Epithelial Transport Systems

Epithelium

sheet of cells connected by tight junctions (O-rings that seal off spaces between cells)

separate mucosal from serosal spaces

transport can be trancellular (through cell—usually active) or paracellular (between cells—diffusion)

leaky vs. tight epithelium—determined by conductance, mostly through paracellular path

leaky: small intesine, renal proximal tubule, acini of exocrine

tight: collecting duct of kidney, urinary bladder, excretory ducts of sweat glands

basolateral and apical sides of epithelial sheet differ

Na⁺/K⁺ ATPase is on the basolateral surface, allowing for secondary active transport across apical
apical channels regulated by:

flow of ions can create transepithelial voltage potential

missing in CF patients
determined by charge transport across apical membrane (basolateral can easily transmit charge)

Four Examples of Secondary Active Transport – Recharged by the Basolateral Na⁺/K⁺ Pump

(1) nutrient absorption by Na⁺/glucose symporter (electrogenic – net Na⁺ influx)
(2) NaCl secretion in exocrine glands, gut, respiratory – Cl^- channels (electrogenic due to Cl^- efflux)
(3) NaCl absorption in gall bladder, small int., proximal tubule – Na⁺/H⁺ antiport and Cl^-/H⁺ symport (electoneutral)
(4) NaCl absorption in large intestine – steroid sensitive Na⁺ channels – mineralcorticoid opens, amilaride closes (electrogenic)

Four Transporters in the Stomach – Effect is Secretion of HCl Through the Epithelium (to pH=2.0)

(1) Cl^-/HCO3^- antiporter on basolateral (net transport of HCl into cell, since HCO₃^- export = H⁺ import)
(2) Cl^- channel on apical membrane (non-CFTR, secretes Cl^-)
(3) K⁺/H⁺ antiporter ATPase (unique to this cell type, secretes H⁺)
(4) K⁺ channel (removes the K⁺ imported by the K⁺/H⁺ antiporter)