ADRENERGIC DRUGS

As their name suggests, these drugs resemble sympathetic nerve stimulation in their effects; they may be divided into two groups on the basics of their chemical structure.

1. Catecholamines: -these are compounds which have the catechol nucleus.

Catecholamines have a direct action on sympathetic effectors cells through interactions with receptor sites on the cell membrane.

The group includes adrenaline, noradrenaline, dopamine, isoprenaline, and dobutamine.

–     Noncatecholmines: – lack the catechol nucleus.

They may directly act on the receptors or may indirectly release the physiologic catecholamines-

e.g. ephedrine, phenylephrine, amphetamine

Adrenergic drugs, like cholinergic drugs, can be grouped by mode of action and by the spectrum of receptors that they affect.

1. Direct mode of action: directly interact with and activate adrenoreceptors, e.g., adrenaline and noradrenaline
2. Indirect mode of action: their actions are dependent on the release of endogenous catecholamines. This may be
   1. Displacement of stored catecholamies from the adrenergic nerve endings, e.g., amphetamine, tyramine
   1. Inhibition of reuptake of catecholamines already released, e.g. cocaine, tricyclic antidepressants

Both    types   of   sympathomimetics,   direct   and    indirect,   ultimately           cause  activation of adrenoreceptors leading to some or all characteristic effects of the catecholamines.

Organ-system Effects of Activation of the Adrenergic System

1. CVS:
   1. Heart: increased rate and force of contraction, increased cardiac output, myocardial demand, and AV conduction
   1. Blood Vessels and Blood pressure: constriction of blood vessels in the skin and mucous membranes

• Dilatation of skeletal muscle vessels
  • Adrenaline increases systolic and decreases diastolic blood pressure at low doses but increases both at higher doses
  • Noradrenaline increases both systolic and diastolic blood pressure

• Smooth Muscle:
  • Bronchi: relaxation.
  • Uterus: relaxation of the pregnant uterus
  • GIT: relaxation of wall muscles and contraction of sphincters
  • Bladder: relaxation of detrusor muscle; contraction of sphincter and trigone muscle
• Eye: mydriasis; reduction of intraocular pressure in normal and glacucomatous eyes
• Respiration: Bronchodilatation; relief of congestion; mild stimulation of respiration
• Metabolic:    Increased    hepatic    glycogenolysis;    decreased    peripheral    glucose intake; increased free fatty acids in the blood (lipolysis)
• CNS: excitement, vomting, restlessness
• Skeletal muscle: facilitation of neuromuscular transmission and vasodilatation Drugs Acting on the Adrenergic Receptor Subtypes

Adrenaline stimulates all the four receptor subtypes.

Noradrenaline stimulates both alpha receptors and beta₁ but has very poor affinity for beta₂ receptors. Labetalol blocks all beta receptors as well as some alpha receptors.