Autonomic Nervous System Pharmacology Review

Pharmacological Profile of Bethanechol as a Muscarinic Agonist

Bethanechol is a specialized drug that functions as a muscarinic agonist, primarily targeting the bladder. Its main clinical application is for the treatment of urinary retention, specifically in post-operative patients who have not voided for an extended period, such as over $10$ hours. The physiological action of Bethanechol involves stimulating the bladder to contract while simultaneously increasing salivation and gastrointestinal (GI) mobility. A useful memory trick for Bethanechol is "Bathroom = WET," highlighting its effects on promoting excretion and body secretions.

Atropine: Clinical Use in Bradycardia and Muscarinic Antagonism

Atropine is a muscarinic antagonist that acts across multiple organs. It is primarily used to treat bradycardia, especially in symptomatic patients who may present with a heart rate ($\text{HR}$) as low as $35$ and clinical symptoms such as dizziness. When administered, Atropine induces a faster heart rate ($\text{HR} \uparrow$), leads to pupillary dilation, and causes the mouth to become dry. These effects are summarized in the mnemonic: "Faster Heart = Dry side effects."

Epinephrine and the Management of Anaphylactic Emergencies

Epinephrine is an agonist that acts upon a broad spectrum of receptors, specifically $\alpha_1$, $\beta_1$, and $\beta_2$. It affects the heart, lungs, and blood vessels. Its most critical use is in the treatment of anaphylaxis, a condition where a patient experiences acute respiratory distress and cannot breathe. Epinephrine works to open the lungs, elevate blood pressure ($\text{BP} \uparrow$), and increase the heart rate ($\text{HR} \uparrow$). Given its vital role in critical care, the memory aid is "Epi = Emergency."

Norepinephrine and Endogenous Catecholamines in Septic Shock

Norepinephrine is classified as an agonist with a receptor preference where $\alpha_1$ activity exceeds $\beta_1$ ($\alpha_1 > \beta_1$). It is one of the "three catecholamines" that the body naturally produces, along with Epinephrine and Dopamine. Its primary action is concentrated on the blood vessels, causing powerful vasoconstriction to significantly increase blood pressure. It is used clinically for septic shock, particularly when a patient has a dangerously low blood pressure, such as $60/30$. The mnemonic used to remember its function is "NO pressure? + Norepi \rightarrow BP goes up."

Dose-Dependent Action of Dopamine in Cardiogenic Shock

Dopamine is an agonist that exhibits unique dose-dependent effects across the kidneys, heart, and blood vessels. At low doses, it primarily affects the kidneys; at medium doses, it focuses on the heart; and at high doses, it acts on the blood vessels. This medication is the drug of choice for cardiogenic shock, which occurs when a patient has a weak heart. The pharmacological goal is to tailor the dose to achieve the necessary cardiovascular support.

Phenylephrine for Nasal Congestion and Hypotension

Phenylephrine acts as a selective $\alpha_1$ agonist targeting the blood vessels. It is utilized to treat nasal congestion and hypotension. In patients who cannot breathe due to a stuffed nose, Phenylephrine works by shrinking swollen nasal vessels. It also provides a systemic increase in blood pressure ($\text{BP} \uparrow$). The simplified mnemonic for this drug is "Phenyl-Free = free your nose."

Albuterol: Targeted Beta-2 Agonism for Respiratory Relief

Albuterol is a $\beta_2$ agonist that specifically targets the lungs to induce bronchodilation. Its primary use is for managing asthma and Chronic Obstructive Pulmonary Disease (COPD), particularly during acute asthma attacks characterized by wheezing. Albuterol works to open the lungs and facilitate easier breathing. Students utilize the memory trick "Albuterol Air = open lungs" to recall its effect.

Prazosin and Alpha-1 Antagonism for Hypertension

Prazosin is an $\alpha_1$ antagonist that acts on the blood vessels to treat high blood pressure. By antagonizing the $\alpha_1$ receptor, it causes blood vessels to relax, which directly results in a reduction of blood pressure ($\text{BP} \downarrow$). This clinical outcome is captured in the mnemonic "Prazosin pressure reduced."

Doxazosin and Tamsulosin for Prostate and Pressure Management

Doxazosin is an $\alpha_1$ antagonist that serves "double duty" by targeting both the blood vessels and the prostate (noted as affecting both pressure and prostatic tissue). It is indicated for patients suffering from both hypertension (HTN) and Benign Prostatic Hyperplasia (BPH), as it lowers blood pressure while relaxing the prostate. Tamsulosin is a more specific $\alpha_{1A}$ antagonist dedicated to prostate management in BPH. It works by relaxing the prostate and the bladder neck, making it effective for patients with an enlarged prostate and a weak urine stream.

Selective and Non-Selective Beta Blockade: Metoprolol and Propranolol

Metoprolol is a selective $\beta_1$ antagonist, often referred to as a cardiac-specific "heart ($\heartsuit$) drug," used to treat HTN, angina, and heart failure by decreasing heart rate and contractility. Propranolol, however, is a non-selective antagonist of both $\beta_1$ and $\beta_2$ receptors, meaning it affects both the heart and the lungs. It is used for HTN, migraines, tremors, and performance anxiety. Because of its $\beta_2$ block, Propranolol must be avoided in patients with asthma to prevent bronchoconstriction. The mnemonic for this distinction is "Propranolol = Heart and lung = bad for asthma," whereas "Metoprolol Beta 1 = heart drug."