OBJECTIVE 3: TO UNDERSTAND THE GENERAL ELECTRICAL PROPERTIES OF THE TUBULAR EPITHELIA.

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A: The transport and permeability properties of the basolateral membrane, the apical membrane and the paracellular pathway interact in a complex manner to set up a transepithelial electrical gradient (V_te) that varies along the length of the nephron.

B. The major conductance present in almost all tubular cell membranes is a potassium channel. That, together with the Na-K-ATPase, establishan electrical gradient such that the interior of the cell is electrically negative with regard to the exterior.

C. The presence of channels for other ions and electrogenic transport mechanisms in the apical and basolateral membranes modify the transmembrane electrical gradients. Since the presence of these channels and mechanisms differ between the two membranes of each cell, the electrical gradients across the apical and basolateral membrane differ and this creates a transepithelial electrical gradient. For example, the principal cells in the collecting tubule possess K⁺ channels in both the apical and basolateral membrane and Na-K-ATPase in the basolateral membrane. There is also a Na⁺ channel in the apical membrane only (Fig. 3-4). The Na⁺ channel permits Na⁺ to enter the cell and this tends to depolarize the apical membrane to a voltage that is less negative (-30 mV in Fig. 3-4) than that of the basolateral membrane (-80 mV). This results in a transepithelial potential difference (-50 mV) with the lumen more negative than the interstitium.

D. The conductance of the paracellular pathway is higher than that of the cellular pathway, thus the transepithelial electrical gradient tends to drive a current flow (ion flux) through the paracellular pathway. In the example in Fig. 3-4, an anion flux out of the lumen or a cation flux into the lumen would occur. This passive flux tends to minimize the transepithelial potential difference and also would tend to increase the apical potential difference and reduce the basolateral potential difference.

E. The magnitude of V_te depends upon the individual properties of the apical and basolateral membrane and the magnitude of the conductance of the paracellular pathway. The conductance of that pathway is very high in the proximal tubule and falls progressively from there to the collecting duct. Basically, the lower the conductance of the paracellular path, the higher the electrical gradient, in other words, current flow through the paracellular pathway tends to shunt V_te. Thus, the proximal tubule with its high conductance has only a small transepithelial electrical gradient. The distal tubule and collecting tubule have a high electrical gradient partly because of the low conductance of the paracellular path.

Fig. 3-4. An example of the generation of a transepithelial electrical gradient by a tubular cell.

F. The magnitude of the passive flux of ions driven by the electrical gradient is proportional to the product of the gradient (V_te) and the conductance (g_te). This product and thus the passive flux of ions through the paracellular path is much higher in the proximal tubule than in the distal or collecting tubule.

QUESTIONS:  
10. Why may the apical membrane potential differ from the basolateral membrane potential? What is the origin of the transepithelial electrical gradient.?

11. What effect does V_te have on ion fluxes through the paracellular pathway? What is the effect of those fluxes on V_te and on V_a and V_b?

12. In the late proximal tubule the tubular fluid Cl concentration exceeds that of the interstitium and drives a passive flux of Cl through the paracellular pathway. What will be the effect of this flux on V_te?

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