Neuronal metabolism of catecholamines in pithed and electrically stimulated rats

The aim of the present study was to investigate a possible physiological equilibrium, assessed by statistically significant correlations, between pre-junctional mechanisms that regulate both release and reuptake of norepinephrine (NE) and post-junctional mechanisms that participate in the regulation of the smooth muscle cell and thus in the regulation of blood pressure. This study was carried out in pithed and electrically stimulated (2.5 Hz) rats to obtain an experimentally controlled release of NE. A radio-enzymatic assay was used to measure plasma concentrations of NE, index of NE release and levels of its deaminated metabolites 3,4-dihydroxyphenyl glycol (DHPG) and 3,4-dihydroxymandelic acid (DOMA). DHPG is known to be an index of NE reuptake and deamination while the physiological significance of DOMA remains unclear. Our results demonstrate a statistically significant correlation between plasma NE concentration and blood pressure on one hand, and between plasma NE and DHPG concentrations on the other. These correlations support our working hypothesis and suggest a physiological equilibrium between pre- and post-junctional phenomena in the control of blood pressure. During 2.5-Hz stimulation, plasma DHPG concentration was increased while plasma DOMA remained unchanged. This is consistent with activation of the reduction pathway and the consequent formation of DHPG by aldehyde reduction, while the oxidation pathway mediated by aldehyde dehydrogenase does not appear to play a major role in the presynaptic metabolism of reuptaken NE in the present experimental conditions. Further investigations are needed to investigate the apparent dissociation between the two enzymatic pathways involved in the second step of the deamination process.