Prostaglandins regulate acid-induced cell-mediated bone resorption

Abstract

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 microM indomethacin to inhibit prostaglandin synthesis. We measured net calcium efflux and medium PGE(2) levels. Compared with neutral pH medium, acid medium led to an increase in net calcium flux and PGE(2) 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 PGE(2). Net calcium flux was correlated directly with medium PGE(2) (r = 0.879, n = 29, P < 0.001). Exogenous PGE(2), 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 PGE(2) 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 PGE(2) synthesis.

Effect of metabolic and respiratory acidosis on intracellular calcium in osteoblasts

“…Resp acidosis has minimal, if any, effects on urine Ca excretion (27,49,57). In vitro models of Met acidosis increase net Ca efflux from bone through a marked increase in levels of PGE 2 leading to increased osteoclastic bone resorption (47). Again, in vitro isohydric Resp acidosis has minimal effects on bone resorption and levels of PGE 2 (20).…”

“…For neutral conditions (Ntl), the partial pressure of medium carbon dioxide (PCO 2) was maintained by gentle bubbling with 5% CO2 in air (AirGas, Rochester, NY) for at least 15 min before perfusion to achieve a PCO2 of ϳ40 mmHg. To model Met acidosis, the medium pH was lowered to ϳ7.10 by the addition of 1 ml of 2.4 M HCl to 100 ml Ntl medium to lower the medium [HCO 3 Ϫ ] as previously reported (5,7,10,13,15,47,48). To model Resp acidosis, the pH was lowered to ϳ7.10 by increasing the PCO 2 of the medium through gentle bubbling with 10% CO2 in air (AirGas) as previously reported (5,7,15,20,32).…”

Cortisol Inhibits Acid-Induced Bone Resorption In Vitro

“…Acid medium stimulates osteoblastic PGE 2 production, which mediates the subsequent stimulation of osteoclastic bone resorption (12)(13)(14)35). Cortisol has previously been shown to inhibit prostaglandin production (36) as well as hormonal stimulation of the rate-limiting enzyme that converts arachidonic acid to PGE 2 (prostaglandin G/H synthase, PGHS-2) in neonatal mouse calvariae (37).…”

“…Neonatal (4-to 6-d-old) CD-1 mice (Charles River, Wilmington, MA) were killed, and their calvariae were removed by dissection (7,10,12,29). Exactly 2.8 ml of Dulbecco's modified Eagle medium (DMEM) containing 15% heat-inactivated horse serum, heparin sodium (10 U/ml), and potassium penicillin (100 U/ml) were preincubated at a fixed partial pressure of carbon dioxide (Pco 2 , 40 mmHg) at 37°C for 3 h in 35-mm dishes.…”

“…With prolonged incubation in acidic medium, there is decreased osteoblastic bone formation and increased osteoclastic resorption (9,10), all of which contribute to a net efflux of calcium from bone (6,11). This cell-mediated resorption appears due to increased osteoblastic prostaglandin E 2 (PGE 2 ) synthesis, leading to a suppression of osteoblastic bone formation and subsequent stimulation of osteoclastic bone resorption (12)(13)(14).…”