β-Adrenergic receptor antagonists (β-blockers) are medications used in the treatment of various cardiovascular, neurologic, endocrine, ophthalmologic, and psychiatric disorders. Among all the exposures to cardiovascular agents, β-blocker exposures were the leading cause of poison center calls and ranked among the top three in this class as a cause of severe toxicity and mortality.1
The β-adrenergic receptors are membrane glycoproteins present as three subtypes in various tissues (Table 194-1). These receptors play a critical role in cardiovascular physiology by modulating cardiac activity and vascular tone.
TABLE 194-1Location and Activity of β-Adrenergic Receptors ||Download (.pdf) TABLE 194-1 Location and Activity of β-Adrenergic Receptors
Stimulates renin release
Stimulates aqueous humor production
Inhibits renin release
Inhibits aqueous humor production
Bronchial smooth muscle
Visceral smooth muscle
Increases force of contraction
Stimulates glycogenolysis and gluconeogenesis
Inhibits glycogenolysis and gluconeogenesis
During times of stress (i.e., catecholamine release), β-adrenergic receptor stimulation increases myocardial and vascular smooth muscle cell activity through a sequence of intracellular events (Figure 194-1).2,3
Cardiac myocyte β1-receptor and calcium signaling. Following myocyte depolarization, extracellular calcium (Ca2+) enters the cell via the L-type or voltage-gated calcium channel (L-VDCC) and binds to the ryanodine receptor (RyR) in the sarcoplasmic reticulum, causing an efflux of sequestered Ca2+ out of the sarcoplasmic reticulum into the cytosol. Free Ca2+ binds to troponin that allows the myosin and actin interaction, resulting in contraction of the cardiac myocyte. Binding of a β-agonist to the β1-adrenergic receptor (B1) on the cell surface activates the Gs protein. The Gs protein then activates adenylate cyclase (AC), which converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP). The increased cAMP activates protein kinase A (PKA). Activated PKA serves as further stimulus for the L-VDCC opening. Glucagon independently activates adenylate cyclase. cAMP is metabolized by phosphodiesterase (PDE) into inactive adenosine 5'-monophosphate (5'AMP).
The β-receptor is coupled to a stimulatory Gs protein. This Gs protein stimulates adenylate cyclase, which in turn catalyzes the formation of cyclic adenosine monophosphate, the so-called intracellular second messenger. Increased cyclic adenosine monophosphate ultimately phosphorylates the L-type calcium channel, which leads to channel opening and calcium entry into the cell. Extracellular calcium is then coupled to the ryanodine receptor to carry the calcium current to the sarcoplasmic reticulum, which then releases its stored calcium. This process is termed calcium-induced calcium release. ...