The catecholamines dopamine, L(-)-epinephrine and L(-)-norepinephrine (Figure 1.1-1.3) act in the body as neurotransmitters as well as as hormones. Norepinephrine is the predominant neurotransmitter, whereas epinephrine the major hormone. Catecholamines are released from the nervous system following neuronal depolarization. The ratio of L(-)-epinephrine and L(-)-norepinephrine is about 80:20 in humans. Both transmitters put the body into a condition coined as "fight and flight reaction":
Figure 1.3: L(-)-Norepinephrine
Figure 1.2: L(-)-Epinephrine
Figure: Dopamine
Catecholamines exert their cellular action via binding to membrane proteins,
the so-called adrenergic receptors or adrenoceptors.
The term `adrenoceptor' was
first introduced by AHLQUIST to explain the
different physiological effects of catecholamines[17]. In 1948
Ahlquist [19] further
differentiated 
- and
-receptors
due to their response to
L(-)-epinephrine. In vascular smooth muscles the
degree of contraction induced by these substances follows the order
L(-)-epinephrine 
L(-)-norepinephrine
isoprenaline ( -receptors).
The vaso-dilatatory effect and the positive chronotropic effects (i.e., rise
of heart rate)
follow the order isoprenaline > L(-)-epinephrine
L(-)-norepinephrine ( -receptors).
These types of receptors were further splitted into several subgroups. Table tab:prop summarizes their pharmacological differentiation and physiological effects.
The existence of these receptor subtypes has been
proven by radioligand assays .
It has been shown, that a number of tissues may contain both
-receptors. This distribution is often unbalanced, as table
tabb1b2 shows.
Table 1.1: Distribution of -receptors
