Mechanism Tubular Re-absorption and Secretion (Monday 6/11/17)

Question 1

Describe Proximal Tubule Transport

Answer 1

The proximal tubule is made up of epithelium, seperating the urine/primary filtrate and the blood.

• The proximal tubule cells have tight junctions to form a barrier
• The proximal tubule cells have microvilli on the apical side (facing the filtrate) and a basolateral membrane facing the blood.
• So re-absorption has to happen through transport proteins on either of the two membranes, or between the cells through the tight junctions, called para-cellular transport.

Question 2

What does fractional efflux measure?

Answer 2

Fractional efflux measures how much of a substance is left in the proximal tubule. The rest got re-absorbed. So for something like inulin which doesn’t get reabsorbed it will have a 100% efflux: all left in proximal tubule.

Question 3

Early proximal tubule (the first half of the tubule).

Answer 3

• Sodium-solute co-transporters are used to reabsorb solutes the body doesn’t want to lose (glucose, phosphate, a.a). Sodium- potassium ATPase causes low sodium concentration in cells, so sodium diffuses in from filtrate.
• If damage proximal tubule, this mechanism doesn’t work, so solutes appear in the urine (eg both glucose and phosphate and a.a appearing in urine suggests proximal tubule damage AKA FANCONI syndrome).

Question 4

What happens in the late proximal tubule?

Answer 4

• Chloride can be re-absorbed down its electrochemical gradient between cells. (More cl minus in lumen than blood- initially).
• When all calcium have been moved to blood, there is a voltage setup. Just sodiums left in lumen, which repel themselves, and move to blood
• This is the mechanism for sodium and pottasium absorption in the late proximal tubule. It is passive, and para-cellular.
• In late proximal tubule water is also re-absorbed. Solutes get trapped between cells/ the laterak interspace. This increases the osmalality in cell, causing water to move through cells or aqua-porins.
• Patients with diabetes malietus are always thirsty. Have glucose remaining in the tubule, so the osmotic gradient for water to be re-absorbed is decreased. Less water re-absorbed, increasing urine volume, patient thirsty.
• The order of re-absorption is importants, as passive sodium absorption happens last, so there is a chloride gradient already established, so sodum can be moved passively without using energy.

Question 5

Why is the key to understanding hypertension how sodium is handled?

Answer 5

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Question 6

Sodium/solute transporters in the early proximal tubule: Sodium/bicarbonate transporter SEQUENCE

Answer 6

The re-absorption of sodium and bicarbonate is important to balance sodium and bicarbonate levels.

How it happens:

1. Bicarbonate (HCO3) interacts with secreted proton and forms water and carbon dioxide. Catalysed by carbonic anyhdrase.
2. Carbon dioxide diffuses into cell and reacts with water, forming proton and bicarbonate.
3. The bicarbonate enters blood via sodium-bicarbonate transporter.
4. Sodium-proton pump used to produce proton in cell.

Question 7

Why does the cl- concentration rise in late proximal tubule compared to the proximal tubule?

Answer 7

The reason why the chloride concentration seems to rise in the late proximal tubule compared to the early proximal tubule is that no chloride has been reabsorbed, but water has, so there is the same amount of chloride in less volume, so concentration of chloride rises.

Question 8

How is NaCl re-absorbed in the early proximal tubule?

Answer 8

For Nacl reabsorption we have co-transporters. This transporter is also used to get ride of metabolic waste products.

Na+ enters cell using sodium/proton antiporter. Then is reabsorbed into blood using sodium/potassium anti-porter.

Cl- enters cell using chloride/ formate oxalate lactate ion anti-porter. Chloride moves into blood using chloride/ formate-oxalate-lactate ion anti-transporter.

Formate oxalate lactate ion enters the cell from the blood using formate oxalate lactate/ tricarboxylate antiporter. Or it can use a formate oxalate lactate/ chloride anti-porter to enter a cell.

Question 9

Difference between symporters, co-transporters and anti-porters?

Answer 9

Uniporters are involved in facilitated diffusion and work by binding to one molecule of substrate at a time to move it along its concentration gradient.

Symporters and antiporters are involved in active transport.

Antiporters transport molecules in opposite directions, while symporters transport molecules in the same direction.