Cnidocytes and adjacent supporting cells form receptor-effector complexes in anemone tentacles.

Cnidocytes, the stinging cells of enidarians, discharge enidae (intracellular capsules containing eversible tubules) in response to physical contact combined with the stimulation of specific chemoreceptors. These receptors, occurring in at least two classes, bind N-acetylated sugars and certain amino-compounds, respectively (Thorington and Hessinger, 1988). Colloidal gold coated with bovine submaxillary mucin (mucin-gold) binds exclusively at the surface of the supporting cells which surround enidocytes (Watson and Hessinger, 1986). We now find that mucin-gold sensitizes enidocytes to discharge nematocysts in a dose-dependent manner. In addition, we find that the number of mucin-gold particles appearing at the surface of supporting cells changes over time, and that such changes correlate with the time-course of enidocyte responsiveness. Thus, the discharge of nematocysts by enidocytes may be regulated by the number of receptor-ligand complexes at the surface of the adjacent supporting cells. We conclude that enidocytes and supporting cells constitute receptor-effector complexes. Subsequent to binding at the cell surface, mucin-gold is endocytosed (Watson and Hessinger, 1987a). Multivesicular bodies seem to dispose of the endocytosed mucin-gold at the cell surface rather than via lysosomes. This novel route appears to be the major pathway by which endocytosed mucingold is removed from supporting cells.