THE EFFECTS OF CENTRALLY ADMINISTERED ADENOSINE ON FETAL SHEEP HEART-RATE ACCELERATIONS

OBJECTIVE: We examined the effect of administering a long-acting adeno sine analog, L-2-N6-(phenylisopropyl) adenosine, into the cerebrospina l fluid of the fourth ventricle on fetal sheep heart rate acceleration s. STUDY DESIGN: Pregnant ewes between 123 and 130 days' gestation wer e anesthetized, and the fetal head was exteriorized. Catheters were pl aced in the fourth cerebral ventricle through the foramen magnum and i n the brachial artery to record fetal heart rate. Studies were perform ed in unanesthetized fetuses 4 to 7 days after surgery. Accelerations were defined as a 10 beats/min rise in heart rate for at least 5 secon ds. RESULTS: The mean number of accelerations before administration of L-2-N6-(phenylisopropyl) adenosine was 3.9 +/- 0.7 (mean - SE) per 1 0-minute epoch, decreasing to 2.0 +/- 0.7 and 1.4 +/- 0.8 after instil lation of 0.2 mug and 0.5 mug of L-2-N6-(phenylisopropyl) adenosine, r espectively (p < 0.05). Increasing the L-2-N6-(phenylisopropyl) adenos ine dose to 10.0 mug resulted in loss of heart rate accelerations. Acc elerations were not reduced when theophylline, an adenosine receptor b locker, was given before L-2-N6-(phenylisopropyl) adenosine CONCLUSION : Increasing amounts of a centrally administered adenosine analog prog ressively decreased the number of fetal sheep heart rate accelerations , most probably by suppression of brainstem sympathetic outflow.