Renal failure was surgically induced and, based on serum creatinine, rats were divided into normal, moderate renal failure, and advanced renal failure. In each group, three diets with the same calcium (0.6%) but different phosphorus contents were used: high (1.2%, HPD); moderate (0.6%, MPD); and low (0.2%, LPD) phosphorus. The study diet was given for 14-16 days followed by a 48 h infusion of rat PTH(1-34) (0.11 microg/100 g per hour), a dose five times greater than the normal replacement dose. During the PTH infusion, rats received a calcium-free, low phosphorus (0.2%) diet. In both moderate and advanced renal failure, the PTH level was greatest in the HPD group (p < 0.05) and, despite normal serum calcium values, PTH was greater in the MPD than the LPD group (p < 0.05). Despite phosphorus restriction and normal serum calcium and calcitriol levels in the azotemic LPD groups, the PTH level was greater (p < 0.05) in the LPD group with advanced rather than moderate renal failure. During PTH infusion, the increase in serum calcium was progressively less (p < 0.05) in all groups as renal function declined. Furthermore, despite normal and similar serum phosphorus values at the end of PTH infusion, the serum calcium concentration was less (p < 0.05) in the HPD group than the other two groups and similar in the LPD and MPD groups.

IN CONCLUSION

(1) uremia and phosphorus each had separate and major effects on skeletal resistance to PTH; (2) skeletal resistance to PTH was an important cause of secondary hyperparathyroidism, even in moderate renal failure; (3) during PTH infusion, the dynamics of skeletal resistance to PTH changed because all groups received a low phosphorus diet, and the adaptation to a new steady state was delayed by the degree of renal failure and the previous dietary phosphorus burden; and (4) normal serum phosphorus may not be indicative of body phosphorus stores during states of disequilibrium.

Effect of serum phosphate on parathyroid hormone secretion during hemodialysis

BACKGROUND

Recent studies have demonstrated that a high concentration of phosphate directly stimulates parathyroid hormone (PTH) secretion. High serum levels of phosphate are usually observed in patients with end-stage renal disease. The aim of the present study was to evaluate whether serum phosphate concentration had an acute effect on PTH secretion in hemodialysis patients. The levels of serum phosphate were manipulated during the hemodialysis session by using a phosphate free dialysate or a dialysate with a high content of phosphate.

METHODS

Ten stable hemodialysis patients with PTH values above 300 pg/ml were included in the study. A PTH-calcium curve was obtained during both high phosphate and phosphate free hemodialysis.

RESULTS

The serum phosphate concentration remained high (2.17 +/- 0.18 mM) throughout the high phosphate hemodialysis and decreased progressively to normal levels (1.02 +/- 0.06 mM) during the phosphate free hemodialysis. The serum PTH levels at maximal inhibition by hypercalcemia (minimal PTH) were greater during the high phosphate than the phosphate free hemodialysis (413 +/- 79 vs. 318 +/- 76 pg/ml, P < 0.003). In all patients the values of minimum PTH were greater during the high phosphorus than the phosphorus free hemodialysis. The values of maximally stimulated PTH during hypocalcemia and the set point of the PTH-calcium curve were similar during the high phosphate and the phosphate free hemodialysis.

CONCLUSION

The maintenance of high serum phosphorus levels during hemodialysis prevented, in part, the inhibition of PTH secretion by calcium, which strongly suggests that in hemodialysis patients high serum phosphate contributes directly to the elevation of PTH levels despite normal or high serum calcium concentration.

Disparate effects of mild, moderate, and severe secondary hyperparathyroidism on cancellous and cortical bone in rats with chronic renal insufficiency

The subtotally nephrectomized rat has often been used to investigate the etiology and treatment of secondary hyperparathyroidism (secondaryHPT), but it has been used less frequently to study the effects of secondaryHPT on bone. The recent development of a reliable and specific rat parathyroid hormone (PTH) immunoradiometric assay has provided an opportunity for a thorough investigation of the relationship between circulating, biologically active PTH, and the skeletal abnormalities that occur in chronic renal insufficiency (CRI). Rats were 5/6 nephrectomized (Nx) or sham operated and fed diets with varying levels of Ca and P for 12-14 weeks to induce differing magnitudes of secondaryHPT. Parathyroid gland volume increased by 80%-90% in 5/6 Nx rats in the mild and moderate secondaryHPT groups (2.3- and 7.7-fold higher PTH levels, respectively) and by 3.3-fold in the severe secondaryHPT group (12-fold increase in PTH). The increases in gland volume were caused primarily by cell hyperplasia. Mild secondaryHPT resulted in a 12% decrease in bone mineral density (BMD) across the entire femur, increased osteoclast numbers (N.Oc), unchanged osteoblast numbers (N.Ob), and decreased cancellous bone volume (Cn.BV) in the tibial metaphysis but, apart from increased marrow area, no major changes in cortical bone at the tibio-fibular junction. Moderate secondaryHPT was associated with no changes in femoral BMD, or in tibial Cn.BV, but N.Ob and bone formation rate (BFR) were markedly elevated. Increased periosteal, intracortical, and endocortical BFR and turnover were evident, and contributed to increased cortical porosity (Ct.Po). The changes were exaggerated in the severe secondaryHPT group; BMD was lower in the proximal, but higher in the distal femur, and Cn.BV, N.Ob, N.Oc, and BFR were increased by six-, seven-, three-, and 30-fold, respectively. Endocortical BFR was elevated 31-fold and the extensive Ct.Po (10%) decreased bone strength. However, Ct.Po was not apparent until PTH levels exceeded 500 pg/mL. Thus, in rats with CRI of similar magnitude, progressive secondaryHPT is associated with dramatically different effects on bone. Mild secondaryHPT caused loss of cancellous and endocortical bone, and moderate secondaryHPT tended to maintain both types of osseous tissue, whereas PTH levels >500 pg/mL resulted in substantial cortical bone loss, but cancellous bone gain.

A new analog of 1,25-(OH)2D3, 19-NOR-1,25-(OH)2D2, suppresses serum PTH and parathyroid gland growth in uremic rats without elevation of intestinal vitamin D receptor content

We have previously reported that 19-nor-1,25-(OH)2D2, a new analog of 1,25-(OH)2D3, suppresses parathyroid hormone (PTH) secretion in uremic rats in the absence of hypercalcemia or hyperphosphatemia. In the current study, we examined the effect of 19-nor-1,25-(OH)2D2 on parathyroid gland growth and intestinal vitamin D receptor (VDR) content. After induction of uremia by 5/6 nephrectomy, rats were divided into five experimental groups and received intraperitoneal injections of vehicle, 1,25-(OH)2D3 (2 or 6 ng/rat), or 19-nor-1,25-(OH)2D2 (25 or 100 ng/rat) three times a week for 8 weeks. Twelve normal rats received vehicle and served as the normal control group. During the course of the study, rats were maintained on a 1.0% calcium and 0.8% phosphorus diet. The higher dose of 1,25-(OH)2D3, 6 ng, significantly decreased PTH from 52.7 +/- 10.2 pg/mL in the uremic control group to 25.7 +/- 6.7 pg/mL (P < 0.01). This dose of 1,25-(OH)2D3, however, increased serum levels of both ionized calcium (4.71 +/- 0.05 to 4.85 +/- 0.06 mg/dL; P < 0.05) and phosphorus (4.34 +/- 0.30 to 6.67 +/- 0.63 mg/dL; P < 0.01). Both doses of 19-nor-1,25-(OH)2D2 decreased serum PTH as effectively as 1,25-(OH)2D3 without changes in serum calcium or phosphorus. The 100-ng dose of 19-nor-1,25-(OH)2D2 decreased PTH to 20.7 +/- 3.1 pg/mL (P < 0.01) and suppressed parathyroid gland growth by more than 50%. Both doses of 19-nor-1,25-(OH)2D2 also decreased endogenous 1,25-(OH)2D3 levels compared with uremic control rats (25 ng:30.4 +/- 2.0, P < 0.05, and 100 ng:27.9 +/- 3.2, P < 0.01, v 48.4 +/- 6.6 pg/mL). The 6-ng dose of 1,25-(OH)2D3 elevated intestinal VDR content (138.5 +/- 20.0 fmol/mg protein) compared with animals receiving both doses of 19-nor-1,25-(OH)2D2 (25 ng:84.0 +/- 11.9, P < 0.05, and 100 ng:78.4 +/- 10.9, P < 0.01). This was probably attributable to the marked decrease in endogenous 1,25-(OH)2D3 levels caused by both doses of 19-nor-1,25-(OH)2D2 because intestinal VDR correlated directly with serum 1,25-(OH)2D3 (r = 0.963; P = 0.008). Thus, 19-nor-1,25-(OH)2D2 appears to exert a selective action on the parathyroid glands compared with the intestine. Its low calcemic and phosphatemic properties may result from the decreased endogenous 1,25-(OH)2D3 levels that lead to a reduction in intestinal VDR. This selectivity makes this analog ideal for the treatment of secondary hyperparathyroidism.

High phosphorus diet increases preproPTH mRNA independent of calcium and calcitriol in normal rats

Phosphorus retention favors secondary hyperparathyroidism by decreasing calcitriol synthesis and serum calcium levels. However, a direct effect of high extracellular phosphorus on parathyroid (PTH) function, gene expression, and cell proliferation is still controversial. Normal rats were fed standard (St; 0.6% calcium, 0.6% phosphorus) or high phosphate (HP) diet (0.6% calcium, 1.2% phosphorus) for 18 days. To rule out transient decreases in serum calcium or calcitriol levels, sets of animals were sacrificed at different time periods after the last feeding (2, 4, 8, 12 or 24 hr). The HP diet led to hyperphosphatemia and secondary hyperparathyroidism and maximum differences in PTH levels were observed eight hours after feeding (St 29.4 +/- 15 vs. HP 87.9 +/- 56 pg/ml, mean +/- SD; P = 0.01). High levels induced by the HP diet prevented both hypocalcemia and low calcitriol levels at each study point. The HP diet also promoted a significant increase of PTH mRNA levels that peaked about eight hours after feeding (100% increase). This was confirmed at the cellular level by in situ hybridization. Parathyroid glands from animals fed the HP diet showed a 25% increase in volume with respect to the St diet (P = 0.01), and a typical pattern of hyperplasia was found. Parathyroid vitamin D receptor (VDR) mRNA levels were not modified by the HP diet. In conclusion, parathyroid gene expression per cell and parathyroid cell hyperplasia are stimulated by high dietary phosphorus independently of calcium and calcitriol. This effect is not mediated by alterations in the gene expression of the parathyroid calcitriol receptor. Our findings emphasize the importance of the control of hyperphosphatemia in chronic renal failure patients.

Direct effect of phosphorus on PTH secretion from whole rat parathyroid glands in vitro

Phosphorus retention is an important factor in the development of hyperparathyroidism secondary to renal failure. In vivo manipulation of phosphorus is associated with changes in serum calcium and calcitriol levels which in turn can modify parathyroid hormone synthesis and secretion. The present in vitro study evaluates whether high extracellular phosphorus has a direct effect on parathyroid hormone secretion. Fresh rat parathyroid glands were incubated in a media with phosphorus concentrations of 1, 2, 3, and 4 mM and subsequently exposed to calcium levels ranging from 0.4 to 1.35 mM. In 1.25 mM calcium, the parathyroid hormone secretion rate was similar in 1 and 2 mM phosphorus; however, a phosphorus concentration of 3 and 4 mM produced a 3- and 4-fold increase in the parathyroid hormone secretion, respectively, as compared with 1 mM phosphorus. While in 1 or 2 mM phosphorus an increase in calcium from 0.6 to 1.35 mM reduced parathyroid hormone secretion to 37%, in 4 mM phosphorus the same increase in calcium only inhibited parathyroid hormone secretion to 75%. Furthermore, the addition of arachidonic acid 20 microM, a substrate for inhibitory intracellular signal pathway, to the 4 mM phosphorus-1.35 mM calcium incubation media reduced the parathyroid hormone secretion to 34.5% (p < 0.05). In conclusion, our results indicate that in vitro, high phosphorus directly increases parathyroid hormone secretion.

Relative effects of PTH and dietary phosphorus on calcitriol production in normal and azotemic rats

In moderate renal failure, the serum calcitriol level is influenced by the stimulatory effect of high PTH and the inhibitory action of phosphorus retention. Our goal was to evaluate the relative effect that high PTH levels and increased dietary phosphorus had on calcitriol production in normal rats (N) and rats with moderate renal failure (Nx). Normal and Nx (3/4 nephrectomy) rats were divided into two groups: (1) rats with intact parathyroid glands (IPTG) and (2) parathyroidectomized rats in which PTH was replaced (PTHR) by the continuous infusion of rat 1-34 PTH, 0.022 microgram/hr/100 g body wt, using a miniosmotic Alzet pump. To test the effect of dietary phosphorus, rats received either a moderate (MPD, 0.6% P) or a high phosphorus (HPD, 1.2%) diet for 14 days. The experimental design included pair-fed N and Nx rats with either IPTG or PTHR. Serum calcitriol and PTH levels in N rats fed a MPD were 69 +/- 3 and 40 +/- 5 pg/ml, respectively. In Nx rats on a MPD, serum calcitriol levels decreased only if hyperparathyroidism was not allowed to occur (76 +/- 4 vs. 62 +/- 4 pg/ml in Nx-IPTG-MPD and Nx-PTHR-MPD groups respectively, P < 0.05). Even in N rats on a HPD, high PTH levels (67 +/- 8 pg/ml in the N-IPTG-HPD group) were required to maintain normal serum calcitriol levels (69 +/- 4 vs. 56 +/- 6 pg/ml in Nx-IPTG-HPD and Nx-PTHR-HPD groups, respectively; P < 0.05). In Nx rats on a HPD, the development of secondary hyperparathyroidism (286 +/- 19 pg/ml in the Nx-IPTG-HPD group) prevented a decrease in serum calcitriol levels (68 +/- 7 pg/ml). In contrast, serum calcitriol levels were low in the Nx-PTHR-HPD group (52 +/- 4 pg/ml, P < 0.05), which were deprived of the adaptative increase in endogenous PTH production. In conclusion, our results in rats indicate that in moderate renal failure, an elevated PTH level maintains calcitriol production and overcomes the inhibitory action of phosphorus retention.

The set point of calcium and the reduction of parathyroid hormone in hemodialysis patients

Since in some studies in hemodialysis patients calcitriol treatment has resulted in a reduction of both parathyroid hormone (PTH) levels and the set point of calcium, it has been suggested that that the set point of calcium reflects a reduction in the magnitude of hyperparathyroidism. However, others have maintained that the set point of calcium is primarily an indicator of the serum calcium at which PTH is secreted and may be dissociated form the magnitude of hyperparathyroidism. The present study was designed to evaluate how a reduction in PTH levels associated with an increase in the predialysis (basal) serum calcium would affect the set point of calcium. Two different treatments were used to produce a reduction in PTH that was associated with an increase in predialysis serum calcium. In the first group, hemodialysis patients received 2 micrograms of intravenous calcitriol and were dialyzed with a 3.5 mEq/liter calcium dialysate for six weeks; in the second group, hemodialysis patients were dialyzed with a 4 mEq/liter calcium dialysate and had oral calcium supplementation increased for six weeks. In both groups, low and high calcium studies were performed to determine the PTH-calcium relationship before treatment, at the end of six weeks of treatment, and six weeks after the discontinuation of treatment. In the calcitriol group the predialysis calcium increased form 9.62 +/- 0.34 to 10.56 +/- 0.31 mg/dl, P < 0.05 and the set point of calcium increased from 9.34 +/- 0.23 to 9.79 +/- 0.25 mg/dl, P < 0.05 at the same time as maximally stimulated PTH decreased from 2637 +/- 687 to 1555 +/- 617 pg/ml, P < 0.05. In the high calcium dialysate group, the predialysis serum calcium increased from 9.19 +/- 0.31 to 9.84 +/- 0.28 mg/dl, P < 0.05, and set point of calcium increased form 9.01 +/- 0.28 to 9.39 +/- 0.22 mg/dl, P < 0.05 at the same time as maximally stimulated PTH decreased from 1642 +/- 450 to 1349 +/- 513 pg/ml, P < 0.05. Discontinuation of treatment for six weeks resulted in a return to pretreatment values. In conclusion, our results would suggest that (1) the set point of calcium may not be a reliable indicator of the magnitude of hyperparathyroidism during calcitriol treatment, and (2) PTH secretion may adapt to the ambient serum calcium concentration.

Development of secondary hyperparathyroidism and bone disease in diabetic rats with renal failure

Renal osteodystrophy in diabetic patients on maintenance hemodialysis is characterized by a higher prevalence of low bone turnover and is associated with a relative deficiency of parathyroid hormone (PTH) as compared with non-diabetic hemodialysis patients. The goal of the study was to evaluate how diabetes affected the development of secondary hyperparathyroidism (2 degrees HPT) and bone disease in azotemic rats. Three groups of 5/6 nephrectomized, pair-fed male Wistar rats maintained on a high phosphorus (1.2%) diet were studied: (1) the control group, non-diabetic azotemic rats (NDR); and two streptozotocin-induced diabetic azotemic groups, (2) poorly-controlled diabetic rats (PCDR) which received only enough NPH insulin to maintain the blood glucose between 300 and 400 mg/dl, and (3) well-controlled insulin-treated diabetic rats (IDR) which received a continuous insulin infusion for 14 days via a subcutaneously implanted miniosmotic pump. Serum calcium, phosphorus and creatinine levels were similar among the three groups. Blood glucose levels were greater in the PCDR group than the IDR and NDR groups (358 +/- 11 vs. 83 +/- 9 and 87 +/- 8 mg/dl, respectively; P < 0.001). Rats in the PCDR group weighed less at sacrifice as compared with the IDR and NDR groups (P < 0.05). Serum PTH levels (normal 47 +/- 2 pg/ml) were elevated, but not different among the three groups (136 +/- 34, 147 +/- 21 and 98 +/- 8 pg/ml in the PDCR, IDR and NDR groups, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Secondary hyperparathyroidism in renal failure: the trade-off hypothesis revisited

Our understanding of the mechanism responsible for secondary hyperparathyroidism (HPTH) has advanced significantly since the "trade-off" hypothesis was formulated. It appears that in early renal failure a deficit of calcitriol synthesis is an important factor. However, additional factors, such as a defect of the vitamin D receptor or the newly cloned calcium sensor receptor (BoPCaR1), may be present in the parathyroid cells. As renal failure progresses, the lack of calcitriol becomes more pronounced, inducing HPTH. With advanced chronic renal failure, hyperphosphatemia is an additional important factor in worsening HPTH. In addition, resistance of the parathyroids to calcitriol due to a reduced density of calcitriol receptors also may contribute to HPTH. Finally, uremia per se not only may cause a receptor abnormality in the parathyroid but at the level of the bone it may aggravate the impaired calcemic response to PTH. In conclusion, after reviewing the "trade-off" hypothesis, although some of the original concepts may have been simplistic, most of the factors postulated 30 years ago are still operative in the pathogenesis of secondary HPTH in renal failure.

Direct in vivo assessment of parathyroid hormone-calcium relationship curve in renal patients

Secondary hyperparathyroidism (SHP) is a well documented finding even in the early stages of chronic renal failure (CRF). A sigmoidal relationship, fitting a four parameter model, links PTH secretion rate and calcium concentration changes. To our knowledge, PTH secretory parameters have only been studied in uremic patients who are in dialysis treatment. As a result of these studies, a possible role for derangement in setpoint values (that is, the serum calcium concentration corresponding to the mid-range value on the sigmoidal curve) has been suggested in the pathogenesis of SHP in CRF. Our study was undertaken to gain insight into the calcium-PTH relationship curve in the first course of CRF and to assess whether a change in any of the secretory parameters is related to the beginning of SHP. We studied 27 male renal patients with a variable degree of renal function (creatinine clearance 12 to 164 ml/min) and 9 control subjects. In all patients and controls the following parameters were evaluated: (1) basal 1,25(OH)2 vitamin D, 25(OH)vitamin D, calcitonin (CT), intact PTH; (2) GFR by Cr51EDTA clearance; (3) the sigmoidal PTH-ionized calcium relation curve, by means of a hypocalcemic stimulating test (Na2-EDTA 37 mg/kg body weight/2 hr) and a hypercalcemic test (Ca gluconate giving 8 mg/kg of body weight/2 hr of Ca element), performed on two consecutive days.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorus and protein restriction and parathyroid function in chronic renal failure

Phosphorus retention as a result of chronic renal failure (CRF) induces secondary hyperparathyroidism (HPT II) while supplemented low-phosphorus low-protein diets (LPD) prevent it. The aim of this study was to assess in seven patients with advanced CRF and biological HPT II the effects of a LPD providing daily 5 to 7 mg/kg phosphorus, 0.4 g/kg protein, 300 mg calcium (Ca) and supplemented with amino acids, ketoacids, CaCO3 and vitamin D2, on the relationship between ionized Ca (iCa) and PTH concentrations. Hyper- and hypocalcemia were induced by CaCl2 and Na2-EDTA infusion. After three months of LPD, serum phosphorus decreased from 1.59 +/- 0.15 to 1.26 +/- 0.24 mmol/liter (mean +/- SEM, P < 0.02), basal PTH levels from 251 +/- 25 to 127 +/- 16 pg/ml (P < 0.03), while basal iCa and GFR did not vary. The sigmoidal PTH-calcium curve shifted downward with maximal PTH decreased from 482 +/- 86 to 319 +/- 60 pg/ml (P < 0.02) and minimal PTH from 35 +/- 4 to 21 +/- 4 pg/ml (P < 0.05). On the other hand, the slope of the % maximal PTH-iCa curve, which is an indicator of the sensitivity of the parathyroid cell to changes in iCa concentrations, did not vary significantly. The set point of Ca and calcitriol levels were not modified. These results demonstrate a direct inhibition of PTH secretion over a wide range of iCa concentration by LPD in patients with advanced CRF and mild HPT II over a three months period. This effect is independent of changes in plasma calcitriol levels.

Comparison of two parathyroid hormone assays for the rat: the new immunoradiometric and the older competitive binding assay

A competitive binding assay for parathyroid hormone (PTH INS) has been used since 1986 to measure PTH in rats. During the past year an immunoradiometric assay for the measurement of PTH (PTH IRMA) in the rat was developed. The purpose of the present study was to compare results obtained with the PTH INS and IRMA and to provide a framework for comparison for investigators who have used the PTH INS in previous studies. A total of 99 rats were studied; 27 rats had normal renal function, and 72 rats had surgically induced renal failure. In the azotemic rats, the magnitude of hyperparathyroidism was varied by changing the calcium and phosphorus composition of the diet. The correlation between the two PTH assays in the 99 rats was r = 0.98, p < 0.001. For the 27 rats with normal renal function, the correlation even within the narrow range of normal PTH values was significant, r = 0.71, p < 0.001. In the 72 azotemic rats, in which the highest INS PTH value was approximately 17 times normal, the correlation between the two PTH assays was r = 0.98, p < 0.001. The PTH IRMA provides distinct advantages, such as extended standard range, shortened incubation time, increased sensitivity, and technical simplicity, but our results indicate that the PTH INS provided an accurate measurement of PTH. Furthermore, our results should provide investigators who have used the PTH INS in previous studies with a framework for comparison with studies in which the PTH IRMA was used.

The calcemic response to PTH in the rat: effect of elevated PTH levels and uremia

Secondary hyperparathyroidism (2 degrees HPT) is a consistent finding in renal failure. A decreased calcemic response (CR) to parathyroid hormone (PTH) contributes to the development of 2 degrees HPT. Since parathyroidectomy (PTX) corrects the decreased CR to PTH in azotemic animals, down-regulation of PTH receptors induced by an elevation of PTH has been advanced as an important factor in the development of 2 degrees HPT. The goal of the study was to determine in azotemic rats whether a progressive reduction of PTH improves the CR to PTH and whether the maintenance of normal PTH levels corrects the CR to PTH. Seven groups of pair-fed rats were studied. Three groups of rats had normal renal function (NRF groups) and received either a high phosphorus (HPD-NRF), a moderate phosphorus (MPD-NRF), or a low phosphorus (LPD-NRF) diet. Three azotemic (NX) groups received similar diets (HPD-NX, MPD-NX and LPD-NX groups) in order to vary the magnitude of 2 degrees HPT. A PTX was performed in a fourth azotemic group (PTX-NX) to induce the complete absence of PTH. After 14 to 16 days on the maintenance diets, the CR to PTH was determined with a 48 hour infusion of 1-34 rat PTH.(ABSTRACT TRUNCATED AT 250 WORDS)