Oral calcium load test: diagnostic and physiologic implications in hyperparathyroidism.
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Expression of the vitamin D receptor (VDR) in the parathyroid glands is decreased in secondary hyperparathyroidism associated with chronic renal failure by undefined mechanisms. In the present study, we examined the effects of hyperparathyroidism and dietary calcium and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] on the expression of VDR in rat parathyroid glands. Vitamin D-deficient rats were maintained on diets containing 0.02% Ca (-D, LCD), 0.4% Ca (-D, NCD), or 2.0% Ca (-D, HCD) for 6 weeks. Serum ionized Ca (ICa) in the rats on the three diets ranged from 2.5-5.2 mg/dl. Serum PTH ranged from 22-590 pg/ml and correlated inversely with ICa (r = -0.835; P < 0.001). Rats with the highest ICa had normal PTH values, suggesting that vitamin D deficiency per se does not lead to hyperparathyroidism. VDR messenger RNA (mRNA) levels in the parathyroid glands correlated positively with ICa (r = 0.845; P < 0.001) and negatively with PTH (r = -0.716; P < 0.001). VDR mRNA levels in the rats fed the -D, HCD were 6 times higher than those receiving -D, LCD and the same as those in rats fed a normal (Purina) diet. Thus, prevention of hyperparathyroidism with high dietary calcium prevented the drop in VDR expression. Treatment of the rats on all three diets with 0, 25, or 100 ng 1,25-(OH)2D3, ip, 48 and 12 h before death dose dependently increased ICa and decreased PTH, as expected, and also increased parathyroid gland VDR mRNA. This coordinate regulation of VDR mRNA by calcium and 1,25-(OH)2D3 was also observed in the kidney, but intestinal VDR mRNA was not stimulated by dietary calcium or 1,25-(OH)2D3. Analysis of covariance for parathyroid gland VDR mRNA and ICa for the three doses of 1,25-(OH)2D3 revealed no significant independent effect of 1,25-(OH)2D3 on VDR mRNA, suggesting that the up-regulation of VDR expression by 1,25-(OH)2D3 in the parathyroid glands may be mediated primarily by increasing serum calcium.
Parathyroid gland
Parathyroid chief cell
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We performed 6-h human PTH-(1-34) infusions in 8 control subjects, 10 subjects with primary hyperparathyroidism, and 7 men with idiopathic hypercalciuria. We measured serum calcium, serum 1,25-dihydroxyvitamin D, urinary calcium, and fractional phosphate excretion. The PTH-induced rise in serum 1,25-dihydroxyvitamin-D was significantly smaller in the hyperparathyroid patients than in either the controls or the hypercalciuric patients. The rise in serum calcium was similar in all 3 groups. The hyperparathyroid subjects had higher basal fractional phosphate excretion than the other two groups. PTH failed to increase fractional phosphate excretion in the hyperparathyroid individuals, whereas there was a statistically significant increase in the other two groups. PTH was without significant effect on urinary calcium excretion in any of the three groups. There were no discernible differences between the responses of the hypercalciuric patients and those of the normal subjects. These findings suggest that while responses to PTH are normal in hypercalciuria, some hyperparathyroid patients are resistant to exogenous PTH. This resistance is limited to specific arms of the PTH response pathway and may not involve PTH receptors.
Urinary calcium
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Calcium absorption declines with age. Because 1,25-dihydroxyvitamin D (1,25(OH)2D) is the major hormone controlling calcium absorption, changes in vitamin D metabolism may account for the malabsorption of aging. Serum levels of 1,25(OH)2D have been reported to either decrease or remain unchanged with age. To assess the effect of aging on renal production of 1,25(OH)2D, we evaluated the response of renal 25OHD 1 alpha hydroxylase to human parathyroid hormone (hPTH(1-34) stimulation in 119 women ages 25-83 years. In this population, baseline serum 25OHD and 1,25(OH)2D values did not significantly change with age, but serum iPTH (r = 0.44; p < 0.001) and serum creatinine (r = 0.31; p < 0.01) increased with age. However, the stimulatory activity of hPTH(1-34) on the renal production of 1,25(OH)2D declined with age (r3 = -0.36; p < 0.001) and was most apparent after age 75, being 50% less than that of younger women. Besides age, the production of 1,25(OH)2D was found to be dependent on baseline serum iPTH (r = -0.31; p < 0.0001). Administration of hPTH(1-34) led to suppression of endogenous PTH, and suppressibility of endogenous PTH declined with age (r = 0.53; p < 0.0001). The increase in serum PTH and decreased suppressibility of PTH with age could be due to mild secondary hyperparathyroidism. The increase in PTH with age is probably responsible for maintaining normal serum 1,25(OH)2D levels in elderly subjects; however, decreased metabolism of 1,25(OH)2D in the elderly could also maintain normal serum 1,25(OH)2D levels.
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Experimentally diabetic rats have low serum 1,25-dihydroxyvitamin D, intestinal malabsorption of calcium, secondary hyperparathyroidism, and bone loss. To examine the hypothesis that abnormalities similar to those in the diabetic rat might explain human diabetic osteopenia, we studied calcium metabolism in 40 healthy control and 82 diabetic patients aged 18--75 yr [47 untreated: fasting plasma glucose (mean +/- SE), 267 +/- 8 mg/dl; 19 treated but hyperglycemic: glucose 305 +/- 24 mg/dl; 16 treated and in better control: glucose, 146 +/- 8 mg/dl]. Serum total calcium, ionic calcium, immunoreactive parathyroid hormone (Arnaud method, GP-1M and CH-12M antisera), 25-hydroxyvitamin D (Haddad method), and 1,25-dihydroxyvitamin D (Lambert method) concentrations were normal in all 3 groups of diabetics and were not significantly different from values in the control group. We determined absorption of calcium from the intestine by a double isotope method (100 mg Ca carrier; normal range, 40--80%) in 11 control and 13 untreated, uncontrolled diabetics (mean plasma glucose, 285 +/- 17 mg/dl). Absorption of calcium in controls was 60 +/- 3% and in diabetics was 56 +/- 3% (not significantly different). We have found no derangement of calcium metabolism in adults with insulin-requiring juvenile- and adult-onset diabetes regardless of treatment status. The experimental diabetic rat model does not appear to be useful for determining the pathogenesis of adult human diabetic osteopenia.
Hypoparathyroidism
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Increased urinary excretion of cAMP is a common finding in patients with primary hyperparathyroidism. We report a patient with hypercalcemia, primary hyperparathyroidism, vitamin D deficiency and high nephrogenous cAMP that fell to low levels during the course of a protracted illness. Surgical removal of a large parathyroid cystic adenoma was associated with a decrease in plasma calcium. Because of the relatively low nephrogenous cAMP with high plasma iPTH the biological activity of the fluid aspirated from the adenoma was examined. Acute clearance studies were performed in parathyroidectomized rats and their response to the parathyroid fluid was compared with the response of synthetic PTH. Similar phosphaturic responses to PTH and the aspirated fluid were recorded and were preceded by similar increments in nephrogenous cAMP. Thus the discrepancy between the high plasma calcium, high PTH and the low nephrogenous cAMP seen in our patient was related to impaired cAMP production by the renal adenylate cyclase. There was no evidence for a hormone with a different biological activity. The impaired formation of cAMP may reflect a combined result of several factors including downregulation of renal adenylate cyclase, phosphate depletion and vitamin D deficiency state.
Cyclic adenosine monophosphate
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Calcium-sensing by the parathyroids is abnormal in familial benign hypocalciuric hypercalcemia and in primary hyperparathyroidism (1°HPT), but the role of a calcium-sensing defect in uremic secondary hyperparathyroidism (2°HPT) remains controversial. To study the regulation of PTH release by calcium, set point estimates were obtained using the four parameter model during in vivo dynamic tests of parathyroid gland function in 31 patients with 2°HPT, 8 patients with advanced 2°HPT studied shortly before undergoing parathyroidectomy (Pre-PTX), 3 patients with 1°HPT, and 20 subjects with normal renal function (NL); the response to 2-h iv calcium infusions was also evaluated. Neither blood ionized calcium (iCa+2) levels nor the set point for calcium-regulated PTH release differed between 2°HPT and NL; iCa+2 levels and set point values were moderately elevated in Pre-PTX and markedly elevated in 1°HPT. Compared with values obtained in NL, the lowest serum PTH levels achieved during calcium infusions, expressed as a percentage of preinfusion values, were incrementally greater in 2°HPT, Pre-PTX, and 1°HPT, whereas the slope of the relationship between iCa+2 and PTH, expressed as the natural logarithm (ln) of percent preinfusion values, decreased incrementally in 2°HPT, Pre-PTX, and 1°HPT. The inhibitory effect of calcium on PTH release is blunted both in 2°HPT and 1°HPT because of increases in parathyroid gland mass, but a calcium-sensing defect is a late, rather than early, consequence of renal 2°HPT.
Parathyroid gland
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The effect of prolonged (60 days) dietary phosphate restriction on divalent ion metabolism was studied in four patients with moderate renal insufficiency in an effort to delineate the mechanisms of secondary hyperparathyroidism in renal failure. The patients had low normal serum phosphorous and normal vitamin D metabolite levels but had evidence of disturbances in target organs for vitamin D, including impaired intestinal absorption of calcium, reduced calcemic response to PTH, low serum ionized calcium levels, and, consequently, elevated PTH levels. After dietary phosphate restriction, there was marked improvement or normalization of intestinal absorption of calcium, calcemic response to PTH, and ionized calcium and PTH levels. There was also a significant rise (44%) in serum 1,25-dihydroxyvitamin D levels. The data suggest that intracellular phosphorous retention, which may develop as renal insufficiency ensues, may interfere with the action and production of 1,25-dihydroxyvitamin D. This leads to defective intestinal calcium absorption and reduced calcemic responses to PTH. As a result, hypocalcemia develops, causing secondary hyperparathyroidism. Our data assign a critical role for a disturbance(s) in vitamin D metabolism in genesis of the hyperparathyroidism of renal failure.
Osteomalacia
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Parathyroid hormone (PTH) is strongly concerned with the pathogenesis of urinary stones. PTH is mainly regulated by the serum calcium concentration and not by other hormones, as is usually the case. We studied whether PTH is also regulated by adrenocorticotrophic hormone (ACTH) or not. ACTH (0.25 mg) was injected intravenously to 17 patients with primary hyperparathyroidism PHP, 7 patients with urolithiasis, 7 patients with malignant hypercalcemia, and 6 control subjects. Serum calcium was significantly increased in only PHP. The serum calcium increase rate showed a significant positive correlation with serum alkaline phosphatase, and a negative correlation with the preinjected serum calcium. PTH was slightly increased in all four groups. Serum cortisol and ACTH concentrations were not significantly different among the groups. PTH concentration in a culture medium of parathyroid tissues increased after ACTH addition. Serum calcium was significantly increased after ACTH injection in an adrenalectomized rat, and decreased in a parathyroidectomized rat. From our data and those of others, it appears that ACTH acts on the adrenal glands to decrease the serum calcium concentration, and might act directly on the parathyroid gland or bones to increase it.
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Homeostasis
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