Adrenergics And Adrenergic Blockers

Andrenergics are medications that stimulate alpha1-receptors and beta-adrenergic receptors. Alpha1-receptors are located in the smooth muscle of vascular (vessels) tissues. Beta2-adrenergic receptors are in the smooth muscle of the lungs, arterioles of skeletal muscles, and the uterine muscles. Adrenergics also stimulate the dopaminergic receptor located in the renal, mesenteric, coronary, and cerebral arteries to dilate and increase blood flow. Dopamine is the only adrenergic that can activate this receptor.

Adrenergic blockers inactivate these receptors in three ways:

1. They promote reuptake of the transmitter back into the neuron (nerve cell terminal).

2. Transmitters are transformed or degraded by enzymes making them unable to attach to a receptor. Two enzymes that inactive norepinephrine are monoamine oxidase (MAO) and catechol-o-methyl-transferase (COMIT). MAO is inside the neuron and COMIT is outside the neuron.

3. Transmitters are diffused away from receptors.

Sympathomimetic drugs stimulate andrenergic receptors and are classified into three categories according to its effect on organ cells. These categories are:

1. Direct-acting sympathomimetics—directly stimulate receptors.

2. Indirect-acting sympathomimetics—stimulate the release of norepineph-rine from terminal nerve endings.

3. Mixed-acting sympathomimetics—have the effect of both direct-acting sympathomimetics and indirect-acting sympathomimetics. They simulate the adrenergic receptor sites and stimulate the release of norepinephrine from terminal nerve endings. Ephedrine is an example of a mixed-acting sympathomimetic and is used to treat idiopathic orthostatic hypotension and hypotension resulting from spinal anesthesia. Ephedrine also stimulates beta2-receptors to dilate bronchial tubes and is used treat mild forms of bronchial asthma.

Many adrenergic medications stimulate more that one adrenergic receptor site. For example, epinephrine (Adrenalin) acts on alpha1-, beta1-, beta2-receptor sites. These receptor sites include an increase in blood pressure, pupil dilation, increase in heart rate (tachycardia), and bronchodilation.

Epinephrine (Adrenalin) is used to treat cardiogenic and anaphylactic shock because it increases blood pressure, heart rate, and airflow through the lungs through bronchodilation. Because it affects three different receptors, it lacks selectivity.

Alpha-adrenergic blockers inhibit the response at the alpha-adrenergic receptor sites. There are two types of alpha-adrenergic blockers: selective and nonselective blockers. Both types decrease symptoms of benign prostatic hypertrophy (BPH) (enlarged prostate) and promote vasodilation and treat peripheral vascular disease such as Raynaud's disease.

Doxazosin (Cardura) is a selective alpha^blocker and phentolamine (Regitine) is a nonselective alpha adrenergic blocker. Both can be used to treat hypertension.

However, alpha-adrenergic blockers can cause orthostatic hypotension (drop in blood pressure when an individual stands up), dizziness, and reflex tachycardia. They are not as frequently prescribed as beta-blockers.

Beta-adrenergic blockers (see chart)—also known beta blockers—decrease heart rate and decrease blood pressure resulting in bronchoconstriction. Therefore, beta-adrenergic blockers should be used with caution for patient's who have COPD or asthma.

Other Adrenergics

Ephedrine HCl (alphap betap beta2)

PO 25-50 mg tid/qid; SC/IM: 25-50 mg; IV: 10-25 mg PRN; maximum dose 150 mg/24 hr; effective for relief of hay fever, sinusitis, and allergic rhinitis; may be used for treating mild cases of asthma; Pregnancy category C: PB UK; half life 3-6 hours.

Norepinephrine bitartrate (Levophed) (alphap betax):

Potent vasoconstrictor; IV: 4 mg in 250-500 mL of D5W or NSS infused initially 8-12 ^g/min; then 4 ^g/min; titrated according to blood pressure; Pregnancy category CD PB: UK; half life: UK.

Metaraminol bitartrate (Aramine) (alphap betax)

V/Inf: 15-100 mg in 500 mg D5W at a rate adjusted according to blood pressure; Pregnancy category C; PB UK; half life UK.

Dobutamine HCl (Dobutrex) (betax)

To treat cardiac decompensation due to depressed myocardial contractility which may result from organic heart disease, cardiac surgery. IV: 2.5-20 ^g/kg/min initially; increase dose gradually; maximum dose of 40 ^g/kg/min. Pregnancy category: C; PBUK; half life 2 min.

Dopamine HCl (Intropin) (alphap betax) IV/Inf

1-5 ^g/kg/min initially; gradually increase 5-10 ^g/kg/min to a maximum of 50 ^g/kg/min; it does not decrease renal function in doses <5 ^g/kg/min. Pregnancy category C; PB: UK; half life 2 min.

There are also two types of beta-adrenergic blockers: selective and nonselective. For example, metoprolol tartrate (Lopressor) is a selective beta-adrenergic blocker that blocks betax receptors to decrease pulse rate and decrease blood pressure. Propranolol NCl (Inderal) is a non-selective beta-adrenergic blocker that blocks both betax and beta2 receptors resulting in a slower heart rate, decreased cardiac output, and lower blood pressure.

Other Beta Blockers

Doxazosin mesylate (Cardura) alpha1

Mild to moderate hypertension; PO: 1 mg/d; titrate dose up to maximum of 16 mg/d; maintenance 4-8 mg/day. Pregnancy category C; PB 95%; half life 3 h.

Carvedilol (Coreg) alpha1, beta1, beta2

Use for hypertension and mild to moderate heart failure; can be used with a thiazide diuretic; PO: 6.25 mg bid; may increase to 12.5 mg bid to maximum 50 mg/d; Pregnancy category: C; PB UK; half life 7-10 h.

Labetalol (Normodyne) alpha1, beta1, beta2

Mild to severe hypertension; angina pectoris; PO: 100 mg bid; dose may be increased to a maximum of 2.4 g/day. IV: 20 mg OR 102 mg/kg; repeat 20-80 mg at 10-min interval to maximum dose of 300 mg/day. Pregnancy category C; PB 50%; half life 6-8 hours.

Nadolol (Corgard) beta1, beta2

Management of hypotension and angina pectoris. Contraindicated in bronchial asthma and severe COPD. PO: 40-80 mg/d; maximum 320 mg/day. Pregnancy category: C; PB 30%; half life 10-24 h.

Selective Beta Adrenergics

Metoprolol tartrate (Lopressor)—beta1

Management of hypertension, angina pectoris, postmyocardial infarction. Hypertension: PO: 50-100 mg/d in 102 divided doses; maintenance 100-450 mg/d in divided doses to maximum of 450 mg/d in divided doses. Myocardial infarction: IV: 5 mg q2 min x 3 doses, then PO: 100 mg bid. Pregnancy category C; PB 12%; half life 3-4 h.

Atenolol (Tenormin)— beta1

Mild to moderate hypertension and angina pectoris. May be used in combination with antihypertensive drugs. PO: 25-100 mg/d. Pregnancy category: C; PB 6-16%; half life 6-7h Esmolol HCl (Brevibloc)—beta1. Supraventricular tachycardia, atrial fibrillation/ flutter, and hypertension. Contraindications: heart block, bradycardia, cardiogenic shock, uncompensated CHF; IV: Loading dose 500 ^/kg/min for 1 min; then 50 ^g/kg/min for 4 min. Pregnancy category C; PB UK; half life 9 min.

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