Parathyroid Hormone Infusion Research Articles

Peptide aldehyde inhibitors of cathepsin K inhibit bone resorption both in vitro and in vivo.

We have shown previously that cathepsin K, a recently identified member of the papain superfamily of cysteine proteases, is expressed selectively in osteoclasts and is the predominant cysteine protease in these cells. Based upon its abundant cell type-selective expression, potent endoprotease activity at low pH and cellular localization at the bone interface, cathepsin K has been proposed to play a specialized role in osteoclast-mediated bone resorption. In this study, we evaluated a series of peptide aldehydes and demonstrated that they are potent cathepsin K inhibitors. These compounds inhibited osteoclast-mediated bone resorption in fetal rat long bone (FRLB) organ cultures in vitro in a concentration-dependent manner. Selected compounds were also shown to inhibit bone resorption in a human osteoclast-mediated assay in vitro. Chz-Leu-Leu-Leu-H (in vitro enzyme inhibition Ki,app = 1.4 nM) inhibited parathyroid hormone (PTH)-stimulated resorption in the FRLB assay with an IC-50 of 20 nM and inhibited resorption by isolated human osteoclasts cultured on bovine cortical bone slices with an IC-50 of 100 nM. In the adjuvant-arthritic (AA) rat model, in situ hybridization studies demonstrated high levels of cathepsin K expression in osteoclasts at sites of extensive bone loss in the distal tibia. Cbz-Leu-Leu-Leu-H (30 mg/kg, intraperitoneally) significantly reduced this bone loss, as well as the associated hind paw edema. In the thyroparathyriodectomized rat model, Cbz-Leu-Leu-Leu-H inhibited the increase in blood ionized calcium induced by a 6 h infusion of PTH. These data indicate that inhibitors of cathepsin K are effective at reducing osteoclast-mediated bone resorption and may have therapeutic potential in diseases of excessive bone resorption such as rheumatoid arthritis or osteoporosis.

Dysregulation of calcium homeostasis after severe burn injury in children: Possible role of magnesium depletion

Objective: To determine the cause and extent of hypocalcemia observed in children after severe burns. Design: We studied 10 children with burns covering 57% ± 17% (SD) body surface area, ages 9.6 ± 4.7 years, who were admitted consecutively during a 6-month period. Diet supplied a minimum of 2.7 gm/m 2 of calcium, 0.3 gm/m 2 of magnesium, and 2.2 gm/m 2 phosphate. Blood specimens were obtained daily for 10 ± 5 days for the following tests: (1) simultaneous analysis for ionized calcium, magnesium, and intact parathyroid hormone (group A); (2) two of these children, randomly selected, had serial 2-hour determinations on a single day (group B); (3) a modified Ellsworth-Howard test, consisting of a 10-minute infusion of synthetic parathyroid hormone 18 ± 10 days postburn and associated changes in urinary cyclic adenosine monophosphate excretion and renal threshold phosphate concentration (group C). Three of these children, when normomagnesemic, also received a standard magnesium infusion to determine magnesium retention (group D). Data were analyzed with chi-square, regression analysis, and nonparametric testing as appropriate. Results: All patients showed sustained hypocalcemia and hypomagnesemia; intact parathyroid hormone response was inappropriately low and response to synthetic parathyroid hormone infusion was blunted. Lowest ionized calcium levels were associated with hypomagnesemia. Conclusion: Hypoparathyroidism and blunted renal response to parathyroid hormone suggest that magnesium depletion may contribute to their pathogenesis. Magnesium repletion and monitoring are recommended. (J Pediatr 1997;131:246-51)

Renal regulation of phosphate excretion in endotoxaemic rats.

1. Maintenance of phosphate homeostasis is essential for energy producing and oxygen delivery systems, particularly, when the energy requirements are increased in certain conditions, such as septicaemia. We investigated the phosphaturic response to parathyroid hormone (PTH) in endotoxin (ETx)-treated rats in order to clarify the renal regulation of phosphate excretion during endotoxaemia. 2. Wistar rats that had undergone thyroparathyroidectomy were challenged with either Escherichia coli ETx (n = 8) or saline vehicle (n = 9). Thirty-minute renal clearance tests were done before and after PTH infusion. Rats infused with saline instead of PTH served as time controls for the ETx- (n = 7) and saline-treated (n = 8) rats. 3. In time control rats, ETx administration enhanced phosphate excretion progressively and this was associated with an obvious increase in the level of kidney adenosine 3', 5'-cyclic mono-phosphate (P < 0.005) compared with levels following saline vehicle administration. However, this phosphaturia in late-phase endotoxaemia was not observed in rats infused with PTH; ETx, but not saline vehicle, blunted the PTH-mediated increase in phosphate excretion (P < 0.005). Increased urinary noradrenaline and constant dopamine excretion were observed in endotoxaemic rats. Endotoxin administration produced marked metabolic acidosis and hypocapnia in comparison with the administration of the saline vehicle. 4. To test whether renal tubular sensitivity to parathyroid hormone related-protein (PTHrP) was enhanced during endotoxaemia, phosphaturic response to PTHrP in ETx- (n = 7) and saline-treated rats (n = 7) was examined. Parathyroid hormone related-protein infusion produced phosphaturia in both groups. However, the severity of the phosphaturia after PTHrP infusion was less in ETx-than in saline-treated rats. 5. In summary, although ETx administration causes a progressive increase in phosphate excretion in the absence of PTH, this is overcome by the antiphosphaturic effect of ETx, attenuating PTH-mediated phosphaturia after PTH infusion.

Osteopetrosis

Congenital osteopetrosis is a group of disorders resulting in decreased osteoclastic function and hence decreased bone resorption. Various medical treatments have been attempted to ameliorate the osteopetrotic condition. A calcium-deficient diet has limited further sclerosis in some patients. Prednisone therapy has improved haematological function in some patients, but has not resulted in a reduction in bone mass. Calcitrophic hormones, such as parathyroid hormone (PTH) infusions and oral calcitriol, stimulate osteoclastic activity, and calcitriol in particular has stimulated osteoclastic bone resorption in some patients with osteopetrosis. Bone marrow transplantation, although curative, is limited by paucity of donors, risk of graft-versus-host disease and relapse of the disease. The demonstration of defective leucocyte superoxide production in osteopetrotic patients and the premise that osteoclasts appear to arise from the granulocyte macrophage lineage have led to attempts at treating osteopetrosis with immunomodulators. Since treatment with recombinant interferon-γ-1b (interferon γ-1b, IFNγ-1b) has resulted in increased level of superoxide generation and clinical improvement in chronic granulomatous disease, a similar strategy has been employed using IFNγ-1b to treat patients with osteopetrosis. IFNγ-1b has been demonstrated to increase osteoclastic bone resorption and leucocytic function. Long term therapy with IFNγ-1b by subcutaneous injection 3 times weekly resulted in marked clinical improvement, a decreased incidence of infections, a decreased trabecular bone mass, and an increased marrow space resulting in improved haemopoiesis. The therapy has been associated with few adverse effects, mainly fever and diarrhoea which have been managed with a reduction in IFNγ-1b dosage. The low-calcium diet occasionally results in hypocalcaemic tetany, which may be corrected by increased dietary calcium intake. Thus, IFNγ-1b has a distinct place in the therapeutic armamentarium for patients with osteopetrosis and is a feasible treatment option in such patients.

Phosphaturic effect of L-NMMA in the presence of parathyroid hormone.

The objective of this study was to examine the effect of NG-monomethyl-L-arginine (L-NMMA) on phosphate excretion in the presence and absence of parathyroid hormone (PTH). Renal clearances were obtained before and during infusion of L-NMMA (15 mg/kg bolus and 500 micrograms.kg-1.min-1 infusion) in Sprague-Dawley rats with intact parathyroid glands (n = 6), in thyroparathyroidectomized (TPTX) rats receiving a constant infusion of PTH-(1-34) (0.01-0.03 U.kg-1.min-1) (n = 11) throughout the experiment, or in TPTX rats, that received an acute infusion of PTH-(1-34) (33 U/kg bolus and 1 U.kg-1.min-1 infusion) after L-NMMA infusion alone (n = 7). In rats with intact parathyroid glands, L-NMMA increased the fractional excretions of phosphate (FEPi) and sodium (FENa) and mean arterial pressure (MAP) (delta 8.6 +/- 1.5%, delta 0.62 +/- 0.1%, and delta 26.7 +/- 4.9 mmHg, respectively; P < 0.05). In TPTX rats receiving a constant infusion of PTH, L-NMMA again increased FEPi, FENa, and MAP (delta 9.5 +/- 3.6%, delta 1.1 +/- 0.4%, and delta 28.4 +/- 4.5 mmHg, respectively; P < 0.05). However, in TPTX rats, L-NMMA alone did not increase FEPi (delta 0.9 +/- 0.3%), whereas the subsequent infusion of PTH with L-NMMA increased FEPi (delta 15.6 +/- 3.1%; P < 0.05). In an additional group of intact and TPTX rats, the fractional excretion of lithium (FELi) was measured as an index of proximal reabsorption. L-NMMA increased FELi in intact rats (delta 13.2 +/- 2.6%; P < 0.05), but not in TPTX rats (delta 4.2 +/- 3.3%). In conclusion, L-NMMA increases phosphate excretion in association with increases in MAP and FENa, and this phosphaturic effect is dependent on the presence of PTH.

Effect of parathyroid hormone (hPTH[1-34]) infusion on serum 1,25-dihydroxyvitamin D and parathyroid hormone in normal women.

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.

Effect of parathyroid hormone on renal calbindin-D28k.

The present investigation was conducted to examine the effects of parathyroid hormone (PTH) and parathyroid hormone related peptide (PTHrP) on renal calbindin-D28k in rats. Four groups of studies were performed: (1) parathyroidectomy (PTX) or a sham operation followed by infusion of 1,25-dihydroxyvitamin D (1,25[OH]2D) or vehicle; (2) infusions of PTH(1-34), PTH(1-84), 1,25(OH)2D, or vehicle; (3) infusion of PTHrP(1-34), PTHrP (1-86), PTH(1-34), or vehicle; and (4) injections of calcium or vehicle. PTX reduced renal calbindin-D28k levels even when plasma concentrations of 1,25(OH)2D were kept constant by infusion of 1,25(OH)2D. Infusions of PTH(1-34), PTH(1-84), and 1,25(OH)2D all increased renal calbindin-D28k and plasma calcium, whereas PTHrP(1-34) and PTHrP(1-86) increased renal calbindin-D28k before an increase of plasma calcium took place. Hypercalcemia induced by the injection of calcium did not affect the levels of renal calbindin-D28k. The present data suggest that PTH and PTHrP exert a direct effect on renal calbindin-D28k, which is not mediated by changes of 1,25(OH)2D or calcium.

The interaction of PTH and dietary phosphorus and calcium on serum calcitriol levels in the rat with experimental renal failure

Renal failure results in decreased calcitriol production, a key factor in the development of secondary hyperparathyroidism. Phosphorus accumulation and high parathyroid hormone (PTH) levels, both inherent to renal failure, have different effects on calcitriol production; moreover, dietary calcium loading may have a separate inhibitory effect on calcitriol production. This study was designed to evaluate the relative effects of PTH and dietary phosphorus and calcium on serum calcitriol levels. Renal failure was surgically induced and rats were divided into normal, moderate renal failure, and advanced renal failure based on the serum creatinine. Each group was subdivided and received either a high-phosphorus diet (HPD, 0.6% Ca, 1.2% P) or high-calcium diet (HCaD, 1.2% Ca, 0.6% P) for 14-16 days to determine the relative effects of dietary calcium and phosphorus loading on serum calcitriol. In addition the effect of PTH and phosphorus on calcitriol stimulation was determined with a 48-h PTH infusion combined with either a low (0.16%) or high (1%) phosphorus diet; both diets had negligible calcium (< 0.05%). With decreasing renal function, PTH increased and was greater in rats fed the HPD than the HCaD; serum calcitriol decreased as renal function decreased and was lower in normal rats and rats with moderate renal failure fed a HCaD (P < 0.01). The calcitriol response to a PTH infusion decreased as renal function decreased (P < 0.05) but was greater on a low- (0.16%) than a high- (1%) phosphorus diet (P < 0.05). Dietary calcium loading either directly decreases serum calcitriol or acts by modifying the stimulatory effect of PTH; the stimulatory effect of PTH on serum calcitriol is modified by dietary phosphorus; in moderate renal failure, serum calcitriol levels depend on a complex interaction between PTH and dietary calcium and phosphorus; and in advanced renal failure, serum calcitriol levels are low and are difficult to stimulate, presumably because of the loss of renal mass.

Abnormalities in hepatic lipase in chronic renal failure: role of excess parathyroid hormone.

Post-heparin hepatic lipase activity is reduced in chronic renal failure (CRF). This could be due to reduced synthesis, decreased activity, and/or impaired secretion of the enzyme. Further, the factor(s) responsible for such derangements are not elucidated. We examined hepatic lipase metabolism in normal, 6-wk-old CRF rats, CRF-PTX (parathyroidectomized) rats, and CRF and normal rats treated with verapamil (CRF-V, normal-V) using liver homogenate, hepatic cell culture for 8 h, and in vitro liver perfusion. The Vmax of hepatic lipase in liver homogenate was significantly (P < 0.01) reduced and the Km was significantly (P < 0.01) increased in CRF rats, but the values were normal in CRF-PTX, CRF-V, and normal-V rats. Culture of hepatic cells for 8 h was associated with an increase in hepatic lipase activity but the increment in CRF rats was significantly (P < 0.01) lower than that of normal, CRF-PTX, CRF-V, and normal-V rats. Both parathyroid hormone (PTH)-(1-84) and 1-34 inhibited the production of hepatic lipase in cultured cells from normal, CRF-PTX, CRF-V, and normal-V rats. The expression of the mRNA of the hepatic lipase was significantly reduced in CRF animals with the ratio between it and that of house keeping gene G3DPH being 15 +/-3% compared to 40 +/- 1.3% in normal, 44+/-2.9% CRF-PTX, 44 +/- 5.4% in CRF-V, and 39 +/- 3.9% in normal-V rats. Infusion of heparin to the in vitro hepatic perfusion system increased the activity of hepatic lipase in the effluent in all groups of rat except in CRF animals. Infusion of PTH-(1-34) in dose of 10(-6) M into the liver perfusion system inhibited the increase in post-heparin hepatic lipase activity. The data show that in CRF (a) the mRNA of hepatic lipase is downregulated, and hepatic lipase production, activity and release are impaired, (b) that this is due to the state of secondary hyperparathyroidism of CRF since both acute and chronic excess of PTH were associated with these abnormalities, (c) and that prevention of excess PTH by PTX of CRF rats or blocking the effect of PTH by treatment with verapamil corrected the derangement in hepatic lipase metabolism.

Effect of parathyroid hormone on antioxidant enzyme activity and lipid peroxidation of erythrocytes in five-sixths nephrectomized rats.

The purpose of the study was to investigate the effects of parathyroid hormone (PTH) infusion on antioxidant enzyme activity and lipid peroxidation of erythrocytes in five-sixths nephrectomized (Nx) rats. Five-sixths Nx rats had a higher osmotic fragility in red blood cells (RBC). Thyroparathyroidectomy (TPTX) effectively decreased the abnormality of osmotic fragility in RBC in Nx rats. PTH infusion in Nx-TPTX rats markedly increased the osmotic fragility in RBC. Total glutathione was measured by using the enzyme-recycling method. We found elevated glutathione levels in RBC of five-sixths Nx rats, but this elevation could be inhibited by TPTX and recovered by PTH infusion in Nx-TPTX rats. Five-sixths Nx rats had a lower glutathione peroxidase activity in RBC, but TPTX or PTH infusion was not found to alter the decrease of the glutathione peroxidase activity in RBC of five-sixths Nx rats. These rats had a higher activity in RBC superoxide dismutase as compared with sham-operated controls (p < 0.05), but the higher activity in RBC superoxide dismutase in Nx rats had been inhibited by TPTX. PTH infusion recovered the higher activity in RBC superoxide dismutase in five-sixths Nx-TPTX rats. Nx rats were not found to alter the activity of catalase in RBC. Neither could TPTX or PTH infusion in Nx rats influence the activity of catalase in RBC. A high lipid peroxidation in RBC was found in five-sixths Nx rats, namely, increased formation of malondialdehyde (MDA) in RBC had been induced to produce lipid peroxidation by H2O2, but neither TPTX nor PTH infusion could inhibit or enhance the increase of lipid peroxidation in RBC of Nx rats. These results indicate that PTH infusion did not increase the susceptibility to lipid peroxidation in RBC of five-sixths Nx rats. Thus, the increased osmotic fragility in RBC induced by PTH infusion may not result from the reduction in the RBC defense mechanism against free radical toxicity.

22-Oxacalcitriol does not interfere with parathyroid hormone-induced phosphaturia or cyclic-AMP excretion.

The vitamin D analogue, 22-oxacalcitriol [22-oxa-1,25(OH)2 vitamin D3], has pleiotropic effects similar to or greater than calcitriol but has markedly fewer calcemic and phosphatemic effects. To test the hypothesis that the lesser phosphatemic effect of 22-oxacalcitriol is due, at least in part, to a lack of interference with the phosphaturic effect of parathyroid hormone, acute clearance experiments were performed in parathyroidectomized rats receiving continuous 1-34 parathyroid hormone (PTH) infusion together with 22-oxacalcitriol (200 pmol.100 g body weight-1.min-1) or vehicle. In contrast to the previously reported inhibitory effect of calcitriol on PTH-induced phosphaturia, fractional excretion of phosphorus increased similarly in both groups, from 0.05 +/- 0.01 to 0.26 +/- 0.02 (p < 0.01) in the vehicle-infused animals and from 0.04 +/- 0.01 to 0.24 +/- 0.02 (p < 0.01) in the 22-oxacalcitriol-treated rats (p between groups not significant [n.s.]). Urinary cyclic AMP excretion also increased similarly, from 45.5 +/- 5.2 to 101.6 +/- 21.6 (p < 0.01) and from 45.4 +/- 5.6 to 102.6 +/- 16.7 pmol/min (p < 0.01), respectively (p between groups n.s.). In search for a nongenomic mechanism that might account for the disparate effects of 22-oxacalcitriol and calcitriol, OK cells, which are reminiscent of the mammalian proximal tubule cell, were stimulated with calcitriol and 22-oxacalcitriol and free intracellular calcium concentration was determined. At high concentrations, calcitriol caused a dose-dependent increase in [Ca2+]i; 22-oxacalcitriol had no effect on [Ca2+]i at any concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone decreases in vivo insulin effect on glucose utilization.

Hyperparathyroidism is associated with impaired glucose tolerance, and parathyroidectomy may improve carbohydrate homeostasis. It has been suggested that parathyroid hormone (PTH) suppresses insulin secretion but it is unclear whether it also interferes with the peripheral action of insulin. To evaluate in vivo effects of PTH on insulin-mediated glucose utilization, 15 male Sprague Dawley rats were continuously infused with rat PTH (1-34) using an Alzet miniosmotic pump at a rate of 0.03 nm/hour. Controls were infused with the vehicle alone. Following 5 days of PTH infusion, plasma calcium (Ca) levels were higher in the PTH-infused rats (12.3 +/- 0.2 versus 9.9 +/- 0.1 mg/dl, P < 0.01). On the 5th day, glucose (700 mg/kg) and insulin (0.175 U/kg) were given as a bolus infusion through the left femoral vein, blood samples were obtained from the right femoral vein, and plasma glucose and insulin were measured at basal (0 minutes) and at 2, 5, 10, and 20 minutes postinfusion. Basal, nonfasting glucose levels were higher (166 +/- 4 versus 155 +/- 4 mg/dL, P < 0.04) in the PTH-infused rats but their insulin levels were similar to those of controls (6.5 +/- 0.6 versus 5.6 +/- 0.5 ng/ml). Postinfusions and maximal (2 minutes) glucose and insulin levels were similar in both groups. However, although insulin levels were similar in both groups at all measured time points, glucose levels at 20 minutes were higher in the PTH-treated rats (205 +/- 13 versus 173 +/- 9; P < 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of urinary phosphate excretion in camels

Urinary inorganic phosphorus (P) excretion was measured in 16 adult female non-pregnant, non-lactating daily watered camels. They were randomly divided into four groups of four animals. Groups 1, 2, and received either an i.v. infusion of phosphate or synthetic human parathyroid hormone (PTH) or PTH-related peptide (PTHrP), respectively. The fourth group was used as a control. Intravenous P loading induced a significant increase in phosphatemia and in P renal clearance. Both PTH and PTHrP increased calcemia and decreased phosphatemia. They had no significant effect on urinary calcium excretion, but they increased P renal clearance and phosphaturia. Thus, the regulation of urinary P excretion in normally watered camels looks similar to that already described in other ruminants.

Interaction of the renal nerves and prostaglandins on the phosphaturic response to PTH in phosphate-deprived rats

Previous studies demonstrated that catecholamines modulate the phosphaturic response to parathyroid hormone (PTH) in normal rats. The present study was performed to determine the effect of unilateral renal denervation (DNX) and the interaction with prostaglandin synthesis on the blunted phosphaturic response to PTH in phosphate-deprived rats. One week before the acute experiment, rats were anesthetized, and the left kidney was denervated or sham surgery was performed. Rats were fed either a low-phosphate diet (LPD, 0.07%) or a normal-phosphate diet (NPD, 0.7%) for 2 days before the experiment. All rats were thyroparathyroidectomized (TPTX). Control clearances were taken from the left kidney 2 h after TPTX. PTH (33 U/kg + 1 U.kg-1.min-1) was infused for 45 min, and then the urine collections were repeated. In phosphate-deprived rats with an innervated kidney, PTH infusion resulted in a blunted phosphaturic response [changed fractional excretion of phosphate (delta FEPi) of 9.2 +/- 3.7%, n = 9] compared with rats fed NPD (delta FEPi 45.7 +/- 9.3%, n = 6) or those in the phosphate-deprived group with renal DNX (delta FEPi 23.6 +/- 5.0%, n = 12). Indomethacin pretreatment (3 mg/kg) markedly attenuated the phosphaturic response to PTH in phosphate-deprived rats with a denervated kidney (delta FEPi, 3.2 +/- 1.3%, n = 7) but not in animals fed an LPD with innervated kidneys or in rats fed an NPD. Infusion of Iloprost (2 ng.kg-1.min-1), a stable prostaglandin I2 analogue, in indomethacin-treated phosphate-deprived rats enhanced the phosphaturic response to PTH in rats with a denervated kidney (delta FEPi 17.3 +/- 3.5%).(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphaturic effect of PTH during hypoxia and hypocapnia in rats

This study examined the effect of acute hypoxia or hypocapnia on renal phosphate excretion in thyroparathyroidectomized rats. Hypoxia is usually accompanied by a secondary hypocapnia due to hypoxic hyperventilation. Respiratory alkalosis has been described as blunting the phosphaturic effect of parathyroid hormone (PTH). In the present study, to know the effect of hypoxia on renal phosphate excretion in the absence of hypocapnia, the rats were ventilated mechanically, and arterial PCO2 levels were controlled. The rats were divided into three groups depending on the arterial PO2 and PCO2 levels: 1) hypoxic normocapnic group; 2) normoxic normocapnic group; 3) normoxic hypocapnic group. Hypoxia was achieved by ventilating with 10% oxygen, and hypocapnia by hyperventilating with 25-30% oxygen. PTH infusion significantly increased fractional excretion of phosphate (FEPi) from 4.1 +/- 0.9 (mean +/- SE) to 37.7 +/- 2.6% in the hypoxic group (n = 7), from 1.4 +/- 0.3 to 27.4 +/- 2.5% in the normoxic group (n = 8), and from 1.5 +/- 0.4 to 19.5 +/- 1.2% in the hypocapnic group (n = 10). The change of FEPi (delta FEPi) after PTH infusion during hypoxia was significantly greater (33.6 +/- 2.1%) than that during normoxia (26.1 +/- 2.4%, p < 0.05). In contrast to this, hypocapnia blunted the phosphaturic response to PTH (18.0 +/- 1.1% delta FEPi, p < 0.05). Urinary adenosine 3', 5'-cyclic monophosphate (cAMP) increased similarly after PTH infusion in all three groups. To test whether the enhanced phosphaturic effect of PTH during hypoxia and the blunted phosphaturic effect of PTH during hypocapnia are due to steps beyond the production of cAMP, cAMP was administered to the three groups. Cyclic AMP infusion displayed greater phosphaturia in the hypoxic group (n = 6, 30.0 +/- 1.4%) and less phosphaturia in the hypocapnic group (n = 7, 11.3 +/- 1.8%) as compared the the normoxic group (n = 6, 24.1 +/- 1.0%). In conclusion, acute hypoxia enhances the phosphaturic effect of PTH, whereas acute hypocapnia attenuates the phosphaturic effect of PTH.

Effects of infusion of parathyroid hormone and primary hyperparathyroidism on formation and breakdown of type I collagen

The influence of chronic and acute exposure to parathyroid hormone (PTH) on formation and breakdown of type I collagen, using two recently developed radioimmunoassays for serum PICP (the carboxyterminal propeptide of type I procollagen) and serum ICTP (the carboxyterminal telopeptide of type I collagen), have been evaluated. Fasting morning values were obtained from 18 women with primary hyperparathyroidism (HPT) and an equal number of age-matched, healthy controls. A 24-hour infusion of synthetic human parathyroid hormone (PTH 1-38) was performed in 14 healthy females. The patients with HPT had higher values for serum ICTP than the controls (6.0 +/- 3.0 and 4.1 +/- 2.1 micrograms/liter; P < 0.05), whereas the serum PICP concentrations were not different (170 +/- 72 and 151 +/- 65 micrograms/liter; n.s.). During infusion of PTH in healthy subjects, there was an increase of the serum ICTP concentrations (from 3.6 +/- 1.3 to 4.4 +/- 1.8 micrograms/liter; P < 0.001) whereas those of serum PICP decreased (from 185 +/- 78 to 118 +/- 42 micrograms/liter; P < or = 0.0001). The increase of serum ICTP during infusion of PTH was positively related to the increase of serum calcium and other indices of bone resorption, i.e., fasting urinary excretions of hydroxyproline and calcium. The decrease of serum PICP was also related to the changes of serum ICTP and hydroxyproline in urine, serum calcium, and alkaline phosphatase but not to osteocalcin, an established marker of osteoblastic activity. The findings support the fact that serum ICTP is a valuable method for evaluating bone resorption and is also easy to perform.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphaturic effect of parathyroid hormone in the spontaneously hypertensive rat

The Okamoto spontaneously hypertensive rat (SHR) has been reported to have altered phosphate metabolism. Hypophosphaturia in the presence of increased serum parathyroid hormone (PTH) levels has been reported in the SHR. Therefore it has been postulated that the SHR may be hyporesponsive to the phosphaturic effect of endogenous PTH. In addition, the SHR exhibits enhanced renal sympathetic nerve activity. Recent studies demonstrated that stimulation of the renal adrenoreceptors decreases the phosphaturic response to PTH infusion. Thus a hyporesponsiveness to PTH in the SHR may be due in part to higher renal sympathetic tone. The present study determined the phosphaturic effect of a pharmacological dose of PTH (33 U/kg bolus and 1 U.kg-1.min-1 infusion) in the thyroparathyroidectomized SHR compared with its normotensive control, the Wistar Kyoto (WKY) rat. Three groups of clearance experiments were performed on male 10- to 14-wk-old SHR and WKY rats. In the first group of rats, the fractional excretion of phosphate (FEPi) in response to PTH infusion was 35.4 +/- 4.2% in the SHR (n = 6) and 26.2 +/- 3.0% in the WKY rat (n = 6), NS. In the second group, all animals underwent acute unilateral renal denervation (DNX). The FEPi in response to PTH was 35.3 +/- 1.5% in the innervated (INN) kidney of the SHR (n = 10) compared with 27.9 +/- 2.5% in the INN kidney of the WKY rat (n = 11), and 39.1 +/- 1.9% in the DNX kidney of the SHR compared with 30.5 +/- 2.0% in the DNX kidney of the WKY rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum carboxy-terminal propeptide of human type I procollagen in patients with primary hyperparathyroidism: studies in basal conditions and after parathyroid surgery.

This study was carried out in order to evaluate serum carboxy-terminal propeptide of human type I procollagen (PICP) in patients with primary hyperparathyroidism and to examine its changes following parathyroidectomy. Seventeen patients (four males and 13 famels, aged 53.8 +/- 3.1 SEM years) were studied in basal conditions; six patients also were investigated after successful parathyroid surgery. Mean serum PICP values of patients with primary hyperparathyroidism (194.5 +/- 27 SEM micrograms/l) were significantly higher (p < 0.001) with respect to those found in normal subjects. However, deviations from the norm (Z score values) were significantly less with respect to deviations of serum osteocalcin, alkaline phosphatase and urinary hydroxyproline/creatinine ratio. Following parathyroidectomy, it was possible to observe a discrepancy between markers of bone resorption and those of bone formation. The former tend to decrease, while the latter either do not show any significant change (serum alkaline phosphatase and serum osteocalcin) or increase (serum procollagen). The results of our investigation indicate that in basal conditions the assay of serum procollagen may be of clinical value but it would be better to use it in combination with other biomarkers of skeletal remodelling. The results obtained after parathyroidectomy are the opposite of those obtained following parathyroid hormone infusion and should be ascribed to the effect of acute hormone deficiency on collagen synthesis. The positive biochemical uncoupling following surgery might lend support to the rise of bone mineral density consistently reported in the first few months following parathyroidectomy.

Parathyroid hormone/adenylate cyclase coupling in vascular smooth muscle cells.

Parathyroid hormone (PTH) has been implicated in hypertension, but PTH infusion results in vasodilation. PTH activates adenylate cyclase in vascular smooth muscle, but little is known about the factors that regulate PTH receptor/adenylate cyclase coupling in vascular cells. To characterize hormone-receptor signaling, we measured cyclic AMP levels in rat arterial smooth muscle cells in culture exposed to PTH (bovine 1-34). PTH yielded time- and concentration-dependent increases in cyclic AMP levels. Compared with isoproterenol, PTH was more potent, with a threshold at 2 x 10(-9) versus 5 x 10(-8) mol/L and half maximal responses at 10(-8) versus 2.4 x 10(-7) mol/L. PTH-induced increases in cyclic AMP were independent of extracellular calcium, cyclooxygenase metabolites, phospholipase C, and protein kinase C because PTH-induced increases in cyclic AMP were not prevented by variations in extracellular calcium, indomethacin, angiotensin II, vasopressin, and protein kinase C activators or inhibitors. PTH/adenylate cyclase coupling was G protein-dependent because increases in cyclic AMP were prevented by preincubation with cholera toxin but not with pertussis toxin. Prolonged exposure to PTH resulted in time- and concentration-dependent homologous desensitization of cyclic AMP responses. Desensitization occurred proximal to G protein/adenylate cyclase because after prolonged PTH, responses to forskolin and cholera toxin remained intact. Desensitization was independent of protein kinase A and receptor sequestration because cyclic AMP responses remained after prolonged exposure to forskolin and pretreatment with phenylarsine oxide, colchicine, and cytochalasin D. We conclude that in vascular smooth muscle cells, PTH is coupled to adenylate cyclase through a cholera toxin-sensitive G protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of acute hypoxia on phosphate excretion in rats

This study evaluated the effect of acute hypoxia on renal handling of phosphate in rats in the presence and absence of parathyroid hormone (PTH). Hypoxia causes respiratory alkalosis in spontaneously breathing humans and animals. Respiratory alkalosis has been reported to induce a blunted phosphaturic response to PTH. In this study, to avoid the confounding effect of hypocapnia accompanying the hypoxia on phosphate excretion, the rats were ventilated mechanically, and arterial PCO2 levels were controlled. Rats were divided into two main groups depending on the arterial PO2 levels: a hypoxic group (n = 16) and a normoxic group (n = 18). Hypoxia was produced by ventilating with 10% oxygen, and hypocapnia was produced by hyperventilation. In response to PTH, the hypoxic rats without hypocapnia showed a greater increase in fractional excretion of phosphate (FEPi; 37.7 +/- 2.6%, mean +/- SE) compared with normoxic rats (27.4 +/- 2.5%, P < 0.02). During hypocapnia, there was no difference in FEPi between hypoxic and normoxic groups (21.2 +/- 1.5 and 19.5 +/- 1.2%, respectively), and both groups showed a significantly blunted phosphaturic response to PTH compared with normocapnia (P < 0.05 and P < 0.01, respectively). Urinary adenosine 3',5'-cyclic monophosphate (cAMP) increased similarly after PTH infusion between each group. To test whether the phosphaturic effect of PTH in hypoxia and the blunted phosphaturic effect of PTH in hypocapnia are due to steps beyond the generation of cAMP, the phosphaturic response to cAMP infusion was evaluated in 1) hypoxic and normocapnic rats (n = 6), 2) normoxic and normocapnic (control) rats (n = 6), and 3) normoxic and hypocapnic rats (n = 7).(ABSTRACT TRUNCATED AT 250 WORDS)