Overview Atropine blocks vagally-mediated HCl and intrinsic factor secretion (parietal cells) does not block vagally-mediated gastrin secretion (G cells) because mediator of vagal effect is gastrin-release peptide (GRP), not acetylcholine (ACh) Gastrin ↑ gastric H+ secretion by 2 mechanisms (1) gastrin stimulates enterochromaffin-like (ECL) cells in gastric mucosa to release histamine, which diffuses to nearby parietal cells and binds H2 receptors, stimulating H+ release (primary mechanism) (2) gastrin binds CCKB receptors on parietal cells, stimulating H+ release (secondary mechanism)