13 Metoprolol

Figure 7 Examples of class 2 antiarrhythmics with the prototypical b-adrenoceptor blocker propranolol, and two other blockers with some selectivity for the ^-adrenoceptor (cardiac form).

An alternative description for partial agonism is intrinsic sympathetic activity (ISA) although the former is the preferred pharmacological term. This is the best term for an antagonist that actually stimulates b-adrenoceptor when they are not being stimulated by their neurotransmitter norepinephrine, or by epinephrine from the adrenal medulla, but block when such stimulation occurs. Thus b-blockers that are partial agonists only provide b-adrenoceptor blockade when activity of the sympathetic system on the heart is high.

b-Blockers also vary in their pharmacokinetic characteristics in terms of metabolism, half-life, and excretion. Since the catecholamines, norepinephrine from sympathetic nerves and epinephrine from the adrenal medulla, are well-identified arrhythmogens, blocking their actions provides prophylactic protection against catecholamine-induced arrhythmias. Indirect antiarrhythmic effects of b-adrenoceptor blockers in acute myocardial infarction may also involve elevation of serum potassium concentrations by b-adrenoceptor blockade since moderate elevations of serum potassium are antiarrhythmic.

Effectiveness: b-Adrenoceptor blockers are also used routinely in postmyocardial infarction patients who can tolerate them. They reduce mortality by about 15%, although such protection may involve various mechanisms including antiarrhythmic actions, prevention of reinfarction, and beneficial effects in heart failure. They are also used to treat atrial and ventricular arrhythmias associated with sympathetic nervous system activation, or excess thyroid hormone secretion (more rarely), as well as slowing ventricular rate in atrial fibrillation. Toxicity: The adverse effects of b-adrenoceptor blockers are mostly due to b-adrenoceptor blockade. These include asthma due to blockade of the relaxant actions of catecholamines on bronchial smooth muscle, intermittent claudication (muscle pain associated with exercise), excessive bradycardia, and AV node blockade.

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