We examined whether leminoprazole (an acid pump inhibitor) enhances the secretion and synthesis of gastric mucus in vitro. Mucosal cells were prepared from male rabbit stomachs, and the amounts of secreted and synthesized mucus were determined by the [3H] glucosamine labeling method. Exposure of gastric mucosal cells to leminoprazole at 1 to 100 microM for 8 hr caused significant increases in the secretion and synthesis of mucus in a dose-related manner, but omeprazole was not effective. The stimulatory effects of leminoprazole on secretion and synthesis were observed 4 hr and 6 hr later, respectively. In contrast, both 16,16-dimethyl prostaglandin E2 and nitroprusside, a nitric oxide (NO) generator, caused marked increases in mucus secretion and synthesis even after 2-hr incubation. The effects of leminoprazole on gastric mucus were strongly inhibited by NO synthase inhibitors such as NG-nitro-L-arginine methyl ester and NG-monomethyl-L-arginine and by the NO scavenger oxyhemoglobin. However, neither indomethacin (an inhibitor of cyclooxygenase), aminoguanidine (an inhibitor of inducible nitric oxide synthase) nor cycloheximide (a protein synthesis inhibitor) could suppress its effects. These results suggest that leminoprazole stimulates the secretion and synthesis of gastric mucus after a lag period, probably through NO produced by constitutive NO synthase.