Drugs blocking adrenoreceptors

Август 1, 2022

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Drugs blocking adrenoreceptors

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3. Adrenoblockers – drugs blocking adrenoreceptors and preventing the effects of the mediator and catecholamines that circulate
4. Classification of adrenoblockers (AB) Nonselective α1a2 AB:—Phentolamine, Nicergoline Selective α1 - AB: Prazosin, Doxazosin, Tamsulosin Selective
5. Blockade of vasoconstrictor α1 (also α2) receptors reduces peripheral resistance and causes pooling of blood in
6. Nasal stuffiness and miosis result from blockade of α receptors in nasal blood vessels and in
7. Phentolamine is used as a quick and short acting α blocker for diagnosis and intraoperative management
8. Side effects: tachycardia partially due to the block of presynaptic α₂-AR; orthostatic collapse, dizziness, exacerbation of
9. The effect of epinephrine on blood pressure after administration of alpha-blockers α-adrenoblocker
10. Nicergoline dilates blood vessels of the brain, improves cerebral circulation. Indications for the use: disorders of
12. Prazosin blocks all subtypes of α1 receptor. It dilates blood vessels, lowers blood pressure, dilates veins,
13. Terazosin and Doxazosin are long acting drugs (24 h). They are used for the treatment of
15. Tamsulosin is uroselective α1A blocker. It is effective in improving BHP symptoms, because α1A subtype predominate
16. Non-selective β1, β2- AB without intrinsic sympathomimetic activity : Timolol, Propranolol, with intrinsic sympathomimetic activity: Pindolol
17. Propranolol decreases heart rate, force of contraction, cardiac output. It inhibits atrioventricular conduction. The effects become
19. antiarrhythmic effect: propranolol reduces the automatism of the sinus node, automatism and conductivity of the atrioventricular
20. Antihypertensive effect. P. blocks β1-AR of the heart and reduces cardiac rate and intensity, decreases stroke
21. The effect of epinephrine on blood pressure after administration of β-AB β-adrenoblocker
22. Indication for use: angina pectoris, hypertension supraventricular tachycardia, atrial fibrillation, tachycardia in mitral stenosis, thyrotoxicosis.
23. Side effects: cardiac failure, cardiac block, bronchospasm, increase in peripheral vessels tone (numbness of limbs, cold
25. Lipophilic substances penetrate the BBB, reduce fear, anxiety, panic, aggression, prevent the activating effect of the
27. Cardioselective b. are more potent in blocking cardiac (β1) than bronchial (β2) receptors. However, selectivity is
28. Partial agonistic (intrinsic sympathomimetic) action (acebutolol). These drugs themselves activate β1 and/or β2 receptors submaximally. The
29. Pharmacokinetics Lipophilic (propranolol, metoprolol) are metabolized in the liver, undergoes presystemic metabolism, pass through the BBB
30. Nebivolol is a highly selective β1 blocker. It also acts as a NO donor, produces vasodilatation
31. Carvedilol is a β1 + β2 + α1 adrenoceptor blocker. It reduces the work of the
32. Reserpine is an alkaloid of the Rauwolfia plant. It impairs the process of norepinephrine storage in
34. R. causes a gradual decrease in arterial pressure. The maximal effect is observed after several days.
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Adrenoblockers – drugs blocking adrenoreceptors and preventing the effects of the

mediator and catecholamines that circulate in the blood and other adrenomimetics.
Sympatholytics – drugs inhibiting the transmission of mediators from adrenergic fiber terminals: Adelphan (Reserpine + Dihydralazine + hydrochlorothiazide)

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Classification of adrenoblockers (AB)
Nonselective α1a2 AB:—Phentolamine, Nicergoline
Selective α1 - AB: Prazosin,

Doxazosin, Tamsulosin
Selective α2 - Yohimbine

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Blockade of vasoconstrictor α1 (also α2) receptors reduces peripheral resistance and

causes pooling of blood in capacitance vessels → venous return and cardiac output are reduced → fall in BP.
Reflex tachycardia occurs due to fall in mean arterial pressure and increased release of NA due to blockade of presynaptic α2 receptors.

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Nasal stuffiness and miosis result from blockade of α receptors in

nasal blood vessels and in radial muscles of iris.
Intestinal motility is increased.
Tone of smooth muscle in bladder trigone, sphincter and prostate is reduced by blockade of α1 receptors (mostly of the α1A subtype) → urine flow in patients with benign hypertrophy of prostate (BHP) is improved.

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Phentolamine is used as a quick and short acting α blocker

for diagnosis and intraoperative management of pheochromocytoma, disorders of peripheral blood circulation (endarteritis, Raynauld’s disease), hemorragic or cardiogenic shock with spasm of arteriols, hypertensive crises.
Pheochromocytoma (tumor of the adrenal medulla) produces large amounts of epinephrine, which leads to substanial increase of the arterial pressure.

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Side effects: tachycardia partially due to the block of presynaptic α₂-AR;

orthostatic collapse, dizziness, exacerbation of peptic ulcer, dyspepsia, itching, skin hyperemia. edema, allergic reactions.

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The effect of epinephrine on blood pressure after administration of alpha-blockers
α-adrenoblocker

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Nicergoline dilates blood vessels of the brain, improves cerebral circulation.
Indications

for the use:
disorders of cerebral circulation (atherosclerosis of cerebral vessels, post-stroke conditions, diabetic retinopathy, dystrophic eye diseases), migraine,
acute and chronic peripheral vascular disorders (endarteritis, trophic wounds and ulcers, bedsores).
Side effects: marked decrease in blood pressure, dizziness, dyspeptic disorders (nausea, diarrhea, abdominal pain), a feeling of heat, allergic reactions, drowsiness or insomnia.

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Prazosin blocks all subtypes of α1 receptor. It dilates blood vessels,

lowers blood pressure, dilates veins, decreases venous return of blood to the heart, declines preload of the heart; reduces platelet aggregation, reduces the content of atherogenic lipoproteins in the blood.
Prazosin is effective orally. Effect of a single dose lasts for 6–8 hours.
Indications: hypertension and hypertensive crisis, chronic heart failure

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Terazosin and Doxazosin are long acting drugs (24 h). They are

used for the treatment of hypertensive disease, benign hypertrophy of prostate.
They block α1 receptors in bladder trigone and prostatic smooth muscle, there by improve urine flow, reduce residual urine in bladder.

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Tamsulosin is uroselective α1A blocker. It is effective in improving BHP

symptoms, because α1A subtype predominate in the bladder base and prostate.
Side effects: the effect of the first dose, tolerance, swelling, dizziness, ejaculation disorder, headache, palpitation.

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Non-selective β1, β2- AB
without intrinsic sympathomimetic activity : Timolol, Propranolol,

with intrinsic sympathomimetic activity: Pindolol
2. Selective β1 -AB: Metoprolol, Bisoprolol, Nebivolol
with intrinsic sympathomimetic activity: Acebutolol
3. α and β blockers: Carvedilol, Labetalol

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Propranolol decreases heart rate, force of contraction, cardiac output. It inhibits

atrioventricular conduction. The effects become prominent under sympathetic overactivity (exercise, emotion).
Cardiac work and oxygen consumption are reduced. Total coronary flow is reduced (blockade of dilator β receptors) in the subepicardial region, while perfusion of the subendocardial area (which is the site of ischaemia in angina patients) is not affected.

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antiarrhythmic effect:
propranolol reduces the automatism of the sinus node, automatism and

conductivity of the atrioventricular node, conductivity in Purkinje fibers, eliminates the influence of the sympathetic system on the heart, has a weak direct membrane-stimulating effect.
So, rate of diastolic depolarization in ectopic foci is reduced.

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Antihypertensive effect.
P. blocks β1-AR of the heart and reduces cardiac

rate and intensity, decreases stroke and minute heart volume, and decreases systolic blood pressure.
Diastolic pressure firstly increased, but P. blocks βAR of juxtaglomerular apparatus of the kidneys and decreases renin secretion. P. blocks presynaptic vascular β-AR and reduces release of the mediator.
P. inhibits the Central links of the vasoconstrictor reflex.

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The effect of epinephrine on blood pressure after administration of β-AB
β-adrenoblocker

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Indication for use:
angina pectoris,
hypertension
supraventricular tachycardia, atrial fibrillation, tachycardia

in mitral stenosis, thyrotoxicosis.

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Side effects:
cardiac failure, cardiac block, bronchospasm,
increase in peripheral vessels tone

(numbness of limbs, cold hands and feet),
increased hypoglycemia in patients with diabetes mellitus, increased blood levels of atherogenic lipoproteins,
drowsiness, lethargy, reduced reactions rate,
edema, dyspepsia, increasing the tone of the uterus

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Lipophilic substances penetrate the BBB, reduce fear, anxiety, panic, aggression, prevent

the activating effect of the Central nervous system on the cardiovascular system. They can be used for fear of public speaking, at stress.
Propranolol inhibits adrenergically provoked tremor.
Propranolol and some other β blockers reduces secretion of aqueous humor; intraocular pressure is lowered.

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Cardioselective b. are more potent in blocking cardiac (β1) than bronchial

(β2) receptors. However, selectivity is only relative and is lost at high doses.
Their features are:
Lower propensity to cause bronchoconstriction,
Less interference with carbohydrate metabolism and less inhibition of glycogenolysis during hypoglycaemia—safer in diabetics.
Lower incidence of cold hands and feet, Raynaud’s phenomenon.
No/less deleterious effect on blood lipid profile.

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Partial agonistic (intrinsic sympathomimetic) action (acebutolol). These drugs themselves activate β1

and/or β2 receptors submaximally.
The benefits of this property:
Bradycardia and depression of contractility at rest are not prominent, but exercise tachycardia is blocked.
They can be used in elderly patients, with sick sinus.

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Pharmacokinetics
Lipophilic (propranolol, metoprolol) are metabolized in the liver, undergoes presystemic metabolism,

pass through the BBB
Lipo / hydrophilic (bisoprolol, nebivolol, carvedilol) are eliminated by kidneys and liver.
Hydrophilic (atenolol) are not metabolized in the liver, excreted mainly by the kidneys.

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Nebivolol is a highly selective β1 blocker. It also acts as

a NO donor, produces vasodilatation and improves endothelial function, which may delay atherosclerosis.
It has not effect on plasma lipids and on carbohydrate metabolism.
In contrast to older β blockers, hypotensive response to nebivolol has a rapid onset. It is used in hypertension and CHF.

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Carvedilol is a β1 + β2 + α1 adrenoceptor blocker. It

reduces the work of the heart, dilates blood vessels, reduces the secretion of renin, reduces preload and postload of the heart.
It is used in hypertension, coronary artery disease, chronic heart failure.
Side effects: headache, bronchospasm, allergic reactions.
It is used 1-2 times a day.

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Reserpine is an alkaloid of the Rauwolfia plant. It impairs the

process of norepinephrine storage in the vesicles, which leads to a reduction in its concentration in the varicosities. The main part of mediator is deaminated.
The drug lowers norepinephrine concentrations in the heart, vessels, adrenal medulla, CNS. It inhibits CNS and has calming effect, promotes the developing of sleep.

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R. causes a gradual decrease in arterial pressure. The maximal effect

is observed after several days. The drug reduces cardiac output and decreases peripheral vascular resistance and pressor reflex.
It can cause bradycardia, bronchospasm, an increase in the secretory and motor activity of the gastrointestinal tract and miosis, drowsiness, weakness, depression, extrapyramidal disorders, increase in appetite.
In the treatment of hypertension reserpine is prescribed together with vasodilators and diuretics to enhance the therapeutic effect and reduce side effects.