Gastrin is a peptide hormone which is soluble in the water. This hormone plays an important role in the secretion of HCI in stomach. Gastric acid secretion also helps the digestion of protein, the absorption of iron and calcium. In the first, Gastrin activates a particular G protein by attaching to a cell surface receptor in the plasma membrane. Then, the activated G protein binds to another membrane enzyme, phospholipase C (PLC). This enzyme-substrate complex carries out a hydrolysis reaction where hydrolyze phosphatidylinositol- 4,5-bisphosphate (PiP2) to inositol-1,4,5-triphosphate (IP3) and 1,2- diacylglycerol (DAG).
In the second phase, inositol-1,4,5-triphosphate (IP3), reacts with the endoplasmic reticulum and stimulates the release of Ca2+ ion into the cytoplasm. In the same time, 1,2- diacylglycerol (DAG), which was released into the cytoplasm, diffuses through the plasma membrane, where interacts with protein kinase C. The Ca2+ just released from the endoplasmic reticulum and DAG cooperation stimulates that kinase to phosphorylate a protein which in turn causes HCI secretion into the stomach. 
1.Gastric secretion. Schubert ML. Curr Opin Gastroenterol. 2005 Nov;21(6):636-43. Review. PMID:16220038 [PubMed - indexed for MEDLINE]
- Schubert, ML (2005). Gastric secretion..