Cellular Mechanisms of Ion Pumping in Freshwater Fish Gills

What are the cell-membrane proteins and other cellular mechanisms that enable the gill epithelium of a freshwater fish to take up Na⁺ and Cl^– ions from pond or stream water? This important question is still under active investigation. Nonetheless, some of the principal attributes of the ion-pumping mechanisms are well understood, as you will learn by reading Box Extension 5.2.

Figure A presents two models for the pumping of Na⁺ across the gill epithelium from pond water to the blood plasma of a fish. Before we discuss these models, it is important to recognize that for any postulated model to hold promise of being correct, it must propose ways for the epithelial cells to stay in steady state; for instance, a model that involved the endless buildup of K⁺ in a cell could not possibly be correct because no cell can survive that sort of nonsteady-state condition.

Figure A Two models of the cellular mechanism of active Na⁺ uptake across the epithelial cells of freshwater fish gills

According to the model in Figure A1, two principal membrane proteins are involved in Na⁺ transport: the ubiquitous Na⁺–K⁺-ATPase in the basolateral membrane, and a countertransport protein for Na⁺ and H⁺ in the apical membrane. ATP-bond energy is invested in the process by the Na⁺–K⁺ pump, which lowers the intracellular Na⁺ concentration and helps make the inside of the epithelial cell negative (see Figure 5.12a). Na⁺ enters the cell across the apical membrane by carrier-mediated diffusion, following its electrochemical gradient. The Na⁺ binds to the countertransporter during this process, and the countertransporter moves H⁺ outward in exchange, thereby maintaining charge balance. Once Na⁺ is in the cell, it makes the next step to the blood by being pumped by the Na⁺–K⁺ pump. K⁺ channels in the basolateral membrane allow the K⁺ pumped into the cell by the Na⁺–K⁺ pump to return to the blood.

According to the model in Figure A2, ATP-bond energy is invested principally by a strongly electrogenic proton pump (a V-type H⁺-ATPase) in the apical membrane. By pumping H⁺ out without simultaneously pumping out other ions, this pump produces negative charge on the inside of the apical membrane. The negativity establishes an electrochemical gradient favoring the inward diffusion of Na⁺, which enters by way of a Na⁺ channel. In this model, Na⁺ and H⁺ are exchanged in a 1:1 ratio because the motive force for Na⁺ entry originates from H⁺ exit. After Na⁺ is in the cell, it continues its journey to the blood by being transported by the Na⁺–K⁺ pump, much as in the model in Figure A1. The source of H⁺ for both models in Figure A is the reaction of the metabolic waste CO₂ with water (see Figure 5.15).

Currently, evidence exists for both of the models for Na⁺ pumping shown in Figure A. Both mechanisms may be employed in some cases, and some species may prove to emphasize one mechanism whereas others emphasize the other.

Regarding the mechanism of the Cl^– pump, a countertransporter protein in the apical membrane that carries out a strict 1:1 exchange of Cl^– and HCO₃^– is the best-understood element. Figure B shows this countertransporter and one model of how it may be employed.

Figure B A model of active Cl^– uptake across the apical membranes of cells in the gill epithelium of freshwater fish  Although active transport of protons out of the cell is depicted in the apical membrane, an alternative model is that it occurs in the basolateral membrane. (See Figure A for a key to symbols.)

According to this model, the energy for countertransport comes from an electrochemical gradient for HCO₃^– across the apical membrane. The gradient favors outward HCO₃^– diffusion, which drives the countertransporter protein, bringing Cl^– into the cell against the Cl^– electrochemical gradient (a form of secondary active transport). Steady formation of HCO₃^– inside the cell—which maintains the HCO₃^– electrochemical gradient—is driven by a proton pump that uses ATP-bond energy to pump H⁺ outward and thus steadily pull the CO₂/HCO₃^– reaction to the left by mass action, as shown. The outward pumping of H⁺ may also lower the local HCO₃^– concentration on the outside of the apical membrane because extruded protons will react there with HCO₃^–. The steady formation of HCO₃^– in the cell and steady depletion of HCO₃^– outside the cell maintain the electrochemical gradient for outward HCO₃^– transport across the apical membrane, according to this model.