Electrogenic ion pump

Lauger, P. (1991). Electrogenic Ion Pumps. Sinauer Associates, Inc., Sunderland, Mass. 

Animal cells (notably neurons, sensory cells and muscle cells) are made excitable in part through the operation of ion pumps that variously keep cytosolic concentrations of Na+, Cl- and Ca2+ low and cytosolic K+ concentration high. It should be noted that the cytosolic free concentration of Ca2+ is extremely low (0.1 (jtM in resting cells and about 10 (xM in excited cells) as compared to cytosolic concentrations of Na+, CP and K+ of about 10, 10 and 100 mM, respectively. The transmembrane potential (v tm) of animal cells is typically about —0.1 volt (V) (potential difference inside with respect to the outside), this being substantially due to internal constituents, selective membrane permeability and the operation of electrogenic ion pumps. Changes in the permeability of the cell membrane (plasma membrane, PM) to particular ions causes a change in v tm as described below. 

Lauger, P. Electrogenic Ion Pumps Sinauer Associates Sunderland, MA, 1991. 

P Lauger. Electrogenic Ion Pumps. Sunderland, MA Sinauer Associates, 1991. 

The electrogenic ion pump in the plasma membrane of animal cells is the Na+/K+-ATPase. As shown in Fig. 12, three Na+ ions are transported out of the cell and two K+ ions are pumped in for each ATP that is hydrolyzed. Since three positively charged ions are exported, but only two imported, the Na+/K+-ATPase is electrogenic. The trans... 

Lauger, R, Electrogenic Ion Pumps (Distinguished Lecture Series of the Society of General Physiologists, Volume 5), Sinauer Associates, Sunderland, MA, 1991. 

Figure 2. Sodium and chloride uptake across an idealised freshwater-adapted gill epithelium (chloride cell), which has the typical characteristics of ion-transporting epithelia in eukaryotes. In the example, the abundance of fixed negative charges (muco-proteins) in the unstirred layer may generate a Donnan potential (mucus positive with respect to the water) which is a major part of the net transepithelial potential (serosal positive with respect to water). Mucus also contains carbonic anhydrase (CA) which facilitates dissipation of the [H+] and [HCO(] to CO2, thus maintaining the concentration gradients for these counter ions which partly contribute to Na+ import (secondary transport), whilst the main driving force is derived from the electrogenic sodium pump (see the text for details). Large arrow indicates water flow...

Gibb, L.E. Eddy, A.A. (1972). An electrogenic sodium pump as a possible factor leading to the concentration of amino acids by mouse ascites-tumour cells with reversed sodium ion concentration gradients. Biochem. J. 129, 979-981. 

Figure 9.29 Some mammalian (left) and microbial (right) membrane transport systems. (A) Primary electrogenic mechanisms (pumps) creating either a Na+ or a H+ gradient. (B) Secondary active transport systems of the symport type, in which the entry of a nutrient S into the cell is coupled with the entry of either the sodium ions or protons. (D) Various passive ion movements, possibly via channels or uniports. (Reproduced by permission from Serrano R. Plasma Membrane ATPase of Plants and Fungi. Boca Raton CRC Press, 1985, p. 59.)...

In order for an organelle lumen or an extracellular space (e.g., the lumen of the stomach) to become acidic, movement of protons must be accompanied either by (1) movement of an equal number of anions (e.g.. Cl ) in the same direction or by (2) movement of equal numbers of a different cation in the opposite direction. The first process occurs in lysosomes and plant vacuoles whose membranes contain V-class H ATPases and anion channels through which accompanying Cl ions move (Figure 7-10b). The second process occurs in the lining of the stomach, which contains a P-class H /K ATPase that is not electrogenic and pumps one H outward and one inward. Operation of this pump is discussed later in the chapter. 

It is electrogenic. However, the gastric H+,K+-ATFase exchanges H3O+ for K+ and cleaves ATF with formation of a phosphoenzyme. It belongs to the family of F-type ion pumps that includes the mammalian Na+,K+-ATFase (Fig. 8-25) and Ca +-ATFase (Fig. 

If chloride is available, the electrogenic chloride pump makes the mucosal surface negative and accounts for the short-circuit current, as Hogben and Villegas had shown, but when chloride is unavailable, the independent hydrogen ion pump uncovered by Heinz and Durbin makes the mucosal surface positive and generates the short-circuit current. This was the generally accepted opinion about 1960-65. 

Fig. 3 shows a single sodium pump in the peritubular cell membrane since the Na extrusion mechanism is not necessarily linked with K uptake into the cell. The intracellular [K ] can all be accounted for on the basis of a passive electro-chemical equilibrium distribution of potassium. The peritubular membrane is known (Giebisch, 1961) to be highly permeable to K" ion and to exhibit a high K selectivity. The observation that the peritubular membrane of the Necturus proximal tubule is characterized by = E may be explained in one of two ways. Either that the PD across the peritubular cell membrane is mainly due to a K" " diffusion potential, or that the peritubular cationic pump is an electrogenic Na pump which by its operation generates E. Then a passive influx occurs to the point where Ej = E. ... 

FIGURE 12 Some ion pumps in the plasma membrane. The Na+/K+-ATPase of animal cells uses the energy of ATP hydrolysis to move three Na+ ions out of the cells and two K+ ions in, which results in the generation of ion gradients and a membrane potential. Plant, yeast, and fungal cells do not have a Na+/K+-ATPase, but instead have a H+-ATPase, as the electrogenic pump. The plasma membrane also contains a Ca +-ATPase that pumps Ca + out of cells to help keep the intracellular Ca + concentration low. 

Besides the remarks made above, which might notably weaken one s confidence in the electrogenic proton pump theory, let us stress some further complications. First, it is impossible to perform direct proton permeability measurements by the use of tritium in radiotracer methods. Second, several processes might be correlated to the external pH change, like effects caused by CO2 transport, 0H efflux, HCO5 uptake, the permeability of H and ions via pores, and kinetic control of all ionic pumps. 

Such considerations have led us to adopt a less restrictive modellistic approach . For the diagnostic of steady state electrogenic ion transfer, we have used therefore a computation which starts from the Nernst-Planck equation, avoids a separation between diffusion and ion pumping, both appearing at once as antagonistic and complementary processes (see Equation 6), and which handles all transferable chemical species possibly involved in ion transfer across membrane without selecting a priori a main transport process. 

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