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Diagram to explain how β-adrenoceptors antagonists produce clinically useful effects
It suffices to note that b blockers have their main role outside the central nervous system. They competitively block receptor occupancy of b receptors, from agonists like catecholamines. Some bind irreversibly to beta receptors and are therefore used only in experimental settings. Some beta antagonists constitutively reduce the activity of beta receptors. These kind are known as inverse agonists. Most beta receptors blockers are structurally similar to isoproterenol
Harley,1983
Would we treat uncomplicated hypertension in 68year old patient with moderate asthma and renal impairment with atenolol, metoprolol or propranolol? Briefly explain the reasons for your decision. (4 marks)
It is believed that beta blockers work in reducing hypertension in the long term possibly partly by reducing the amount of renin released by the kidneys and partly by action on the central nervous system.in the short term, they cause a rise in the peripheral vascular resistance. Renin release is usually triggered by an active sympathetic nervous system.
Currently, no drug acts purely selectively on b1 receptors without b2 effects. It is worth noting that atenolol and metoprolol act with greater affinity on b1 receptors than b2. The b2 receptors in the respiratory system are best avoided in asthmatic patients because they are the ones responsible for increasing airway resistance. Therefore it is advisable to use no beta active drug.
Unless absolutely necessary, where the benefits outweigh the risks, atenolol and metoprolol can prove useful when the antihypertensive effects are needed urgently in a hypertensive asthmatic patient.
Atenolol has an oral bioavailability higher than propranolol suggesting that it is cleared less by the hepatocytes than kidneys.
Propranolol is contraindicated in asthma patients because it has b2 effects which can trigger an asthma attack. Propranolol is acts equally on both b1 and b2 receptors. Propranolol is extensively cleared by the liver therefore there would be no direct link between its administration and the renal impairment.
Again, hypertensive patients usually respond well to treatment with beta blockers alone but usually, diuretics are used as adjunct therapy or sometimes a vasodilator is substituted for a diuretic.
As noted, the hypertension is uncomplicated; therefore we would use atenolol or metoprolol along with a diuretic because of the impaired renal function. It would not be appropriate to use propranolol in whatever dose.
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