High-density localization of ANP receptors in chondrocytes of eel gill cartilage

Very high levels of atrial natriuretic peptide (ANP) receptor were found in a quite unexpected tissue, i.e., the chondrocyte of the eel gill cartilage. ¹²⁵I-labeled ANP binding assay indicated that the eel gill contains large amounts of the receptor and its levels are much higher (>10- fold) than those in the other tissues examined, including the brain, kidney, head kidney including steroidogenic interrenal cells, and gut. Autoradiography using gill sections revealed dense localization of ANP receptors in the chondrocytes of the cartilage as well as moderate localization in the parenchymal cells. Biochemical and pharmacological characterization of the chondrocyte receptor indicated that most of the receptors are type C receptors, having a broad ligand specificity and a reduced M(r) of 68,000. Because a hyaline cartilage in which chondrocytes reside lacks blood vessels, and all metabolic exchange is by slow diffusion through the matrix, the chondrocyte localization of the type C ANP receptor is notable and may stimulate not only chondrocyte research but also studies on physiological significance of the type C receptor, which is generally considered to be merely a clearance (silent) receptor.