(USMLE topics) Adrenergic receptors and drugs, agonists and antagonists, mechanisms of action.

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Adrenergic drugs are medications that stimulate (sympathomimetic) or inhibit (sympatholytic) adrenergic receptors. Adrenergic receptors mediate the action of noradrenaline, also known as norepinephrine; and adrenaline, also known as epinephrine. Adrenergic neurotransmission is responsible for the body’s sympathetic response the “fight or flight” state which dilates pupils, increases heart rate and respiratory rate, diverts blood flow to the muscles, inhibits activities that are not essential in emergency, and releases stored energy. Adrenergic receptors are also active in the central nervous system, in processes such as memory and alertness.
There are several types of adrenergic receptors, all of which are Gprotein coupled, but they differ in several aspects:
They couple with different Gproteins, leading to different downstream signalings, and hence different cellular responses.
They differ in sensitivity to different drugs.
While several receptors may coexist in the same tissue, there is usually one that predominates and is mainly responsible for the tissue’s adrenergic response. For example: alpha1 receptor predominates in peripheral vascular smooth muscle – its activation induces vasoconstriction; beta1 is prominent in the heart it increases heart rate and cardiac contractility when activated; beta2 activation results in bronchodilation in the lungs; and alpha2 reduces sympathetic outflow in the brainstem. Alpha2 can also act at the presynaptic neuron, where it inhibits neurotransmitter release, as a feedback mechanism.
Most adrenergic drugs act directly at the receptors, only a few act indirectly by promoting neurotransmitter release, or by preventing its degradation.
Nonspecific drugs are those that can bind to several receptors. Nonspecific agonists include epinephrine, norepinephrine and dopamine. Their relative activity via different receptors depends on the dose administered. For example, epinephrine has a greater affinity for beta receptors in small doses, but can bind to alpha receptors equally well at higher doses. At low levels, epinephrine preferentially binds to vascular beta2receptor and causes vasodilation. As the concentration of epinephrine increases, lower affinity alphareceptors begin to bind epinephrine, producing vasoconstriction. Because there are more alphareceptors than betareceptors in peripheral blood vessels, alphamediated vasoconstriction eventually overrides betamediated vasodilation. Thus, at higher pharmacologic doses, epinephrine induces vasoconstriction via alpha receptors; increases heart rate, cardiac contractility via beta1 receptor; and dilates bronchi via beta2 receptor. Epinephrine is the treatment of choice for cardiac arrest, anaphylaxis, and severe croup.
Specific drugs target only a certain type of receptor:
Alpha1 specific agonists induce smooth muscle contraction and are used as vasopressors for treatment of shock, hypotension; as nasal decongestants; or to dilate pupils.
Alpha1 antagonists, on the other hand, are used to treat hypertension, and to relax smooth muscle within the prostate for treatment of benign prostatic hyperplasia.
Alpha2 agonists act on alpha2 receptors in the brainstem to reduce sympathetic tone, and are used to treat hypertension. Stimulation of peripheral alpha2 receptors may initially cause vasoconstriction, but it is quickly overridden by the central effect.
Beta1 agonists increase heart rate and contractility, and are indicated for treatment of cardiogenic shock and heart failure.
Beta2 agonists relax smooth muscles. They are used to dilate bronchi, for treatment of asthma, obstructive pulmonary disease, and anaphylaxis. Some are used to relax uterine smooth muscle to delay preterm birth.
Beta antagonists, or beta blockers, are used for the treatment of hypertension, ischemic heart disease, obstructive cardiomyopathy, and arrhythmias.