In recent years, it has become apparent that neurons have associated with them specific transport systems for the particular transmitter which they manufacture and release. Thus neurons that release norepinephrine (NE) possess a specific transport system capable of transporting NE from the external medium into the neurons. Similar uptake systems for dopamine and 5-HT are associated with neurons containing these transmitters (Iversen, 1967, 1970, 1971, 1973; Snyder et al., 1970). In cholinergic neurons, there is an uptake system for the precursor substance choline (Snyder et al., 1973). Apart from the transport of biogenic amines into the neurons themselves, other mechanisms exist to mediate the transport of these amines across the membranes of various nonneuronal cells which are associated with synapses using amine transmitters. Furthermore, within the neuronal cytoplasm there are other uptake systems to transfer the amines from the cytoplasm into synaptic vesicles, which act as concentrated amine storage reservoirs. The existence of these numerous amine transport systems is perhaps not surprising, since the biogenic amines are highly polar substances which do not readily penetrate biological membranes by simple diffusion.

Uptake Processes for Biogenic Amines

Membrane-bound catechol-O-methyltransferase: a reevaluation of its role in the O-methylation of the catecholamine neurotransmitters.

Long before the mechanism of action of the indirectly acting sympathomimetic amines was understood, it was clear that the amines of this group differ pharmacologically. For instance, while cocaine potentiates the effects of adrenaline (FroHlich and Loewi, 1910) it antagonizes the effects of tyramine (Tainter and Chang, 1927). A parallel to this “cocaine paradox” was observed after chronic sympathetic denervation, a procedure which potentiates the effects of adrenaline and antagonizes those of tyramine (Burn and Tainter, 1931; Bulbring and Burn, 1938).

Classification of Sympathomimetic Amines

There are no methods available for the direct determination of the concentration of noradrenaline in the “biophase”, i.e., in the very small part of the extracellular space in the immediate neighbourhood of the receptors of adrenergically innervated organs. Hence, any discussion of the factors influencing the concentration of the amine at the receptors has to be based on indirect evidence. If the direct approach is impossible, one can gain some insight from recording responses (or changes in responses) of the effector organ to known concentrations of noradrenaline. However, any interpretation involves two crucial assumptions, namely that, in the absence of conclusive evidence to the contrary, the interaction between drug and receptor as well as the function of the link between receptor activation and eventual response remains unaffected by the change in experimental conditions which is being studied. In many experimental situations the validity of these assumptions remains questionable, since it has not been possible to obtain positive evidence.

Factors Influencing the Concentration of Catecholamines at the Receptors

A kinetic analysis of the extraneuronal uptake and metabolism of catecholamines

The sensitivity of various isolated organs to catecholamines was tested before and after block of catechol-O-methyl transferase (COMT) by U-0521 (3′,4′-dihydroxy-2-methyl propiophenone; 18 μg/ml for 20 min). 
Isolated Nictitating Membrane of the Cat. The sensitivity to catecholamines was increased by inhibition of COMT whenever the experimental conditions resulted in a high potency of the amine, i.e., when the ED50 of the amine (in the absence of U-0521) was below about 10−6 M. Thus, block of COMT potentiated the effects of (−)-noradrenaline and (−)-adrenaline after denervation (or in the presence of cocaine) but not on muscles with an intact adrenergic innervation; it also increased the sensitivity to theβ-effects but not to theα-effects of isoprenaline. No potentiation was observed for dopamine which has a low potency even after denervation. The relation between potency of the catecholamine and the degree of the sensitizing effect of U-0521 was not due to saturation of COMT or of access to the enzyme. Measurements in denervated muscles of the production of O-methylated metabolites from (±)-noradrenaline-H3 (added to the bath for 20 min in concentrations of about 10−7 to 10−4 M), and of the extraneuronal accumulation of noradrenaline-H3 did not reveal any saturation of either the enzyme or the extraneuronal accumulation of the amine. When block of COMT resulted in supersensitivity to a catecholamine, the time required to reach steady-state responses was usually increased. This is consistent with the view that block of the enzyme impaired a site of loss. Block of COMT failed to produce any substantial potentiation of the effects of the indirectly acting amines, tyramine and mephentermine. O-methylation of the released transmitter seems to occur after the amine has reached the receptors of the effector cells. 
Isolated Strips of Cat and Bat Spleen. Results were quantitatively similar, since block of COMT potentiated the effects of (−)-noradrenaline and (−)-adrenaline in the presence but not (or only very little) in the absence of cocaine. However, the degree of supersensitivity after block of COMT (in the presence of cocaine) was smaller than in the nictitating membrane. In the cat spleen, cocaine causes a small but significant degree of supersensitivity to isoprenaline but not to methoxamine. 
Isolated Sat Aorta, Block of COMT did not increase the effect of cocaine. It is suggested that differences in the morphology of the adrenergic innervation contribute to the observed organ differences.

K. H. Graefe

Papers

The influence of block of catechol-O-methyl transferase on the sensitivity of isolated organs to catecholamines