55-140 Lecture Notes - Pinocytosis, Paracellular Transport, Cotransporter

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Published on 1 Feb 2013
Course
Professor
Renal handling of Na+, K+, & Cl- and Glucose—Guggino
I. Intro
a. 180 L/day enter glomerulus 120 L/day reabsorbed in Proximal Tubule 35 L
reabsorbed in Loop of henle of remaining 18L, concentrated to 1-2L of urine
in Distal tubule & collecting duct
II. Epithelia
a. 180 L/day enter glomerulus 120 L/day reabsorbed in Proximal Tubule 35 L
reabsorbed in Loop of henle of remaining 18L, concentrated to 1-2L of urine
in Distal tubule & collecting duct
b. Structural Polarity
i. Epithelia is made of polar cells => different components on apical and
basolateral surfaces!
ii. Occluding zone (= “tight jxn”) between cells fxns as a barrier to prevent
passage of plasma membrane proteins
iii. Specialized extracellular compartments (microvilli, etc) concentrate
solutes
iv. Key components
1. substances move across cell in 1 main direction
2. transport proteins different on apical & basal membranes
3. must sort transporters to proper membrane
4. must maintain transporters in proper place
c. Functional Polarity (absorption—to blood/body vs. secretion—out of bl./body)
i. Ions enter via channels or carrier-mediated transport systems
ii. Na-K pump—creates Na gradient which supplies energy for transport
1. usually located on blood (basal) side
2. Fxn: 1) exit step for Na+, 2) generator of ion gradient essential for
solute entry mechanism
iii. Eg: Absorption of Na : Na channel is on apical membrane and Cl diffuses
passively through a paracellular transporter (between cells)
iv. Eg: Secretion of Na : Na& Cl are cotransported into cell from basal
membrane => Cl channel to apical surface, Na diffuses out through
paracellular channel
III. Water Transport
a. Water Flow
i.
)(
π
= PLJ pw
ii. Where Lp= hydraulic water permeability
iii. ΔP = hydrostatic pressure distance ~= 0 (most epithelial tissues)
iv. Δπ= osmotic pressure difference
b. Solute Coupled H2O movement
i. Apical solute entry raises osmolality
ii. Basolateral Na-K pumps move solutes to restricted intercellular space
iii. This small gradient => H2O follows (b/c Lp is very high)
c. Highly Water Permeable Epithelium
i. See enlarged surface area (for absorption) in the form of microvilli (aka
brush border) & high permeability to H2O (large Lp)
1
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