Mechanisms of elevation of adenosine levels in anoxic hepatocytes

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Abstract
Previous work has shown that normoxic isolated rat hepatocytes continuously produce adenosine from AMP and that the nucleoside is not catabolized further but immediately rephosphorylated by adenosine kinase [Bontemps, Van den Berghe and Hers (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 2829-2833]. We now report the effect of anoxia on adenosine production and on the AMP/adenosine substrate cycle. In cell suspensions incubated in O2/CO2, the adenosine concentration was about 0.4 microM. It increased 30-fold in cells incubated in N2/CO2 or with 5 mM KCN, and 20-fold in cells incubated with 2 mM amytal. Adenosine production, measured in hepatocytes in which adenosine kinase and adenosine deaminase were inhibited by 5-iodotubercidin and deoxycoformycin respectively, was about 18 nmol/min per g of cells in normoxia; it increased about 2-fold in anoxia, although AMP increased 8-16-fold in this condition. From studies with inhibitors of membrane 5′-nucleotidase and of S-adenosylhomocysteine hydrolase, it was deduced that adenosine is produced by the latter enzyme and by cytosolic 5′-nucleotidase in normoxia, and by cytosolic and membrane 5′-nucleotidases in anoxia. Unlike in normoxic hepatocytes, inhibition of adenosine kinase by 5-iodotubercidin neither elevated the adenosine concentration nor enhanced total purine release from adenine nucleotides in cells treated with N2/CO2 or KCN; it had only a slight effect in cells treated with amytal. This indicates that recycling of adenosine is suppressed or profoundly inhibited in anoxia. The rate of accumulation of adenosine in anoxia was several-fold lower than the rate of its rephosphorylation upon reoxygenation. It is concluded that the elevation of adenosine in anoxic hepatocytes is much more dependent on decreased recycling of adenosine by adenosine kinase than on increased production by dephosphorylation of AMP.