Monoamine deamination in the brain and heart of rats exposed to toxic hyperbaric oxygenation

Abstract

Biogenic amines are known to participate in the convulsant response of electrical, chemical, and hyperoxic genesis. For instance, an increase in sensitivity to convulsants was accompanied by a fall in the brain level of all biogenic amines [i]. Conversely, elevation of the noradrenalin (NA) and dopamine (DA) levels in the brain after injection of dopa and of monoamine oxidase inhibitors (MAO) followed a parallel course with elevation of the threshold of chemical and electrical convulsions [12]. In rats predisposed to audiogenic convulsions, the serotonin (5-HT) and NA levels were considerably depressed in various brain structures, whereas the DA level was raised by comparison with these parameters in animals with low excitability [9]. Activity of MAO, one of the most important enzymes of biogenic amine metabolism: has been shown [4,6] to be considerably modified when the oxygen pressure is raised~ the velocity of oxidative deamination of the principal neutotransmitter monoamines, 5-HT and NA, is reduced, whereas at the same time the enzyme acquires the property of oxidizing substrates which under normal conditions areuncharacteristic, namely GABA, putrescine, aminosugars, and other substances [4]. A similar effect, namely MAO transformation, has also been described under other extremal conditions [6], and it can be regarded as the enzyme's response to stress stimuli. However, in the publications cited above, the velocity of the deamination reaction for most MAO substrates was determined purely as an indicator of specific activity, using a single (saturating for "normal" conditions) concentration of the substrate, and a single time point, namely the 60th minute of exposure to the toxic action of oxygen. Yet it is possible that saturation curves of enzymes under stress conditions are modified, and this makes it essential to study the deamination kinetics of monoamines during exposure to hyperoxia to be investigated by the use of different concentrations. Another important aspect of the investigation is a comparative study of changes in monoamine metabolism in the presence of a raised oxygen pressure in the CNS and peripheral organs, especially those which suffer under these conditions, namely the heart.