SUBCELLULAR-LOCALIZATION OF PROSTAGLANDIN-E2 RECEPTORS IN THE GASTRIC-MUCOSA

Gastric mucosal PG E2 receptors are the common antisecretory working point of all prostanoid types and may also be involved in ''protective' effects. We investigated the subcellular localization of these receptors, as measured by displaceable H-3-PG E2 binding, and identified different organelles by monitoring the activities of specific marker enzymes. Porcine mucosal homogenates were subdivided by differential centrifugation into fractions PI (1000xg), P2 (20000xg), P3 (300000xg) and the supernatant S1. P3 was further fractionated over a series of sucrose step gradients. Mitochondria and lysosomes were enriched in P2 (maximum specific activities of cytochrome-c-oxidase of beta-glucosidase, beta-glucuronidase, beta-galactosidase, respectively). Plasma membranes (alkaline phosphatase, gamma-glutamyl-transpeptidase, 5-nucleotidase), tubulovesicles (H+/K+-ATPase) and rough endoplasmic reticulum (NADPH-cytochrome-c-reductase) were mainly found in P3, which also contained the majority of H-3-PG E2 binding sites. In contrast, prostanoid binding was barely detectable in SI. Density fractionation of P3 revealed that H-3-PG E2 binding sites shared a similar sedimentation profile with plasma membranes and tubulovesicular markers. No or negative correlation was found with lysosomes, rough endoplasmic reticulum and mitochondria. We conclude that mucosal PG E2 receptors are predominantly located at the cell surface. This supports the view that prostanoids inhibit gastric secretion through membrane receptors, but gives no clue for intracellular ''protective'' working points.