Rapid recovery of plasma ionized calcium after acute induction of hypocalcaemia in parathyroidectomized and nephrectomized rats. (9/3417)

BACKGROUND: Plasma ionized calcium (Ca2+) is extremely tightly regulated in normal mammals. Even a small decline in Ca2+ is followed by a fast and steep increase of the parathyroid hormone (PTH) secretion and the current understanding of the calcium homeostasis indicates that PTH is the main factor responsible for this tight minute-to-minute regulation of the normal plasma Ca2+ concentration. However, experiments from our laboratory and some clinical experiences points towards the existence of factors, other than PTH, involved in the rapid minute-to-minute calcium homeostasis. Thus, the aim of the present study was to examine whether PTH plays an important role in the rapid upregulation of plasma Ca2+ after induction of hypocalcaemia in the rat. METHODS AND RESULTS: I. Parathyroidectomy (PTX) was performed in seven rats; 60 min later no PTH was detectable in the circulation. Then by a brief infusion of EGTA plasma Ca2+ was reduced from 1.26+/-0.02 to 0.86+/-0.02 mmol/l, P<0.001. Despite there being no PTH in the circulation plasma Ca2+ increased significantly to 0.97+/-0.02 mmol/l already 10 min after discontinuation of the EGTA infusion, P<0.04, and plasma Ca2+ was normalized within another 2 h. II. To evaluate a possible role of renal Ca2+ handling in the rapid upregulation of plasma Ca2+ a group of eight rats had acute PTX and bilateral nephrectomy (NX) performed; 60 min later plasma Ca2+ was reduced from 1.18+/-0.01 to 0.86+/-0.02 mmol/l by an EGTA infusion. Despite there being no PTH and no kidneys present plasma Ca2+ increased significantly already 10 min after discontinuation of EGTA to 0.96+/-0.02 mmol/l, P<0.02. After another 1.5 h the plasma Ca2+ reached the levels of the PTX/NX control rats. III. In order to exclude a possible action of receptor-bound PTH, which may have lasted for more than 1 h, seven rats were PTX 24 h before the induction of hypocalcaemia. Basal plasma Ca2+ was significantly reduced to 1.07+/-0.01 mmol/l, P<0.01. Then plasma Ca2+ was further reduced to 0.79+/-0.03 mmol/l by EGTA. Ten minutes after discontinuing EGTA plasma Ca2+ increased to 0.91+/-0.02 mmol/l, P<0.03 and 60 min later plasma Ca2+ reached the level of the control PTX rats. Normal rats with intact parathyroid glands had an exactly similar response of plasma Ca2+ to EGTA as that of 24 h PTX rats, but at significantly higher levels of plasma Ca2+ with a fall from 1.28+/-0.01 to 0.96+/-0.03 mmol/l and again a significant increase of plasma Ca2+ to 1.13+/-0.03 (P<0.001) 10 min after discontinuation of EGTA. After another hour basal levels were reached. CONCLUSIONS: Despite there being no PTH in the circulation a rapid increase of plasma Ca2+ occurs immediately after a brief induction of hypocalcaemia. The kidneys are not responsible for this phenomenon. The present results suggest the existence of a mechanism other than the effect of PTH, which is responsible for the rapid minute-to-minute regulation of plasma Ca2+ in the rat.  (+info)

Dual mechanisms of regulation of Na/H exchanger NHE-3 by parathyroid hormone in rat kidney. (10/3417)

Parathyroid hormone (PTH) is a potent inhibitor of mammalian renal proximal tubule sodium absorption via suppression of the apical membrane Na/H exchanger (NHE-3). We examined the mechanisms by which PTH inhibits NHE-3 activity by giving an acute intravenous PTH bolus to parathyroidectomized rats. Parathyroidectomy per se increased apical membrane NHE-3 activity and antigen. Acute infusion of PTH caused a time-dependent decrease in NHE-3 activity as early as 30 min. Decrease in NHE-3 activity at 30 and 60 min was accompanied by increased NHE-3 phosphorylation. In contrast to the rapid changes in NHE-3 activity and phosphorylation, decrease in apical membrane NHE-3 antigen was not detectable until 4-12 h after the PTH bolus. The decrease in apical membrane NHE-3 occurred in the absence of changes in total renal cortical NHE-3 antigen. Pretreatment of the animals with the microtubule-disrupting agent colchicine blocked the PTH-induced decrease in apical NHE-3 antigen. We propose that PTH acutely cause a decrease in NHE-3 intrinsic transport activity possibly via a phosphorylation-dependent mechanism followed by a decrease in apical membrane NHE-3 antigen via changes in protein trafficking.  (+info)

Expression of 25(OH)D3 24-hydroxylase in distal nephron: coordinate regulation by 1,25(OH)2D3 and cAMP or PTH. (11/3417)

Previous studies using microdissected nephron segments reported that the exclusive site of renal 25-hydroxyvitamin D3-24-hydroxylase (24OHase) activity is the renal proximal convoluted tubule (PCT). We now report the presence of 24OHase mRNA, protein, and activity in cells that are devoid of markers of proximal tubules but express characteristics highly specific for the distal tubule. 24OHase mRNA was undetectable in vehicle-treated mouse distal convoluted tubule (DCT) cells but was markedly induced when DCT cells were treated with 1,25 dihydroxyvitamin D3 [1,25(OH)2D3]. 24OHase protein and activity were also identified in DCT cells by Western blot analysis and HPLC, respectively. 8-Bromo-cAMP (1 mM) or parathyroid hormone [PTH-(1-34); 10 nM] was found to potentiate the effect of 1, 25(OH)2D3 on 24OHase mRNA. The stimulatory effect of cAMP or PTH on 24OHase expression in DCT cells suggests differential regulation of 24OHase expression in the PCT and DCT. In the presence of cAMP and 1, 25(OH)2D3, a four- to sixfold induction in vitamin D receptor (VDR) mRNA was observed. VDR protein, as determined by Western blot analysis, was also enhanced in the presence of cAMP. Transient transfection analysis in DCT cells with rat 24OHase promoter deletion constructs demonstrated that cAMP enhanced 1, 25(OH)2D3-induced 24OHase transcription but this enhancement was not mediated by cAMP response elements (CREs) in the 24OHase promoter. We conclude that 1) although the PCT is the major site of localization of 24OHase, 24OHase mRNA and activity can also be localized in the distal nephron; 2) both PTH and cAMP modulate the induction of 24OHase expression by 1,25(OH)2D3 in DCT cells in a manner different from that reported in the PCT; and 3) in DCT cells, upregulation of VDR levels by cAMP, and not an effect on CREs in the 24OHase promoter, is one mechanism involved in the cAMP-mediated modulation of 24OHase transcription.  (+info)

Phosphate depletion in the rat: effect of bisphosphonates and the calcemic response to PTH. (12/3417)

BACKGROUND: The removal of phosphate from the diet of the growing rat rapidly produces hypercalcemia, hypophosphatemia, hypercalciuria, and hypophosphaturia. Increased calcium efflux from bone has been shown to be the important cause of the hypercalcemia and hypercalciuria. It has been proposed that the increased calcium efflux from bone is osteoclast mediated. Because bisphosphonates have been shown to inhibit osteoclast-mediated bone resorption, this study was performed to determine whether bisphosphonate-induced inhibition of osteoclast function changed the biochemical and bone effects induced by phosphate depletion. METHODS: Four groups of pair-fed rats were studied: (a) low-phosphate diet (LPD; phosphate less than 0.05%), (b) LPD plus the administration of the bisphosphonate Pamidronate (APD; LPD + APD), (c) normal diet (ND, 0.6% phosphate), and (d) ND + APD. All diets contained 0.6% calcium. A high dose of APD was administered subcutaneously (0.8 mg/kg) two days before the start of the study diet and on days 2, 6, and 9 during the 11 days of the study diet. On day 10, a 24-hour urine was collected, and on day 11, rats were either sacrificed or received an additional APD dose before a 48-hour parathyroid hormone (PTH) infusion (0.066 microgram/100 g/hr) via a subcutaneously implanted miniosmotic pump. RESULTS: Serum and urinary calcium were greater in the LPD and LPD + APD groups than in the ND and ND + APD groups [serum, 11.12 +/- 0.34 and 11.57 +/- 0.45 vs. 9.49 +/- 0.17 and 9.48 +/- 0.15 mg/dl (mean +/- SE), P < 0.05; and urine, 8.78 +/- 2.74 and 16.30 +/- 4.68 vs. 0.32 +/- 0.09 and 0.67 +/- 0.28 mg/24 hr, P < 0.05]. Serum PTH and serum and urinary phosphorus were less in the LPD and LPD + APD than in the ND and ND + APD groups (P < 0.05). The calcemic response to PTH was less (P < 0.05) in the LPD and LPD + APD groups than in the ND group and was less (P = 0.05) in the LPD + APD than in the ND + APD group. Bone histology showed that phosphate depletion increased the osteoblast and osteoclast surface, and treatment with APD reduced the osteoblast surface (LPD vs. LPD + APD, 38 +/- 4 vs. 4 +/- 2%, P < 0.05, and ND vs. ND + APD, 20 +/- 2 vs. 5 +/- 2%, P < 0.05) and markedly altered osteoclast morphology by inducing cytoplasmic vacuoles. CONCLUSIONS: (a) Phosphate depletion induced hypercalcemia and hypercalciuria that were not reduced by APD administration. (b) The calcemic response to PTH was reduced in phosphate-depleted rats and was unaffected by APD administration in normal and phosphate-depleted rats, and (c) APD administration markedly changed bone histology without affecting the biochemical changes induced by phosphate depletion.  (+info)

Randomised controlled trial of recombinant human growth hormone in prepubertal and pubertal renal transplant recipients. British Association for Pediatric Nephrology. (13/3417)

AIMS: To evaluate the efficacy (height velocity (HV), change in height standard deviation score (delta HSDS)), and safety (glomerular filtration rate (GFR), incidence of rejection, and calcium and glucose metabolism) of recombinant human growth hormone (rhGH) treatment after renal transplantation. DESIGN: A two year randomised controlled trial. SUBJECTS: Fifteen prepubertal and seven pubertal children: mean (SD) age, 13.0 (2.6) and 15.2 (2.4) years, respectively; mean (SD) GFR, 51 (30) and 48 (17) ml/min/1.73 m2, respectively. Six prepubertal and three pubertal children were controls during the first year; all received rhGH in the second year. RESULTS: In the first year, mean (SE) HV and delta HSDS in the prepubertal treated group increased compared with controls: 8.1 (0.9) v 3.7 (0.6) cm/year and 0.6 (0.1) v -0.3 (0.2), respectively. In the pubertal treated group, mean (SE) HV and delta HSDS were also greater: 10.1 (0.6) v 3.9 (1.3) cm/year and 0.6 (0.1) v -0.1 (0.2), respectively. Comparing all treated and control children, there was no significant change in GFR: treated group, mean (SE) 9.9 (5.4) ml/min/1.73 m2 v control group, -1.6 (7.6) ml/min/1.73 m2. There were also no differences in the incidence of rejection in the first year: eight episodes in 13 patients v five episodes in nine patients, respectively. Phosphate, alkaline phosphatase (ALP), parathyroid hormone (PTH), and fasting insulin concentrations rose during the first year of treatment, but not thereafter. In the second year of treatment, HV remained above baseline. CONCLUSION: Treatment with rhGH improves growth in prepubertal and pubertal children with renal transplants, with no significant change in GFR or the incidence of rejection. Phosphate, ALP, PTH, and insulin increased during the first year of treatment.  (+info)

Elevated parathyroid hormone 44-68 and osteoarticular changes in patients with genetic hemochromatosis. (14/3417)

OBJECTIVE: To determine whether the osteoarticular changes associated with genetic hemochromatosis could be explained by metabolic parathyroid hormone (PTH) disorders. METHODS: The study involved 210 patients with liver iron overload syndromes. Osteoarticular changes were numerically scored as the number of damaged joints. PTH 1-84 and 44-68 were assayed. RESULTS: An increase in serum PTH 44-68 levels was found in one-third of untreated patients who had no calcium or PTH 1-84 abnormalities. Serum PTH 44-68 levels correlated positively with serum ferritin levels. In multivariate analyses, the number of affected joints correlated positively with age, serum PTH 44-68 levels, and serum ferritin levels. CONCLUSION: Liver iron overload syndromes, especially genetic hemochromatosis, are associated with elevated circulating levels of PTH fragments containing the 44-68 region, which appears to play a role in osteoarticular changes. This increase seems to be a consequence of iron overload.  (+info)

Maturational disturbance of chondrocytes in Cbfa1-deficient mice. (15/3417)

Cbfa1, a transcription factor that belongs to the runt-domain gene family, plays an essential role in osteogenesis. Cbfa1-deficient mice completely lacked both intramembranous and endochondral ossification, owing to the maturational arrest of osteoblasts, indicating that Cbfa1 has a fundamental role in osteoblast differentiation. However, Cbfa1 was also expressed in chondrocytes, and its expression was increased according to the maturation of chondrocytes. Terminal hypertrophic chondrocytes expressed Cbfa1 extensively. The significant expression of Cbfa1 in hypertrophic chondrocytes was first detected at embryonic day 13.5 (E13.5), and its expression in hypertrophic chondrocytes was most prominent at E14.5-16.5. In Cbfa1-deficient mice, whose entire skeleton was composed of cartilage, the chondrocyte differentiation was disturbed. Calcification of cartilage occurred in the restricted parts of skeletons, including tibia, fibula, radius, and ulna. Type X collagen, BMP6, and Indian hedgehog were expressed in their hypertrophic chondrocytes. However, osteopontin, bone sialoprotein, and collagenase 3 were not expressed at all, indicating that they are directly regulated by Cbfa1 in the terminal hypertrophic chondrocytes. Chondrocyte differentiation was severely disturbed in the rest of the skeleton. The expression of PTH/PTHrP receptor, Indian hedgehog, type X collagen, and BMP6 was not detected in humerus and femur, indicating that chondrocyte differentiation was blocked before prehypertrophic chondrocytes. These findings demonstrate that Cbfa1 is an important factor for chondrocyte differentiation.  (+info)

Expression of parathyroid hormone-related peptide messenger ribonucleic acid in developing kidney. (16/3417)

BACKGROUND: Parathyroid hormone (PTH)-related peptide (PTHrP), originally identified as a causative agent of hypercalcemia of malignancy, has been implicated in the regulation of growth and differentiation of endochondral bone, hair follicle, and breast as an autocrine/paracrine factor. Although some experiments indicate that PTHrP works as a growth factor for primary renal cells in vitro, the role of PTHrP in the kidney in vivo is not yet known. METHODS: We examined the amounts of PTHrP and PTH/ PTHrP receptor (PTHR) mRNA in the mouse kidney developmental process by reverse transcription-polymerase chain reaction, and investigated which cells produce PTHrP and PTHR in vivo by in situ hybridization. RESULTS: We observed high levels of PTHrP mRNA during mouse kidney maturation. PTHrP mRNA was expressed in the collecting duct, urothelium of the pelvis, and immature elements in the cortex of the developing kidney, including the S-shaped body, ureteric bud, and glomerulus. However, the expression of PTHR mRNA was lower during maturation than after the completion of the maturation process, and it was not detected in the collecting duct, urothelium of the pelvis, or nephrogenic zone in embryonic day 16 or 0-day-old mouse kidneys. CONCLUSION: These findings suggest that PTHrP has a role in mouse kidney maturation or glomerular development.  (+info)