The ionic concentrations in the mitochondria‐rich or chloride cell of Fundulus heteroclitus

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Abstract
The intracellular concentrations of sodium, potassium, and chloride ions, together with total phosphorous, in the cytoplasm of the chloride cells of the opercular epithelium of Fundulus heteroclitus have been estimated by X‐ray emission analysis of bulk frozen hydrated tissue. The estimated concentrations are Na, 171 ± 12; Cl, 155 ± 6; K, 62 ± 4 mE/kg water, together with 80 ± 6 mM P/kg water. It is shown that these high concentrations are consistent, with the Silva model of chloride secretion in which a sodium‐chloride linked carrier, driven by the inward diffusion of sodium ions, concentrates chloride in the cytoplasm. The intracellular concentration of chloride is such that if the potential across the apical membrane is greater than 34 mV, inside negative, there will be a net efflux of chloride ions out of the cell. If the linkage between sodium and chloride on the carrier is one to one, it is shown that energy dissipation is minimal when the product of the cytoplasmic concentrations of sodium and chloride approaches the product of those in the plasma and it would be energetically disadvantageous to maintain low intracellular concentrations of these ions. In practice, however, optimum concentrations are not attainable because of other intracellular solutes that contribute substantially to the osmotic pressure.