Adrenergic refers to physiological responses related to adrenaline and noradrenaline, two neurohormones that are also known commonly as epinephrine and norepinephrine, respectively. Adrenergic drugs either stimulate activity in tissues that respond to epinephrine and norepinephrine (adrenergic agonists) or inhibit epinephrine and norepinephrine influence (adrenergic antagonists). Epinephrine and norepinephrine are released from the adrenal gland and reach the heart, kidneys, and various other tissues and organs via the systemic circulation (see Chapter 18).
Norepinephrine is also the neurotransmitter typically found at the junction between sympathetic postganglionic neurons and peripheral tissues. Consequently, medical practitioners administer most of the adrenergic agonists to augment sympathetic responses, while they administer adrenergic antagonists to attenuate sympathetic-induced activity. In fact, adrenergic agonists are sometimes referred to as sympathomimetic, and antagonists are referred to as sympatholytic because of their ability to increase and decrease sympathetic activity, respectively.
This chapter focuses specifically on the adrenergic drugs that primarily influence activity in the sympathetic nervous system through their effect on adrenergic synapses. As a result, the drugs are categorized according to a common mode of action rather than according to common clinical applications. Most of the drugs introduced in this chapter will also appear elsewhere in the text when they are classified according to their use in treating specific problems. For instance, the beta-selective adrenergic antagonists, or beta blockers, are collectively introduced in the section “Beta Antagonists.” Individual beta blockers are also discussed in subsequent chapters with regard to their use in specific problems such as hypertension (see Chapter 21), angina pectoris (see Chapter 22), cardiac arrhythmias (see Chapter 23), and congestive heart failure (see Chapter 24).
Adrenergic drugs are used to treat a variety of disorders, ranging from severe cardiovascular and respiratory problems to symptoms of the common cold. Therefore, many of the patients you will see in physical therapy and occupational therapy may be taking adrenergic agonists or antagonists. In this chapter, the discussion is limited to the basic pharmacodynamic mechanisms, clinical applications, and adverse effects of these drugs. The relevance of specific adrenergic drugs to physical rehabilitation is addressed in more detail in subsequent chapters, where their use is categorized according to certain disorders (hypertension, angina, asthma, etc.).
Many adrenergic agonists and antagonists exert their effects by binding directly to the appropriate postsynaptic receptor. Because a great deal of the specificity (or lack of specificity) of these drugs depends on the drug-receptor interaction, the first section is a brief review of the adrenergic receptor classes and subclasses.
ADRENERGIC RECEPTOR SUBCLASSIFICATIONS
Adrenergic receptors can be divided into two primary categories: alpha and beta receptors (see Chapter 18). Each category can then be subdivided, so that five receptor subtypes are commonly identified: alpha-1, alpha-2, beta-1, beta-2, and beta-3.1 Alpha-1 receptors have been further categorized as alpha-1a, -1b, and -1d receptors, and alpha-2 receptors ...