Adenosine metabolism in neurons and astrocytes in primary cultures

Metabolic fate of [8-14C]adenosine was studied in primary cultures of either astrocytes or neurons from the mouse brain. In astrocytes the main metabolic route was the formation of nucleotides. Thus, synthesis of adenosine triphosphate (ATP) amounted to about 0.2 nmol X min-1 X mg-1 protein. The deamination occurred less rapidly. The total rate of formation of inosine was difficult to establish because a considerable amount of labeled inosine accumulated in the medium. The initial incorporation of radioactivity into inosine in the medium was extremely rapid, probably because of the action of an ectoenzyme. However, the labeling of inosine in the medium also continued to increase slowly throughout the incubation, maybe as a result of release of intracellularly formed inosine. The total inosine formation rate during the incubation amounted to at most 0.1 nmol X min-1 X mg-1. Hypoxanthine was formed at a corresponding rate but was released to a lesser extent. In neurons much less label was incorporated into ATP. The major metabolite was inosine, formed intracellularly at a rate of 0.2 nmol X min-1 X mg-1. In addition, there was an immediate rapid labeling of inosine (and to a lesser extent hypoxanthine) in the medium, again suggesting the action of an ectoenzyme. Neither neurons nor astrocytes released a measurable amount of nucleotides to the medium. The cellular differences in adenosine metabolism are probably of relevance for the interpretation of adenosine metabolism in brain in situ. The ectoenzyme may be of importance for rapid termination of the neuromodulator activity of adenosine, and the rapid nucleotide formation in astrocytes is in agreement with a high metabolic activity of these cells.