The effects of corticotrophin-releasing factor and two antagonists on breathing movements in fetal sheep.

1. The respiratory effects of corticotrophin-releasing factor (CRF) and the CRF antagonists alpha-helical CRF 9-41 (alpha hCRF) and [DPhe 12, Nle 21-38] rCRF (12-41) (DPhe CRF) have been studied in unanaesthetized fetal lambs of 125-140 days gestation. 2. CRF when given as a 10 micrograms bolus followed by a 5 micrograms h-1 infusion into a lateral cerebral ventricle caused prolonged continuous fetal breathing movements which were stimulated in both amplitude and frequency but which did not persist during hypoxia. 3. Lower doses of CRF (20 ng bolus followed by 10 ng h-1) increased the amplitude but not the frequency of fetal breathing movements which did not become continuous. 4. At higher doses (20 micrograms bolus followed by 10-15 micrograms h-1) CRF induced cerebral convulsions which were also associated with fetal breathing movements of increased amplitude and frequency. 5. The CRF antagonists alpha hCRF and DPhe CRF both inhibited fetal breathing movements and induced a prolonged apnoea which was resistant to the stimulatory effects of 5-6% hypercapnia. 6. We conclude that CRF stimulates breathing movements in the fetal lamb. The finding that administration of the CRF antagonists alone cause apnoea suggests that CRF may have a tonic role in the regulation of fetal breathing movements.