Calcium-sensing receptors

It is now known that variations in extracellular calcium concentration exert diverse physiologic effects in a variety of tissues that are mediated by a calcium-sensing receptor (CaSRs). In parathyroid tissue, the CaSR represents the molecular mechanism by which parathyroid cells detect changes in blood ionized calcium concentration, modulate parathyroid hormone (PTH) secretion accordingly, and thus maintain serum calcium levels within a narrow physiologic range. In the kidney, the CaSR regulates renal calcium excretion and influences the transepithelial movement of water and other electrolytes. More generally, activation of the CaSR represents an important signal transduction pathway in intestine, placenta, brain, and perhaps bone. Some of these actions involve cell cycle regulation, changes that may be relevant to understanding the pathogenesis of parathyroid gland hyperplasia in secondary hyperparathyroidism caused by chronic kidney disease. The CaSR represents an appealing target for therapeutic agents designed to modify parathyroid gland function in vivo, offering the prospect of novel therapies for selected disorders of bone and mineral metabolism. Other receptors capable of responding to extracellular calcium ions also have been identified, but the functional importance of these interactions remains to be determined.     

FGF23 and the parathyroid glands

Fibroblast growth factor 23 (FGF23) is a phosphatonin that is secreted by osteocytes and osteoblasts in response to hyperphosphatemia and 1,25-dihydroxyvitamin D (1,25D). It acts on its receptor complex, Klotho–FGFR1c (fibroblast growth factor receptor 1 c-splicing form), in the distal convoluted tubule to repress renal phosphorus reabsorption in the proximal tubule and suppress the renal synthesis of 1,25D. Klotho–FGFR1c is also expressed in the parathyroid glands. FGF23 acts on the receptor complex in the parathyroid glands to decrease parathyroid hormone (PTH) gene expression and PTH secretion through activation of the MAPK pathway. In chronic kidney disease (CKD), both FGF23 and PTH are increased, implying resistance of the parathyroid glands to FGF23. There is a decrease in the Klotho–FGFR1c complex in the parathyroid glands in both experimental CKD and in patients with end-stage renal disease. In addition, in advanced experimental CKD, FGF23 has a decreased ability to inhibit PTH expression.     

Cinacalcet HCl: a novel treatment for secondary hyperparathyroidism in stage 5 chronic kidney disease

BACKGROUND: Chronic kidney disease (CKD) alters the regulation of calcium and phosphate homeostasis, leading to secondary hyperparathyroidism, metabolic bone disease, soft tissue calcifications, and other metabolic derangements that have a significant impact on morbidity and mortality. The parathyroid gland is the central organ responsible for regulating these adaptive responses. Suppression of parathyroid hormone (PTH) secretion, hypertrophy, and hyperplasia are a major goal of treatment of CKD. METHODS: Current literature was reviewed and combined with the author's experience to address a number of issues regarding the optimal treatment of secondary hyperparathyroidism in hemodialysis patients. RESULTS: The calcium sensing receptor (CASR) is the most important factor regulating parathyroid gland function, and allosteric modulators of CASR, called calcimimetics, provide a novel drug therapy to suppress PTH secretion. The current use of active vitamin D analogues to suppress PTH is often limited by hypercalcemia and hyperphosphatemia. Clinical trials of cinacalcet HCl, the first calcimimetic to be approved for treatment of secondary hyperparathyroidism in CKD, have demonstrated suppression of circulating PTH levels without increments in the calcium-phosphorus (Ca x P) product, making it easier to achieve the stringent management guidelines proposed for subjects with CKD by the National Kidney Foundation Kidney Disease Outcomes Quality Initiative (K/DOQI). CONCLUSION: The management of disordered calcium and phosphate homeostasis in CKD patients is evolving based on our knowledge of the major importance of the calcium sensing receptor (CASR) in controlling parathyroid gland function and the potent actions of calcimimetics to target CASR. The purpose of this presentation is to provide an overview of the role of the CASR in regulation of parathyroid gland function, to examine the mechanisms whereby calcimimetics target the CASR, and to review the clinical trials that support the use of cinacalcet HCl for the treatment of secondary hyperparathyroidism in stage 5 chronic kidney disease (CKD).     

22-Oxacalcitriol upregulates p21(WAF1/Cip1) in human parathyroid glands. A preliminary report.

In the era of 22-oxacalcitriol (OCT), newly synthesized 1 alpha,25-dihydroxyvitamin D(3) analogue, against secondary hyperparathyroidism, the indications of parathyroidectomy (PTx) has been restricted. Recent investigations on animal models have revealed the inhibitory effects on PTH secretion after OCT treatment, whereas there has been no evidence about human parathyroid glands. A 38-year-old man with a 19-year history of hemodialysis was performed PTx after the failure of OCT treatment. Expressions of proliferative nuclear cell antigen (PCNA), calcium-sensing receptor (CaSR), vitamin D receptor (VDR), p53 and p21(WAF1/Cip1) were analyzed by Western blotting and immunohistochemistry on resected parathyroid glands. We confirmed up-regulations of CaSR and VDR, which contribute the reduction of serum PTH, by OCT treatment. Concomitant up-regulation of p21(WAF1/Cip1) but not p53, especially in nodular hyperplasia, can be considered to induce cell cycle arrest of the parathyroid cells, but not cytocidal effect of OCT.     

Vitamin D treatment in chronic kidney disease

Activated vitamin D continues to be the major treatment for suppressing parathyroid hormone (PTH) levels in dialysis patients who have secondary hyperparathyroidism. Active vitamin D compounds are distinguished by their ability to bind with high affinity to vitamin D receptors (VDRs) not only in the parathyroid glands, but in cells throughout the body. Because of recent data showing that pulsatile, intravenous vitamin D treatment (calcitriol or paricalcitol) confers a survival advantage in the dialysis population, there is new interest in understanding the systemic effects of VDR activation, particularly in the predialysis stages of chronic kidney disease (CKD), where high mortality rates from cardiovascular disease have recently been documented. Previous underutilization of calcitriol treatment to control PTH levels in stages 3 and 4 CKD was often due to concerns about its potential for accelerating the progression of CKD as a consequence of hypercalcemia, hypercalciuria, or hyperphosphatemia. Vitamin D analogs with selective VDR activity (such as paricalcitol) have great potential for preventing parathyroid hyperplasia and bone loss in early CKD without adversely affecting kidney function. Whether they also reduce cardiovascular morbidity and mortality in early CKD, as they appear to do in dialysis patients, remains to be determined.     

Calcimimetic agents and secondary hyperparathyroidism: rationale for use and results from clinical trials

Calcimimetic agents are small organic molecules that act as allosteric activators of the calcium sensing receptor (CaSR). In parathyroid cells, they lower the threshold for receptor activation by extracellular calcium ions and diminish parathyroid hormone (PTH) secretion. Calcimimetic compounds thus represent a novel way of controlling excess PTH secretion in clinical disorders such as secondary hyperparathyroidism (SHPT) due to chronic renal failure. Clinical trials have documented that the calcimimetic agent cinacalcet hydrochloride effectively lowers plasma PTH levels without increasing serum calcium or phosphorus concentrations in adult hemodialysis patients with SHPT. Serum phosphorus levels and values for the calcium-phosphorus ion product in serum often decline as plasma PTH levels fall during treatment. Experimental evidence suggests that calcimimetic agents may also impede the development of parathyroid gland hyperplasia, an integral component of SHPT due to chronic renal failure. Calcimimetic agents have considerable potential, therefore, as part of new therapeutic strategies for SHPT.     

Secondary hyperparathyroidism: Pathophysiology, histopathology, and medical and surgical management

It is generally accepted that morphological changes of the parathyroid glands appear early in renal failure. When diffuse hyperplasia develops into a nodular type, the cells grow monoclonally and proliferate aggressively, with abnormal suppression of parathyroid hormone (PTH) secretion under high extracellular calcium. Based on histopathological and pathophysiological findings, patients with nodular hyperplasia in renal hyperparathyroidism might be refractory to medical treatment, including calcitriol pulse therapy. Thus, parathyroid surgery is indicated for individuals developing hypercalcemia, elevated PTH levels, and/or bone disease, who cannot be effectively treated medically. The detection of enlarged parathyroid glands by image diagnosis is another criterion for surgery. In our experience, parathyroidectomy is an effective treatment; however, the timing of the operation is important, because skeletal deformity and vessel calcification cannot be expected to diminish even after successful surgery. Technically, it is important to identify all parathyroid glands and, in autotransplantation, to use an adequate amount of suitable, tissue, namely, a diffuse type of hyperplastic tissue, to guarantee satisfactory postoperative function.     

Decreased Parathyroid Klotho Expression Is Associated With Persistent Hyperparathyroidism After Kidney Transplantation

Although successful kidney transplantation usually corrects hyperparathyroidism, the condition persists in some patients. The present study was designed to determine whether Klotho or fibroblast growth factor 23, the key regulator of parathyroid hormone, is involved in persistent hyperparathyroidism in kidney transplant recipients (KTRs). Nineteen hyperplastic parathyroid glands were obtained from end-stage renal disease (ESRD) patients and KTRs; 6 normal parathyroid glands were used as controls. We compared the expression of Klotho, fibroblast growth factor receptor 1 (FGFR1) and calcium-sensing receptor (CaSR) in the KTRs and ESRD patients. Expressions of Klotho, FGFR1, CaSR and vitamin D receptor, as evaluated by immunohistochemistry, were quantified as the number of positive cells per unit area. The Klotho, FGFR1 and CaSR expressions in parathyroid glands of the post–kidney transplantation (PSKT) and the ESRD groups were significantly decreased compared with normal controls. In the ESRD group, Klotho expression and number of proliferating cell nuclear antigen–positive cells in the parathyroid gland were significantly decreased in parathyroid adenomas as compared with parathyroid hyperplasia. The expression of FGFR1 and CaSR in the parathyroid glands was significantly increased in the PSKT compared with the ESRD group. There was no significant difference in Klotho expression between the PSKT and ESRD groups. Incomplete recovery of Klotho levels in the parathyroid gland may play a role in the pathogenesis of tertiary hyperparathyroidism after kidney transplantation.     

Cinacalcet HCl attenuates parathyroid hyperplasia in a rat model of secondary hyperparathyroidism

BACKGROUND: Secondary hyperparathyroidism (HPT) in chronic kidney disease (CKD) is a physiologic response to kidney failure characterized by elevated serum parathyroid hormone (PTH) levels and parathyroid gland enlargement. Calcimimetic agents acting through allosteric modification of the calcium-sensing receptor (CaR) can attenuate parathyroid hyperplasia in rats with secondary HPT. The present study explores the effects of the calcimimetic cinacalcet HCl on parathyroid hyperplasia, apoptosis, and PTH secretion in a rat model of secondary HPT. METHODS: Cinacalcet HCl was gavaged daily (1, 5, or 10 mg/kg) for 4 weeks starting 6 weeks post-5/6 nephrectomy. After dosing, hyperplasia was determined using parathyroid weight and proliferating cell nuclear antigen (PCNA) immunochemistry. Apoptosis was determined using in situ techniques. Serum PTH((1-34)) and blood chemistries were determined throughout the course of the study. RESULTS: Administration of cinacalcet HCl (5 or 10 mg/kg) significantly reduced the number of PCNA-positive cells and decreased parathyroid weight compared with vehicle-treated 5/6 nephrectomized rats. There was no difference in apoptosis from cinacalcet HCl-treated or vehicle-treated animals. Serum PTH and blood ionized calcium levels decreased in cinacalcet HCl-treated animals compared with vehicle-treated controls. CONCLUSION: The results confirm previous work demonstrating that calcimimetic agents attenuate the progression of parathyroid hyperplasia in subtotally nephrectomized rats, extending earlier observations to now include cinacalcet HCl. These results support a role for the CaR in regulating parathyroid cell proliferation. Therefore, cinacalcet HCl may represent a novel therapy for improving the management of secondary HPT.     

Thyroid,parathyroid hormones and calcium homeostasis

The thyroid gland secretes thyroxine (T4) and triiodothyronine (T3) in response to thyroid-stimulating hormone release from the anterior pituitary gland. Iodine is essential for the synthesis of thyroid hormones. T4 and T3 increase the basal metabolic rate, heat production, and help to maintain normal growth and development. Serum calcium levels are under very tight control. The majority of calcium is found in bones. Calcium and phosphate levels are maintained by four hormones – parathyroid hormone (PTH), calcitonin, vitamin D and fibroblast growth factor 23. PTH is produced by the parathyroid glands and its secretion is determined by serum calcium levels.     

Regulation of parathyroid function in chronic kidney disease (CKD)

In chronic kidney disease (CKD), several abnormalities in bone and mineral metabolism develop in the majority of patients. The parathyroid plays a very important role in regulating bone and mineral metabolism; thus, control of parathyroid function is one of the main targets of the management of CKD-mineral and bone disorder (CKD-MBD). In the development of secondary hyperparathyroidism, it has recently been suggested that fibroblast growth factor 23 (FGF23) plays a crucial role, both as a phosphaturic factor and as a suppressor of active vitamin D (1,25D) production in the kidney. FGF23 is originally secreted to prevent hyperphosphatemia in CKD, but this occurs at the expense of low 1,25D and hyperparathyroidism (“trade-off” hypothesis revisited). Furthermore, recent data suggest that FGF23 could be another useful marker for the prognosis of hyperparathyroidism, because a high serum level may reflect the cumulative dose of vitamin D analogues previously administered. We have also demonstrated that severe hyperparathyroidism was associated with the production and secretion of a new form of parathyroid hormone (PTH) molecule, which can be detected by third-generation assays for PTH, but not by the second-generation assays. For the regression of already established nodular hyperplasia, the more advanced type of parathyroid hyperplasia, it is certainly necessary, in the near future, to develop new agents that specifically induce apoptosis in parathyroid cells. Until such agents are developed, prevention and early recognition of nodular hyperplasia is mandatory for the effective and safe management of hyperparathyroidism in CKD.     

The Use of Calcimimetics for the Treatment of Secondary Hyperparathyroidism: A 10 Year Evidence Review

Until the discovery of calcimimetics, the management of secondary hyperparathyroidism (SHPT) relied exclusively on treatment with phosphate binders, vitamin D derivatives or surgical parathyroidectomy with limited success. The therapeutic use of calcimimetic agents, together with a better understanding of the pivotal role of the calcium‐sensing receptor (CaSR) in the physiological regulation of parathyroid gland function, substantially advanced the management of hyperparathyroidism in dialysis practice. Calcimimetics bind selectively to the CaSR receptor in parathyroid tissue and enhance the inhibitory effect of extracellular calcium ions on parathyroid hormone (PTH) secretion, thereby reducing PTH levels even when serum calcium concentrations are normal or low. The availability of calcimimetic agents for clinical use has opened a new era in the management of patients with SHPT. Indeed, calcimimetic compounds have been shown to reduce PTH levels and to lower serum calcium concentrations in all forms of hyperparathyroidism, including primary hyperparathyroidism (PHPT) and parathyroid carcinoma. Such findings underscore the critical importance of the CaSR as a therapeutic target in this family of clinical disorders. New calcimimetic agents are being developed that have the potential to offer improved efficacy and safety compared with currently available calcimimetic compounds.     

Calcium-sensing receptor expression and parathyroid hormone secretion in hyperplastic parathyroid glands from humans

In uremic patients, severe parathyroid hyperplasia is associated with reduced parathyroid calcium-sensing receptor (CaR) expression. Thus, in these patients, a high serum Ca concentration may be required to inhibit parathyroid hormone (PTH) secretion. This study compares the magnitude of reduction in CaR expression and the degree of the abnormality in Ca-regulated PTH release in vitro. A total of 50 glands from 23 hemodialysis patients with refractory hyperparathyroidism were studied. Tissue slices were incubated in vitro to evaluate (1) the PTH secretory output in a normal Ca concentration (1.25 mM) and (2) the PTH secretory response to high (1.5 mM) and low (0.6 mM) Ca concentration. Tissue aliquots were processed for determination of CaRmRNA expression. The results showed that, corrected for DNA, parathyroid tissue with lowest CaR expression secreted more PTH than that with relatively high CaR expression (146 +/- 23 versus 60 +/- 2 pg/microg DNA; P < 0.01). Furthermore, glands with low CaR expression demonstrated a blunted PTH secretory response to both the inhibitory effect of high Ca and the stimulatory effect of low Ca. The study also showed that the larger the gland, the lower the CaRmRNA expression. Thus, large parathyroid glands produce a large amount of PTH not only as a result of the increased gland size but also because the parathyroid tissue secretory output is increased. These abnormalities in PTH regulation are related to low CaR expression.     

Parathyroidectomy in dialysis patients: Indications,methods, and consequences

Secondary hyperparathyroidism, characterized by increased PTH synthesis and secretion, is often seen in advanced stages of chronic kidney disease. Excessive proliferation of parathyroid cells leads to the development of diffuse hyperplasia that subsequently progresses to nodular histology. Refractory hyperparathyroidism occurs when parathyroid glands fail to respond to medical therapy. Parathyroidectomy (PTX), surgical resection of parathyroid glands, is usually performed in cases of persistent serum levels of PTH above 1000 pg/mL associated with hypercalcemia or when hyperparathyroidism is refractory to conservative therapy. Parathyroidectomy can be carried out using different procedures: subtotal PTX or total PTX with or without parathyroid autotransplantation. Parathyroid surgery may have undesirable consequences due to PTH oversuppression, such as the development of adynamic bone disease; hungry bone syndrome is quite common after this surgery. However, PTX improves survival and parameters of mineral metabolism. Parathyroidectomy needs to be considered in those patients with severe hyperparathyroidism with a poor response to pharmacological treatment and with distinct undesirable effects of PTH on bone and mineral metabolism parameters.     

Diseases of the parathyroid gland in chronic kidney disease

During the past few years, remarkable advances have been made in the understanding and the management of parathyroid diseases in patients with chronic kidney disease (CKD). One of the important insights is the identification of fibroblastic growth factor 23, which has greatly reshaped our understanding of secondary hyperparathyroidism (SHPT). The recent introduction of calcimimetic cinacalcet hydrochloride has led to a major breakthrough in the management of SHPT. Recognition of circulating molecular forms of parathyroid hormone (PTH) is also a major milestone in the accurate assessment of parathyroid function in CKD. Primary hyperparathyroidism should also be considered in patients with CKD, because it can cause various renal manifestations and can also occur as a sporadic disease in these patients. Hypoparathyroidism is occasionally seen in dialysis patients in the setting of diabetes mellitus and malnutrition–inflammation complex syndrome, as well as after parathyroidectomy for advanced SHPT. For patients with adynamic bone disease due to hypoparathyroidism and/or skeletal resistance to PTH, teriparatide, a PTH analog, may have potential for improving bone metabolism and reducing the risk of fracture. In this review, we summarize our current knowledge on diseases of the parathyroid gland in CKD patients, with a particular focus on recent work in the field.     

Cyclooxygenase 2 promotes parathyroid hyperplasia in ESRD

Hyperplasia of the PTG underlies the secondary hyperparathyroidism (SHPT) observed in CKD, but the mechanism underlying this hyperplasia is incompletely understood. Because aberrant cyclooxygenase 2 (COX2) expression promotes epithelial cell proliferation, we examined the effects of COX2 on the parathyroid gland in uremia. In patients with ESRD who underwent parathyroidectomy, clusters of cells within the parathyroid glands had increased COX2 expression. Some COX2-positive cells exhibited two nuclei, consistent with proliferation. Furthermore, nearly 78% of COX2-positive cells expressed proliferating cell nuclear antigen (PCNA). In the 5/6-nephrectomy rat model, rats fed a high-phosphate diet had significantly higher serum PTH levels and larger parathyroid glands than sham-operated rats. Compared with controls, the parathyroid glands of uremic rats exhibited more PCNA-positive cells and greater COX2 expression in the chief cells. Treatment with COX2 inhibitor celecoxib significantly reduced PCNA expression, attenuated serum PTH levels, and reduced the size of the glands. In conclusion, COX2 promotes the pathogenesis of hyperparathyroidism in ESRD, suggesting that inhibiting the COX2 pathway could be a potential therapeutic target.     

The intact nephron hypothesis: the concept and its implications for phosphate management in CKD-related mineral and bone disorder

Mechanistic understanding of secondary hyperparathyroidism, vascular calcification, and regulation of phosphate metabolism in chronic kidney disease (CKD) has advanced significantly in the past five decades. In 1960, Bricker developed the 'intact nephron hypothesis', opening the door for hundreds of investigations. He emphasized that 'as the number of functioning nephrons decreases, each remaining nephron must perform a greater fraction of total renal excretion'. Phosphate per se, independent of Ca2+ and calcitriol, directly affects the development of parathyroid gland hyperplasia and secondary hyperparathyroidism. Vitamin D receptor, Ca2+ sensing receptor, and Klotho-fibroblast growth factor (FGF) receptor-1 complex are all significantly decreased in the parathyroid glands of patients with CKD. Duodenal instillation of phosphate rapidly decreases parathyroid hormone release without changes in calcium or calcitriol. The same procedure also rapidly increases renal phosphate excretion independently of FGF-23, suggesting the possibility of an 'intestinal phosphatonin'. These observations suggest a possible 'phosphate sensor' in the parathyroid glands and gastrointestinal tract, although as yet there is no proof for the existence of such a sensor. Evidence shows that phosphate has a key role in parathyroid hyperplasia by activating the transforming growth factor-α-epidermal growth factor receptor complex. Thus, control of serum phosphorus early in the course of CKD will significantly ameliorate the pathological manifestations observed during progressive deterioration of renal function.     

PTHrP enhances the secretory response of PTH to a hypocalcemic stimulus in rat parathyroid glands

BACKGROUND: The secretion of parathyroid hormone (PTH) from the parathyroid glands might be regulated by autocrine/paracrine factors, and a feedback regulatory mechanism of PTH on the secretion of PTH has been suggested. Because of the existence of a common receptor between PTH and PTH-related peptide (PTHrP), the aim of the present study was to examine the possible effects of PTHrP 1-40 and 1-86 on PTH secretion in rats. METHODS: In vivo, the effect of PTHrP on Ca++-regulated PTH secretion was examined by the induction of hypocalcemia and hypercalcemia by an infusion of EGTA and Ca++, with and without PTHrP. The eventual effects of PTHrP on the peripheral metabolism of PTH were examined by infusion of human PTH (hPTH) with and without PTHrP. hPTH was measured by an intact hPTH assay not cross reacting with rat PTH or PTHrP. To examine whether near physiological levels of circulating PTH have an autoregulatory effect in vivo on PTH secretion from the parathyroid gland, an acute reduction of the circulating PTH was induced by an acute unilateral parathyroidectomy (UPTX). PTH secretion from the remaining parathyroid gland was followed in response to EGTA-induced hypocalcemia. In vitro investigations on the effect of PTHrP 1-40 on PTH secretion from whole rat parathyroid glands incubated in media containing a calcium concentration of 0.6 or 1.35 mmol/L were performed to confirm whether the effect of PTHrP was directly on the gland. The rat PTH assay was examined for cross reaction with PTHrP. RESULTS: In vivo, the same rate of decrease of plasma Ca++ was induced in the experimental groups. The maximal response of PTH to hypocalcemia (218 +/- 16 pg/mL, N = 6) was significantly enhanced by PTHrP 1-40 (525 +/- 79 pg/mL, N = 6) and by PTHrP 1-86 (465 +/- 29 pg/mL, N = 6, P < 0.001). No effect of PTHrP on PTH secretion was found during normocalcemia or hypercalcemia. UPTX resulted in a 50% reduction of PTH secretion, and no compensatory increase of PTH was observed. PTHrP had no effect on the metabolism of PTH. In vitro, low-Ca++-induced PTH secretion was significantly augmented by 300% (P < 0.01) when the medium contained PTHrP 1-40. PTHrP did not cross react with the PTH assay. CONCLUSIONS: PTHrP significantly enhanced the low-Ca++-stimulated PTH secretion in vivo and in vitro. An autocrine/paracrine role of PTHrP in the parathyroid glands is suggested. An autoregulatory effect of circulating PTH on the PTH secretion from parathyroid glands seems unlikely. The "maximal secretory capacity" of the parathyroid glands induced by hypocalcemia in vivo and in vitro is not the maximum, as PTH secretion can be increased even further, by several-fold.     

Hyperplasia of the parathyroid gland without secondary hyperparathyroidism

BACKGROUND: Low dietary phosphorus (P) prevents parathyroid gland (PTG) hyperplasia and the development of secondary hyperparathyroidism (SH) in uremic rats. The present study explores the effects of P restriction on parathyroid hormone (PTH) synthesis and secretion and PT cell growth in rats with established SH and PTG hyperplasia. METHODS: Normal and 5/6 nephrectomized rats were fed a high P (0.8%) diet. After two weeks, the normal rats and half of the uremic rats were sacrificed (U-HP) while the remaining uremic rats were switched to a low P (0.2%) diet (U-HP-LP). RESULTS: High dietary P induced a significant increase in serum P, PTH, and PTG weight, but not ionized calcium compared to normal animals fed the same diet (N-HP). P restriction returned serum P and PTH to normal levels by one week. In contrast, PTG size did not regress and glands remained enlarged for up to eight weeks with no evidence of apoptosis. Ribonuclease protection assay and metabolic labeling studies demonstrated similar PTH/actin mRNA ratios and 35S-labeled PTH among the three groups. Intracellular intact PTH was higher in U-HP and U-HP-LP rats compared to N-HP animals with no differences between the two uremic groups. PTG-PTH content correlated only with PTG weight, and serum PTH only with serum P. The PTG secretory response to calcium remained intact. CONCLUSIONS: In established chief-cell hyperplasia, P restriction restores normal serum PTH levels without affecting PTG hyperplasia, PTH synthesis, PTG cytosolic PTH or the PTH secretory response to calcium, suggesting an impaired exocytosis of PTH.