Pharmacology
Summary
In the sympathetic nervous system, the ultimate effects of sympathetic stimulation are mediated by the release of norepinephrine from nerve terminals. This activates adrenergic receptors on post-synaptic sites. In response to stimuli such as stress, the adrenal medulla releases epinephrine, which is transported in the blood to target tissues. Essentially, epinephrine acts as a hormone, whereas norepinephrine acts as a neurotransmitter. The drugs that mimic the actions of these two are called sympathomimetic drugs.
Sympathomimetics include phenylephrine, epinephrine, norepinephrine, dobutamine, terbutaline, and albuterol, and can grouped based on the spectrum of receptors they activate. Key receptors include alpha-1, alpha-2, beta-1, and beta-2, each leading to specific effects on organ systems based on the coupled G protein (Gq, Gi, Gs, and Gs respectively). The end-organ effects range from regulation of blood pressure through vasoconstriction or vasodilatation, to regulating smooth muscle cells. Sympathomimetic drugs are essential in situations like shock treatment and in cases of heart failure, asthma and COPD, where potent activation of alpha and beta receptors can be life-saving.
Lesson Outline
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FAQs
Alpha-1 activation triggers several physiological effects, such as: (1) Increasing peripheral arterial resistance through vasoconstriction at small arteries, arterioles, and precapillary sphincters, (2) Boosting mean arterial pressure (MAP), (3) Enhancing venous return via venoconstriction, (4) Inducing mydriasis (pupillary dilation) at the pupillary dilator muscle, and (5) Causing urethral sphincter and prostatic smooth muscle contraction, leading to urinary retention.
Beta-1 receptors are predominantly found on cardiac myocytes, including the SA and AV nodes. Activation of these receptors triggers an increase in intracellular calcium via increased cyclic AMP, leading to a rise in heart rate, a boost in cardiac contractility, and a subsequent enhancement of the cardiac output.
Phenylephrine, an alpha-1 agonist, has multiple uses. It is often used to treat nasal congestion through alpha-1 mediated vasoconstriction. It induces mydriasis by activating alpha-1 receptors at the pupillary dilator muscle, and increases mean arterial pressure by increasing systemic vascular resistance via alpha-1 activation. It also increases systolic pressure through alpha-1 arteriolar constriction and diastolic pressure via alpha-1 venous constriction. However, it can cause reflex bradycardia in response to alpha-1 elevation in MAP.
Dobutamine is primarily a beta-1 agonist, and because of its beta-1 activations, it can prove beneficial in treating cardiogenic shock. Dobutamine increases heart rate, contractility, and cardiac output through its beta-1 activity
Specific beta-2 agonists, such as terbutaline and ritodrine, are known to relax the uterine smooth muscle, which may help to prevent premature labor.
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