Antagonists with partial agonist activity
Acebutolol and pindolol [PIN-doe-lole] are not pure antagonists; instead, they have the ability to weakly stimulate both β1 and β2 receptors
Figure : Comparison of agonists, antagonists, and partial agonists of β adrenoceptors.
and are said to have intrinsic sympathomimetic activity (ISA). These partial agonists stimulate the β receptor to which they are bound, yet they inhibit stimulation by the more potent endogenous catecholamines, epinephrine and norepinephrine. The result of these opposing actions is a much diminished effect on cardiac rate and cardiac output compared to that of β-blockers without ISA.
b. Decreased metabolic effects: Blockers with ISA minimize the disturbances of lipid and carbohydrate metabolism that are seen with other β-blockers.
2. Therapeutic use in hypertension:
β-Blockers with ISA are effective in hypertensive patients with moderate bradycardia, because a further decrease in heart rate is less pronounced with these drugs. Carbohydrate metabolism is less affected with acebutolol and pindolol than it is with propranolol, making them valuable in the treatment of diabetics. [Note: The b blockers with ISA are not used as antiarrhythmic agents due to their partial agonist effect.]
Figure : summarizes some of the indications for βblockers.