Membrane and Transepithelial Potential

00:01 How do we measure membrane potential? In a normal, symmetrical, or general cell, we put a little recording electrode within the cell and have a secondary electrode in a bath or in the fluid around it.

00:17 That gives us our membrane potential.

00:20 Epithelial tissue is different and that it has a number of different membrane potentials.

00:26 The apical membrane has a membrane potential, the basolateral membrane has a membrane potential, and there’s a membrane potential across the whole epithelial cell itself.

00:40 So we have three membrane potentials to take into account: the apical, the basolateral, and the complete endothelial cell’s membrane potential from one side to the other.

00:54 So let’s go through some examples to practice this because this is a very different way of looking at voltages.

01:02 It’s both across the membrane, across the second membrane, and then across the whole cell line.

01:09 If we have an apical membrane voltage of let’s say minus 60 millivolts, we have a basolateral voltage of about minus 80 millivolts, therefore, there is a 20-millivolt difference between those two sides.

01:27 So, it’s possible to involve ions differently that might want to move across the whole entire tissue that don’t necessarily want to move across one membrane or the other.

01:40 Let’s keep going through some examples of this.

01:45 Let’s use the transport of sodium and chloride as an example.

01:50 If we have a voltage that is set up by a sodium-potassium ATPase, we have sodium traveling in across the apical membrane and then maybe potassium traveling across the basolateral.

02:05 It can set up a voltage difference across the membrane and that voltage difference will want to drive chloride across through those tight junctions.

02:18 Without the transepithelial voltage difference, the chloride would not want to travel in this direction.

02:24 So it’s traveling towards a negative charge on the apical membrane to a positive charge on the basolateral.

02:33 The positive charge is set up by positively charged ions leaving the basolateral side.

02:39 A second example is looking at sodium chloride secretion.

02:45 Here, we are going to use the sodium-potassium ATPase as the initial setup on the basolateral membrane.

02:53 We also have another channel that it’s going to cotransport three things: sodium, chloride, and potassium.

03:01 These are sometimes known as NCCK channels.

03:06 This allows the co-transport of all three of these ions into the cytosol.

03:11 Chloride then is allowed to travel across the apical membrane and that sets up a negative charge across the apical membrane.

03:21 If we look at this, that will want to pull sodium across with it.

03:29 Because sodium will want to travel from a positive to a negative, and therefore, this transepithelial gradient allows for this negative pull across the transepithelial surface or the leaky tight junctions.