Role of adenosine in regulation of brain temperature in fetal sheep

OBJECTIVE: The purpose of this study was to test adenosine's possible suppression of heat production by the ovine fetal brain during acute hypoxia. STUDY DESIGN: Hypoxia was induced by occluding the umbilical cord for 5 minutes in 8 fetal sheep with and without an adenosine receptor blocker, theophylline, and fetal brain and core temperatures were recorded. RESULTS: In 8 untreated fetuses, card occlusion induced severe but reversible hypoxemia (decrease in carotid arterial PO2 from 23 +/- 1 to 5 +/- 1 mm Hg (P < .01), a 2.3-fold increase in plasma adenosine, and an increase in body core temperature of 0.19 degrees C +/- 0.03 degrees C (P < .01), yet brain temperature remained constant. However, after cord occlusion with prior and continuing administration of theophylline, brain temperature did not increase as hypothesized but rather decreased, suggesting marked reductions in cerebral metabolic rate. This response occurred despite similar degrees of hypoxemia and similar increases in plasma adenosine and body core temperature. CONCLUSION: We conclude that the temperature of the fetal ovine brain remains constant or decreases during severe reversible hypoxemia by mechanisms other than those dependent on adenosine.