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      Oral Feeding Acutely Down-Regulates Serum PTH in Hemodialysis Patients

      , ,

      Nephron Clinical Practice

      S. Karger AG

      Parathyroid hormone, Phosphorus, Diet, Calcium

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          Abstract

          Background: Changes in serum parathyroid hormone (PTH) within minutes are known only to be mediated by changes in ionized calcium. Recent animal studies show ingestion of a low phosphorus meal can lower serum PTH within 15 min, before changes in serum ionized calcium or phosphorus occur, suggesting a rapid gastrointestinal signal may regulate PTH. Methods: Eight hemodialysis patients with secondary hyperparathyroidism were admitted twice to a metabolic unit and ate a high and low phosphorus meal after an overnight fast. Serum PTH, total and ionized calcium, phosphorus, pH, and glucose were measured at 0, 15, 30, 60, 120 and 240 min. In the second protocol, we examined the possible role of volume or glucose changes in rapid PTH suppression by administering intravenous saline and glucose after an overnight fast to 6 patients, with similar testing. Results: Intact PTH decreased 24% from 419 ± 331 at baseline to 312 ± 221 pg/ml (p = 0.002) 15 min after a meal. Total and ionized calcium and pH did not change, glucose rose by 15 min, and phosphorus changed only after 60–90 min. During the second protocol, saline and glucose infusions failed to change PTH. Conclusions: In dialysis patients, a glucose-containing meal, with or without phosphorus, rapidly suppresses serum PTH ∼25% within 15 min. This effect is not mediated by changes in ionized calcium, phosphorus, pH, glucose, or insulin. These data suggest there may be an as yet unknown enteral signal that rapidly suppresses PTH.

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          Most cited references 20

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          Phosphorus restriction prevents parathyroid gland growth. High phosphorus directly stimulates PTH secretion in vitro.

          Dietary phosphorus (P) restriction is known to ameliorate secondary hyperparathyroidism in renal failure patients. In early renal failure, this effect may be mediated by an increase in 1,25-(OH)2D3, whereas in advanced renal failure, P restriction can act independent of changes in 1,25-(OH)2D3 and serum ionized calcium (ICa). In this study, we examined the effects of dietary P on serum PTH, PTH mRNA, and parathyroid gland (PTG) hyperplasia in uremic rats. Normal and uremic rats were maintained on a low (0.2%) or high (0.8%) P diet for 2 mo. PTG weight and serum PTH were similar in both groups of normal rats and in uremic rats fed the 0.2% P diet. In contrast, there were significant increases in serum PTH (130 +/- 25 vs. 35 +/- 3.5 pg/ml, P < 0.01), PTG weight (1.80 +/- 0.13 vs. 0.88 +/- 0.06 microg/gram of body weight, P < 0.01), and PTG DNA (1.63 +/- 0.24 vs. 0.94 +/- 0.07 microg DNA/gland, P < 0.01) in the uremic rats fed the 0.8% P diet as compared with uremic rats fed the 0.2% P diet. Serum ICa and 1,25-(OH)2D3 were not altered over this range of dietary P, suggesting a direct effect of P on PTG function. We tested this possibility in organ cultures of rat PTGs. While PTH secretion was acutely (30 min) regulated by medium calcium, the effects of medium P were not evident until 3 h. During a 6-h incubation, PTH accumulation was significantly greater in the 2.8 mM P medium than in the 0.2 mM P medium (1,706 +/- 215 vs. 1,033 +/- 209 pg/microg DNA, P < 0.02); the medium ICa was 1.25 mM in both conditions. Medium P did not alter PTH mRNA in this system, but cycloheximide (10 microg/ml) abolished the effect of P on PTH secretion. Thus, the effect of P is posttranscriptional, affecting PTH at a translational or posttranslational step. Collectively, these in vivo and in vitro results demonstrate a direct action of P on PTG function that is independent of ICa and 1,25-(OH)2D3.
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            Direct effect of phosphorus on PTH secretion from whole rat parathyroid glands in vitro.

            Phosphorus retention is an important factor in the development of hyperparathyroidism secondary to renal failure. In vivo manipulation of phosphorus is associated with changes in serum calcium and calcitriol levels which in turn can modify parathyroid hormone synthesis and secretion. The present in vitro study evaluates whether high extracellular phosphorus has a direct effect on parathyroid hormone secretion. Fresh rat parathyroid glands were incubated in a media with phosphorus concentrations of 1, 2, 3, and 4 mM and subsequently exposed to calcium levels ranging from 0.4 to 1.35 mM. In 1.25 mM calcium, the parathyroid hormone secretion rate was similar in 1 and 2 mM phosphorus; however, a phosphorus concentration of 3 and 4 mM produced a 3- and 4-fold increase in the parathyroid hormone secretion, respectively, as compared with 1 mM phosphorus. While in 1 or 2 mM phosphorus an increase in calcium from 0.6 to 1.35 mM reduced parathyroid hormone secretion to 37%, in 4 mM phosphorus the same increase in calcium only inhibited parathyroid hormone secretion to 75%. Furthermore, the addition of arachidonic acid 20 microM, a substrate for inhibitory intracellular signal pathway, to the 4 mM phosphorus-1.35 mM calcium incubation media reduced the parathyroid hormone secretion to 34.5% (p < 0.05). In conclusion, our results indicate that in vitro, high phosphorus directly increases parathyroid hormone secretion.
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              Calcium-ion-sensing cell-surface receptors.

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                Author and article information

                Journal
                NEC
                Nephron Clin Pract
                10.1159/issn.1660-2110
                Nephron Clinical Practice
                S. Karger AG
                1660-2110
                2006
                June 2006
                13 March 2006
                : 103
                : 3
                : c106-c113
                Affiliations
                Renal Division, Department of Internal Medicine, and the Chromalloy American Kidney Center at Washington University School of Medicine, St. Louis, Mo., USA
                Article
                92019 Nephron Clin Pract 2006;103:c106–c113
                10.1159/000092019
                16534234
                © 2006 S. Karger AG, Basel

                Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

                Page count
                Figures: 3, Tables: 3, References: 33, Pages: 1
                Product
                Self URI (application/pdf): https://www.karger.com/Article/Pdf/92019
                Categories
                Original Paper

                Cardiovascular Medicine, Nephrology

                Diet, Phosphorus, Calcium, Parathyroid hormone

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