ß-adrenergic receptors

• Order of agonist potency

  • Isoproterenol > epinephrine > norepinephrine

• ß-receptors are divided into two major categories: ß₁ and ß₂.

  • ß₁ receptors myocardium.

  • ß₂ receptors smooth muscle and most other sites.

• A ß₃ receptor has been found that is strongly activated by norepinephrine compared to epinephrine and may explain "atypical" pharmacological properties of adipose tissue. The ß₃ -receptor is not blocked by propranolol, classified as a non selective beta-receptor blocker.The ß₃ -receptor is not blocked by propranolol, classified as a non selective beta-receptor blocker.

• Activation of ß₁, ß₂ and ß₃ receptors increases adenylyl cyclase activity (G_s mediated) resulting in a rise of intracellular cAMP.

  • Cardiac inotropic effects result from increases in Ca²⁺ concentration, due to:

    • phosphorylation of L-type Ca²⁺ channels

    • phosphorylation of sarcolemmal Ca²⁺ pumps

    • direct action G_s action on the L-type channel

  • Effects on the liver lead to activation of glycogen phosphorylation

• ß₂ receptor activation mediates relaxation of vascular smooth muscle

  • ß₂ receptor activation mediates relaxation of G.I. smooth muscle. alpha₂ adrenergic receptor activation acts presynaptically to reduce Ach release and promote G.I. smooth muscle relaxation. The alpha₂ receptor effect is the more important.

Category: Pharmacology Notes