Absorption Across the Epithelium

Study Notes

Victorian transport describes substance movement across entire epithelial sheets, not just cell membranes.
This occurs in various tissues including intestines, renal tubules, exocrine glands, gallbladder, choroid plexus, and placenta.
Transport involves active transport across one side of a cell, followed by either simple or facilitated diffusion across the opposite side.
This unidirectional transport is achieved by different transporter families on opposite cell faces.
Solute carriers (SLCs) facilitate influx in secondary active or facilitated transport.
ATP-binding cassette (ABC) transporters facilitate efflux in active transport.
Examples include absorption from the intestine to blood, from blood to bile, across kidney tubules, and from blood to other body cells.

Physiological factors influence absorption rate:
- Temperature: higher temperatures increase diffusion rates.
- Surface area: larger surface areas allow faster absorption.
- Diffusion distance: shorter distances facilitate quicker absorption.
Pharmacological considerations:
- Stomach acidity affects acid-labile drugs, potentially impacting absorption. This may necessitate enteric-coated tablets.
- Gastrointestinal motility affects drug absorption. Drugs might be taken on an empty stomach to optimize absorption.
- Presence of efflux transporters, such as P-glycoprotein, can reduce drug absorption. Dosage adjustments might be needed with concurrent inhibitors of P-glycoprotein.
Factors related to the substance itself:
- Size and molecular weight: smaller substances absorb more quickly.
- Ionization: non-ionized (lipophilic) drugs absorb more easily across cell membranes than ionized (hydrophilic) drugs. Ionization depends on the drug's pKa and the surrounding pH.
Drug interactions: Cardiac glycosides like digoxin can have interactions with other drugs impacting P-glycoprotein-mediated transport, affecting absorption and elimination.
Examples include macrolide antibiotics (e.g., erythromycin) and anti-epileptic drugs (e.g., carbamazepine), which can affect Digoxin absorption in terms of increasing absorption or eliminating it in urine.