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Pharmacology > Adrenergic Agonists > Flashcards

Adrenergic Agonists Flashcards

1
Q

Adrenergic Agonists (sympathomimetics)

A

Drugs which activate adrenergic receptors

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2
Q

Adrenergic receptors

A

Receptors which mediate the effects of norepinephrine (the neurotransmitter of sympathetic nervous system) & epinephrine.

(Norepinephrine and epinephrine are synthesized from aminoacid tryrosine.)

Adrenergic receptors are located in sympathetic nerve endings.

  • Most of them located on post synaptic membrane, but there are also presynaptic adrenoreceptors (mainly alpha 2 adrenoreceptors)
  • Extra synaptic adrenoreceptors ( alpha 2 and beta 2 adrenoreceptors especially in vessels) respond circulating epinephrine.
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3
Q

Mechanism of elimination of norepinephrine from adrenergic synapses

A
  • reuptake to presynaptic nerve ending
  • inactivation in synaptic cleft by COMT
  • inactivation in presynaptic nerve ending by MAO
  • diffusion to surrounding tissues and finally to blood
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4
Q

Subtypes of adrenergic receptors

A

Alpa and beta adrenoreceptors:
Alpha: 1,2
Beta: 1,2,3

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5
Q

Alpha 1:

A
  1. Vascular smooth muscles(skin,mucosa,kidney): Vasoconstriction
  2. Vascular smooth muscles(skin,mucosa,kidney): Constriction
  3. Eye (radial muscle of iris): Constriction, dilation of pupil
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6
Q

Alpha 2:

A
  1. Sympathetic nerve endings (presynaptic): Decreased release of norepinephrine
  2. Brainstem (nucleus tractus solitarii): Decreases sympathetic nervous system activity
  3. Vascular smooth muscle: Vasoconstriction
  4. Platelets: Platelet aggregation
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7
Q

Beta 1

A
  1. Heart: Increased HR and contractility
  2. Juxtaglomerular cells: Stimulation of renin secretion
    I
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8
Q

Beta 2

A
  1. Bronchial smooth muscle: Bronchodilation
  2. Vascular smooth muscles( vessels in skeletal muscles): vasodilation
  3. GIT and GUT smooth muscles: Decreased motility of GIT. Relaxed detrusor of urinary bladder
  4. Uterus: relaxation
  5. Liver: Increase of glycogenolysis
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9
Q

Beta 3

A
  1. Adipose tissue: Increase of lipolysis

2. Detrusor of urinary bladder: relaxation

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10
Q

Mechanisms of signal transduction in adrenoceptors:

A

• for alpha-1 adrenoceptors - activation of Gq proteins leading to stimu-
lation of phospholipase C, which contributes to the formation of DAG
and IP

• for alpha-2 adrenoceptors - activation of Gi proteins, which inhibit
adenylyl cyclase leading to decrease of intracellular cAMP

• for all subtypes of beta adrenoceptors - activation of Gs proteins,
which stimulate adenylyl cyclase leading to increase of intracellular cAMP.

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11
Q

Classification of adrenergic agonists by mechanism of action:

A
  1. Directly acting agonists:

bind to adrenoceptors and activate them.

  • alpha
  • beta
  • alpha/beta

2.Indirectly acting agonists:

do not act directly on the receptors but increase the level of norepinephrine in the synaptic cleft by

  • stimulation of its release (ex: amphetamine) or
  • inhibition of reuptake (ex: cocaine).
  1. Mixed acting:

induce both above-mentioned effects:

  • direct activation of adrenoceptors and
  • stimulation of the release of norepinephrine to the synaptic cleft (ex: ephedrine).
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12
Q

Classification of adrenergic agonists by the chemical structure:

A

• catecholamines -

  1. epinephrine
  2. norepinephrine
  3. dopamine
  4. isoprenaline (isoproterenol)
  5. dobutamine

• non-catecholamines - all other adrenergic receptor agonists.

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13
Q

Distinctive features of catecholamines:

A
  • short duration of action
  • they are not absorbed from GIT
  • Poor penetration to CNS
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14
Q

Alpha / Beta Adrenergic Receptor Agonists

Alpha / Beta Adrenomimetics

A

These drugs activate both alpha and beta adrenoceptors.

  • They include epinephrine and norepinephrine.
  • Mixed acting (ephedrine) and indirectly acting adrenergic receptor agonists (amphetamine) also activate both alpha and beta adrenoceptors.
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15
Q

Difference between epinephrine and norepinephrine in affinity to subtypes of adrenergic receptors:

A
  • Epinephrine binds equally to alpha- I and alpha-2, beta-1 and beta-2 adrenergic receptors (alpha-1 = alpha-2, beta- 1 = beta-2)
  • Norepinephrine has high affinity to alpha-1, alpha-2, beta-1 adrenoceptors and very low affinity to beta-2 adrenoceptors (alpha-1 = alpha-2, beta-1 > beta-2).
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16
Q

Effects of Epinephrine and Norepinephrine:

A

Epinephrine:

  • Systolic blood pressure: significant increase
  • Diastolic blood pressure: moderate increase/decrease (in high doses)
  • Mean blood pressure: moderate increase
  • Heart rate: moderate increase
  • Heart contractility: significant increase
  • Bronchial muscles: relaxation

Norepinephrine:

  • Systolic blood pressure: significant increase
  • Diastolic blood pressure: significant
  • Mean blood pressure: significant increase
  • Heart rate: moderate decrease
  • Heart contractility: moderate increase
  • Bronchial muscles: no effect
17
Q

Effects explained:

A

Epinephrine:

  • Epinephrine has a complicated influence on the vessels: constricts the
    vessels of the skin and mucosa, which contain mainly alpha adrenoceptors,
    and dilate vessels of skeletal muscles, where beta-2 adrenoceptors predom-
    inate.
    -When epinephrine is administered in lower doses, vasodilator effect
    mediated by beta-2 adrenoceptors predominates, so the diastolic blood
    pressure decreases.
    -In higher doses, epinephrine binds also to alpha adrenoceptors, and vasoconstrictor effect is more significant, so diastolic blood pressure increases.

Norepinephrine:

-Decrease of the heart rate caused by norepinephrine is of a reflex origin:
vasoconstriction causes increase of blood pressure, stimulation of baroreceptors in aortic arc and activation of the center of nervus vagus, which leads to decrease of the heart rate.

18
Q

Application of epinephrine

A

• anaphylactic shock (IM or IV);
• cardiac arrest (direct injection into the heart);
• prolongation of local anesthesia (added to solutions of local anesthet-
¡cs);
• bleeding (as a topical hemostatic agent);
• bronchial asthma (SC).
The beneficial effect of epinephrine in anaphylactic shock is explained
by:
a) constriction of peripheral vessels (through alpha adrenoceptors)
leading to the increase of blood pressure and decrease of swelling
b) relaxation of bronchial muscles (through beta-2 adrenoceptors) and, as a con-
sequence, relieving dyspnea.

19
Q

Application of norepinephrine

A

Norepinephrine is of limited value:

sometimes it is used as an intravenous infusion in acute hypotension (shock).

20
Q

Application of ephedrine

A

-Most of the effects of the ephedrine (alkaloid of the plant Ephedra) are
similar to the action of epinephrine.
-The differences of ephedrine in comparison with epinephrine are:
a) a longer duration of the action
b) a high bioavailability after oral administration
c) a mild stimulant effect on the CNS.
- Ephedrine is sometimes used in bronchial asthma.

21
Q

Application of Amphetamine

A

Amphetamine is mainly known as a psychostimulant drug

22
Q

Alpha Adrenergic Receptor Agonists

Alpha Adrenomimetics

A
  1. Phenylephrine:
    - Alpha-1
    - Acute hypotension, as a mydriatic agent in examination of retina

(The main effect is a constriction of peripheral vessels and, as a result,
the increase of both systolic and diastolic blood pressure (due to the activa-
tion of alpha-1 and, to some degree, alpha-2 receptors). Dilation of the
pupil (mydriasis) develops due to activation of alpha-1 adrenoceptors
in the radial muscle of the iris)

  1. Midodrine:
    - Alpha-1
    - Chronic hypotension, orthostatic hypotension
  2. Xylometazoline,oxymetazoline, naphazoline:
    - Preferentially alpha-2 in CNS 1 in PNS
    - As nasal decongestants

(The mechanism of the beneficial effect. vasoconstriction leading to the decrease of the volume of the nasal mucosa.)

  1. Clonidine, methyldopa:
    -Alpha-2
    -Arterial hypertension
    (leads to the decrease of the blood pressure.
    This effect is of central origin; activation of alpha-2 adrenoceptors of the
    brainstem (nucleus tractus solitarii) the causes decrease of sympathetic
    outflow from the central nervous system, which results in the decrease
    of the peripheral vascular resistance and the heart rate. Other effects of ac-
    tivation of the central alpha-2 adrenoceptors are sedation and analgesia)
    Side effects of clonidine:dry mouth,dizziness,depression
  • These drugs bind to both alpha-1 and alpha-2 adrenoceptors but have
    a higher affinity to alpha-2 adrenoceptors.
23
Q

Classification of Beta Adrenergic Receptor Agonists

Beta Adrenomimetics

A
  1. non-selective (beta-1 / beta-2) adrenergic receptor agonists:
    - iso-prenaline - INN (isoproterenol - USAN)
  2. selective beta- 1 adrenergic receptor agonists:
    - dobutamine
  3. selective beta-2 adrenergic receptor agonists:
    - salbutamol - INN (albuterol - USAN),
    - fenoterol,
    - salmeterol,
    - hexoprenalin
  4. selective beta-3 adrenergic agonists:
    - mirabegron.
24
Q

Effects & application of non-selective beta-adrenomimetic isoprenaline:

A

• the increase of the heart rate, automaticity and contractility (through
beta-1 adrenoceptors); cardiac arrhythmias can be caused as the most com-
mon adverse effect.

• slight increase of the systolic blood pressure (due to the increase of car-
diac output), decrease of the diastolic and the mean blood pressure (due
to dilation of vessels in the skeletal muscles through beta-2 adrenoceptors)

• the relaxation of bronchial smooth muscles (through beta-2 adreno-
ceptors).

Application: sublingually in bradyarrhythmias (especially in atrioventricular block).

25
Q

Effects & application of selective beta- 1 adrenomimetic dobutamine

A

Increase of cardiac contractility

Application: in acute heart failure ( as IV infusion)

26
Q

Effects & application of Selective beta-2 adrenomimetics salbutamol, fenoterol, salmeterol:

A

widely used in bronchial asthma (mainly as aerosols).
- Salmeterol is a long-acting drug (the duration of the action of salmeterol is 12 hours vs 4 hours for salbutamol), so it is used for the prevention of asthma attacks.
* An advantage of selective beta-2 adrenergic receptor agonists is a lack of signifi-
cant influence on the heart (low risk of cardiac arrhythmias).
- prevention of preterm labor (salbutamol IV, hexoprenalin IV). Mechanism of a beneficial effect is the relaxation of myometrium through beta-2 adrenoceptors.

27
Q

Effects & application of Selective beta-3 adrenergie receptor agonist mirabegron:

A

is used in overactive urinary bladder as an alternative to the selective antagonists of muscarinic Ms-receptors.

28
Q

Adverse effects of adrenergic agonists:

A
  • vasoconstriction and hypertension (alpha adrenergic receptors agonists)
  • tachycardia, hypotension, tremor (beta adrenergic receptor agonists)
29
Q

Dopamine Receptor Agonists

A

There are 5 types of dopamine receptors:

  • D1 receptors are located in mesenteric and renal arteries, they mediate the vasodilator effect
  • D2 - D5 receptors are located only in the CNS
30
Q

Effects and applications of dopamine:

A

• low doses (1.5-3.5 mcg/kg/min) - activation of D1 receptors and
beta-1 adrenoceptors : increase of the heart rate and contractility, decrease
of the peripheral vascular resistance.
Application: used in acute heart failure

• high doses (> 3.5 mcg/kg/min) - activate alpha adrenoceptors also:
increase of the peripheral vascular resistance and arterial blood pressure.
Application: used in acute hypotension (shock, collapse)

31
Q

Fenoldopam

A

Agonist of peripheral dopamine D1 receptors:

  • causes dilation of renal and mesenteric vessels, decreases blood pressure,
  • used in hypertensive crises (especially with renal failure).
32
Q

Bromocriptine

A

agonist of dopamine receptors, which can penetrate to the CNS.
- used in Parkinson’s disease and some endocrinologic disorders (acromegaly).

Pharmacology (3 decks)

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