Tubular Reabsorption Flashcards
What are the two potential routes for reabsorption from the tubular lumen?
Paracellular - diffusion between cells
Transcellualr - diffusion across the cell (from apical to basolateral membrane)
How is sodium reabsorbed in the cortical collecting duct?
Move down their concentration gradient into the cell from the lumen through sodium channels
Actively transported across the basolateral membrane via the Na-K ATPase
How is glucose reabsorbed in the proximal tubule?
Cotransported with sodium across the luminal membrane (secondary active transport)
Exits the basolateral membrane via facilitated diffusion
*Overall process ultimately depends on the primary active Na-K ATPase pump in the basolateral membrane
What is the transport maximum?
Limit to the amounts of material the active transport systems in the renal tubule can transport per unit of time
Due to saturation of carriers
What is the glucose titration curve?
Describes the mass flows of filtration, reabsorption, and excretion of glucose over the range of plasma glucose concentrations
What is splay?
The appearance of glucose in the urine before threshold is reached
What is threshold?
plasma concentration at which glucose first appears in the urine
Describe glucose clearance?
Normally, the clearance of glucose is 0
However, when the plasma glucose threshold is exceeded, plasma begins to be cleared and increases progressively with increasing plasma glucose concentrations
What is renal glycosuria?
Glucose in the urine as a result of a defective or missing transport mechanism
Tg = 0 or very low
What is Diabetes mellitus (as it relates to kidney function)?
Glucosuria due to lack of insulin
Tg = normal
How does pregnancy affect glucose reabsorption?
Glucosuria due to glomerular hyperfiltration (large increase in GFR)
Tg = normal
Generally, how do the kidneys handle amino acids?
Major role is conservation, zero clearance
Actively reabsorbed, negligible quantities excreted
Kidneys do not primarily regulate the plasma concentrations of amino acids
Describe the carriers used to reabsorb amino acids?
Lysine, Arginine, Ornithine, Cystine, Histidine carrier
Glutamic and Aspartic acid carrier
Everything else by one or more other carriers
What organic anions are actively reabsorbed?
Citrate
a-KG
B-hydroxybutyrate
Vit C
How do the kidneys handle proteins?
Very small amount is filtered (0.6%)
Rebasorbed in the proximal tubule by endocytosis
Kidney is the major site of catabolism of many plasma proteins
What factors cause the low filtration rate of proteins?
Steric hinderance
Viscous drag
Electrical Hinderance
How are peptides reabsorbed?
Small linear peptides are completely filterable
They are catabolized to amino acids within the proximal tubule and then the amino acids are actively reabsorbed
What are the renal effects of parathyroid hormone (PTH)?
Promotes calcitriol formation in the kidney
Increases Ca reabsorption
Increases phosphate excretion
What are the three components of total plasma calcium?
Protein bound (40%) - does not filter
Ionized and biologically active (50%) - filters
Complexed with anions (10%) - filters
What is the relationship between plasma H+ and ionized calcium?
As plasma H+ concentration, so does the concentration of ionized calcium
This reflects the availability of protein binding sites
In acidosis, more H+ gets buffered by protein, so bound calcium is displaced
What is the effect of the ECF calcium concentration on kidney reabsorption of calcium?
Low [Ca] in the ECF causes the release of PTH from the parathyroid gland, resulting in an increase in calcium reabsorption in the kidney
The reverse is also true
Describe calcium reabsorption in the proximal tubule
Paracellular pathway - driven by solvent drag
Transcellular pathway - uptake across the brush border via an ion channel and exits the basolateral membrane via Ca-ATPase
Describe calcium reabsorption in the thick ascending limb of Henle
Paracellular pathway - driven by the transepithelial electrochemical gradient for Ca
Describe calcium reabsorption in the distal tubule
Transcellular pathway - Ca enters the cell via ion channels
Inside the cell, Ca binds to calmodulin, and the complex diffuses across the cell to deliver Ca to the BL membrane
Ca is transported across the BL membrane by a 3Na - 1Ca antiporter
How does the kidney handle phosphate?
90% of plasma phosphate is freely filtered
90% of the filtered load is actively reabsorbed
Linear relationship
How is phosphate reabsorbed in the proximal tubule?
Apical transport - 3Na-1P symporter
Phosphate leaves the BL membrane via a P-anion antiporter
How does PTH regulate phosphate homeostasis?
Inhibits reabsorption of phosphate by the proximal tubule
Stimulates the endocytic removal of NPT2 (phosphate symporter) from the apical membrane
How does dietay phosphate intake regulate phosphate homeostasis?
Increased intake increases excretion and vice versa
Changes in intake alter the activity and number of NPT2 transporters
