The parietal cell in the stomach contain receptors for acetylcholine(M3), gastrin(CCK-B), and histamine(H2). There are also cells present near the gastric parietal cells called Enterochromaffin like cells (ECL) which contain receptors for acetylcholine and gastrin.
The main function of parietal cells is to secrete H+ ions in the parietal cell canaliculi and that of the ECL cells is to secrete gastrin.
All the three chemicals, acetylcholine(Ach), gastrin and histamine cause H+ ion secretion from the parietal cells through the H+,K+ ATPase enzyme(The proton pump) located on the canalicular surface.
Gastrin mainly cause acid secretion indirectly by causing secretion of histamine from ECL cells. The gastrin released from ECL cells then bind to parietal cells and cause H+ secretion.
Acetylcholine on the other hand mainly cause H+ secretion by directly binding to the parietal cells.
Mechanism of action of Gastrin and Acetylcholine:
Mechanism of action of histamine: