General and Systematic Veterinary Pharmacology

ACTIVE TRANSPORT

• Like facilitated diffusion, active transport also involves a specialized carrier molecule. However, in active transport a drug molecule is taken up by a specialized carrier molecule in the membrane and the cell expends energy to move the drug molecules across or to “reset” the carrier molecule for the next transport movement.
• Unlike diffusion in which the direction of net drug movement is determined by the concentration gradient, active transport can move drug molecules against the concentration gradient (from areas of lower concentration to areas of higher concentration).
• Glucose entry within cell is facilitated diffusion while passage across gastric mucosa and excretion by proximal renal tubular cells is active transport.
• Drugs related to normal metabolites are actively absorbed from the gut by aromatic amino acid transport processes.
• Drugs actively transported may potentially reach very high  concentrations within cells that they exert a toxic effect eg. aminoglycoside antibiotics.
• When the energy production of the cell is disrupted (such as toxicities), active transport of drug molecules across biological membranes may be prevented.
• Nonspecific active transport of drugs and their metabolites occurs in renal tubules and hepatic sinusoids separately for organic acids and organic bases. Certain drugs have been found to be actively transported into the brain and choroid plexus also.
• Examples for active transport are levodopa crossing the blood brain barrier, secretion some drugs into the bile and secretion of many organic acids and bases by renal tubular cells.