Transport and metabolism of adenosine in relation to the transcriptional activity of hnRNA genes in Chironomus salivary gland cells.

The transport and metabolism of adenosine in explanted salivary gland cells of Chironomus tentans have been investigated. The adenosine transport is rapid and reaches a maximum velocity within seconds after administration. Nevertheless, a transmembrane equilibrium in adenosine concentrations could never be attained because of the efficiency of the intracellular trapping reaction. Only about 10% or less of the extracellular adenosine concentration could be maintained intracellularly. The rapidity of adenosine phosphorylation did not allow us the assessment of transport kinetics with any degree of accuracy. At lower external [3H]adenosine doses, [3H]ATP was the predominating metabolite, yielding a [3H]ATP/[3H]AMP ratio of 2.5-3.5, while at higher concentrations the [3H]ATP/[3H]AMP ratio was lowered to below 0.9. The [3H]AMP fraction derived from [3H]adenosine-treated cells was not uniform, but rather it consisted of 3H-labeled 5'AMP, 3'AMP and 2'AMP isomers. Whereas the accumulation of 3H-labeled 5'AMP and ATP attained steady-state levels after 30-60 min of incubation at higher exogenous adenosine concentrations, the content of 3H-labeled 3'AMP and 2'AMP continuously and linearly increased. The data indicate that the metabolism of adenosine to 2'AMP and 3'AMP represents a salvage pathway operating at unphysiological adenosine levels and that the well-known inhibitory effect of adenosine on polymerase-II-promoted RNA transcription is not exerted by its phosphorylated metabolites.