Ion exchanges through respiratory and chloride cells in freshwater- and seawater-adapted teleosteans

The present study discusses the respective roles of the chloride and respiratory cells in branchial ion exchange in freshwater- and seawater-adapted teleosts. In the gill, two distinct epithelia are defined according to their blood irrigation and cellular characteristics: the primary lamellar epithelium containing mostly chloride cells is functionally connected to the venous compartment; the secondary lamellar epithelium consisting of respiratory cells is irrigated by the arterioarterial circulation. The fundamental reorganization occurring in the chloride cells during adaptation to seawater consists of the appearance of leaky ultrastructure related to an increase in the ionic and nonelectrolyte permeabilities of the primary lamellar epithelium. The physiological functions of respiration, excretion, and the maintenance of acid-base balance occur through the secondary lamellar epithelium both in freshwater and seawater fish. The increase of the exchange diffusion across the secondary lamellar epithelium on seawater adaptation is directly related to the increase in the external salinity. The study of adrenergic control in the teleost gill emphasizes the functional advantage to the fish during adaptation to seawater of the anatomic separation of the osmoregulatory primary lamellar epithelium.