VPT 311: General and Systemic Veterinary Pharmacology (2+1)

METHYLXANTHINES

• Methylxanthines include the alkaloids caffeine, theobromine and theophylline drugs, which have been used as CNS stimulants, diuretics, heart stimulants and smooth-muscle relaxants.
• It is the smooth muscle relaxant property, which is responsible for the bronchodilator effect of the xanthines.
• The smooth-muscle relaxant action is attributed to inhibition of phosphodiesterase, the intracellular enzyme that inactivates cAMP.
• The CNS stimulant action of these drugs also contributes to improved respiratory function, as also the improvement in circulatory and renal function in cardiac asthma of dogs.
• Derivatives of theophylline with superior pharmacokinetic properties are now preferred.
• Aminophylline, diprophylline and etamiphylline given by slow intravenous, intramuscular or subcutaneous injection are effective in reducing airway resistance in acute bronchoconstriction.
• Aminophylline is more soluble than theophylline and is better absorbed.