09.04 Receptors

Drugs can act at receptor to mimic the effect of the endogenous substance, and this is known as agonism. Conversely, drugs can act at receptors to block the effects of endogenous substances, and this is antagonism. The sites of actions of drugs can be limited, if the drug shows selectivity for a receptor. Examples from the sympathetic nervous system are to be used to explain this. Noradrenaline (released from the sympathetic nervous system) and adrenaline (released from the adrenal medulla) stimulate all the peripheral adrenoceptors to have widespread effects. They can act on the β₂-adrenoceptors in the lung to cause bronchodilation, and they can act on the β₁-adrenoceptors of the heart to increase heart rate and force. The selective β₂-adrenoceptor agonist salbutamol has no effect at α-adrenoceptors, and is selective for β₂- over β₁-adrenoceptors. Salbutamol is the standard selective β₂-adrenoceptor agonist used to cause bronchodilation in an asthma attack. The β₁-selective adrenoceptor antagonist atenolol is selective, not specific, for β₁-adrenoceptors, and have some effects at β₂-adrenoceptors. The selective β₁-blockers are used in the treatment of hypertension.