Veterinary Neuropharmacology

α1 - adrenergic receptor agonists

α₁ - ADRENERGIC RECEPTOR AGONIST

Acts predominantly on α₁ adrenergic receptors.

• Phenylephrine
  • It is a direct acting non-catecholamine with powerful α₁ adrenergic agonistic activity.
  • It has negligible β agonistic and CNS actions.
  • Chemically phenylephrine differs from adrenaline in lacking 4-OH group on the benzene ring.
  • It is less potent than nor adrenaline on α₁, but has long duration of action.
• Pharmacological effects
  • It produces peripheral vasoconstriction with resulting increase in BP and small decrease in COP.
  • Trough phenylephrine has no direct effect on myocardium, it produces reflex bradycardia.
  • It decreases cutaneous, renal splanchnic blood flow, but increases the coronary blood flow.
  • It causes mydriasis and reduces intraocular pressure.
• Pharmacokinetics
  • Given orally is rapidly metabolized in the GI tract and does not produce any response. Hence, given parenterally by IV or IM routes.
  • It is mainly metabolized in the liver and partly its action is terminated by uptake into tissue.
• Methoxamine
  • It is a directly acting sympathomimmetic with relatively selective α receptor action. But unlike phenylephrine, methoxmine slightly higher doses has some β receptor blocking action. This β- receptor blocking effect makes it less prone to induce cardiac arrhythmias.
  • Methoxamine is primarily used in the treatment of hypotensive states. In contrast, to other adrenergic drugs, methoxamine can be used during general anesthesia with halogenated hydrocarbons.
  • Some selective α-adrenergic receptor agonists: Oxymetazoline, Xylometazoline, Naptazoline.