Flashcards in SNS Antagonists Deck (40)
Describe the effects of each type of adrenoceptor.
GI tract relaxation
Inhibition of transmitter release
Contraction of vascular smooth muscle
Heart – increased heart rate + contractility
Kidneys – increased renin release
GI tract relaxation
Relaxation of visceral smooth muscle
State five adrenoceptor antagonists including the receptors that they block.
Labetalol = alpha 1 + beta 1 (more beta 1 (4:1))
Phentolamine = alpha 1 + alpha 2
Prazosin = alpha 1
Propranolol = beta 1 + beta 2
Atenolol = beta 1
State four main clinical uses of SNS antagonists and false transmitters.
State three elements that contribute to hypertension.
Peripheral vascular tone
What is the main control of blood pressure?
Sympathetic drive to the kidneys via beta 1 receptors
This triggers renin release from the kidneys (leads to an increase in angiotensin II and aldosterone)
What are beta-1 selective blockers called?
What effect is responsible for most of the anti-hypertensive effect of beta-blockers?
Beta 1 blockade in the kidneys – this reduces renin release from the kidneys
What effect does beta-1 blockade have on the heart?
Decrease in heart rate
Decrease in cardiac output
How does the effect of beta-blockers on the heart change?
The effect of beta blockade on the heart disappears with chronic treatment as the heart begins to reset itself
What is the effect of presynaptic beta 1 receptors?
They have a positive facilitation effect on the synthesis and release of neurotransmitter
State four conditions in which you would not give a patient a betablocker. Explain each of them.
Asthma – blockade of beta 2 receptors in the lungs can take away the beta 2 mediated bronchodilation, which can be fatal in asthmatics
Cardiac Failure – these patients rely on a certain degree of sympathetic drive to the heart to maintain adequate cardiac output
COPD – same reason as asthma
Diabetes – beta blockade masks the symptoms of hypoglycaemia (e.g. tremors, palpitations, sweating) and beta 2 blockade also inhibits hepatic glycogenolysis
State some other unwanted actions of beta-blockers.
What effect does propranolol have on heart rate, cardiac output and blood pressure?
It has little effect on these parameters at rest
It decreases all of these parameters when exercising
Why would you still not give a cardioselective beta-blocker to anasthmatic patient?
Selectivity is dependent on concentration
What are the effects of labetalol?
Acts more on beta 1 than alpha 1
It lowers blood pressure by reducing total peripheral resistance and it induces a change in heart rate and cardiac output (beta 1 mediated)
What is the main mediator of total peripheral resistance?
Alpha 1 mediated vasoconstriction
What are the effects of alpha blockade?
Vasodilation causing a fall in TPR and hence a fall in blood pressure
NOTE: blocking the sympathetic drive can cause postural hypotension
What reflex will be triggered by alpha blockade?
It triggers a baroreceptor mediated reflex tachycardia to increase heart rate and cardiac output
State some of the problems with non-selective alpha blockers (e.g. phentolamine).
Alpha 1 blockade will cause vasodilation and a fall in TPR and blood pressure
But the alpha 2 blockade will mean that you take away the negative effect of alpha 2 on the synthesis and release of noradrenaline so more noradrenaline will be released from the nerve terminal
This enhances the reflex tachycardia
Side effects include increased GIT motility and diarrhoea
Phentolamine is no longer used
What are the effects of prazosin?
Highly selective alpha-1 antagonist
Leads to vasodilation and a fall in blood pressure
You get less tachycardia than with non-selective alpha blockers because the negative effect of alpha 2 on noradrenaline release has not been removed
NOTE: postural hypotension is troublesome
Prazosin also increases HDL and decreases LDL so is becoming more popular as an anti-hypertensive
Describe the mechanism of action of methyldopa.
Methyldopa is taken up by noradrenergic neurones and is decarboxylated and hydroxylated for form alpha-methyl noradrenaline
This is not deaminated by MAO so accumulates more than noradrenaline in the synapse
Alpha-methyl noradrenaline displaces NA from the synaptic vesicles
It is less effective than NA on alpha 1 receptors so does not cause as much vasoconstriction
It is more effective than NA on alpha 2 receptors thus reducing noradrenaline release
It also affects the CNS by inhibiting sympathetic outflow
State some other benefits of methyldopa other than its effect as an anti-hypertensive.
It does not have any adverse effects on foetus’ despite crossing the placenta
It maintains renal and CNS blood flow so it is used in patients with renal insufficiency or cerebrovascular disease
Adverse effects: dry mouth, sedation, postural hypotension, male sexual dysfunction
It is RARELY used
What are arrhythmias usually caused by and why?
Myocardial ischaemia – damage to the heart muscle can result in re-entry of impulses that messes up the heart rhythm
What can precipitate or aggravate arrhythmias?
An increase in sympathetic drive to the heart via beta 1 receptors
What part of the heart’s electrical circuit depends heavily on sympathetic activity?
AV conductance depends heavily on sympathetic activity
What effect do beta antagonists have on the refractory period of the AV node? How does this help in dealing with arrhythmia?
Beta-blockers INCREASE the refractory period of the AV node
It can interfere with AV conduction in atrial tachycardias and slow downventricular rate
What class of drugs are beta-blockers?
Class II anti-arrhythmics
What is propranolol particularly effective at treating?
It is particularly successful in arrhythmias that occur during exercise or mental stress
Chest pain that occurs when the oxygen supply to the myocardium is insufficient for its needs