Prostaglandins regulate acid-induced cell-mediated bone resorption

Nancy S. Krieger, Walter R. Parker, Kristen M. Alexander, and David A. Bushinsky

Department of Medicine, Nephrology Unit, University of Rochester School of Medicine, Rochester, New York 14642


    ABSTRACT
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Metabolic acidosis induces bone calcium efflux initially by physicochemical dissolution and subsequently by cell-mediated mechanisms involving inhibition of osteoblasts and stimulation of osteoclasts. In rat kidney, acidosis increases endogenous prostaglandin synthesis, and in bone, prostaglandins are important mediators of resorption. To test the hypothesis that acid-induced bone resorption is mediated by prostaglandins, we cultured neonatal mouse calvariae in neutral or physiologically acidic medium with or without 0.56 µM indomethacin to inhibit prostaglandin synthesis. We measured net calcium efflux and medium PGE2 levels. Compared with neutral pH medium, acid medium led to an increase in net calcium flux and PGE2 levels after both 48 h and 51 h, a time at which acid-induced net calcium flux is predominantly cell mediated. Indomethacin inhibited the acid-induced increase in both net calcium flux and PGE2. Net calcium flux was correlated directly with medium PGE2 (r = 0.879, n = 29, P < 0.001). Exogenous PGE2, at a level similar to that found after acid incubation, induced net calcium flux in bones cultured in neutral medium. Acid medium also stimulated an increase in PGE2 levels in isolated bone cells (principally osteoblasts), which was again inhibited by indomethacin. Thus acid-induced stimulation of cell-mediated bone resorption appears to be mediated by endogenous osteoblastic PGE2 synthesis.

metabolic acidosis; prostaglandin E; osteoblasts; calcium


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

METABOLIC ACIDOSIS INCREASES urine calcium excretion (8, 21, 38), without a corresponding increase in intestinal calcium absorption (27, 39), resulting in a net loss of body calcium (8, 39). Bone has been implicated as the source of this additional urinary calcium, as the vast majority of body calcium is located within the mineral phases of bone (59). Indeed, depletion of bone mineral has been found in mammals made acidemic (4, 33).

In vitro studies have supported the hypothesis that metabolic acidosis induces loss of bone calcium (2, 3, 5-7, 9-20, 24-26, 36, 47). Metabolic acidosis stimulates net calcium efflux from bone through both early physicochemical and later cell-mediated mechanisms (2, 3, 5-7, 10, 11, 14, 16, 19, 24-26, 36, 47). Acutely, in the first 3 h, calcium efflux is primarily due to physicochemical bone mineral dissolution (2, 11, 14, 19, 26); however, by 24-48 h, cell-mediated calcium release predominates (3, 5-7, 10, 16, 24, 25, 36, 47). Using neonatal mouse calvariae in an organ culture system, we have previously demonstrated that the cell-mediated calcium release observed in response to a model of metabolic acidosis results from both an inhibition of osteoblastic activity and a stimulation of osteoclastic activity (5, 16, 36). Osteoblastic collagen synthesis and alkaline phosphatase activity both were decreased after incubation in acidic medium compared with controls incubated in neutral pH medium (5, 16, 36). The immediate early response gene egr-1 was inhibited by acidosis (26) as were the bone matrix genes, matrix gla protein and osteopontin (24, 25). Release of osteoclastic beta -glucuronidase, a lysosomal enzyme whose release correlates with osteoclast-mediated bone resorption, was increased during culture in acid medium (5, 16, 36).

Acidosis has multiple effects on cells, one of which is to increase levels of prostaglandins (1, 22, 23, 28). In both toad bladder (22, 23) and rat kidney (28), PGE2 levels have been shown to increase in response to metabolic acidosis. In newborn pig brain, acidosis increases the level of prostaglandins that are associated with vasodilatation (1). An increase in prostaglandin levels by bone cells would be important, as prostaglandins are potent local stimulators of bone resorption and appear to mediate bone resorption induced by a variety of cytokines and growth factors (23, 43).

In this study we tested the hypothesis that incubation of neonatal mouse calvariae in acidic medium would lead to an increase in medium prostaglandins and that this increased level is responsible, at least in part, for acidosis-induced bone resorption. The results of this study support our hypothesis. We demonstrate that incubation of bone cells and calvariae in acidic medium both lead to an increase in the level of medium PGE2. In calvariae there is a parallel increase in net calcium efflux and PGE2 levels, and inhibition of PGE2 production strongly limits this acidosis-induced bone calcium release.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Primary Bone Cell Culture

Cells were isolated from neonatal mouse calvariae immediately after dissection as described previously (34, 35). The cells were isolated by collagenase digestion of the calvariae and are primarily osteoblasts (35). Briefly, bones were washed in PBS containing 4 mM EDTA for 10 min at 37°C and then incubated in a 25 mM HEPES buffer solution, pH 7.4, containing 2 mg/ml collagenase (Wako Pure Chemicals, Dallas, TX) and 90 µM Nalpha -tosyl-L-lysine chloromethyl ketone for four sequential 20-min digestion periods at 37°C in a shaking water bath. At the end of each digestion, released cells were collected and resuspended in HEPES buffer containing 1 mM MgSO4, and all four digests were pooled for plating on 60-mm Primaria culture dishes (Falcon) in DME supplemented with 15% heat-inactivated horse serum. Medium was changed every 2-3 days.

At confluence, control medium was replaced with preincubated neutral (pH 7.4), acidic (pH 7.1), or very acidic medium (pH 6.8), with or without the prostaglandin inhibitor indomethacin (0.56 µM). At the end of the incubation, medium was immediately assayed for PGE2.

Organ Culture of Bone

Exactly 2.8 ml of DME containing 15% heat-inactivated horse serum was preincubated at PCO2 = 40 mmHg at 37°C for 3 h in 35-mm dishes (2, 3, 5, 6, 9-20, 24-26, 36, 47). Calvariae were dissected from 4- to 6-day-old neonatal mice, and just prior to adding the bones, 1 ml of medium was removed to determine preincubation medium pH and PCO2 and calcium. Medium pH and PCO2 were determined with a blood-gas analyzer (Radiometer model ABL 30), and Ca was determined by fluorometric titration (Calcette, Precision Systems). At the end of each incubation period, medium was removed and analyzed for pH, PCO2, and calcium. After the 24-48 h and the 48-51 h incubation, medium was also immediately analyzed for PGE2. The concentration of medium bicarbonate (HCO3-) was calculated from medium pH and PCO2 as described previously (2, 12). Net calcium flux was calculated as Vm([Ca]f - [Ca]i), where Vm is the medium volume (1.8 ml) and [Ca]f and [Ca]i are the final and initial medium Ca concentrations, respectively.

Calvarial Treatment Groups

Some calvariae were incubated in neutral or acidic medium in the presence or absence of indomethacin (0.56 µM) for 24 h. Calvariae were transferred to similar, preincubated fresh medium and cultured for an additional 24 h. Calvariae were again transferred to similar, preincubated fresh medium and cultured for a final 3 h. Other calvariae were incubated in neutral pH medium with or without PGE2 at concentrations of 5 and 50 ng/ml.

Prostaglandin Levels

The level of medium PGE2 in the isolated cells and calvariae was determined immediately after the end of the incubation using an enzyme immunoassay kit (Cayman Chemical, Ann Arbor, MI). Quantitation of the assay was done using a Dynatech model MR700 microplate reader and the Immunosoft computer program.

Statistical Analysis

All values are expressed as means ± SE. Tests of significance were calculated by ANOVA with the Bonferroni correction for multiple comparisons and regression analysis using conventional computer programs (BMDP; University of California, Los Angeles, CA) on a personal computer. P < 0.05 was considered significant.


    RESULTS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Bone Cells

Prostaglandin levels. To determine whether acidosis leads to an increase in prostaglandin levels, we measured medium PGE2 levels from isolated primary bone cells incubated in neutral pH or acidic medium. We chose to measure PGE2, as it is the most potent known metabolite of arachidonic acid that stimulates bone resorption in mouse calvariae (43, 55, 58). Confluent cultures were incubated for 24 h in neutral (pH 7.43 ± 0.01, PCO2 = 38.9 ± 0.5 mmHg, HCO3- = 25.0 ± 0.1 mM) or acidic (pH 7.09 ± 0.01, PCO2 = 39.3 ± 0.1, HCO3- = 11.3 ± 0.1) or very acidic medium (pH 6.82 ± 0.04, PCO2 = 39.3 ± 0.8, HCO3- = 6.1 ± 0.8), each with or without 0.56 µM indomethacin. We and others have shown that this concentration of indomethacin completely inhibits prostaglandin production in bone cells (37, 51). Compared with cells incubated in neutral medium, incubation of cells in acidic and very acidic medium led to a significant dose-dependent increase in the medium PGE2 level; indomethacin inhibited PGE2 production in neutral, acidic, and very acidic medium (Fig. 1). In the cultured cells not treated with indomethacin, the level of medium PGE2 was correlated inversely with initial medium pH (r = -0.620, n = 22, P = 0.002).


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Fig. 1.   Acid-induced change in medium PGE2 levels in primary calvarial cell cultures. Primary cells were isolated from neonatal mouse calvariae and cultured in neutral pH medium until confluent. At confluence, medium was again changed, and some bone cell cultures were continued in neutral medium (pH 7.4) and others in acidic medium (pH 7.1) or very acidic medium (pH 6.8). Indomethacin (Indo, 0.56 µM final concentration) was added to the medium of some bone cell cultures in each of these groups. After 24 h of incubation the concentration of medium PGE2 was immediately determined. Data are the means ± SE for 6-8 bone cell cultures in each group. *P < 0.001 vs. pH 7.4, no Indo. +P < 0.001 vs. pH 7.1, no Indo. oP < 0.001 vs. pH 6.8, no Indo.

Calvariae

Initial medium pH, PCO2, and HCO3-. To determine whether increased prostaglandin levels are responsible for acid-induced calcium release from bone, we incubated calvariae under neutral pH or acidic conditions in the presence or absence of indomethacin. During each of the three time periods, 0-24 h, 24-48 h, and 48-51 h, the acidic medium had a significantly lower pH, due to a decrease in medium HCO3-, than the neutral medium (Table 1). There were no differences in initial medium pH or initial medium HCO3- in any of the acid groups, nor were there differences in initial medium pH or initial medium HCO3- in any of the neutral groups. The PCO2 did not differ in any group during any of the three time periods.

                              
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Table 1.   Initial pH, PCO2 and [HCO3-]

Net calcium efflux and medium prostaglandin levels. Compared with incubation in neutral medium, incubation in acidic medium resulted in an increase in net calcium efflux from calvariae at the conclusion of the 24 h incubation (Table 1). Indomethacin partially inhibited the net calcium efflux in both the neutral and acid incubations.

Although acid-induced calcium efflux during the first 24 h is due primarily to physicochemical calcium release (2, 11, 14, 19, 26), we have previously shown that calcium efflux beyond 48 h is due, almost entirely, to cell-mediated mechanisms (3, 5-7, 10, 16, 24, 25, 36, 47). At the conclusion of the 24-48 h incubation there was a marked increase in net calcium efflux from calvariae incubated in acidic medium compared with those incubated in neutral medium (Fig. 2). There was a parallel increase in the concentration of medium PGE2 from calvariae incubated in acidic, compared with neutral, pH medium. During this time period indomethacin completely inhibited calcium efflux from calvariae incubated in neutral medium and partially inhibited net calcium efflux from calvariae incubated in acidic medium. Indomethacin completely suppressed any increase in medium PGE2 concentration from calvariae incubated in both neutral and acidic medium.


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Fig. 2.   Acid-induced net calcium efflux and change in medium PGE2 levels in mouse calvariae: 24-48 h time period. Calvariae were incubated in neutral (Ntl) or acidic (Acid) medium for 48 h in the absence or presence of 0.56 µM indomethacin (Indo), with a medium change at 24 h. Initial medium pH, PCO2, and HCO3- for the two 24-h incubations are shown in Table 1. Net calcium efflux and medium PGE2 concentration are shown for the 24-48 h time period. Results are the mean ± SE for 12 pairs of bones in each group. *P < 0.001 vs. Ntl. +P < 0.001 vs. Acid. oP < 0.001 vs. Ntl + Indo.

At the conclusion of the 48-51 h incubation there was again a marked increase in net calcium efflux from calvariae incubated in acidic medium compared with those incubated in neutral medium (Fig. 3). There was also a parallel increase in the concentration of medium PGE2 from calvariae incubated in acidic, compared with neutral, pH medium. During this time period indomethacin completely inhibited net calcium efflux from calvariae incubated in neutral and acidic medium and totally suppressed any increase in the concentration of medium PGE2 in calvariae incubated in both neutral and acidic medium. There was a significant direct correlation between medium PGE2 levels and net calcium flux during this time period (Fig. 4).


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Fig. 3.   Acid-induced net calcium efflux and change in medium PGE2 levels in mouse calvariae: 48-51 h time period. The calvariae shown in Fig. 2 were incubated for an additional 3 h. They were continued in neutral (Ntl) or acidic (Acid) medium in the absence or presence of 0.56 µM indomethacin (Indo). Initial medium pH, PCO2, and HCO3- for this additional 3-h incubation are shown in Table 1. Net calcium efflux and medium PGE2 concentration are shown for the 48-51 h time period. Results are the mean ± SE for 12 pairs of bones in each group. *P < 0.001 vs. Ntl. +P < 0.001 vs. Acid.



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Fig. 4.   Correlation between PGE2 levels and net calcium efflux in cultured mouse calvaria. Calvariae were incubated for 51 h with a medium change at 24 and 48 h in neutral (triangles) or acidic (squares) medium in the absence (solid symbols) or presence (open symbols) of 0.56 µM indomethacin. Initial medium pH, PCO2, and HCO3- for all three incubations are shown in Table 1. Net calcium efflux and medium PGE2 concentration are plotted for the 48-51 h time period. The curve was fit best by a second order polynomial: net calcium flux = -85.01 (PGE2 level)2 + 33.076 (PGE2 level) - 13.80.

Effect of PGE2 on net calcium efflux. To ensure that the concentration of PGE2 assayed in culture medium was sufficient to induce net calcium efflux from calvariae, we incubated calvariae in the absence or presence of exogenously added PGE2. The addition of 5 and 50 ng/ml PGE2 (final concentration) each led to a significant increase in net calcium efflux from cultured calvariae (Fig. 5).


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Fig. 5.   Increase in net calcium efflux induced by exogenously added PGE2. Calvariae were incubated in the absence or presence of 5 or 50 ng/ml PGE2 (final concentration) for 48 h. Results are means ± SE for 5-6 pairs of bones in each group during the final 24 h of the incubation. *P < 0.001 vs. 0. +P < 0.05 vs. 5 ng/ml PGE2.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Metabolic acidosis induces calcium efflux from bone, first due to physicochemical dissolution (2, 11, 14, 19, 26) and then through cell-mediated mechanisms consisting of a decrease in osteoblastic and an increase in osteoclastic activity (3, 5-7, 10, 16, 24, 25, 36, 47). Until the current study, the metabolic pathway by which these alterations in bone cell function were mediated was not known. We now demonstrate that acid-induced, cell-mediated calcium efflux from cultured neonatal mouse calvariae is mediated, at least in part, by an increase in endogenous PGE2. In calvariae cultured in acidic medium, PGE2 levels increase in parallel with net calcium efflux and the increase in both is blocked by the prostaglandin synthesis inhibitor indomethacin. Culture of calvariae in neutral pH medium with an amount of exogenous PGE2 similar to that found in the acidic cultures induces net calcium efflux from bone. The PGE2 appears to be produced in osteoblasts, as incubation of primary bone cells, which consist mostly of osteoblasts, in acidic medium led to a marked increase in medium PGE2 levels. This increase was again totally suppressed by the prostaglandin inhibitor indomethacin. The magnitude of this acid-induced increase in medium PGE2 levels of primary bone cells (Fig. 1) was comparable to that observed in cultured calvariae incubated in acid medium (Fig. 2).

Prostaglandins are potent multifunctional regulators of bone formation and resorption that mediate the response of bone to a variety of stimuli (43, 44). Prostaglandins, especially PGE2, have been shown to stimulate bone resorption in organ culture (32, 55) and to mediate resorption of mouse calvariae in response to a variety of cytokines and growth factors including epidermal growth factor (52), platelet-derived growth factor (51), tumor necrosis factor-alpha (TNF-alpha ) (53) and transforming growth factor-alpha (TGF-alpha ) (9, 54). As indicated above, in this study we chose to measure only PGE2, as it is the most potent known metabolite of arachidonic acid that stimulates bone resorption in mouse calvariae (43, 55, 58)

While this is the first report detailing the role of prostaglandins in mediating acid-induced bone resorption, acidosis has been associated with increased prostaglandin levels in other model systems. In response to chronic metabolic acidosis induced in toads, urinary bladder cellular PGE2 concentration increased, leading to enhanced proton excretion rates (23, 61). The acidosis-induced inhibition of the hydrosmotic response to vasopressin in the toad bladder is also apparently mediated by increased PGE2 synthesis (22). In the rat, acute metabolic acidosis stimulates urinary prostaglandin excretion, which appears to be involved as a negative feedback regulator of renal ammonia synthesis (28).

The acid-induced increase in bone culture medium PGE2 levels could be due to increased osteoblastic prostaglandin synthesis or decreased degradation. Although changes in prostaglandin levels are generally thought to be due to differences in production, previous studies have shown that degradation is altered as a function of pH (48). PGE2 dehydrates in aqueous solution to PGA2 (48). There is increased stability at acidic pH compared with alkaline pH, so it is possible that the increased levels we observed in acidic medium resulted from decreased degradation. However, the small reported differences in degradation, as a function of pH, make this an unlikely possibility. At pH 6 the aqueous stability of PGE2 at 25°C predicts a 4.8% loss over 24 h, whereas at pH 8 the predicted loss is 6.0% (48). At 37°C the degradation would be greater but probably proportionately similar at the two pH values. Given the reported 1.2% difference in the 24-h rate of degradation between pH 6 and pH 8, the narrower range of pH studied in this report (pH 6.9 vs. pH 7.4) would make differences in degradation an unlikely cause for the two- to threefold increase in PGE2 levels observed in acidic medium.

Prostaglandin synthesis is regulated by the release of arachidonic acid from membrane phospholipids through an increase in phospholipase A2 activity. The subsequent conversion of arachidonic acid to specific prostanoids is catalyzed by prostaglandin G/H synthase (PGHS), also called cyclooxygenase (46). There are two forms of PGHS, both of which are expressed in osteoblasts (41). PGHS-1 is constitutively expressed and PGHS-2 is the inducible form of the enzyme (46). PGHS-2 expression is regulated by several bone-resorbing factors, including interleukin-1 (31, 40, 42), parathyroid hormone (30, 31, 56), interleukin-6 (50), TGF-alpha (42), TNF-alpha (30), and basic fibroblast growth factor (29). We have not yet measured the direct effect of acidosis on cellular PGHS-2 RNA or protein levels in calvariae as we have for other immediate early response genes (26). We may anticipate that PGHS-2 RNA will increase in response to acidosis as we found with another immediate early response gene, egr-1 (26). Determination of the effect of acidosis on PGHS-2 levels or activity will help address the question of whether acidosis increases PGE2 levels by an increase in production or by a decrease in degradation.

We do not yet know which intracellular signal transduction pathway(s) is activated by the acid-induced prostaglandin synthesis nor what specific cellular receptor is activated in response to acidosis. cAMP seems to be the primary second messenger mediating PGE2-stimulated bone resorption (45). However, there is also evidence for PGE2-induced mobilization of intracellular calcium and activation of protein kinase C in osteoblasts (57, 60). Medium pH does not appear to be the sole regulator of cell-mediated bone resorption. We have previously shown that if the medium pH is lowered by a reduction in HCO3-, a model of metabolic acidosis, then there is a greater net calcium efflux than with an isohydric reduction of medium pH achieved by increasing the PCO2, a model of respiratory acidosis (3, 13, 17, 18, 47). Analogously, there is increased osteoclastic activity and decreased osteoblastic activity with models of metabolic but not respiratory acidosis (5).

Thus metabolic acidosis induces net calcium efflux from neonatal mouse calvariae initially through physicochemical dissolution and subsequently through cell-mediated mechanisms consisting of an increase in osteoclastic activity and a decrease in osteoblastic activity. The current study suggests that this alteration in cellular activity is due to an increase in the concentration of PGE2, an autocoid known to mediate bone resorption in response to other regulatory agents. From a clinical perspective, studies will be necessary to determine whether prostaglandin inhibition will decrease acid-induced bone resorption.


    ACKNOWLEDGEMENTS

This work was supported in part by National Institutes of Health Grants RO1-AR-46289 and PO1-DK-56788.


    FOOTNOTES

Address for reprint requests and other correspondence: N. S. Krieger, Dept. of Medicine and of Pharmacology and Physiology, Univ. of Rochester School of Medicine, Nephrology Unit, 601 Elmwood Ave., Box 675, Rochester, NY 14642 (E-mail: Nancy_Krieger{at}urmc.rochester.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 6 June 2000; accepted in final form 3 August 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Aalkjaer, C, and Peng H. pH and smooth muscle. Acta Physiol Scand 161: 557-566, 1997[ISI][Medline].

2.   Bushinsky, DA. Net proton influx into bone during metabolic, but not respiratory, acidosis. Am J Physiol Renal Fluid Electrolyte Physiol 254: F306-F310, 1988[Abstract/Free Full Text].

3.   Bushinsky, DA. Net calcium efflux from live bone during chronic metabolic, but not respiratory, acidosis. Am J Physiol Renal Fluid Electrolyte Physiol 256: F836-F842, 1989[Abstract/Free Full Text].

4.   Bushinsky, DA. The contribution of acidosis to renal osteodystrophy. Kidney Int 47: 1816-1832, 1995[ISI][Medline].

5.   Bushinsky, DA. Stimulated osteoclastic and suppressed osteoblastic activity in metabolic but not respiratory acidosis. Am J Physiol Cell Physiol 268: C80-C88, 1995[Abstract/Free Full Text].

6.   Bushinsky, DA. Metabolic alkalosis decreases bone calcium efflux by suppressing osteoclasts and stimulating osteoblasts. Am J Physiol Renal Fluid Electrolyte Physiol 271: F216-F222, 1996[Abstract/Free Full Text].

7.   Bushinsky, DA, Chabala JM, Gavrilov KL, and Levi-Setti R. Effects of in vivo metabolic acidosis on midcortical bone ion composition. Am J Physiol Renal Physiol 277: F813-F819, 1999[Abstract/Free Full Text].

8.   Bushinsky, DA, Favus MJ, Schneider AB, Sen PK, Sherwood LM, and Coe FL. Effects of metabolic acidosis on PTH and 1,25(OH)2D3 response to low calcium diet. Am J Physiol Renal Fluid Electrolyte Physiol 243: F570-F575, 1982[ISI][Medline].

9.   Bushinsky, DA, Gavrilov K, Chabala JM, Featherstone JDB, and Levi-Setti R. Effect of metabolic acidosis on the potassium content of bone. J Bone Miner Res 12: 1664-1671, 1997[ISI][Medline].

10.   Bushinsky, DA, Gavrilov K, Stathopoulos VM, Krieger NS, Chabala JM, and Levi-Setti R. Effects of osteoclastic resorption on bone surface ion composition. Am J Physiol Cell Physiol 271: C1025-C1031, 1996[Abstract/Free Full Text].

11.   Bushinsky, DA, Goldring JM, and Coe FL. Cellular contribution to pH-mediated calcium flux in neonatal mouse calvariae. Am J Physiol Renal Fluid Electrolyte Physiol 248: F785-F789, 1985[Abstract/Free Full Text].

12.   Bushinsky, DA, Krieger NS, Geisser DI, Grossman EB, and Coe FL. Effects of pH on bone calcium and proton fluxes in vitro. Am J Physiol Renal Fluid Electrolyte Physiol 245: F204-F209, 1983[Abstract/Free Full Text].

13.   Bushinsky, DA, Lam BC, Nespeca R, Sessler NE, and Grynpas MD. Decreased bone carbonate content in response to metabolic, but not respiratory, acidosis. Am J Physiol Renal Fluid Electrolyte Physiol 265: F530-F536, 1993[Abstract/Free Full Text].

14.   Bushinsky, DA, and Lechleider RJ. Mechanism of proton-induced bone calcium release: calcium carbonate dissolution. Am J Physiol Renal Fluid Electrolyte Physiol 253: F998-F1005, 1987[Abstract/Free Full Text].

15.   Bushinsky, DA, Levi-Setti R, and Coe FL. Ion microprobe determination of bone surface elements: effects of reduced medium pH. Am J Physiol Renal Fluid Electrolyte Physiol 250: F1090-F1097, 1986[Abstract/Free Full Text].

16.   Bushinsky, DA, and Nilsson EL. Additive effects of acidosis and parathyroid hormone on mouse osteoblastic and osteoclastic function. Am J Physiol Cell Physiol 269: C1364-C1370, 1995[Abstract/Free Full Text].

17.   Bushinsky, DA, and Sessler NE. Critical role of bicarbonate in calcium release from bone. Am J Physiol Renal Fluid Electrolyte Physiol 263: F510-F515, 1992[Abstract/Free Full Text].

18.   Bushinsky, DA, Sessler NE, and Krieger NS. Greater unidirectional calcium efflux from bone during metabolic, compared with respiratory, acidosis. Am J Physiol Renal Fluid Electrolyte Physiol 262: F425-F431, 1992[Abstract/Free Full Text].

19.   Bushinsky, DA, Wolbach W, Sessler NE, Mogilevsky R, and Levi-Setti R. Physicochemical effects of acidosis on bone calcium flux and surface ion composition. J Bone Miner Res 8: 93-102, 1993[ISI][Medline].

20.   Chabala, JM, Levi-Setti R, and Bushinsky DA. Alteration in surface ion composition of cultured bone during metabolic, but not respiratory, acidosis. Am J Physiol Renal Fluid Electrolyte Physiol 261: F76-F84, 1991[Abstract].

21.   Coe, FL, and Bushinsky DA. Pathophysiology of hypercalciuria. Am J Physiol Renal Fluid Electrolyte Physiol 247: F1-F13, 1984[ISI][Medline].

22.   Forrest, JN, Jr., Schneider CJ, and Goodman DBP Role of prostaglandin E2 in mediating the effects of pH on the hydroosmotic response to vasopressin in the toad urinary bladder. J Clin Invest 69: 499-506, 1982[ISI][Medline].

23.   Frazier, LW, and Yorio T. Prostaglandins as mediators of acidification in the urinary bladder of Bufo marinus. Proc Soc Exp Biol Med 194: 10-15, 1990[Abstract].

24.   Frick, KK, and Bushinsky DA. Chronic metabolic acidosis reversibly inhibits extracellular matrix gene expression in mouse osteoblasts. Am J Physiol Renal Physiol 275: F840-F847, 1998[Abstract/Free Full Text].

25.   Frick, KK, and Bushinsky DA. In vitro metabolic and respiratory acidosis selectively inhibit osteoblastic matrix gene expression. Am J Physiol Renal Physiol 277: F750-F755, 1999[Abstract/Free Full Text].

26.   Frick, KK, Jiang L, and Bushinsky DA. Acute metabolic acidosis inhibits the induction of osteoblastic egr-1 and type 1 collagen. Am J Physiol Cell Physiol 272: C1450-C1456, 1997[Abstract/Free Full Text].

27.   Gafter, U, Kraut JA, Lee DBN, Silis V, Walling MW, Kurokawa K, Haussler MR, and Coburn JW. Effect of metabolic acidosis in intestinal absorption of calcium and phosphorus. Am J Physiol Gastrointest Liver Physiol 239: G480-G484, 1980[Abstract/Free Full Text].

28.   Jones, ER, Beck TR, Kapoor S, Shay R, and Narins RG. Prostaglandins inhibit renal ammoniagenesis in the rat. J Clin Invest 74: 992-1002, 1984[ISI][Medline].

29.   Kawaguchi, H, Gronowicz G, Abreu C, Fletcher BS, Herschman HR, Raisz LG, and Hurley MM. Transcriptional induction of prostaglandin G/H synthase-2 by basic fibroblast growth factor. J Clin Invest 96: 923-930, 1995[ISI][Medline].

30.   Kawaguchi, H, Nemoto K, Raisz LG, Harrison JR, Voznesensky O, Alander CB, and Pilbeam CC. Interleukin-4 inhibits prostaglandin G/H synthase-2 and cytosolic phospholipase A2 induction in neonatal mouse parietal bone cultures. J Bone Miner Res 11: 358-366, 1996[ISI][Medline].

31.   Kawaguchi, H, Raisz LG, Voznesensky O, Alander CB, Hakeda Y, and Pilbeam CC. Regulation of the two prostaglandin G/H synthases by parathyroid hormone, interleukin-1, cortisol and prostaglandin E2 in cultured neonatal mouse calvariae. Endocrinology 135: 1157-1164, 1994[Abstract].

32.   Klein, DC, and Raisz LG. Prostaglandins: stimulation of bone resorption in tissue culture. Endocrinology 86: 1436-1440, 1970[ISI][Medline].

33.   Kraut, JA, Mishler DR, Singer FR, and Goodman WG. The effects of metabolic acidosis on bone formation and bone resorption in the rat. Kidney Int 30: 694-700, 1986[ISI][Medline].

34.   Krieger, NS. Demonstration of sodium/calcium exchange in rodent osteoblasts. J Bone Miner Res 7: 1105-1111, 1992[ISI][Medline].

35.   Krieger, NS, and Hefley TJ. Differential effects of parathyroid hormone on protein phosphorylation in two osteoblast-like cell populations isolated from neonatal mouse calvaria. Calcif Tissue Int 44: 192-199, 1989[ISI][Medline].

36.   Krieger, NS, Sessler NE, and Bushinsky DA. Acidosis inhibits osteoblastic and stimulates osteoclastic activity in vitro. Am J Physiol Renal Fluid Electrolyte Physiol 262: F442-F448, 1992[Abstract/Free Full Text].

37.   Krieger, NS, and Stern PH. Potassium effects of bone: comparison of two model systems. Am J Physiol Endocrinol Metab 245: E303-E307, 1983[Abstract/Free Full Text].

38.   Lemann, J, Jr., Adams ND, and Gray RW. Urinary calcium excretion in human beings. N Engl J Med 301: 535-541, 1979[ISI][Medline].

39.   Lemann, J, Jr., Litzow JR, and Lennon EJ. The effects of chronic acid loads in normal man: further evidence for the participation of bone mineral in the defense against chronic metabolic acidosis. J Clin Invest 45: 1608-1614, 1966[ISI][Medline].

40.   Min, Y, Rao Y, Okada Y, Raisz LG, and Pilbeam CC. Regulation of prostaglandin G/H synthase-2 expression by interleukin-1 in human osteoblast-like cells. J Bone Miner Res 13: 1066-1075, 1997[ISI].

41.   Pilbeam, CC, Kawaguchi H, Hakeda Y, Voznesensky O, Alander CB, and Raisz LG. Differential regulation of the inducible and constitutive prostaglandin endoperoxide synthase in osteoblastic MC3T3-E1 cells. J Biol Chem 268: 25643-25649, 1993[Abstract/Free Full Text].

42.   Pilbeam, CC, Kawaguchi H, Voznesensky O, Alander CB, and Raisz LG. Regulation of inducible prostaglandin G/H synthase by interleukin-1, transforming growth factors-alpha and -beta , and prostaglandins in bone cells. Adv Exp Med Biol 400B: 617-623, 1997[ISI].

43.   Raisz, LG. Bone cell biology: new approaches and unanswered questions. J Bone Miner Res 8: S457-S465, 1993[ISI][Medline].

44.   Raisz, LG. Physiologic and pathologic roles of prostaglandins and other eicosanoids in bone metabolism. J Nutr 125: 2024S-2027S, 1995[Medline].

45.   Raisz, LG, and Martin TJ. Prostaglandins in bone and mineral metabolism. In: Bone and Mineral Research Annual 2, edited by Peck WA.. Amsterdam: Elsevier, 1984, p. 286-310.

46.   Smith, WL. Prostanoid biosynthesis and mechanisms of action. Am J Physiol Renal Fluid Electrolyte Physiol 263: F181-F191, 1992[Abstract/Free Full Text].

47.   Sprague, SM, Krieger NS, and Bushinsky DA. Greater inhibition of in vitro bone mineralization with metabolic than respiratory acidosis. Kidney Int 46: 1199-1206, 1994[ISI][Medline].

48.   Stehle, RG. Physical chemistry, stability, and handling of prostaglandins E2, F2alpha , D2, I2: a critical summary. Methods Enzymol 86: 436-464, 1982[ISI].

49.   Stern, PH, Krieger NS, Nissenson RA, Williams RD, Winkler ME, Derynck R, and Strewler GJ. Human transforming growth factor-alpha stimulates bone resorption in vitro. J Clin Invest 76: 2016-2019, 1985[ISI][Medline].

50.   Tai, M, Miyaura C, Pilbeam CC, Tamura T, Ohsugi Y, Koishihara Y, Kubodera N, Kawaguchi H, Raisz LG, and Suda T. Transcriptional induction of cyclooxygenase-2 in osteoblasts is involved in interleukin-6-induced osteoclast formation. Endocrinology 137: 2372-2379, 1997.

51.   Tashjian, AH, Jr, Hohmann EL, Antoniades HN, and Levine L. Platelet-derived growth factor stimulates bone resorption via a prostaglandin-mediated mechanism. Endocrinology 111: 118-124, 1982[Abstract].

52.   Tashjian, AH, Jr, and Levine L. Epidermal growth factor stimulates prostaglandin production and bone resorption in cultured mouse calvaria. Biochem Biophys Res Commun 85: 966-975, 1978[ISI][Medline].

53.   Tashjian, AH, Jr, Voelkel EF, Lazzaro M, Goad D, Bosma T, and Levine L. Tumor necrosis factor-alpha (cachectin) stimulates bone resorption in mouse calvaria via a prostaglandin-mediated mechanism. Endocrinology 120: 2029-2035, 1987[Abstract].

54.   Tashjian, AH, Jr, Voelkel EF, Lazzaro M, Singer FR, Roberts AB, Winkler ME, and Levine L. Alpha and beta human transforming growth factors stimulated prostaglandin production and bone resorption in cultured mouse calvaria. Proc Natl Acad Sci USA 82: 4535-4538, 1985[Abstract].

55.   Tashjian, AH, Jr, Voelkel EF, Levine L, and Goldhaber P. Evidence that the bone resorption-stimulating factor produced by mouse fibrosarcoma cells is prostaglandin E2. J Exp Med 136: 1329-1343, 1972[ISI][Medline].

56.   Tetradis, S, Pilbeam CC, Liu Y, and Kream BE. Parathyroid hormone induces prostaglandin G/H synthase-2 expression by a cyclic adenosine 3',5'-monophosphate-mediated pathway in the murine osteoblastic cell line MC3T3-E1. Endocrinology 137: 5435-5440, 1996[Abstract].

57.   Tokuda, H, Miwa M, Oiso Y, and Kozawa O. Autoregulation of prostaglandin E2-induced Ca2+ influx in osteoblast-like cells: inhibitation by self-induced activation of protein kinase C. Cell Signal 4: 261-266, 1992[ISI][Medline].

58.   Voelkel, EF, Tashjian AH, Jr, and Levine L. Cyclooxygenase products of arachidonic acid metabolism by mouse bone in organ culture. Biochim Biophys Acta 620: 418-428, 1980[ISI][Medline].

59.   Widdowson, EM, and Dickerson JWT Chemical composition of the body. In: Mineral Metabolism, edited by Comar CL, and Bronner F.. New York: Academic, 1964, p. 1-247.

60.   Yamaguchi, DT, Hahn TJ, Beeker TG, Kleeman CR, and Muallem S. Relationship of cAMP and calcium messenger systems in prostaglandin-stimulated UMR-106 cells. J Biol Chem 263: 10745-10753, 1988[Abstract/Free Full Text].

61.   Yorio, T, Page RD, and Frazier LW. Prostaglandin regulation of H+ secretion in amphibian epithelia. Am J Physiol Regulatory Integrative Comp Physiol 260: R866-R872, 1991[Abstract/Free Full Text].


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