Gastric prostaglandin E2 release induced by aluminium hydroxide and aluminium hydroxide-containing antacids in rats. Effect of low doses and citric acid

Suspensions of aluminium hydroxide or a commercial antacid containing aluminium hydroxide (Trigastril) was instilled intragastrically in rats in doses comparable to high and low human therapeutic doses (aluminium hydroxide, 125 mg and 12.5 mg/kg, respectively). Corresponding experiments were carried out with 0.6% citric acid added to the antacid suspensions. Prostaglandin E2 (PGE2) in the gastric content was analysed by radioimmunoassay 6 h after drug administration. Both high and low doses of aluminium hydroxide and Trigastril increased the concentration of PGE2 significantly. Citric acid did not significantly affect the antacid-induced PGE2 release except in combination with a low dose of aluminium hydroxide, with which a significant increase was seen. Release of PGE2 by low doses of antacids implies the possibility that enhanced cytoprotection may be involved in the mechanism by which antacids promote the healing of peptic ulcers.     

Possible role of sulfhydryls in mucosal protection induced by aluminum hydroxide

The involvement of sulfhydryl-containing compounds was investigated in the cytoprotective effect of certain antacids against ethanol-induced gastric mucosal damage in the rat. Not only the protective effect of a sulfhydryl containing compound,N-acetylcysteine, was abolished after pretreatment withN-ethylmaleimid, but also the adaptive cytoprotection induced by 20% ethanol. Pretreatment of the animals with the cyclooxygenase inhibitor indomethacin or with the thiol reagentN-ethylmaleimide significantly diminished the cytoprotection evoked by aluminum hydroxide. However, the protective effect of aluminium hydroxide could only be totally abolished by pretreatment with the combination of indomethacin andN-ethylmaleimide. The present results suggest the additional role of sulfhydryl-containing compounds localized in the gastric mucosa in the cytoprotection induced by a mild irritant or by aluminium hydroxide and aluminium hydroxidecontaining antacids as well.     

Cytoprotection by prostaglandins in rats. Prevention of gastric necrosis produced by alcohol, HCl, NaOH, hypertonic NaCl, and thermal injury.

Oral administration to fasted rats of either absolute ethanol, 0.6 N hydrochloric acid, 0.2 N sodium hydroxide, 25% sodium chloride, or boiling water produced extensive necrosis of the gastric mucosa. Pretreatment with several prostaglandins of the A, E, or F type, either orally or subcutaneously, prevented such necrosis, and the effect was dose-dependent. This property of prostaglandins is called "cytoprotection." The protective effect against oral administration of absolute ethanol was already maximal 1 min after PGE2 given orally, and 15-30 min after PGE2 given subcutaneously. Cytoprotection by prostaglandins is unrelated to the inhibition of gastric acid secretion since, (a) it is maximal at doses that have no effect on gastric secretion, and (b) anti-secretory compounds (cimetidine, methscopolamine bromide) and antacids are not cytoprotective. Although the mechanism of gastric cytoprotection is unknown, prostaglandins appear to increase the resistance of gastric mucosal cells to the necrotizing effect of strong irritants. These results suggest that certain prostaglandins, by a mechanism other than the inhibition of gastric acid secretion, maintain the cellular integrity of the gastric mucosa, and might be beneficial in the treatment of a variety of diseases in which gastric mucosal injury is present.     

Alimentary tract and pancreas. Stimulation of mucosal prostaglandin synthesis in human stomach and duodenum by antacid treatment.

The effect of a low dose antacid treatment on mucosal prostaglandin metabolism was studied in 15 healthy volunteers. A daily dose of 46 mmol (= 138 mval) Al(OH)3 and 42 mmol (= 84 mval) Mg(OH)2 with a titrated in vitro neutralising capacity of 272 mval of H+ was given for three weeks. Total prostaglandin formation and the prostaglandin profile as well as the degradation of PGE2 were assayed by incubating homogenates of endoscopic biopsies from antral and duodenal mucosa with the precursor (14C)arachidonic acid. Total prostaglandin synthesis in antrum (623 (110) pmol/mg protein) and duodenum (432 (72) pmol/mg) was stimulated after three weeks administration of low dose antacids by 176% (p less than 0.05) and 154% (p less than 0.05), respectively. An untreated control group exhibited no significant changes. In contrast, the prostaglandin profile showed only a small increase of the prostacyclin metabolite 6-keto PGF1a (p less than 0.05) at the expense PGD2. PGE2 catabolism was unaffected. This enhanced activity of mucosal prostaglandin cyclooxygenase might represent one possible mechanism of action of a low dose antacid treatment.     

Role of mucosal prostaglandins and DNA synthesis in gastric cytoprotection by luminal epidermal growth factor.

This study compares the effect of epidermal growth factor and prostaglandins (PGE2 or PGI2), applied topically to gastric mucosa, on gastric secretion and formation of ASA-induced gastric ulcerations in rats. Epidermal growth factor given topically in non-antisecretory doses prevented dose-dependently the formation of ASA-induced ulcers without affecting prostaglandin generation but with a significant rise in DNA synthesis in the oxyntic mucosa. The anti-ulcer effect of topical prostaglandins was also accompanied by an increase in DNA synthesis. This study indicates that topical epidermal growth factor, like PGE2 or PGI2, is cytoprotective and that this cytoprotection is not mediated by the inhibition of gastric secretion or prostaglandin formation but related to the increase in DNA synthesis in oxyntic mucosa.     

Putative mechanisms of cytoprotective effect of certain antacids and sucralfate

An investigation of cytoprotective activity of certain antacids and inert particles was carried out by treating ethanol-induced gastric mucosal damage in rats in order to clarify possible mechanisms by which aluminum-containing antacids act. Al(OH)3 inhibited gastric mucosal damage in a dose-related and time-dependent manner. Neither aluminum ions themselves nor the particle size of the Al(OH)3 complex were responsible for the observed cytoprotection, since neither AlCl3, chemically inert Al2O3*C, nor sea sand showed protective effects. Hyperosmolality in the gastric lumen was not a deciding factor in inducing cytoprotection. Silicic acid and titanium dioxide, with superficial charge similar to Al(OH)3 proved to be effective in inhibiting gastric hemorrhagic lesions and releasing PGE2, suggesting that the surface charge of Al(OH)3 may be important in its cytoprotective properties. The same may also be valid for sucralfate. Since antacid-induced cytoprotection was only partly reduced by pretreatment with indomethacin, it is likely that additional mechanisms and mediators other than prostaglandins, such as nonprotein sulfhydryls, also are involved in gastric cytoprotection arising from aluminum-containing antacids.     

Adaptive cytoprotection in cultured rat gastric mucus-producing cells role of mucus and prostaglandin synthesis

In cultured gastric mucosal cells, we investigated whether: (1) adaptive cytoprotection was associated with stimulation of endogenous prostaglandin synthesis; (2) prostaglandins given exogenously were cytoprotective against ethanol-induced gastric mucosal cell damage; and (3) a relationship existed between cytoprotection and mucus release. Cytolysis was quantified by measuring⁵¹Cr release from prelabeled cells. Mucus release was determined by measurement of [³H]glucosamine release. Concentrations of ethanol >12% caused cell damage and increased⁵¹Cr release dose dependently. Pretreatment with low concentrations of ethanol (0.5–1.5%) decreased ethanol-induced⁵¹Cr release, but also decreased prostaglandin E₂ synthesis. Prostaglandin E₂ and 16,16-dimethyl prostaglandin E₂ given exogenously were cytoprotective against ethanol-induced gastric mucosal cell damage. Treatment with low concentrations of ethanol (1.5%) increased mucus release from cultured gastric mucosal cells. However, prostaglandin E₂ and 16,16-dimethyl prostaglandin E₂ did not affect mucus release. We conclude that in cultured gastric mucus-producing cells: (1) adaptive cytoprotection occurs without stimulation of endogenous prostaglandin synthesis but with increase in mucus release; and (2) exogenous prostaglandins are cytoprotective against ethanol-induced gastric mucosal cell damage without stimulating mucus releasein vitro. We postulate that adaptive cytoprotection in cultured gastric mucus-producing cells is not mediated by prostaglandin, but by mucus released in response to a mild irritant.     

The effects of a prostaglandin endoperoxide analogue on canine gastric acid and mucus secretion

The effects of U-46619, a stable analogue of the prostaglandin endoperoxide PGH2, were studied on canine gastric acid secretion, gastric mucosal blood flow, and secretion of mucus into gastric juice and compared to those of PGE2. U-46619 was approximately four and three times as potent as PGE2 in inhibiting acid secretion and stimulating mucus secretion, respectively. When infused at a low dose, U-46619 inhibited acid secretion directly without causing a decrease in the ratio of mucosal blood flow to volume rate of secretion (R), this effect being similar to that observed for PGE2. However, unlike PGE2, U-46619 when administered in a higher dose caused a decline in R while decreasing acid secretion and mucosal blood flow, suggesting a primary restriction of blood flow. The antisecretory effects of arachidonic acid may be due in part to the endogenous formation of prostaglandin endoperoxides as opposed solely to prostaglandin formation. Considering the antisecretory and mucogenic actions of U-46619, nontoxic analogues of prostaglandin endoperoxides may be of value as antiulcer agents.     

Prostaglandins and renin release: the effect of PGI2, PGE2, and 13,14-dihydro PGE2 on the baroreceptor mechanism of renin release in the dog.

We compared the ability of the vasodilator prostaglandins PGI2, PGE2, and 13,14-dihydro PGE2 to release renin when infused into the denervated, nonfiltering canine kidney in vivo. Papaverine was used as a nonprostaglandin vasodilator. All the prostaglandins tested were capable of stimulating renin secretion, with the scale of potency being 13,14-dihydro PGE2 greater than PGI2 greater than PGE2; papaverine had no effect on renin secretion. These results indicate that both PGE2 and PGI2 can stimulate renin secretion but that vasodilation per se is not a stimulus. 13,14-Dihydro PGE2 was included because it is a poorer substrate than PGE2, both for transport into cells and catabolism to inactive products, but has comparable potency to PGE2 when tested in systems with limited ability to catabolize PGE2. The fact that 13,14-dihydro PGE2 was the most potent prostaglandin tested suggests that the effects of PGE2 in our system are reduced by the kidneys' recognized ability to extract and catabolize PGE2. Since PGI2 is less avidly metabolized than PGE2 by the kidney, the differences in observed potency between PGE2 and PGI2 could be largely the result of differences in renal catabolism of the two prostaglandins rather than differences in intrinsic potency. Therefore, both PGE2 and PGI2 are candidates for the endogenous prostaglandin responsible for stimulating renin secretion.     

Antacid action of the aluminum cation: comparison of the in vitro effect of hydroxide and phosphate in open and closed systems

Antacid capacity of two aluminium containing-antacid drugs was evaluated in vitro; the first drug contained aluminium hydroxide (Alternagel), the second, aluminium phosphate (Phosphalugel). The antacid evaluation was performed 1) in a closed system by measuring antacid activity by down titration, 2) by a dynamic evaluation simulating acid secretion and gastric emptying. The results were reported both to the recommended therapeutical dose and to 100 mg aluminium. In static conditions, without gastric emptying, it was shown that aluminium hydroxide and phosphate acted by their buffer capacity in pH range less than or equal to pH 1.5. The therapeutical dose of aluminium phosphate displayed greater antacid activity than aluminium hydroxide, this fact being due to the empiric choice of the doses. With regard to aluminium content, aluminium phosphate activity remained greater than that of aluminium hydroxide although the difference decreased with decreasing pH values. The antacid capacities were related to the emptying outputs. Antacid activity corresponding to 100 mg aluminium was similar in both antacids less than pH 1.5. This effect was dependent on emptying rates. It can be suggested that Al was responsible for antacid activity in both preparations, and that the buffering capacity was supported by the change of aluminium cation in hydrolysis intermediary compounds.     

Synthetic human parathyroid hormone fragment stimulates prostaglandin E2 synthesis by chick calvariae

Synthetic human PTH N-terminal 1-34 peptide [hPTH-(1-34)] stimulates prostaglandin (PG) production by chick calvariae in culture. PGE2 was the predominant PG found by both RIA and HPLC. Stimulation of PGE2 synthesis was significant at 50 ng/ml (1.2 X 10(-8) M) hPTH-(1-34) and was dose dependent at concentrations up to 0.6 microgram/ml (1.4 X 10(-7) M). Continuous exposure of calvariae to hPTH-(1-34) showed that PGE2 production increased significantly by 1 h, reached a maximum at 24 h, and then persisted over 96 h. Indomethacin at concentrations above 5 X 10(-7) M inhibited PGE2 synthesis by both control and hPTH-(1-34)-treated bones. PTH-(1-34) stimulated bone cells to convert arachidonic acid to PGE2, but did not activate the bone to release stored arachidonate. In summary, our results show that hPTH-(1-34) stimulates PGE2 synthesis by chick calvariae. This endogenous PGE2 may be involved in bone remodeling.     

Involvement of endogenous prostaglandins in pancreatic endocrine and exocrine secretion in dog pancreas

The involvement of endogenous prostaglandins (PGs) in pancreatic endocrine and exocrine secretion was investigated, using the isolated and perfused dog pancreas. Spontaneous production of both PGE2 and 6-keto-PGF1 alpha was recorded in venous effluent. Prostaglandin production increased following stimulation with both 10 x 10(-11) and 20 x 10(-11) mol of CCK-8, but was not affected by a 5 x 10(-11) mol infusion. Insulin, glucagon, and amylase release was stimulated by 10 x 10(-11) mol of CCK-8. Indomethacin pretreatment with 10 mg/kg totally abolished endogenous PG production, but failed to suppress an insulin and glucagon response. On the other hand, an amylase response was accelerated by indomethacin pretreatment. Although low dose CCK-8 failed to stimulate endogenous prostaglandin production, a brisk exocrine secretion was not suppressed by indomethacin pretreatment. From the above results, we conclude that endogenous PGs do not appear to play an important role in pancreatic endocrine and exocrine secretion, but might have a cytoprotective effect on the pancreatic acinar cells damaged by CCK-8.     

Salicylate effects on a monolayer culture of gastric mucous cells from adult rats.

Aspirin, acetyl salicylic acid, damages gastric mucosal cells. This effect is considered related to its inhibition of prostaglandin synthesis. On the other hand, sodium salicylate has been reported to be cytoprotective against drug damage to gastric mucosa in vivo. One reason for this difference is that salicylic acid, unlike acetyl salicylic acid does not inhibit prostaglandin synthesis by gastric mucosa in vivo. Previous studies on tissue culture cells from our laboratory have required gastric mucosa from fetal rats; this was time consuming and expensive. The purpose of this study was to develop a primary cell culture of adult rat fundic epithelial cells and to test the effect of sodium salicylate on: (1) prostaglandin (PGE2) production, (2) cell viability, (3) reducing cell damage by sodium taurocholate. Gastric epithelial cells were isolated from adult rat stomachs and cultured on collagen gel. Cells reached confluency on day 4 at which stage fibroblasts were rarely seen (less than 1%). Autoradiographic study showed that cultured cells incorporated [3-H] thymidine into nuclei. In histochemical studies, 94% of the cells contained PAS positive granules (mucous cells). Mucous granules were observed in the cytoplasm of the majority of cells by electronmicroscopy. These cells synthesised prostaglandin E2 as determined by radioimmunoassay. Indomethacin 10(-4) M strongly suppressed PGE2 production after 30 minutes while 10(-3) and 10(-4)M sodium salicylate had no effect. Pharmacologic concentrations of 10 mM sodium salicylate had no effect on PGE2 production at 30 minutes and only weakly inhibited production after one hour incubation. Sodium salicylate up to 30 mM had no effect on cell viability, a concentration of 50 mM being necessary to produce significant cell damage. Sodium salicylate 10 mM significantly protected cells against damage induced by 10 mM sodium taurocholate. We conclude: (i) adult rat gastric mucous epithelial cells can be successfully cultured in vitro; (ii) prostaglandin synthesis is inhibited by indomethacin but not by low doses of sodium salicylate; (iii) sodium salicylate does not damage gastric mucosal cells except at very high concentrations; (iv) sodium salicylate protects against damage to cells induced by sodium taurocholate.     

Cross-talk between cyclooxygenase and nitric oxide pathways: prostaglandin E2 negatively modulates induction of nitric oxide synthase by interleukin 1.

The inflammatory cytokine interleukin 1 beta (IL-1 beta) induces both cyclooxygenase (COX) and nitric oxide synthase (NOS) with increases in the release of prostaglandin (PG) and nitric oxide (NO) by mesangial cells. Recently, activation of the COX enzyme by NO has been described. However, the effects of COX products (PGs) on the NO pathway have not been fully clarified. Thus we determined the effect of COX inhibition and exogenous PGs on NO production and NOS induction in rat mesangial cells. A COX inhibitor, indomethacin, enhanced IL-1 beta-induced steady-state level of the inducible NOS (iNOS) mRNA and nitrite production. The effect of indomethacin was dose dependently reversed by the replacement of endogenous PGE2 with exogenous PGE2, which is the predominant product of the COX pathway in rat mesangial cells. In contrast to PGE2, a stable analog of PGI2, carba prostacyclin, enhanced IL-1 beta-induced iNOS mRNA levels and nitrite production. Forskolin, an activator of the adenylate cyclase, mimicked the effect of carba prostacyclin but not PGE2. These data suggest that (i) endogenous PGE2 downregulates iNOS induction, (ii) this inhibitory effect of PGE2 on iNOS induction is not mediated by activation of adenylate cyclase, and (iii) exogenous PGI2 stimulates COX induction possibly by activation of adenylate cyclase.     

Role of prostaglandin E2 in rat colon carcinoma.

BACKGROUND/AIMS: It has previously been reported that prostaglandin E2 (PGE2) promotes colon carcinogenesis. We therefore studied the effects of long-term administration of prostaglandin E2 (PGE2) on colon carcinogenesis in rats. METHODOLOGY: Rats received intrarectal n-methyl-n-nitrosourea (MNU) or n-methyl-n-nitrosoguanidine (MNNG) to induce the formation of colonic tumors. Rates then received indomethacin (IND) and/or PGE2, or nothing. After 44 weeks (MNU group) or 46 weeks (MNNG group), colon lesions were identified histologically and colonic mucosa PGE2 concentrations were measured by radioimmunoassay. RESULTS: The incidence of colon carcinoma in the control, MNU, MNU + IND, MNU + PGE2, and MNU + IND + PGE2 groups was 0/14, 5/15, 0/14, 4/10, and 2/9, respectively. In the MNNG group, no tumors were observed. Induction of colon carcinomas by MNU was completely inhibited by IND, while exogenous PGE2 blocked the inhibitory effect of IND. However, PGE2 administration did not accelerate colon carcinogenesis. Neither MNU nor MNNG alone resulted in increased colonic mucosal PGE2 concentrations, whereas exogenous PGE2 administration significantly increased mucosal PGE2 concentrations. IND significantly decreased the mucosal concentration of PEG2 with or without PGE2 administration. CONCLUSIONS: These results suggest that endogenous PGE2 in colon mucosa may be adequate to promote colon carcinoma. To block colon carcinogenesis, PGE2 levels in colonic mucosa must be decreased to less than endogenous levels.     

16, 16-dimethyl prostaglandin E2 modulates aflatoxin B1-induced injury of rat hepatocytes in primary culture: Possible role of cAMP

The hepatocellular cytoprotective effects of 16,16-dimethyl prostaglandin E2 (dmPGE2), an analogue of PGE2, were investigated using primary cultures of rat hepatocytes and aflatoxin B1 as the hepatotoxin. Lactic dehydrogenase (LDH) release by hepatocytes was used as an index of hepatotoxicity. When aflatoxin-treated hepatocytes were co-cultured with 16,16-dmPGE2 (0.01-0.5 micrograms/mL) LDH release was significantly reduced and ultrastructural changes of hepatocellular injury were markedly diminished. The magnitude of the cytoprotective effect was not dependent on the concentration of the prostaglandin over the range tested. A significant cytoprotective effect was also induced when hepatocellular cyclic AMP (cAMP) levels were increased by the addition of dibutyl-cAMP. In contrast to 16,16-dmPGE2, PGF2 alpha Tromethamine, an analogue of PGF2 alpha, which does not stimulate cAMP, induced insignificant changes in cytoprotection. These findings indicate that only a low concentration of 16,16-dmPGE2 (> or = 0.01 micrograms/mL) is necessary to induce a maximal hepatocellular cytoprotective effect and suggest that this effect may be dependent on activation of cAMP.     

The mechanism of acute gastric ulcer after induced hemorrhagic shock

Changes in gastric mucosal blood flow were investigated for their relationship to gastric mucosal prostaglandin E2 (PGE2) and noradrenaline (NA) in rats with hemorrhagic shock. The results were as follows: 1) Gastric mucosal blood flow and NA decreased after hemorrhage. Gastric mucosal PGE2 initially increased after exsanguination and then markedly decreased. 2) Administration of NA before hemorrhage resulted in an increase of PGE2. However, the PGE2 value for animals receiving NA after hemorrhage was not different from that of non-NA-treated group. 3) Pre-treatment with PGE2 suppressed the reduction in both gastric mucosal blood flow and NA and the development of ulcer. These results suggest that the increase in gastric mucosal PGE2 in the early stage of shock might represent a phenomenon of adaptation by the adrenergic activation, and the decrease in PGE2 in the late stage might result from impaired synthesis of PGE2 due to persistent hypoxia and might be one of the possible factors in ulcer formation.     

Effects of sulphasalazine and disodium azodisalicylate on colonic PGE2 concentrations determined by equilibrium in vivo dialysis of faeces in patients with ulcerative colitis and healthy controls.

The role of arachidonic acid metabolites and the mode of action of 5-aminosalicylic acid, the active moiety of sulphasalazine and disodium azodisalicylate, in ulcerative colitis remain obscure. Therefore, experiments were performed in which the effects of medication on immunoreactive prostaglandin (PG) E2 concentrations in free faecal water were assessed using the equilibrium in vivo dialysis of faeces. Colonic PGE2 concentrations in patients with active ulcerative colitis (n = 11) ranged from 2035-18,000 pg/ml to be compared with a range of 103-188 pg/ml in healthy volunteers (n = 10; p less than 0.001). In all healthy volunteers PGE2 concentrations decreased slightly (p less than 0.05) after disodium azodisalicylate intake 2 g/day, whereas low dose disodium azodisalicylate (0.25 g/day) caused no change. In patients with ulcerative colitis in complete clinical, sigmoidoscopic, and histologic remission withdrawal of sulphasalazine (2 g/day; n = 6) increased PGE2 concentrations to values above normal levels (p less than 0.05) which returned to pretrial values (p less than 0.05) on disodium azodisalicylate (2 g/day; n = 7). In conclusion, increased PGE2 in free faecal water indicates an abnormality in the colonic mucosa, even in the absence of conventional signs of inflammation. We could not confirm the hypothesis that sulphasalazine and 5-aminosalicylic acid exert their therapeutic effect through promotion of endogenous cytoprotective prostaglandins. In contrast, the observation that raised PGE2 concentrations were normalised by disodium azodisalicylate in patients with inactive ulcerative colitis suggests that subclinical disease activity was decreased by 5-aminosalicylic acid.     

Stimulation of prostaglandin E and testosterone production in rat interstitial cells by a gonadotropin-releasing hormone agonist

The early direct effects of a GnRH agonist analog [D-Ala6]des-Gly10-GnRH N ethylamide (GnRHa) on rat testicular interstitial cells include increased production of prostaglandin E (PGE) and testosterone (T) at 3 h (ED50 values of 0.5 and 0.75 nM, respectively). On the other hand, LH action on testicular function, which is mediated by increased cAMP, involves an increase in T production at 30 min followed by increased PGE formation at 3 h. GnRHa at concentrations of 10(-12)-10(-8) M had no effect on basal or LH-stimulated cAMP production during a 4-h incubation test. The stimulatory effect of GnRHa on PGE, but not on T production, was abolished by the prostaglandin synthesis inhibitor indomethacin (1.5 microns). We conclude that cAMP does not play a role in mediating the direct testicular effects of GnRH on PGE and T production; that PGE is not involved in mediating GnRH-induced T production; and, finally, that increased PGE and T production might be involved in mediating the direct inhibitory and stimulatory testicular effects of GnRH and its agonist analogs.     

Prostaglandin production by calvariae from sham operated and oophorectomized rats: effect of 17 beta-estradiol in vivo

The mechanism by which estrogen inhibits bone resorption in vivo is not known. Since prostaglandin E2 (PGE2) is a potent stimulator of bone resorption in vitro, we tested the hypothesis that estrogens might affect bone indirectly by inhibiting endogenous PGE2 synthesis. Cultured parietal bones from 7- to 9-week-old rats showed a gradual release of immunoreactive PGE2 in vitro. Oophorectomy resulted in a two-fold increase in PGE2 release. Treatment in vivo with low doses of 17 beta-estradiol or high doses of 17 alpha-estradiol 26 h and 2 h prior to sacrifice inhibited bone PGE2 release in vitro. Addition of 17 beta-estradiol to calvariae in vitro did not affect PGE2 release, whereas cortisol inhibited PGE2 release.