beta-1 agonist

**Receptor and mechanism** - β1 receptors are Gs-protein coupled. Activation stimulates adenylyl cyclase, which raises cAMP and activates PKA. - In cardiac myocytes, PKA phosphorylates L-type Ca2+ channels, so Ca2+ influx increases. It also phosphorylates phospholamban, which speeds Ca2+ reuptake into the sarcoplasmic reticulum, and troponin I. - In the SA node, a steeper phase 4 slope (If current) raises the heart rate. - In the AV node, conduction velocity increases and the refractory period shortens. - In juxtaglomerular cells of the kidney, renin release increases. This activates RAAS and raises BP. - In the ciliary body, aqueous humor production increases. Beta-blockers such as timolol lower IOP by blocking this.

**Cardiac effects (the high-yield list)** - Positive chronotropy (↑ HR) - Positive inotropy (↑ contractility) - Positive dromotropy (↑ AV conduction) - Positive lusitropy (faster relaxation) - Net effect: ↑ cardiac output, ↑ systolic BP, and ↑ myocardial O2 demand.

**Key drugs** - **Dobutamine**: mainly β1 agonist with weak β2 and α1 activity. Net effect is ↑ CO with little change or a slight drop in SVR. - Uses: acute decompensated HF, cardiogenic shock, and pharmacologic stress echo/imaging (an alternative to exercise or adenosine/regadenoson). - Adverse effects: arrhythmias, tachycardia, ↑ myocardial O2 demand (angina), hypotension in some patients, and tolerance with prolonged infusion. - **Dopamine** (dose-dependent): - Low dose (~1–2 mcg/kg/min): D1 receptors, renal and mesenteric vasodilation. - Medium dose (~2–10): β1 effects, ↑ inotropy and CO. - High dose (>10): α1 effects, vasoconstriction, ↑ BP. - **Isoproterenol**: nonselective β1 and β2 agonist. - Uses: bradycardia, AV block, and torsades de pointes (as a bridge to pacing). It has also been used in refractory asthma. - Effect: ↑ HR and CO with ↓ SVR from β2 vasodilation, so diastolic BP and MAP fall. - **Norepinephrine/epinephrine**: strong α1 plus β1 activity. Epinephrine also has β2 effects at low doses.

**Relationship to other receptors** - β2 agonists (albuterol, terbutaline) cause bronchodilation, vasodilation, uterine relaxation, and ↑ glucagon/glycogenolysis. They also drive K+ into cells through the Na+/K+-ATPase, which treats hyperkalemia. - Isoproterenol rank order: β1 = β2 >> α. Epinephrine rank order: β > α at low doses, α > β at high doses. Norepinephrine: α1 > α2 > β1.

**Clinical pearls** - Beta-1 agonists are contraindicated or used cautiously in hypertrophic obstructive cardiomyopathy (↑ contractility worsens outflow obstruction), acute MI or active ischemia, and tachyarrhythmias. - Dobutamine stress testing is useful for patients who can't exercise. It unmasks ischemia by ↑ HR and contractility. - Chronic β1 stimulation causes receptor downregulation and desensitization, which is why dobutamine tolerance develops. - Antagonists: selective β1 blockers (metoprolol, atenolol, esmolol, bisoprolol, nebivolol, acebutolol) reverse these effects. "A BEAM" is a mnemonic for the selective β1 blockers: Atenolol, Betaxolol, Esmolol, Acebutolol, Metoprolol (older mnemonic variants differ). - In beta-blocker overdose, glucagon (↑ cAMP through a non-β receptor) is a treatment option. - Mnemonic: β1 = 1 heart (the heart has one of them: β1 → heart); β2 = 2 lungs.

**Step 1 vs Step 2 focus** - Step 1: receptor signaling (Gs → cAMP → PKA), the cardiac and renal effects, and the differences between dobutamine, dopamine, isoproterenol, and epinephrine. - Step 2: choosing the right agent for shock (cardiogenic shock → dobutamine ± norepinephrine; septic shock → norepinephrine first line), and choosing a stress test (dobutamine stress echo when the patient can't exercise).