Heterocycles in Medicine 173Adrenaline and noradrenaline
Adrenaline and noradrenaline are prominent in the cardiovascular system, acting through - and -adrenergic recep-
tors, which exist as a number of subtypes. The interplay of effects from these receptors is quite complex. Many common
drugs are simple carbocyclic analogues of adrenaline but a few of the important compounds are heterocyclic.
Other signifi cant cardiovascular drugs
Amlodipine is one of a number of 1,4-dihydropyridines, the mechanism of action of which involves blocking calcium
channels (they are said to be ‘calcium antagonists’), resulting in a relaxation of vascular smooth muscle. They are use-
ful for hypertension and angina. Diazoxide is a vasodilator used for intravenous administration in hypertensive emer-
gency. Hydralazine and minoxidil are vasodilators used for chronic hypertension, the latter being particularly useful for
cases resistant to other drugs. Minoxidil is also of proven value for the treatment of baldness (alopecia).
Drugs acting specifi cally on the CNS
The CNS contains a wide variety of neurotransmitters and high concentrations of receptors. Mechanisms of action of
many drugs are often complex combinations of receptor-based actions. Some of the most widely used (and abused)
drugs are hypnotics/sedatives, for treatment of insomnia. Barbiturates such as amylobarbitone have been used for
many years but suffer from side-effects and are addictive. Thiopentone, however, is very useful as a short-acting intra-
venous anaesthetic. The benzodiazepines are safer drugs for insomnia and also can be used for treatment of anxiety
and muscle spasms. Zolpidem is a newer more selective hypnotic.