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4 - Tubular Transport I


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What proteins are thought to be involved with paracellular transport?
Compare filtered load and excreted load in reabsorption and in secretion.
*reabsorption: filtered load > excreted load
*secretion: excreted load > filtered load
Why is movement of water from the interstitium to the peritubular capillaries favored?
The oncotic pressure in the capillaries exceeds hydrostatic pressure, thus movement into the capillary is favored. Also, high flow aids in uptake.
What tubular segment does not show transcellular reabsorption of NaCl?
The descending limb of the loop of Henle.
Is movement of Na+ from the lumen across the apical membrane with or against the electrochemical gradient? What is the benefit of this?
It is a passive process that follows the electrochemical gradient, and therefore it can be coupled to drive other secondary transport mechanisms (Glc, H+, Cl-, etc.).
Is movement of Na+ across the basolateral membrane into the interstitium a passive or active process?
It is active and requires a carrier, usually Na+/K+ ATPase.
Is paracellular movement of Na+ a passive or active process?
It is usually passive.
What two segments of the nephron feature an especially leaky epithelia? Compare the paracellular movement in these two segments.
The proximal convoluted tubule (reabsorption) and descending loop of Henle (secretion of Na+).
Which nephron segments feature a tight epithelia? Describe the movement of water through these segments.
Ascending limb of the loop of Henle, distal tubule, and the collecting ducts. Water movement is transcellular and through aquaporins other than AQP-1.
Describe AQP-1: where is it found? How is it controlled?
AQP-1 is found in the PCT and vasa is constitutively expressed and not regulated by ADH.
Describe the transport of water across the distal tubule and collecting ducts.
In these segments water moves transcellular through AQP-2 which is regulated by ADH.
Where is AQP-3 found? How is it regulated?
AQP-3 is found on the basolateral membrane of distal tubule and collecting duct epithelia. It is non-regulated.
Describe the osmolarity of the reabsorbate and the exiting filtrate of the proximal tubule.
Because the proximal tubule reabsorbs large amounts of both water and solute, both the reabsorbate and the exiting fluid is isosmolar to plasma.
Does the loop of Henle reabsorb more water or more solute? Describe the osmolarity of the reabsorbate and the exiting filtrate of this segment.
The loop of Henle reabsorbs more solute than water. As a result, the reabsorbate is hyperosmolar to plasma, whereas the exiting filtrate is hyposmolar.
Do the distal tubule and collecting ducts reabsorb more water or more solute? What determines this?
These segments reabsorb more water than solute, but only when ADH is present. In its absence the converse is true.
Describe the osmolarity of the reabsorbate and the exiting filtrate of the distal tubule and collecting ducts when ADH is present.
Because more water than solute is reabsorbed in this situation, the reabsorbate is hyposmolar to plasma and exiting filtrate is hyperosmolar to plasma.
Name three solutes that are co-transported with the reabsorption of Na+ in the proximal convoluted tubule.
Reabsorption of Cl-, glucose, and amino acids are coupled to the passive uptake of Na+.
What is the maximum activity for the Na+/glucose transporter in the proximal tubule?
How is Na+ moved across the basolateral membrane in the PCT? How is it then removed from the interstitium?
Transport across the basolateral membrane is an active process facilitated by Na+/K+ ATPase. From the interstitium, Na+ is carried with water into the peritubular capillaries. Also there is some paracellular leakage back into the lumen.
In the PCT there are open channels for water, but what powers its movement?
Water moves down small, local, transient osmotic gradients.
What is osmotic diuresis? How does it occur in diabetes?
Failure to reabsorb an osmotic particle causes osmotic diuresis by creating an osmotic pressure that draws fluid into the lumen. In DM, glucose acts as this particle.
Describe the movement of H+, bicarbonate, and CO2 across the apical membrane of the proximal tubule.
H+ is secreted into the lumen via a Na+/H+ antiporter. It combines with bicarbonate (which does not pass the membrane on its own) to form CO2 and water. CO2 diffuses rapidly across the apical membrane into the cell.
How is bicarbonate moved across the basolateral membrane?
Movement is via a Cl-/bicarbonate ATPase which moves HCO3- out and Cl- in.
What percentage of filtered urea is reabsorbed by the proximal tubule? How is this regulated?
The proximal tubule reabsorbs 50% of filtered urea via unregulated passive diffusion.

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