The cellular source and role of
adenosine in
hypoxia-induced coronary vasodilatation was investigated. The endothelial
adenine nucleotides of Langendorff-perfused rat hearts were prelabelled by perfusion with [3H]
adenosine and the changes in specific radioactivities were employed to identify the source of the
adenine compounds released. The contribution of
ecto-5'-nucleotidase was evaluated in perfusions with the inhibitor alpha,beta-methylene
adenosine diphosphate (
AOPCP). Absorbance of the effluent perfusate at 260 nm was monitored continuously as a convenient means of detecting the output of total
purines, and it showed a good correlation with HPLC-measured
purines (r = 0.72, P < 0.001). Coronary flow increased sharply in
hypoxia but tended to decrease after 2 min, while effluent radioactivity and absorbance increased steadily. The radioactivity-to-absorbance ratio and the specific radioactivity of chemically measured total
purines began to increase after 3 min. The changes in effluent concentrations of
adenosine and
inosine were much more prominent than those in free
purines. The specific radioactivity of
adenosine decreased sharply at the onset of
hypoxia which indicates that
hypoxia affects mainly working cardiomyocytes. This also means that endothelial
adenosine release is delayed if compared to coronary vasoregulation. Although the inhibition of
ecto-5'-nucleotidase caused a decrease in the release of
adenosine and
adenine moiety label from the heart it is most likely that
adenosine was mainly derived from intracellular sources, because the
hypoxia-induced increase in the concentration of
adenosine was more excessive than that of
AMP. In addition,
AOPCP decreased the basal work load and coronary flow of the heart, slightly attenuated the
hypoxia-induced flow increase and prevented adenylate loss during
hypoxia. Thus, the data emphasize the role of cardiomyocytes in
adenosine production and coronary vasoregulation.