Effects of gastric acid stimulants and inhibitors on the activities of [formula omitted], Mg 2+-dependent ATPase and carbonic anhydrase in rat gastric mucosa

Abstract

The effects of gastric acid stimulants or inhibitors, in vivo and in vitro, on the activities of HCO 3 −-stimulated , Mg 2+-dependent ATPase ( (HCO 3 −−Mg 2+)-ATPase , ATP phosphohydrolase, EC 3.6.1.3) and carbonic anhydrase in rat gastric mucosa were investigated in order to elucidate the significance of and the functional relationship between these enzymes. 1. 1. Subcutaneous treatment with carbachol (25–400 μg/kg) produced gastric juice secretion in 3-h pylorus-ligated rats. This drug also increased the Mg 2+-dependent ATPase (Mg 2+-ATPase) and carbonic anhydrase activities of the homogenate of rat gastric mucosa. 2. 2. Subcutaneous treatment with gastric acid stimulants, i.e. carbachol, tetragastrin and histamine, stimulated gastric juice secretion in 3-h pylorus-ligated rats. These reagents also increased the Mg 2+-ATPase and carbonic anhydrase activities in the mitochondrial fraction of rat gastric mucosa. 3. 3. Pretreatment with atropine (5 mg/kg, subcutaneously) or acetazolamide (20 mg/kg, subcutaneously) prevented the carbachol-induced increase of Mg-ATPase and carbonic anhydrase activities in the mitochondrial fraction. 4. 4. In vitro effects of gastric acid stimulants and inhibitors: Incubation of Mg 2+-ATPase or (HCO 3 −-Mg 2+)-ATPase with histamine (10 −3 M), carbachol (10 −3 M) or tetragastrin (10 −5 M) had no effect on the activity of the enzyme. These reagents did not stimulate the enzyme activity, directly. Thiocyanate (10 −2 M) inhibited the activity of the enzyme by about 30–40%. Ouabain (10 −2 M) had no effect on the activity. From these results, it was obvious that Mg 2+-ATPase and carbonic anhydrase activities in the mitochondrial fraction of the gastric mucosa correlated with gastric acid secretion. It is likely that carbonic anhydrase is functionally linked to Mg 2+-ATPase in rat gastric mucosa.