Brain Adenosine Concentration During Hypoxia in Rats

We analyzed brain tissue for adenosine and its metabolites, inosine and hypoxanthine, in rats during acute (30 s) hypoxia and during sustained (5 min) hypoxia and hypocarbia. Within 30 s of the onset of hypoxia, adenosine levels were increased from 0.34 +/- 0.08 (SE) to 1.65 +/- 0.33 nmol/g (P less than 0.005), paralleling temporally the changes in cerebral blood flow. During sustained hypoxia and hypocarbia, brain tissue was sampled by a fast (freeze-blow) or slow (in situ) freezing method. With the freeze-blow technique, adenosine concentrations remained stable between arterial partial pressure of O2 (PaO2) greater than 200 and 100 mmHg, doubled at PaO2 = 50 mmHg, and increased sevenfold (P less than 0.005) when PaO2 reached 30 mmHg. No increases in adenosine or its metabolites were noted with the in situ technique. During hypocarbia (arterial CO2 partial pressure less than 20 mmHg), adenosine concentrations increased with both sampling techniques. Freezing times in brain were measured during in situ freezing and were increased during hypoxia and decreased during hypocarbia. In conclusion, 1) adenosine concentrations in brain are increased during hypoxia, and 2) the in situ technique in rat does not appear to be optimal for sampling brain tissue for subsequent measurement of adenosine under conditions where cerebral blood flow is increased.