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Video transcript
So in our last video, we left off here talking about aldosterone and how it works on the principal cell. And there, kind of the two big ideas were losing potassium and gaining salt and water. So that was how it worked on the principal cell. And aldosterone actually works on one other type of cell. It has an important job there, as well. So let me actually just make a little bit of space on our canvas. And I'll start talking about the second type of cell. That is the alpha intercalated cell. So I'm going to draw it in yellow here. And this is our alpha intercalated cell. And the main job of the alpha intercalated cell is to get rid of protons. And remember that protons are basically our way of representing acid. And if there's an alpha intercalated cell, you can probably guess that somewhere along the line, there's also a beta intercalated cell. And the beta intercalated cell does, in many ways, the opposite of the alpha one. And the beta cell tries to hold onto acid. So here, we're going to talk about how the blood or body, rather, gets rid of acid. And in the beta cells, in a future video, we'll talk about how it holds onto acid. So here's a blood vessel as before. This is our peritubular capillary. And it's running alongside of our cell. And blood is going to be, in this case, a little too acidic. So actually, let me move this down just a touch so I can show you. There we go. So in this case, we're going to have, again, the basolateral surface. And this is the apical surface, right? So we have our two surfaces and just to get you visually oriented, I want to make sure I just draw another cell right here. Just so you remember that this is basically where the urine is kind of collecting. Oops, made it too small. So urine is collecting on this side right here. And we obviously have blood on the other side. And, in our case, let's imagine that this blood is getting a little too acidic. More acidic than we would like. So I'm going to draw little protons here. And there are too many of them, right? So it's a little too acidic. Too many protons. And our goal, our blood would love to get rid of some of these, right? We would love to get rid of some of these protons so that the blood is less acidic. And so, here comes into play the alpha intercalated cell. Now all cells are making carbon dioxide and water. And I say making because we're breaking down sugar, right? And sugar at the very core, or at the very end of the process is going to be turned into carbon dioxide and water. So all cells are making carbon dioxide and water. And when these molecules within the cell meet up, there's an enzyme here in the alpha intercalated cell called carbonic anhydrase. And what this does is it helps the carbon dioxide and water form into protons and bicarb. So this is bicarbonate. HCO3 minus. And if you actually count it up, it all adds up. The carbon ends up over with the bicarb. And the oxygens all end up on the bicarb and you're left with just one lonely little proton over here. And bicarb. So the bicarb, I would love if I could somehow get that bicarb over to the blood. That would be fantastic, right? Because then you can neutralize one of those little protons. So there happens to be a little transporter in the basolateral surface. There's a little transporter. And it basically sends across this bicarb over here. So this bicarb ends up on this side. And in exchange for doing that, it takes on a chloride. OK, no big deal. So it takes on a chloride and now that chloride is sitting on the inside. We'll come back to that chloride in just a second. That bicarbonate, because this is where kind of the magic is happening, that bicarbonate is going to bind to that proton and they will do the reverse. They're actually going to go back to water and carbon dioxide. Right? So they're actually going to go back to water and carbon dioxide. And what have I really accomplished here? Well, I'm no longer left with any bicarb in the blood, that's fine. But here's the cool thing, I've actually gotten rid of a proton, right? Because I have water there and I have carbon dioxide there. And this will get, at some point, I can breathe that out. And water is water, so that's great. And so I've basically accomplished something really cool. I've gotten rid of a proton and I've gotten some water and I've gotten carbon dioxide which I can breathe out through the lungs at some point when the blood gets over there. So this is actually fantastic for the blood because I've been able to get rid of some of that acid, which is the whole goal. Now if you think about it, I've been left with a couple of things. I've got that chloride, which I said I would get back to. So let's deal with that right now. Happens to be a little channel for chloride, so chloride can just make its way over to the blood. Well, that's very convenient, right? So you can keep doing this process and that keeps it nice and tidy. But you're still stuck with a proton. I still got this proton. I got to figure out what to do with this proton. So this proton is going to build up, right? If I keep doing this process, let's say I keep driving this process, keep driving this process. I'm going to get lots and lots of little protons, right? I mean I can see how the bicarb is going to help overall eventually cancel all these protons out. But I'm going to keep building up protons over here, right? So for every proton I get rid of in the blood, I build up a proton in my cell. So at the end of the day, I've got to figure out how to get rid of these protons from the cell. OK, so here's where aldosterone comes into play. We have these really nifty little transporters. And they do cost energy, so they're not free of charge. They're going to take energy. Let me draw that in right now, so I don't forget. It's going to take energy, ATP, to get this transporter to work. But what they'll do is they'll basically send a little proton over into the urine. And you can just urinate that out. So your urine will be a little bit more acidic, sure, but that's actually quite helpful because I'd rather have urine that's acidic and then just flush the toilet, get rid of it, rather than having blood that's acidic. Because that's a problem. So that's one transporter that will get the proton across. And, actually, this transporter is going to be driven, it's actually going to be working almost like overdrive when aldosterone is around. So now finally we've gotten to what aldosterone does in these cells. It makes that transporter work really, really well. So you can get rid of protons. And if you can get rid of protons, then this whole process is going to work really nicely. Now there's actually another transporter that gets rid of protons. And I'll draw that right here. And this one is actually not going to take energy. And you're thinking, well, how do you get rid of protons and not even take energy? Well, I'm going to first draw the proton kind of going across. And it took energy the first time around, but here we're actually going to use something different. We're going to use a gradient. So you remember these cells. Actually, let me draw it here. These cells have a lot of potassium. Right? But not too much salt. In fact, all the salt is sitting in the blood, right? So I'll draw the salt sitting over here in the blood. Because that's the main ion for the blood. Now there isn't much salt over there in the cell. And so you can actually allow salt in. You'll say, all right, we'll take a salt molecule in and because salt wants to get in so badly, it actually will drive this transporter and will allow proton to slip out into the urine. So you're basically, instead of using energy, you're using a gradient. And this second transporter is also going to be revved up by aldosterone. So aldosterone is really working on these two transporters and is going to help you get rid of protons. Now, there actually happens to be a couple of other transporters that are important. I'm just going to draw them in here. So there's another one and this one also takes energy. And let me draw in the energy so we don't forget that because that's always important. This one takes energy. And this one is going to, again, allow protons out because that is the whole point of all this, is to try to get protons out of these cells so we can keep using them so the alpha intercalated cell doesn't become just a bag of acid. And instead of sodium coming back with it, this time you're actually going to use potassium. Potassium can come back. But now immediately you can say, well, there's a problem here. Potassium wouldn't want to be in the cell. There's a lot of potassium already there. And that is why this one takes energy. So you can already see why this transport takes energy whereas the one with sodium does not. Because potassium doesn't want to be in the cell. It's actually forced to be in the cell. And actually, that brings up my last point, which is that, you remember we talked about in the principal cells, and in fact, in all cells, we have these sodium potassium pumps that are on this basolateral surface? Remember we talked about that? Well, in the alpha intercalated cell, it's no different. It also has to maintain this sodium potassium gradient. So it's going to drive potassium in. It's going to take two potassiums in. And it's going to force three sodiums out. Right? So three sodiums are going to get forced outside. And that, again, takes energy. So this is also going to take energy. So these are the different transporters in the alpha intercalated cell. But I want you to really try to focus on the key concept. And the concept here is that you're trying to get acid out of the blood. And you do that by bringing extra protons or making extra protons in the intercalated cell. And you saw how that happened. But the trick is, well, what do you do with them? Well, you have to dump them into the urine. And that's the whole concept. Getting acid in the urine is a process that is either going to take energy, like those ATP pumps, or it's going to take some cleverness through a gradient, like that sodium gradient. And two of these things, I'll point to again, this one and this one, are going to be driven by aldosterone. So that's where aldosterone comes into play in these cells.