The sympathomimetic or adrenergic or
adrenomimetic drugs mimic the effects of
adrenergic sympathetic nerve stimulation.
These are the important group of
therapeutic agents which may be used to
maintain blood pressure and in certain cases
of severe bronchial asthma.

The sympathomimetics are classified as
in table 3.1.1.

Mechanism of Action and Adrenoceptors

The catecholamines produce their action by
direct combination with receptors located on
the cell membrane. In 1948, Ahlquist divided
the adrenergic receptors into two main groups

• alpha and beta. The alpha receptor stimulation
  produces excitatory effect and beta receptor
  stimulation usually produces inhibitory effect.

Adrenergic receptors: These are
membrane bound G-protein coupled
receptors which function primarily by
increasing or decreasing the intracellular
production of second messengers cAMP or
inositol triphosphate (IP 3 )/diacylglycerol
(DAG). Adrenergic receptors are classified
into two main groups α and β.

Alpha receptors: There are two major

groups of alpha receptors, α 1 and α 2.

Activation of postsynaptic α 1 receptors

increases the intracellular concentration of

calcium by activation of a phospholipase C

in the cell membrane via G protein. α 2

receptor is responsible for inhibition of renin

release from the kidney and for central a-

adrenergically mediated blood pressure

depression.

Beta receptors: On the basis of the

relative selectivity and potency of both

agonists and antagonists, three types of beta

receptors can be distinguished.

a. Beta 1 receptors have approximately

equal affinity for adrenaline and

noradrenaline and are responsible for

myocardial stimulation and renin

release.

b. Beta 2 receptors have a higher affinity

for adrenaline than for noradrenaline

and are responsible for bronchial

muscle relaxation, skeletal muscle

vasodilatation and uterine relaxation.

Dopamine receptors: The D 1 receptor is

typically associated with the stimulation of

Chapter

1.4

Chapter

3.1

(Adrenergic Agents)(Adrenergic Agents)(Adrenergic Agents)(Adrenergic Agents)(Adrenergic Agents)

SympathomimeticsSympathomimetics