Effect of omeprazole on gastric acid secretion and plasma gastrin

Omeprazole represents a new class of gastric acid inhibitors, which inhibits the H+, K(+)-ATPase in the secretory membrane of the parietal cell. Single oral doses inhibited pentagastrin stimulated acid secretion with an ED50 of 27 mg. Almost complete inhibition could be achieved with a single dose of 80 mg. Acid secretion then slowly returned and reached normal levels after 3-4 days. Omeprazole also inhibited basal, histamine peptone and vagally stimulated acid secretion with similar potency. For clinical use omeprazole has been formulated as enteric coated granules. During repeated once daily dosing with this formulation the degree of acid suppression increased over the first days after the start of treatment but stabilized within about 4 days. Dose-response studies in both DU-patients and healthy subjects have shown that daily doses of 20-40 mg result in 80-100% reduction of stimulated acid secretion when measured 6 h after dose. Due to the long duration of action the inhibition was still 50-80% when measured 24 h after dose. Studies of 24 h intragastric acidity in DU-patients have shown that once daily treatment with 20 mg omeprazole results in a reduction of intragastric acidity by 97% which in the same study was superior to the 57% reduction caused by treatment with ranitidine, 150 mg twice daily. During omeprazole treatment, plasma gastrin increased in relation to the degree of acid suppression. The level of increase of 24 h plasma gastrin during once daily treatment with 20 mg omeprazole was slightly higher than for ranitidine, 150 mg twice daily, but similar to that seen after highly selective vagotomy.