Question

You are studying ion channels and have set up model lipid vesicles containing the three following transporters:

1. a Na+ /H+ antiporter driven by the Na+ concentration gradient.
2. The common 3Na+ /2K+ pump.
3. K+ leak channels. The experiments are designed such that all three transporters are oriented in the model lipid bilayers in the same direction they would be in the plasma membrane of a cell and that concentration gradients in vitro mimic the concentration gradients experienced by cells (e.g., the extracellular Na+ concentration is similar to the concentration outside of the vesicle whereas the concentration in the lumen of the model vesicles is similar to intracellular Na+ levels).

Required:
a. Draw the arrangement and ion flow of the three types of transporters in the model membranes.
b. What do you predict will happen to the pH of the vesicle lumen if you treat the model membranes with the drug oubain that impairs function of the 3Na+/2K+ pump? Briefly explain your answer in terms of what happens with each transporter.

Answer 1

Answer and Explanation:

a) you will find the arrangement and ion flow of the three types of transporters in the model membranes in the attached files.

• K+ leak channels are channel proteins that allow K⁺ to leave the cell in favor of its concentration gradient. It keeps a stable equilibrium by taking out K⁺ that the 3Na+ /2K+ pump brings into the cell. Any alteration in the pump leads to an alteration in the leak channel functioning.

• Na+ /H+ antiporter is an integral membrane protein, and a secondary transporter is responsible for the interchange of H+ and Na+ through the membrane with no use of energy. It regulates pH within the cell. It responds to the Na+ and H+ concentration gradient, determined by the 3Na+ /2K+ pump. An alteration in the pump functioning produces a change in the Na gradient and consequently affects the antiporter interchange action.

• 3Na+ /2K+ pump is an integral protein that transports K+ and Na+ against the electrochemical gradient. A significant amount of energy is used to make the pump function. It brings K+ into the cell and takes Na+ out of it. It keeps the membrane potential and the electrochemical gradient constant. The pump function is associated with the leak channels and the antiporters function.

Under normal conditions, the cell interior has a lower pH than the exterior. The K+ concentration is higher in the interior, while the Na+ concentration is higher in the cell exterior. The interior of the cell is always more electronegative than the cell exterior.

b)

The drug ouabain inhibits the pump. It impedes the Na+ exit and the K+ entrance. It occurs a sharp increase in the Na+ intracellular concentration, which influences the Na+ /H+ antiporter that changes the ions´ direction, taking out Na+ and bringing in H+ to compensate the intracellular Na+ excess. There is also a decrease in the leak channel function, which does not allow K+ to leave the cell.

Whenever the pump is inhibited, there is an increase in the intracellular pH, because Na+ concentration increases in the interior.