Effect of central adenosine on brainstem blood flow in fetal sheep.

The effect of the adenosine analogue R-N6-(phenylisopropyl)adenosine (R-PIA) on blood flow to the medulla and pons was examined in unanaesthetized fetal sheep. Microspheres labelled with isotopes were used to determine blood flow before and after instillation of 0.2 or 0.5 microgram R-PIA into the cerebrospinal fluid of the fourth ventricle. Blood flow to the medulla, which had a mean value (+/- S.E.M.) of 285 +/- 41 ml min-1 (100 g)-1 during the control period, was not changed by the central instillation of R-PIA. Blood flow to the pons was also not affected. These data indicate that central adenosine, which depresses respiratory drive in fetal sheep, acts by mechanisms independent of removal of carbon dioxide from the brainstem.