Bosentan antagonizes the effects of endothelin-1 on rat gastric blood flow and mucosal integrity

Overcompensation stimulation: a mechanism for hormetic effects

Whether hormetic responses result from a direct or an overcompensation type of stimulatory response has been an unresolved and contentious issue in both radiation and chemical toxicology. The goal of the present article is to identify numerous examples of overcompensation stimulation in the biological/biomedical literature and to evaluate their descriptive and quantitative features. The findings provide support for the hypothesis that hormetic dose-response relationships from a broad array of biological models can occur after an initial disruption in homeostasis. The finding also demonstrates the significant role of temporal factors in the assessment of dose response relationships.

Protection against aspirin-induced human gastric mucosal injury by bosentan, a new endothelin-1 receptor antagonist

BACKGROUND

Gastric ulceration induced by aspirin and by non-steroidal anti-inflammatory drugs (NSAIDs) is a major clinical problem. The mechanism of injury is unclear. There is evidence that NSAID-induced injury may cause endothelin activation. Endothelin-induced vasoconstriction has been shown to be capable of causing gastric ulceration.

AIM

To investigate whether acute gastroduodenal injury induced in humans by aspirin can be prevented by the endothelin-1 antagonist, bosentan.

METHODS

Eighteen healthy volunteers each received 5 x 900 mg aspirin every 12 h on three separate occasions (with either placebo, bosentan 700 mg or misoprostol 400 mg). Treatment order was randomized by Latin square design. Subjects were endoscoped and erosions counted before and 90 min after the first and last dose of aspirin. Plasma concentrations of bosentan were measured up to 5 h post-dose.

RESULTS

There was a significant reduction in the mean number of erosions in the aspirin plus bosentan and aspirin plus misoprostol groups after the first dose of aspirin, compared with controls (aspirin plus placebo) (P<0.05). This was not sustained after the fifth dose of aspirin in the aspirin plus placebo and aspirin plus bosentan groups, but was still present in the aspirin plus misoprostol group. The mean plasma concentration of bosentan measured 3.5 h post-dose fell from 4510 (95% CI: 2791-6230) ng/mL after the 1st dose to 2508 (95% CI: 1733-3283) ng/mL after the 5th dose (P = 0.02).

CONCLUSION

Endothelin receptor antagonism by bosentan can protect the gastric mucosa against aspirin damage. After five doses, bosentan levels fell, possibly because of enzyme induction, and protection was no longer evident. Further investigation is needed to assess whether higher doses would be effective.

Anti-ulcerogenic properties of endothelin receptor antagonists in the rat

BACKGROUND

Endothelins have been implicated in gastric mucosal damage in a variety of animal models. Exogenous ET-1 and ET-3 are causally associated with experimental gastric ulcers. Furthermore, clinical reports also show elevated plasma and gastric mucosal endothelin-1 levels in patients suffering from peptic ulcers.

AIM

To study the possibility that endothelin receptor antagonists may have beneficial effects and prevent the development of gastric ulcers. We have tested in rats the orally-active endothelin antagonist bosentan (Ro 47-0203) and Ro 48-5695, which is 10-30 times more potent than bosentan on endothelin receptors.

METHODS

Water immersion restrained stress (WIRS) and indomethacin were used to provoke gastric mucosal damage. Endothelin receptor antagonists were administered orally prior to the induction of gastric damage. The gastric lesion index (mm), assessed macroscopically, and myeloperoxidase (MPO) activity were used as markers of the extent of mucosal injury.

RESULTS

Bosentan at 100 and 30 mg/kg administered orally caused attenuation of gastric damage in the WIRS model by 58% and 42%, respectively. Bosentan also caused complete reduction of MPO activity. In indomethacin-induced gastric damage, 100 mg/kg bosentan attenuated gastric damage by 45% and 61% as measured by the gastric lesion index and MPO activity respectively. Ro 48-5695 was at least 30 times more potent than bosentan in reducing indomethacin-induced mucosal damage and at 3 mg/kg, caused a decrease of 49% in the gastric lesion index and a reduction in MPO activity of 41%. Bosentan and Ro 48-5695 possess weak antisecretory properties as tested in the mouse gastric gland assay, than cannot, alone, account for their anti-ulcer properties.

CONCLUSIONS

Both endothelin receptor antagonists prevented the development of gastric mucosal injury in the rat. Disturbances in the gastric microcirculation are responsible for the development of experimental gastric ulcers. The anti-ulcer properties of these two endothelin antagonists suggest possible new therapeutic approaches to controlling gastric inflammation.

Role of endogenous endothelin peptides in intestinal ischemia-reperfusion injury in rats

In this study, the effects of BQ123 (an ET(A) receptor antagonist), bosentan (a nonselective ET(A)-ET(B) antagonist), and phosphoramidon (an endothelin converting enzyme inhibitor) were investigated on intestinal mucosal lesion formation and changes in tissue PGE2 and LTC4 levels due to intestinal ischemia-reperfusion (I/R) injury in rats. Following 30 min of ischemia, the substances were given via the inferior caval vein, and 10 min later the intestine was subjected to reperfusion for 30 min. The intestinal specimens were evaluated both microscopically and the tissue PGE2 and LTC4 levels were obtained for each group. The histopathologic examination revealed a significant reduction in tissue injury in both BQ123 and phosphoramidon pretreated groups compared with the control group. Bosentan, on the contrary, did not decrease the injury. The pharmacologic examination revealed a significant reduction of PGE2-like activity in both BQ123 and phosphoramidon pretreated groups, compared with the control group, while LTC4-like activity remained unchanged except for an increase in the bosentan pretreated group.

In vitro effects of endothelin-1 on somatostatin and thyrotropin-releasing hormone release from the rat stomach

The effects of endothelin (ET) 1 on the release of somatostatin (SS) and thyrotropin-releasing hormone (TRH) from the rat stomach were studied in vitro. The rat stomach was incubated in medium 199 with 1.0 mg/ml of bacitracin (pH 7.4) for 20 min. The amounts of SS and TRH released into the medium were measured by individual radioimmunoassays. With the addition of ET-1, the release of SS from the rat stomach was inhibited significantly in a dose-related manner, whereas TRH released from the stomach was enhanced significantly. These effects of ET-1 on SS or TRH release were blocked by BQ-485, a blocker of ETA receptor. These findings suggest that ET-1 inhibits SS and stimulates TRH release from the rat stomach in vitro, and that these effects are mediate via ETA receptor.

Role of endogenous endothelin-1 in stress-induced gastric mucosal damage and acid secretion in rats

In rats subjected to 8 h water-immersion stress, gastric and duodenal mucosal hemorrhage and erosions were detected by macroscopic and histopathological examination. Moreover, plasma and gastric mucosal endothelin-1 (ET-1) levels rose appreciably in a time-related manner during water immersion, with a higher concentration detected in gastric mucosa. Thus, the percentage increases in plasma (gastric mucosal) ET-1, relative to basal levels, after 1, 4 and 8 h of water immersion were 86(172), 169(322) and 210(391)%, respectively. Likewise, a marked increase of gastric acid output was demonstrated 30 min after water immersion and lasted for 3 h. Pretreatment with the endothelin ET(A)/ET(B) receptor blocker, bosentan (30 and 100 mg kg(-1)), orally, dose-dependently antagonized gastric and duodenal mucosal damage as indicated by reductions in lesion lengths of 67 and 80%, respectively. Similar protective effects on mucosa were observed when bosentan was given by the intramuscular route. On the other hand, bosentan suppressed the rate of acid output by 30.3+/-2.1% in the stressed rats, but had no such effect in non-stressed animals. Taken together, results from this study implicate the endogenous peptide, ET-1, as a powerful mediator of stress-evoked gastro-duodenal mucosal damage and, moreover, present bosentan as a potential protector against hyperacidity and mucosal erosion that occur as a consequence of stress.

Bosentan, a novel synthetic mixed-type endothelin receptor antagonist, attenuates acute gastric mucosal lesions induced by indomethacin and HCl in the rat: role of endogenous endothelin-1

Endothelin-1 has been reported to be responsible for gastric mucosal damage in various experimental models. We evaluated the role of endogenous endothelin-1 in the pathogenesis of gastric mucosal damage induced by indomethacin and HCl in the rat. Rats were given indomethacin (25 mg/kg) subcutaneously, and 15 min later, 0.2N HCl intragastrically. Gastric mucosal damage, gastric endogenous endothelin-1, and gastric mucosal hemodynamics were measured. The effects of bosentan, a mixed endothelin receptor antagonist, on gastric mucosal integrity and hemodynamics were assessed. Gastric endogenous endothelin-1 was significantly elevated at 20 min, gastric mucosal blood flow began to decrease significantly at 25 min, and gastric damage occupied 52.2% of the total glandular mucosa at 135 min after injection of indomethacin. Intragastric pretreatment with bosentan (5, 10, 30, and 60 mg/kg) significantly attenuated gastric damage, to 26.1%, 7.7%, 3.6%, and 1.6%, respectively, of the total glandular mucosa. Bosentan (60 mg/kg) prevented the initial decrease of blood flow and, even at 135 min, improved blood flow and hemoglobin oxygen saturation significantly. We suggest that indomethacin-induced endogenous endothelin-1 diminishes gastric mucosal blood flow and tissue oxygenation and ultimately causes gastric damage. Endogenous endothelin-1 may play an important role in the pathogenesis of the acute gastric mucosal lesions induced by indomethacin and HCl.