Sympathomimetics (Adrenergic Agents) 133adenylyl cyclase. The important agonist of
dopamine receptors is fenoldopam (D 1 ) and
bromocriptine (D 2 ) and antagonist is
clozapine (D 4 ).
These receptors are distinct from alpha
and beta receptors and are particularly
important in the brain.
Adrenergic drugs can also be classified
into:
a. Direct sympathomimetics: These act
directly on a or/and b adrenoceptors
e.g. adrenaline, noradrenaline, isopre-
naline, phenylephrine, methoxamine,
salbutamol etc.
b. Indirect sympathomimetics: They act
on adrenergic neurones to release
noradrenaline e.g. tyramine.
c. Mixed action sympathomimetics: They
act directly as well as indirectly e.g. ephe-
drine, amphetamine, mephentermine
etc.Pharmacological Action of
Sympathomimetics (Particularly
adrenaline and noradrenaline)
Heart: Direct effects on the heart are
determined largely by β 1 receptors.
Adrenaline increases the heart rate, force of
myocardial contraction and cardiac output
which is associated with increased
metabolism by the myocardium, increased
oxygen consumption and thus decreasing
cardiac efficiency.
Blood vessels: Adrenaline and
noradrenaline constrict the blood vessels of
skin and mucous membranes. Constriction
predominates in cutaneous and mucous
membranes which occur through both α 1
and α 2 receptors. Adrenaline also dilates the
blood vessels of the skeletal muscles on
account of the preponderance of β 2
receptors.
Blood pressure: Because of vasocon-
striction (α 1 ) and vasodilatation (β 2 ) action
of adrenaline, the net result is decrease in
total peripheral resistance. Although
adrenaline increases the systolic blood pres-
sure but simultaneously lowers the blood
pressure by its peripheral action. The rise in
systolic blood pressure is often followed by
decrease in blood pressure, adrenaline in
such doses activates both a and b receptors.
But mean blood pressure rises with increase
in pulse pressure.
Noradrenaline causes rise in systolic,
diastolic and mean blood pressure and does
not cause vasodilatation (because of no
action on β 2 receptors) and increase in
peripheral resistance due to its a action.
Isoprenaline causes rise in systolic blood
pressure (because of β 1 cardiac stimulant
action) but marked fall in diastolic blood
pressure (because of b 2 vasodilatation action)
but mean blood pressure generally falls.
GIT: Adrenaline causes relaxation of
smooth muscles of GIT and reduce its
motility. Relaxation of smooth muscles of
GIT can be brought about by both alpha and
beta stimulants. It decreases the tone,
frequency and amplitude of contraction of
smooth muscles.
Respiratory system: The presence of β 2
receptors in bronchial smooth muscle causes
relaxation and activation of these receptors
by β 2 agonists cause bronchodilatation.
Among catecholamines, adrenaline and
isoprenaline are potent bronchodilators due
to its β 2 action but not noradrenaline.