PhysiologyWeb Logo  Search
PhysiologyWeb Loading...

PhunnyPhysiology
Resting Membrane Potential -
Physiological Significance of the Membrane Potential
In the previous section, we learned that cells expend energy to maintain the intracellular concentrations of K+ and Na+ (through the activity of the Na+/K+ ATPase) in order to ensure the maintenance of the resting membrane potential. In this section, we wish to better understand the physiological significance of the membrane potential. Indeed, the normal value of the membrane potential is essential for many physiological processes. In particular, we will use two examples to highlight why it is important to maintain the resting membrane potential at a sufficiently negative value.
Role of the membrane potential in allowing for normal action potential generation
In a later lecture on the neuronal action potential, we will see that the action potential (i.e., electrical impulse) is responsible for much of communication in the nervous system, and that it involves a rapid reversal of the membrane potential such that the potential inside of the cell transiently becomes positive with respect to the outside before it returns back to the resting value (see action potential figure). We will also see that in order for a neuron to generate an action potential, the membrane potential has to depolarize from the resting value of around −70 mV to the threshold value of around −50 mV. A similar situation is also at play in muscle cells (skeletal, cardiac, and smooth). This is because it is at the threshold voltage that voltage-gated Na+ and K+ channels become activated. Thus, the normal generation of action potentials depends on a normal physiological value of the resting membrane potential. Importantly, the voltage-gated Na+ channels of neurons, skeletal muscle, and cardiac muscle cells enter an inactivated, non-conducting state after opening, and cannot reenter the open state without first recovering from inactivation (see figure). Recovery from inactivation is a voltage-dependent process that takes place at membrane potential values more negative than the threshold voltage. Thus, the return of the membrane potential to the resting value is critical for allowing these channels to continue to function properly. If due to a pathophysiological condition (such as elevated extracellular K+ levels), the resting membrane potential approaches the threshold potential for neuronal and muscle voltage-gated Na+ channels, the channels would enter the inactive state, from which they cannot recover. The consequence of this will be devastating for the organism in that neurons can no longer fire and cardiac myocytes can no longer contract to pump blood through the circulatory system. It should be clear that this situation will result in the death of the organism!
Contribution of the membrane potential to the concentrative capacity of secondary active transporters
Another example that highlights the importance of the resting membrane potential is the influence of the value of the membrane potential on the activity of electrogenic transport proteins. As described in a previous lecture on secondary active transport, Na+-coupled cotransporters use the energy stored in the electrochemical gradient of Na+ to drive ions and molecules against an electrochemical or a concentration gradient across cell membranes. One example presented was the Na+/glucose cotransporter (SGLT), which couples the simultaneous cotranslocation of 2 Na+ ions and 1 glucose molecule across the apical membrane (i.e., brush border membrane) of epithelial cells in the small intestine (leading to glucose absorption) and kidney proximal tubules (leading to glucose reabsorption) (Fig. 1). In the small intestine, this process is essential for the absorption of glucose contained in ingested food across the wall of the small intestine. Once inside the cell, glucose leaves the cell down a concentration gradient via the activity of a facilitative glucose transporter (GLUT) present in the basolateral membrane. After transport across the wall of the small intestine, glucose enters the circulation via mucosal capillaries. In the kidneys, glucose is filtered out of the glomerular capillaries to enter the Bowman's space and later the lumen of the proximal tubule. In healthy individuals, all of the glucose present within the lumen of the proximal tubule is reabsorbed across the wall of the proximal tubule. Reabsorbed glucose then enters the peritubular capillaries to reenter the circulation.
Transepithelial glucose transport in the small intestine and kidney proximal tubules.
Figure 1. Transepithelial glucose transport in the small intestine and kidney proximal tubules.
Transport of glucose from the lumen across the apical membrane and into the cell is carried out by the Na+/glucose cotransporter (SGLT). SGLT is a secondary active transporter and, thus, couples the downhill movement of Na+ down its electrochemical gradient to the uphill movement of glucose against a concentration gradient. Transport of glucose out of the cell across the basolateral membrane is carried out by the facilitative glucose transporter GLUT.
Ion/substrate coupling stoichiometry of the Na+/glucose cotransporter (SGLT).
Figure 2. Ion/substrate coupling stoichiometry of the Na+/glucose cotransporter (SGLT).
For every cycle of its transport, the Na+/glucose cotransporter (SGLT) translocates 2 Na+ ions and 1 glucose molecule across the plasma membrane. Because net charge is translocated across the plasma membrane during the transport cycle, the process is electrogenic and, thus, sensitive to the voltage difference across the plasma membrane (see text for additional details). SGLT is found in the apical membrane (i.e., brush border membrane) of epithelial cells in the small intestine and kidney proximal tubules (see Fig. 1).
Because 2 Na+ ions are moved during every transport cycle of the Na+/glucose cotransporter, the process is electrogenic and, thus, sensitive to the voltage difference across the plasma membrane (Fig. 2). As shown in a previous lecture, basic thermodynamic principles can be used to derive the equation that describes the thermodynamic equilibrium for transport (Equation 1). At thermodynamic equilibrium, no net transport of Na+ and glucose takes place across the plasma membrane. The membrane voltage at which the transporter is in thermodynamic equilibrium is referred to as the reversal potential (Vrev).
Equation for the thermodynamic reversal potential (Vrev) of the Na+/glucose cotransporter (SGLT) Eq. 1
where, Vrev is the reversal potential for Na+/glucose cotransport, R is the gas constant, T is the absolute temperature, zNa is the valence of Na+ (zNa = +1), F is the Faraday constant, [Na+]o and [Na+]i are the extracellular and intracellular concentrations of Na+ respectively, and [Glucose]o and [Glucose]i are the extracellular and intracellular concentrations of glucose respectively.
Similar to the concepts we described for the electrochemial driving force acting on ions, the resting membrane potential is rarely at the reversal potential for any given transport process and, therefore, the transporter is not at equilibrium at the resting potential of most cells. The magnitude of the difference between Vm and Vrev contributes to the driving force that acts on the transport process. In general for Na+-coupled transporters, if Vm is more negative than Vrev (i.e., Vm < Vrev), then the transporter works to translocate Na+ and glucose into the cell (referred to as the forward mode of transport). If Vm is more positive than Vrev (i.e., Vm > Vrev), then the transporter works to translocate Na+ and glucose out of the cell (referred to as the reverse mode of transport). Thus, the direction of transport can change (i.e., forward or reverse) and depends on the driving force acting on the transport process. Of course, there will be no net transport if Vm = Vrev. Under normal physiological concentrations, SGLT works in the forward mode to accumulate glucose inside the cell.
Since Vm is rarely at the Vrev for any given transport process, and since the intracellular and extracellular concentrations of Na+ are fairly well regulated within narrow limits (see table of ion concentrations), by working in the forward mode to transport glucose into the cell, the Na+/glucose cotransporter continues to accumulate glucose inside the cell in order to adjust the intracellular glucose concentrations such as to make its Vrev equal to Vm. Thus, the equation shown above can be rewritten to examine the concentrative capacity of the transporter (Equation 2). In this case, the concentrative capacity of SGLT can be described as the ratio of intracellular glucose concentration to extracellular glucose concentration. Therefore, the concentrative capacity simply provides a measure of how well a transporter can accumulate its substrate in the cell against a concentration gradient. Given that the intracellular and extracellular concentrations of Na+ are fixed by cellular homeostatic processes, it can be seen in Equation 2 that the membrane potential very strongly influences the concentrative capacity of the Na+/glucose cotransporter. This can also be shown graphically (Fig. 3).
Equation describing the concentrative capacity of the Na+/glucose cotransporter (SGLT) Eq. 2
Graphical representation of the effect of the membrane potential (Vm) on concentrative capacity of the Na+/glucose cotransporter (SGLT).
Figure 3. Graphical representation of the effect of the membrane potential (Vm) on concentrative capacity of the Na+/glucose cotransporter (SGLT).
The effect of the membrane potential (Vm) is shown on the concentrative capacity of the Na+/glucose cotransporter (SGLT). The concentrative capacity of SGLT is shown as the ratio of the intracellular to extracellular glucose concentration (see Equation 2). The higher this ratio, the greater is the concentrative capacity of the transporter. Concentrative capacity increases exponentially as the membrane potential becomes more negative. At a typical membrane potential of −50 mV, SGLT can accumulate glucose inside the cell to a level approximately 4,000 times higher than the extracellular concentration of glucose. To generate this plot, normal physiological values were used: T = 37 °C + 273 = 310 K, [Na+]o = 145 mM, and [Na+]i = 15 mM. Standard values were used for R and F (see fundamental physical constants).
It can be seen in Fig. 3 that the membrane potential provides significant free energy for glucose accumulation inside epithelial cells of the small intestine and kidney proximal tubules. For a typical cell with the resting membrane potential of −50 mV, the ratio of intracellular to extracellular glucose concentration is approximately 4,000. That is to say that SGLT can concentrate glucose inside the cell to a level approximately 4,000 times higher than the concentration outside the cell. Clearly, this represents active transport of glucose into the cell. Specifically, this is Na+-coupled secondary active transport because secondary active transporters use the energy stored in ion gradients and not through the hydrolysis of ATP as utilized by primary active transporters. In the small intestine, this has physiological significance for glucose absorption from food within the lumen of the small intestine. In the kidney proximal tubules, this contributes to complete glucose removal from the lumen before the ultrafiltrate moves beyond the proximal tubules (to enter the loop of Henle).






Posted: Saturday, February 15, 2014
Last updated: Friday, March 11, 2016