Interaction of cyclooxygenase isoenzymes, nitric oxide, and afferent neurons in gastric mucosal defense in rats

The cyclooxygenase (COX)-2 inhibitors 5,5-dimethyl-3-(3-fluorophenyl)-4-(4-methylsulfonyl) phenyl-2(5II)-furanone( DFU) (0.02-2 mg/kg) and N-[2-(cyclohexyloxy)-4-nitrofenyl]-methane-sulfonamide (NS-398) (0.01-1 mg/kg), the COX-1 inhibitor 5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethylpyrazole (SC-560) (0.05-5 mg/kg), and dexamethasone (1 mg/kg) were studied in rats challenged with intragastric acid (300 mM HCl). All compounds induced severe gastric damage when rats were treated concurrently with the inhibitor of constitutive and inducible nitric-oxide (NO) synthase N-G-monomethyl-L-arginine methyl ester (L-NAME) (3 or 40 mg/kg). DFU and NS-398 caused significantly less damage in rats receiving the selective inhibitor of inducible NO synthase N-(3-(aminomethyl) benzyl) acetamidine (1400W) (0.3 mg/kg). The COX-1 inhibitor SC-560 induced moderate damage in the acid-challenged stomach even without suppression of NO, but damage was aggravated by L-NAME. The COX-3 inhibitor phenacetin (400 mg/kg) did not injure the gastric mucosa despite suppression of NO. Furthermore, DFU, NS-398, SC-560, and dexamethasone caused severe injury in the acid-challenged stomach of rats pretreated with capsaicin to ablate afferent neurons. The mucosal damage induced by the COX-1 inhibitor, the COX-2 inhibitors, and dexamethasone in L-NAME- or capsaicin-treated rats was reversed by coadministration of 16,16-dimethyl- prostaglandin E-2 (2x8 ng/kg). Gross mucosal damage was paralleled by histology. Our results support the concept that endogenous NO, prostaglandins, and afferent neurons act in concert in the regulation of gastric mucosal integrity. The prostaglandins necessary for mucosal defense in the face of NO suppression, and afferent nerve ablation can be derived either from COX-1 or COX-2. The data do not propose a protective role for a phenacetin-sensitive COX-3. Our findings suggest that not only COX-1 but also COX-2 has important functions in the maintenance of gastric integrity.