A summary of small molecule movements across apical and basolateral membranes in different parts of the kidney.

1. Kidney  Apical Basolateral
Tubular fluid OUT
 IN Tubular wall cell  IN  OUT Function
segment Membrane Membrane
PCT (early) CO2 production via Na+ SGLT Glu CO2 + H2O ↔ H+ + 2 K+ Na+/K+ pump Na+ - reabs of 70% of NaCl
carbonic anhydrase H+ NHE Na+ HCO3- GLUT2 Glu - reabs of 100% of glucose
(CO2 has diffused in) Various HCO3- HCO3- movement of ions creates osmotic
transporters gradient for water absorption
PCT (late) CO2 production via SGLT-1 2 Na+ 2 K+ Na+/K+ pump 3 Na+ - now against greater gradient
carbonic anhydrase Glu GLUT 1 Glu - more Cl- abs. than in earlier part
KCC K+, Cl- - movement of ions creates osmotic
H+ NHE: Na+ gradient for water absorption
-
Cl channels Cl-
PCT Na+ 2 K+ Na+/K+ pump* 3 Na+ - reabs of K+
anions KCC K+ Cl- - largely paracellularly (passive)
- *K+ held particularly low between
proximal tubule cells, more
pumps
PCT NH3 diffuses out to H+ NHE3 Gln ↔ NH4+ + α-KG 2 K+ Na+/K+ pump 3 Na+ - Production of HCO3-
be converted to NH4+ α-KG ↔ 2 HCO3- NBC-1 Na+ - If necessary
NH4+ ↔ NH3 + H + 3HCO3- - NH3/ NH4+
Cl- BC HCO3-
PCT CO2 production via H+ ATP-pump CO2 + H2O ↔ H+ + 2 K+ Na+/K+ pump 3 Na+ - Reabsorption of 80 % of HCO3-
thick LOH carbonic anhydrase H+ NHE3 Na +
HCO3- NBC-1 Na+
(CO2 has diffused in) 3HCO3-
Cl- BC HCO3-
+ + -
thick LOH NKCC2 Na K 2Cl 2 K+ Na+/K+ pump 3 Na+ - reabs of K+
K+ K+ channel Cl- channel Cl- - trans- and paracellularly
K+ channel K+
thick LOH NH4+ from PCT NKCC2 Na+NH4+2Cl- NH4+ ↔ NH3 + H+ NH3 - NH3 diffuses to medullary interstit.
DCT CO2 production via H+ ATP-pump K+ Type A intercalated cells: Cl- BC HCO3- - Reabsorption of HCO3-ause
CD carbonic anhydrase H+ ATP-pump CO2 + H2O ↔ H+ + Cl- channel Cl- - Cl- recycles
HCO3-
(CO2 has diffused in)
DCT H+ ATP-pump K+ K+ channel K+ - reabs of K+
CD
DCT ENaC Na+ Principal cell: Aldosterone 2 K+ Na+/K+ pump 3 Na+ - K+ regulation
+
CD K SK initiates gene expression K+ channel K+ - K+ from blood to tubular fluid!!
of membrane proteins - Excretion if necessary
DCT TRPV5 Ca2+ Ca2+ binds calbindin to 2 K+ Na+/K+ pump 3 Na+ - Ca2+ regulation
CD TRPV6 Ca2+ cross the cells ATP-pump Ca2+ - Na+ cycles
K+ K+ channel 3 Na+ NCX Ca2+
CD H+ ATP-pump K +
CO2 + H2O ↔ H + +
- H+ pumped to tubular fluid to
H+ ATP-pump HCO3- retain NH3 as NH4+
(CO2 has diffused in)
CD NH3 trapped as NH4+ - NH3 diffuses from medulla to CD
Medulla/ NKCC2 Na+K+2Cl- 2 K+ Na+/K+ pump 3 Na+ - Make medulla hypertonic
thick channels K+ Cl- - K+ recycles
LOH K+ K+ channel (interstitium!)
- less blood vessels!!!
GLUT1: passive? Also takes up Vit C GLUT2: passive glucose transporter (GLUT4: insulin regulated glucose transporter, not shown here)
SGLT: sodium glucose transporter (2* active) NHE: sodium hydrogen exchanger (2*) KCC: potassium chlorine co-transporter (2*)
NKCC2: sodium potassium 2 chloride cotransporter ENaC: epithelial sodium channel SK: small Ca2+ activated K+ channel
NBC-1: Sodium-bicarbonate cotransporter NCX: Sodium calcium exchanger BC: Bicarbonate-chloride cotransporter (named so by me)
PCT/DCT: proximal / distal convoluted tubule LOH: loop of Henle CD: collecting duct