Chloride Secretion by the Chloride Cells of the Isolated Opercular Epithelium of Marine Fish

Abstract

Sea water teleosts drink a highly concentrated NaCl solution and utilize the secretory epithelium of their gills to keep a homeostatic concentration of these electrolytes in their blood, the kidney having a secondary role (Homer Smith, 1930). The secretory epithelium of the gills consists mainly of the chloride cells, which were predicted and discovered by A. Keys in the 1930’s (Keys, 1931; Keys and Uilmer, 1932). By means of vascular perfusion in an isolated gill preparation Keys demonstrated a reduction in salts in the perfusate consistent with an increase in the medium bathing the outside of the gills. The anatomical work with Wilmer demonstrated the presence of large, complex “secretory like” cells in the gills of eels that degenerated or were reduced in number and size durincr adaptation to fresh water. In spite of opinion to the contrary (Bevelander, 1935; 1936) modern physiological and anatomical methods have confirmed Ancel Keys findinas. However, the actual drivina forces responsible for the movements of salts could not be determined conclusively until very recently. The reason for the delay in a aood biophysical demonstration of the chloride secretion of these cells was the need for an in vitro preparation containing chloride cells that permitted the application of the Ussing methodology (Ussing and Zehran, 1951) to a chloride cell rich flat epithelial membrane. This type of membrane rich in chloride cells are found in the epithelium lining the opercular flap of teleosts (Burns and Copeland, 1950). The opercular epithelium can be dissected, mounted as a membrane and actual ionic fluxes, electrical potentials and short circuit current determined to explain the function of these cells (Degnan, Karnaky, and Zadunaisky, 1977). In this presentation the information obtained up to now is presented focusing on the opercular epithelium of Fundulus heteroclitus. Reference to other more recently described opercular epithelia are presented also in the text.