SPECIFIC PROTON PUMP INHIBITORS E3810 AND LANSOPRAZOLE AFFECT THE RECOVERY PROCESS OF GASTRIC-SECRETION IN RATS DIFFERENTLY

After a single subcutaneous administration (30 mg/kg) of proton pump inhibitor 2-[{4-(3-methoxypropoxy)-3-methylpyridin-2-yl}-methylsulfinyl]-1H-benzimidazole sodium salt (E3810), or lansoprazole in rats, time courses of inhibitory and recovery processes of acid secretion in vivo and pump enzyme activity in isolated microsomes were measured. The acid secretion rate which reflects H+,K+-ATPase activity in the secretory canalicular (apical) membrane was compared with that in the microsomal fraction which consists mostly of resting, intracellularly-pooled tubulovesicles. We found that the canalicular pump was first inhibited, followed by slow inhibition of the microsomal pump enzyme activity, with the rate of the latter process depending on the inhibitors. It took 2.5 hr for the half-maximal inhibition of the microsomal pump in E3810-treated rats, and 6 hr in lansoprazole-treated rats. The acid secretion and the microsomal enzyme activity completely recovered within 48 hr after the administration of E3810, but recovered by only 20% even 96 hr after the administration of lansoprazole. Incubation with dithiothreitol of isolated microsomes obtained from E3810-treated rats reactivated the enzyme activity, but not from rats treated with lansoprazole. These results suggest that dissociation of inhibitor from the pump and/or intracellular transport of the pump is affected differently by these inhibitors. Furthermore, it is possible that the half life of the proton pump protein is much longer (greater than 96 hr) than the previously proposed value of 30-48 hr.