An estimation of the parathyroid hormone secretion rate in vitamin D deficient rats

Setting up a parathyroid multidisciplinary team meeting: one year's experience, outcomes and learning points

OBJECTIVE

A parathyroid multidisciplinary team meeting was set up at East Sussex Healthcare Trust, from November 2014 to November 2015, in order to improve and streamline services for patients with parathyroid pathology.

METHODS

Data were collected on all new referrals for hyperparathyroidism, and on the outcomes for each patient discussed at the meeting, including the number of operations and management outcomes. A survey was sent out to the members of the multidisciplinary team meeting to determine their perception of its effectiveness.

RESULTS

Seventy-nine new referrals were discussed throughout the year; 43 per cent were recommended for surgery, 41 per cent had a trial of conservative or medical management before re-discussion, and 16 per cent required further imaging. Ninety-two per cent of patients underwent an ultrasound, single-photon emission computed tomography/computed tomography or nuclear medicine (sestamibi) scan prior to the meeting. All ultrasound scans were performed by a consultant radiologist.

CONCLUSION

The multidisciplinary team meeting has been successful, with perceived benefits for patients, improved imaging evaluation and efficiency of referral pathways, leading to more appropriate patient management.

The calcium-sensing receptor: a key factor in the pathogenesis of secondary hyperparathyroidism

Serum calcium levels are regulated by the action of parathyroid hormone (PTH). Major drivers of PTH hypersecretion and parathyroid cell proliferation are the hypocalcemia and hyperphosphatemia that develop in chronic kidney disease patients with secondary hyperparathyroidism (SHPT) as a result of low calcitriol levels and decreased kidney function. Increased PTH production in response to systemic hypocalcemia is mediated by the calcium-sensing receptor (CaR). Furthermore, as SHPT progresses, reduced expression of CaRs and vitamin D receptors (VDRs) in hyperplastic parathyroid glands may limit the ability of calcium and calcitriol to regulate PTH secretion. Current treatment for SHPT includes the administration of vitamin D sterols and phosphate binders. Treatment with vitamin D is initially effective, but efficacy often wanes with further disease progression. The actions of vitamin D sterols are undermined by reduced expression of VDRs in the parathyroid gland. Furthermore, the calcemic and phosphatemic actions of vitamin D mean that it has the potential to exacerbate abnormal mineral metabolism, resulting in the formation of vascular calcifications. Effective new treatments for SHPT that have a positive impact on mineral metabolism are clearly needed. Recent research shows that drugs that selectively target the CaR, calcimimetics, have the potential to meet these requirements.

Urinary excretion of cyclic AMP in cadmium-intoxicated rats

To obtain further information on the negative calcium balance caused by Cd, the factors associated with serum calcium and phosphorus homeostasis other than inhibition of intestinal calcium absorption were studied by using urinary cyclic 3',4'-adenosine monophosphate (cAMP). In rats exposed to Cd for 30 d, the levels of urinary excretion of cAMP after treatment with parathyroid hormone (PTH), parathyroidectomy (PTX), or 1 alpha-hydroxycholecalciferol (1 alpha-OH-D3) showed almost the same patterns as those of control rats: the response of urinary cAMP to treatment with PTH was not influenced by continuous oral administration of Cd. On the other hand, in rats exposed to Cd for 90 d without the other three treatments, the amount of urinary excretion of cAMP was markedly higher than in control rats. In PTX rats exposed to 90 d of Cd, urinary cAMP was unchanged, but it was markedly increased when the parathyroid was intact, with or without treatment with PTX. This phenomenon indicated hyperparathyroidemia in response to continuous oral administration of Cd for 90 d. The negative calcium balance with hyperparathyroidemia occurred after continuous oral administration of Cd and developed via increased urinary excretion of calcium. Urinary excretion of cAMP in Cd-exposed rats was unaffected by the administration of 1 alpha-OH-D3.

Blood Ca2+ modulates responsiveness of renal 25(OH)D3-1 alpha-hydroxylase to PTH in rats

To clarify whether extracellular Ca2+ modulates renal 25-hydroxyvitamin D3 [25(OH)D3]-1 alpha-hydroxylase, thyroparathyroidectomized rats were infused with 15 mM CaCl2, 20 mM EGTA, and/or 2.5 U/h parathyroid hormone (PTH), and blood Ca2+, serum 1,25-dihydroxyvitamin D [1,25(OH)2D], and renal 1 alpha-hydroxylase activity were determined. Rats with CaCl2, EGTA, or PTH infusion (group 1) exhibited low blood Ca2+, serum 1,25(OH)2D, and 1 alpha-hydroxylase activities. Infusion of CaCl2 alone (group 2) caused a significant increase in blood Ca2+ and a reduction in serum 1,25(OH)2D and 1 alpha-hydroxylase compared with group 1. Administration of PTH alone (group 3) markedly elevated blood Ca2+, serum 1,25(OH)2D, and 1 alpha-hydroxylase activity. When EGTA was infused along with PTH (group 4), blood Ca2+ was significantly reduced compared with group 3, and serum 1,25(OH)2D and renal 1 alpha-hydroxylase were further elevated. In contrast, when CaCl2 was infused with PTH (group 5), blood Ca2+ was higher than that in group 3, and serum 1,25(OH)2D and 1 alpha-hydroxylase activities were significantly reduced compared with group 3. No significant difference in serum inorganic phosphate or urinary cAMP excretion was observed by CaCl2 or EGTA infusion in both PTH-treated and nontreated rats. These results demonstrate that extracellular Ca2+ modulates the responsiveness of renal 1 alpha-hydroxylase to PTH as well as the base-line activity of the enzyme in the absence of PTH. These effects of extracellular Ca2+ on renal 1 alpha-hydroxylase may serve to offer an efficient way of regulating 1,25(OH)2D production and serum 1,25(OH)2D concentration by altering the responsiveness of 1 alpha-hydroxylase to PTH and possibly other stimulations depending on the demand for Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Animal model of primary hyperparathyroidism

An experimental model of hyperparathyroidism was developed in the rat to simulate primary hyperparathyroidism in humans. In this model thyroparathyroidectomized (TPTX) or parathyroidectomized (PTX) animals were infused for 6 days with an amount of bovine synthetic parathyroid hormone (PTH)-(1-34) fragment to restore plasma calcium levels to normal (0.7 U X h-1) or with PTH at twofold (1.4 U X h-1) or threefold (2.1 U X h-1) this basal level. Animals infused with 2.1 U X h-1 of bovine PTH-(1-34) exhibited hypercalcemia, hypophosphatemia, a reduction in theoretical renal threshold for phosphate and an increase in 1,25-dihydroxyvitamin D plasma levels that were approximately threefold the control value. In addition, these animals demonstrated nephrocalcinosis and changes of bone histology that were typical of the findings in patients with primary hyperparathyroidism. In contrast, in animals infused at 1.4 U X h-1, plasma calcium, phosphate, and theoretical renal threshold for phosphate remained within normal limits, but plasma 1,25-dihydroxyvitamin D was increased above control, suggesting that increased activity of 1 alpha-hydroxylase may be the most sensitive index of increased PTH levels. This animal model permits sustained elevation of PTH plasma levels at basal or pathologically elevated levels and should provide an effective means by which to evaluate the consequences of chronic hyperparathyroidism on epithelial function, bone, and other organ systems.

The effect of vitamin D deficiency on the cochlear potentials and the perilymphatic ionized calcium concentration of rats

Sensorineural hearing loss has been frequently reported in patients with renal failure but its etiology has not yet been established. Disturbance of Ca metabolism is present in renal failure and seems to cause hearing loss. The purpose of the present study was to determine whether the disturbance of Ca metabolism has any effect on cochlear function. The cochlear potentials were measured in 19 rats fed with a vitamin D deficient diet. The pathological findings showed prolongation of N1 latency with unchanged N1 amplitude and pseudothreshold, depression of CM amplitude and elevation of the CM pseudothreshold. The latencies of narrow-band APs were prolonged in the entire cochlear partition. Ca2+ concentration in perilymph was 3.2 X 10(-4) M (n = 4) in vitamin D deficient rats and 7.4 X 10(-4) M (n = 4) in the controls. These findings were milder than those obtained in surgically induced renal failure. It was concluded that although vitamin D deficiency is one cause of hearing loss in renal failure, other major factors must be involved. The authors postulate that hearing loss in vitamin D deficiency is mainly attributable to the depression of the Ca2+ concentration in perilymph.

Evidence that blood ionized calcium can regulate serum 1,25(OH)2D3 independently of parathyroid hormone and phosphorus in the rat

This study asks whether arterial blood ionized calcium concentration (Ca++) can regulate the serum level of 1,25-dihydroxy-vitamin D3 [1,25(OH)2D3] independently of serum phosphorus and parathyroid hormone (PTH). We infused either PTH (bovine 1-34, 10 U/kg body wt/h) or saline into awake and unrestrained rats for 24 h, through a chronic indwelling catheter. PTH raised total serum calcium and arterial blood ionized calcium, yet serum 1,25(OH)2D3 fell from 35 +/- 6 (mean +/- SEM, n = 10) with saline to 12 +/- 3 pg/ml (n = 11, P less than 0.005 vs. saline). To determine if the decrease in serum 1,25(OH)2D3 was due to the elevated Ca++, we infused PTH into other rats for 24 h, along with varying amounts of EGTA. Infusion of PTH + 0.67 micron/min EGTA reduced Ca++, and 1,25(OH)2D3 rose to 90 +/- 33 (P less than 0.02 vs. PTH alone). PTH + 1.00 micron/min EGTA lowered Ca++ more, and 1,25(OH)2D3 increased to 148 +/- 29 (P less than 0.01 vs. saline or PTH alone). PTH + 1.33 micron/min EGTA lowered Ca++ below values seen with saline or PTH alone, and 1,25(OH)2D3 rose to 267 +/- 46 (P less than 0.003 vs. all other groups). Thus, during PTH infusion lowering Ca++ with EGTA raised 1,25(OH)2D3 progressively. There were no differences in serum phosphorus concentration or in arterial blood pH in any group infused with PTH. The log of serum 1,25(OH)2D3 was correlated inversely with Ca++ in all four groups infused with PTH (r = -0.737, n = 31, P less than 0.001), and also when the saline group was included (r = -0.677, n = 41, P less than 0.001). The results of this study indicate that serum 1,25(OH)2D3 may be regulated by Ca++ independent of PTH and serum phosphorus levels in the rat. Since 1,25(OH)2D3 regulates gastrointestinal calcium absorption, there may be direct feedback control of 1,25(OH)2D3, by its regulated ion, Ca++.

Vitamin D deficiency and renal calcium transport in the rat

To examine the role of vitamin D in the renal tubular handling of calcium, clearance studies were performed in three groups of rats: group A rats fed a standard vitamin D-deficient diet (Ca 0.45%, P 0.3%) for 6 wk, were hypocalcemic with secondary hyperparathyroidism; group B rats fed the same diet as in group A but with high calcium (Ca 1.4%) and 20% lactose, were normocalcemic and without secondary hyperparathyroidism; group C rats fed the same diet as in group A but supplemented with 25 U of vitamin D3 orally twice a week, were normocalcemic, vitamin D-replete, and euparathyroid. After thyroparathyroidectomy (TPTX), each rat was infused intravenously with an electrolyte solution that contained a fixed concentration of calcium (0-30 mM) with or without parathyroid hormone (PTH; 0.75 or 2.5 U/h) at a rate of 3 ml/h. Urinary calcium excretion and serum calcium concentrations were measured between 16 and 19 h of the infusion, and the apparent threshold of calcium excretion was determined. The threshold of calcium excretion was lower in vitamin D-deficient TPTX rats (groups A and B) than in vitamin D-replete TPTX rats (group C), and not different between group A and group B. Administration of PTH at a dose of 0.75 U/h increased the threshold of calcium excretion by approximately 0.6 mM in group C, but did not alter the threshold either in group A or group B. Administration of a higher dose of PTH (2.5 U/h) raised the threshold similarly in both group A and group B to the extent comparable with that in group C, when it was given 0.75 U/h of PTH. These results demonstrate that the renal threshold of calcium excretion is decreased in the vitamin D-deficient rats independent of the secondary hyperparathyroidism, and that the higher dose of PTH was necessary to raise the calcium threshold in vitamin D-deficient rats. Thus, present study indicates the presence of dual effects of vitamin D on renal tubular handling of calcium; the one is to facilitate renal calcium reabsorption and the other is to enhance the responsiveness of the tubule to PTH.

Maintenance of normocalcemia by continuous infusion of the synthetic bovine parathyroid hormone (1-34) in parathyroidectomized rats

This work was conducted to estimate the replacement dose of the synthetic bovine parathyroid hormone [PTH(1-34)] that is required for maintenance of serum calcium (Ca) in parathyroidectomized (PTX) rats. Male rats were PTX and used in this study only if serum Ca was reduced to at least 7 mg/dl. We found that a solution of 2% cysteine, 150 mM NaCl, and 1 mM HCL was superior to 20 mM acetic acid for maintenance of biological activity of PTH (1-34) in situ during the period of hormone infusion studied. The PTH dose-calcemic response relationship was investigated using PTH in doses of 0.6, 1, and 3 U/h. The infusion of 1 U PTH per hour raised Ca to the normal level, whereas rats infused with 0.6 U/h were hypocalcemic and 3 U/h resulted in marked hypercalcemia. To extend this observation we carried out an infusion of 1 U PTH per hour for 14 days. We found that this infusion rate of bovine PTH (1-34) provided a relatively stable level of serum calcium with modest fluctuation from normocalcemic to somewhat hypercalcemic levels for the entire 14-day period of PTH infusion. Serum calcitonin was also elevated during the infusion period and then returned to the initial level when PTH treatment was stopped. After the minipumps containing PTH were removed, the serum Ca dropped rapidly to 5 mg/dl, which was significantly lower than the control (vehicle-infused) or initial values of serum Ca (7 mg/dl). Infusion of PTH at 3 U/h for 4 days did not produce this rebound hypocalcemia after the pumps were removed. Serum Ca in those experiments returned to the initial level after hormone treatment was discontinued.