Effects of the alpha 2-adrenergic agonist clonidine and its antagonist idazoxan on the fetal lamb.

1. The alpha 2-adrenergic agonist clonidine was given by aortic injection to three groups of unanaesthetized fetal lambs in utero near term. One group was intact, the second had the brain stem transected just above the pons, and the third had bilateral section of the carotid sinus nerves and cervical vagi. 2. Clonidine had similar effects in all three groups. Electrocortical activity entered a high-voltage, low-frequency episode: breathing, neck and limb movements ceased; arterial pressure remained unchanged or increased; heart rate fell or remained unchanged, and the variation in both heart rate and blood pressure was much reduced. This state lasted 10-20 min, and was followed by a period of up to 4 h during which the cycling of electrocortical activity was rapid and irregular. 3. The alpha 2-adrenergic antagonist idazoxan (0.5-2.0 mg I.A.) blocked all the actions of clonidine. When given alone it usually induced a short period of low-voltage electrocortical activity and stimulated breathing movements. These effects were present in both the intact and brain-stem-transected groups, though the stimulation of breathing was significantly reduced by brain-stem transection. There were no consistent effects on heart rate or blood pressure. 4. The effects of clonidine on fetal heart rate and electrocortical activity were similar to those described in adults, but it also had inhibitory effects, not present in adults, on fetal breathing and somatic movements.