ADRENERGIC RECEPTORS The sites in the body on which adrenaline and comparable stimulants of the sympathetic nervous system act. Drugs which have an adrenalinelike action are described as being adrenergic. There are five different types of adrenergic receptors, known as alpha,, alpha2, beta,, beta2 and beta, respectively. Stimulation of alpha receptors leads to constriction of the bronchi, constriction of the blood vessels with consequent rise in blood pressure and dilatation of the pupils of the eyes. Stimulation of beta, receptors quickens the rate and output of the heart, while stimulation of beta2 receptors dilates the bronchi. Beta3 receptors are now known to mediate so-called non-shivering ther-mogenesis, a way of producing heat from specialized fat cells particularly relevant to the human infant.
For long it had been realized that in certain cases of asthma adrenaline had not the usual beneficial effect of dilating the bronchi during an attack; rather it made the asthma worse. This was due to its acting on both the alpha and beta adrenergic receptors. A derivative, isoprenaline, was therefore produced which acted only on the beta receptors. This had an excellent effect in dilating the bronchi, but unfortunately also affected the heart, speeding it up and increasing its output - an undesirable effect which meant that isoprenaline had to be used with great care. In due course drugs were produced, such as salbutamol, which act predominantly on the beta 2 adrenergic receptors in the bronchi and have relatively little effect on the heart.
The converse of this story was the search for what became known as beta-adrenoceptor-blocking drugs, or beta-adrenergic-blocking drugs. The theoretical argument was that if such drugs could be synthesized, they could be of value in taking the strain off the heart - for example: stress —» stimulation of the output of adrenaline -> stimulation of the heart -> increased work for the heart. A drug that could prevent this train of events would be of value, for example, in the treatment of angina pectoris. Now there is a series of beta-adrenoceptor-blocking drugs of use not only in angina pectoris, but also in various other heart conditions such as disorders of rhythm, as well as high blood-pressure. They are also proving of value in the treatment of anxiety states by preventing disturbing features such as palpitations. Some are useful in the treatment of migraine