AUTONOMIC NERVOUS SYSTEM-PART-VII

DIRECT ACTING ALPHA, BETA AGONISTS

These drugs acting on the adrenergic receptors directly. They mimic the actions of noradrenaline.

1.Epinephrine(Adrenaline)

Epinephrine act on alpha-1,alpha-2;and beta-1,beta-2 receptors in a proportionate manner.

In low dosage, it is effective to stimulate the beta-receptors and at high doses, it is effective on alpha receptors. But the alpha stimulation is predominant and persistent at high doses.

Effects of Epinephrine

1.It increases heart contractility(inotropic);heart rate(chronotropic);vasoconstrictions of arterioles in the skin,viscera,and mucus membrane (apha-1);

2.Bronchodilation due to stimulation of beta-2 receptors.

3. Increased glycogenolysis and release of glucagon and a reduced release in insulin leads to hyperglycemia.

Uses

1. To relieve bronchospasm as a secondary symptom in Asthma and Anaphylactic shock.

2.Used in anaphylactic shock and cardiac arrest to increase the electrical conductivity through the pacemaker AV and SA nodes and Purkinje fibers (Dronotropic effects)

3.In conjunction with local anesthetics to prolong the local effects by local vasoconstriction effects.

4. To achieve hemostasis.

Side effects

1.Arrhythmias

2.Hypertension

3.Palpitations

4.Dizziness, anxiety, headache

5.Myocardial Infarction due to heavy workload on the heart.

6.Pulmonary edema.

Epinephrine Reversal 

Interestingly if epinephrine is given alone it increases the systemic blood pressure due to its alpha vasoconstriction effects. If given along with an alpha-blocker such as phenoxybenzamine it will lower the blood pressure due to its beta smooth muscle-relaxing effects. This action is technically known as Adrenaline Reversal.

2.Nor Epinephrine-Nor Adrenaline 

Noradrenaline the main prototype nerve transmitter stimulates alpha-1,alpha-2, and beta-1 receptors with a stronger affinity for alpha receptors than for beta receptors.

Its main physiologic effects are vasoconstriction, Reflux bradycardia

It is one of the last line agents in the treatment of shock.

Side effects

1.Tissue hypoxia secondary to potent vasoconstriction

2.Decreased perfusion to the kidneys

3.Tissue necrosis due to extravasation during I.V. administration.

4.Arrhythmia

3.Dopamine

This drug is synthesized in the CNS, sympathetic ganglia and adrenal medulla.

Dopamine act on alpha-1,beta-1, and beta-2 receptors with a greater affinity towards dopamine and beta receptors. It also acts on its own receptors D-1 and D-2 receptors located in the peripheral mesenteric and renal vascular beds.

Dopamine at its low dose stimulates dopamine receptors, at moderate dose beta receptors and at high dose alpha-1 receptors. Dopamine is highly polar, hydrophilic, and does not cross the blood-brain barrier.

Uses

1. Treatment of shock-It raises blood pressure by increasing the heart rate by stimulating the beta-1 receptors

2. In acute renal failure, dopamine is used to increase renal blood flow.

3.Treatment of acute congestive heart failure.

Administered through I.V.

Side effects

1.Decreased renal perfusion a higher doses

2.Arrhythmias

3.Tachycardia

4.Hypertension

5. Tissue necrosis may occur if dopamine extravasates during I.V.infusion.