Let's continue our reabsorption.
Now, we are gonna talk about the distal tubule and the collecting ducts.
For here, we need to think about the various segments of the nephron again.
So remember, we start it off in Bowman's space or the glomerular capsule,
we went into the proximal convoluted tubule, the proximal straight,
then we went into the thin descending limb,
around the hairpin turn, thin ascending limb,
thick ascending limb and then into the distal convoluted tubule,
to the cortical collecting ducts
and then down to the medullary collecting ducts.
Let's begin with calcium.
Calcium is a divalent cation.
It will travel across the apical membrane by itself
meaning that it does not require cotransport.
The transporter that is important for this process is a TRPV5 channel.
This TRPV5 channel allows calcium across the apical membrane
and calcium can't be in high concentration in the cytosol because it is cytotoxic.
Therefore we need to bind it to something, and that substance is calbindin.
So, calbindin mops up the free calcium within the cyctosol
and then translocate it over towards the basolateral membrane.
It can then be released from the calbindin
and moved across the basolateral membrane by the calcium ATPase,
or be exchanged with sodium.
Either of these processes will get the calcium across the basolateral membrane
into the interstitial space.
What governs the control of how much calcium gets moved across the apical membrane,
mainly involves parathyroid hormone.
So parathyroid hormone, when it's bound,
allows for both the up-regulation of adenylyl cyclase
increasing also phospholipase C activity.
These involve a protein cascade that phosphorolizes these TRPV5 channels,
allowing for more calcium to move across the apical membrane,
so parathyroid hormone up-regulates the calcium transport.
Another divalent cation magnesium also gets transported across the apical membrane.
This is done without co-transport
and therefore, it is a single ion traveling through that channel,
and that channel is a TRPM6 channel.
These particular channels will allow the transport across the apical membrane.
Otherwise, things are very similar to the way calcium is reabsorbed.
Chloride does not travel through the cell to a great degree.
It travels through the paracellular pathway.
It primarily uses an electrochemical gradient.
Therefore, since chloride is a negatively charged anion,
it will want to travel towards the positive pole.
In this case the positive pole is on the basolateral side of the epithelial tissue.
A couple of transporters are available in the distal convoluted tubule,
this involve a chloride channel and then it exchanges with bicarb.
These are, however, only located in intercalated cells
which are not too numerous in the distal convoluted tubule.
So most of the time it uses paracellular pathway
rather than the intercalated cell reabsorption transcellularly.
Sodium and potassium are transported across the apical and basolateral membranes.
In this case, again, the basolateral membrane has the sodium and potassium ATPase.
They are ENaC channels which stands for epithelial sodium channel
that will allow for sodium to travel across the apical membrane.
And then there are ROMK channels that allow potassium to travel the opposite direction of sodium.
So in these principal cells, it is important to think about, when sodium is being reabsorbed,
potassium is being secreted.