Differentiation of the Chloride Extrusion Mechanism During Seawater Adaptation of A Teleost Fish, the Cichlid Sarotherodon Mossambicus

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Abstract
Opercular membranes isolated from the freshwater-adapted euryhaline teleost, Sarotherodon mossambicus, and mounted in Ussing-style chambers, have low conductance and current and do not actively transport chloride. In contrast, membranes isolated from seawater-adapted S. mossambicus have high conductance and generate large currents representing net chloride extrusion. Full development of this chloride secretion process requires 1-2 weeks, the time-course of which provides the first unambiguous measurement of changes in net extrarenal salt secretion associated with a teleost's adaptation to seawater. Tissues from seawater-adapted fish contain typical chloride cells, when observed with the electron microscope, which appear as large cells with fluorescence microscopy after staining with dimethylaminostyrylaethylpyridiniumiodine. These cells are absent from the freshwater tissue, although rudimentary chloride cells are present, appearing as small cells with fluorescence microscopy. Following seawater transfer, the number of chloride cells increases only during the first 3 days. Subsequent chloride cell hypertrophy is highly correlated with the quantity of chloride extrusion. These data strongly implicate the chloride cell as the salt-secretory cell-type. When cortisol was injected into freshwater fish, chloride cell density increased but chloride secretion was not activated. It appears that development of salt extrusion involves increased numbers (controlled, at least in part, by cortisol) and differentiation of chloride cells, including activation of membrane active-transport sites. The opercular membrane from S. mossambicus provides a valuable model for studying these physiological and morphological events.