Gosmanova: Prox Tube Dysfunction, Kidney in HTN

Question 1

How is the proximal tubule well adapted for reabsorption?

Answer 1

• Enhanced surface area:
  1. Epithelial cells have microvilli (brush border)
  2. Basolateral surface thrown into folds
• Rich in mitochondria, which are needed to provide sufficient energy for reabsorption

Question 2

What are the structural and functional divisions of the PT?

Answer 2

• Structural
  1. 1st 2/3: proximal convoluted tubule (PCT)
  2. Last 1/3: proximal straight tubule (parse recta) -> cortical and medullary segments
• Functional (some differences in transporter proteins)
  1. S1: initial short segment of PCT
  2. S2: remaining PCT and cortical parse recta
  3. S3: medullary parse recta

Question 3

What are the functions of the proximal tubule?

Answer 3

• Reabsorption of filtered water, electrolytes, and organic compounds
• Secretion of organic compounds, including some drugs (S2, S3)
• Hormonal function: final pathway in the synthesis of active Vit D (1,25-dihydroxy-vitamin D or calcitrol)

Question 4

What is the difference between transcellular and paracellular absorption?

Answer 4

Question 5

What are the types of transport mechanisms in the PT? Provide some examples.

Answer 5

• Primary active transport: movement of a substance against electrochemical gradient, requires energy -> example: Na/K ATPase
• Secondary active transport: does not use energy directly, but depends on primary active transport
  1. Antiport (exchanger): Na/H
  2. Symport (co-transporter): Na/Pi
• Passive transport: simple diffusion requiring electrochemical gradient (ex: K+ paracellular), facilitated (carrier-mediated) diffusion, diffusion via membrane channel (or pore; ex: aquaporins)
• Pinocytosis: form of endocytosis (ex: insulin uptake via PT cells)

Question 6

How is bicarbonate reabsorbed? What about Na? How are these related?

Answer 6

Question 7

How is PT reabsorption regulated?

Answer 7

• Glomerulotubular balance: intrinsic tubular ability to INC reabsorption rate in response to INC tubular load/inflow
• Arterial pressure (pressure natriuresis): INC in peritubular capillary hydrostatic pressure DEC net reabsorption of Na and water
• Hormonal regulation:
  1. Angtiotensin II: regulates NaCl and water reabsorption and H+ excretion
  2. Parathyroid hormone and FGF23 -> regulate Pi excretion