Teleost chloride cell. II. Autoradiographic localization of gill Na,K-ATPase in killifish Fundulus heteroclitus adapted to low and high salinity environments.

Abstract

The specific binding and inhibitory action of [3H]ouabain were employed to localize transport Na,K-ATPase in the euryhaline teleost gill, a NaCl-transporting osmoregulatory tissue in which both enzyme activity and transepithelial transport vary with environmental salinity. In killifish fully adapted to 10%, 100%, or 200% seawater, the gills were internally perfused and externally irrigated in situ. After suitable internal or external exposure to [3H]ouabain, individual gill arches were excised for Na,K-ATPase assay, measurement of radiolabel binding, or quantitative high-resolution autoradiography. Internal exposure to 50 /xM ouabain resulted in essentially complete enzyme inhibition, and binding paralleled the increases in enzyme activity at higher salinities; in contrast, external exposure gave minimal and erratic results consistent with leakage of external ouabain into interstitial fluid.[3H]Ouabain autoradiographs demonstrated that, irrespective of exposure or salinity, most of the gill binding was associated with chloride cells. These cells increased in size and number with salinity and, at the subcellular level, the distribution pattern for bound ouabain was always identical to that for the amplified basal-lateral (tubular system) membrane. The combined physiologicmorphologic results constitute final direct proof that chloride cells are the primary site of gill Na,K-ATPase. More important, they provide convincing evidence for unexpected increases in basal-lateral enzyme at higher salinities and thus raise a fundamental objection to the long-postulated role of the Na pump in secretory NaC1 transport.Of various biochemical mechanisms thought to play important roles in salt-transporting epithelia, none has received more attention than the ouabain-sensitive, sodium-and potassium-dependent adenosine triphosphatase (Na,K-ATPase). The main role of this membrane-bound enzyme, associated with most animal cells, is thought to be the Na pump involved in maintenance of ion gradients

Differential regulation of sodium–potassium pump isoforms during smolt development and seawater exposure of Atlantic salmon

“…This is probably due to the semi-quantitative nature of immunocytochemistry, which can uncover the number of cells, but not the quantity of NKA pumps within each cell. Previous work indicates that the density of ionocyte NKA pumps is higher in seawater-than in freshwater-adapted fish (Karnaky et al, 1976). It seems likely that the amount of NKAα1b in each ionocyte increases progressively during smolt development, and indicates that the quantitation of cell numbers underestimates the amount of NKAα1b that is available for rapid activation after seawater exposure.…”

Ultrastructural Changes in Gill Epithelia of a Yellowtail, Seriola quinqueradiata, Exposed to Sea Bloom

“…Ultrastructural changes in the chloride cell have been observed for various species of teleosts; mullet (8,26), lamprey (21,23), pupfish (11), lungfish (28), killifish (12) and others. Transfer of the mullet to fresh water resulted in an increase in the number of cellular extensions from the apical membrane of chloride cells and transfer back to sea water decreased them (8).…”

Active chloride transport in the in vitro opercular skin of a teleost (Fundulus heteroclitus), a gill‐like epithelium rich in chloride cells

“…Currently, the models for NaCl secretary mechanisms by the teleost gill Cl-cell propose an important role for Na+, K+-activated ATPase, either directly (Maetz, 1969(Maetz, , 1971 or indirectly (Kirschner, 1977;Silva, Solomon, Spokes & Epstein, 1977 (Karnaky et al 1976). Since the gill and opercular epithelium chloride cells are identical in ultrastructure (Karnaky & Kinter, 1977), it is reasonable to assume that this ATPase is located in the tubular membranes of the opercular epithelium chloride cells.…”

“…Nature of ion transport across the isolated opercular epithelium of fish adapted to low and high salinities In F. heteroclitus gill, chloride cell size and number and specific activity of Na+, K+-activated ATPase increase with adaptation to increasing salinities from 10 % seawater to 200 % seawater (Karnaky et al 1976). In striking contrast, the number of opercular epithelium chloride cells of this same species remains relatively constant with adaptations ranging from freshwater to 200 % seawater (Karnaky & Kinter, 1977).…”