G cells

G cells are found in the gastric antral glands (also present in the duodenum) and are classified as APUD (amine precursor uptake and decarboxylation) cells. They release gastrin into the bloodstream in response to: (1) vagal stimulation via gastrin-releasing peptide (GRP, not ACh directly), (2) peptides and amino acids (especially phenylalanine and tryptophan) in the gastric lumen, and (3) gastric distention. Gastrin acts on CCK-B (gastrin) receptors on ECL cells to stimulate histamine release, and on parietal cells to promote H+/K+ ATPase activity and acid secretion; it also promotes growth of gastric mucosa. Gastrin secretion is inhibited by low antral pH (<3) via somatostatin release from D cells (negative feedback loop) and by secretin. Clinically, G cell hyperplasia or gastrinoma (Zollinger-Ellison syndrome, often associated with MEN1) leads to excessive gastrin secretion, causing recurrent peptic ulcers, diarrhea, and malabsorption due to unregulated acid hypersecretion. Diagnosis involves elevated fasting serum gastrin levels and secretin stimulation test (paradoxical increase in gastrin in ZES). Understanding G cell physiology is essential for grasping gastric acid regulation, peptic ulcer disease pathophysiology, and pharmacologic targets (e.g., proton pump inhibitors, H2 blockers) that modulate acid secretion pathways influenced by gastrin.