Module 4 Section 5 (Tubular Secretion)

Question 1

Describe the secretion of hydrogen and potassium ions.

Answer 1

Hydrogen ions:
- H can be secreted in the proximal, distal, and collecting tubules.
- The extent depends on the acidity of the plasma. • When too much H is present in the plasma, more H undergoes tubular secretion.
• Conversely, when plasma H is low, tubular secretion of H decr.
- H secretion is a key role in the regulation of acid-base balance

Potassium ions:
- They undergo both tubular reabsorption and tubular secretion.
- K is freely filtered at the glomerulus, but then actively reabsorbed in the proximal tubule.
• The majority of the K is reabsorbed in an unregulated fashion.
- However, its secretion is variable and subject to regulation.
- If the plasma conc of K is elevated, K is actively secreted in the distal and collecting tubules, and vice versa.
• In this manner, the kidneys are actively involved in regulating plasma K levels.
- K ion secretion in the distal and collecting tubules is an active process dependent upon the Na+ K+ ATPase pump.
• The Na conc gradient was maintained by pumping Na out across the basolateral membrane in exchange for K. Since most of the K was reabsorbed in the proximal tubules, this makes a conc gradient such that K moves down its concentration gradient and passes through K channels in the luminal membrane into the tubular fluid.

Question 2

Describe why it is necessary to secrete organic anions and cations.

Answer 2

The proximal tubule contains 2 types of secretory carriers, one for organic anions and another for organic cations.

These organic ion secretory systems are important for 3 main reasons:

1) Increasing Excretion:
- Actively adding more organic ions to the tubular fluid, in addition to that which was filtered, can incr the amount of the organic ion excreted compared to glomerular filtration alone.
- This is particularly important for blood-borne chemical messengers such as norepinephrine, histamine, and prostaglandins in order to reduce or limit their biological activity.

2) Excrete Poorly Soluble Organic Ions:
- Many organic ions are not very soluble (they are hydrophobic) and circulate within the plasma bound to carrier proteins.
- Since large proteins aren’t filtered, only the fraction that isn’t carrier-bound can enter the glomerular filtrate.
- Tubular secretion further removes the small, unbound fraction of the organic ions, which causes even more “unloading” of the organic ion from its carrier molecule.
• In this manner, organic ions highly bound to carrier proteins can be excreted.

3) Removal of Foreign Compounds:
- In addition to endogenous organic ions, many foreign organic ions such as food additives, drugs, pesticides, environmental pollutants, and more, need to be removed from the body.
- The kidneys routinely remove these compounds, but there are no regulatory mechanisms in place to incr their removal if necessary.

Question 3

Where does tubular secretion occur?

Answer 3

Like, tubular reabsorption, it also occurs via transepithelial transport.

Question 4

What does tubular secretion involve?

Answer 4

It involves the movement of substances from the peritubular capillaries -> the tubule lumen, and thus is an additional pathway, other than glomerular filtration, for the removal of substances from the body.

Question 5

Various substances undergo tubular secretion. What are they?

Answer 5

The most important are hydrogen ions, potassium ions, and organic anions and cations, many of which are foreign to the body

Question 6

Using what you have just learned about K ion secretion into the lumen, answer the following question.

Since K secretion is linked with Na reabsorption by the Na+ K+ ATPase pump, why isn’t K secreted throughout the Na reabsorbing segments of the tubule, instead of taking place only in the distal parts of the nephron?

Answer 6

The answer lies in the location of the passive K channels.

In the distal and collecting tubules, the K channels through which K passively diffuses are concentrated in the luminal membrane, providing a route for K pumped into the cell to exit into the lumen, thus being secreted.

However, in the reabsorbing tubular segments, these channels are located primarily in the basolateral membrane. As a result, K pumped into the cell from the interstitial space simply moves back out into the interstitial space through these channels.
- This K recycling permits the ongoing operation of the Na+ K+ ATPase pump to accomplish reabsorption with no local net effect on K+.

Question 7

Several factors can alter the rate of K secretion. What are they?

Answer 7

Na+ and K+

• A rise in plasma K directly stimulates the release of aldosterone from the adrenal cortex.
• Aldosterone will incr Na reabsorption, which in turn means more K will be secreted.
• This relationship b/w Na reabsorption and K secretion means that stimuli such as a decr plasma Na, decr ECF volume, or a decr in arterial BP can inadvertently stimulate abnormal K secretion to the extent that K depletion occurs.

Effect of H+ secretion:
- Another important factor that regulates K secretion is the acid-base status of the body.
- The Na+ K+ ATPase pump on the basolateral membrane of the distal sections of the nephron can readily substitute H for K.
- B/c there is a limited number of pumps, when the interstitial conc of one of these ions incr, it can decr the rate at which the other is transported.
- When the plasma is too acidic, H -> into the interstitial space and are transported into the epithelial cell where they passively move into tubular fluid.
• The consequence is that less K is secreted, which can lead to an inappropriate high levels of K.

Check graph on slide 7

Question 8

Overall, the proximal tubule is responsible for most of the reabsorption, as it transfers much of the filtered water and needed solutes back into the blood in an unregulated fashion. The distal and collecting tubules determine the final amounts of H2O, Na+, K+, and H+ that will be eliminated by the body through process that are subject to control.

Using what you have learned regarding the reabsorption of solutes in the proximal tubule, distal tubule, and collecting duct, indicate if the following statements are true or false.

1) In the proximal tubule, roughly 67% of Na+ is actively reabsorbed.
2) No filtered glucose is reabsorbed within the proximal tubule.
3) All H2O is reabsorbed within the distal tubule and collecting duct.
4) There are variable amounts of PO43-reabsorbed within the proximal tubule.

Answer 8

1) In the proximal tubule, roughly 67% of Na+ is actively reabsorbed = True
2) No filtered glucose is reabsorbed within the proximal tubule = False
3) All H2O is reabsorbed within the distal tubule and collecting duct = False
4) There are variable amounts of PO43-reabsorbed within the proximal tubule = True