The baroreceptor-heart rate reflex in ACTH hypertension in sheep

ACTH administration in sheep produces an adrenally dependent rise in blood pressure. Cardiac output and heart rate are usually increased. The precise mechanisms involved in the genesis of the hypertension are unclear. This study examines the sensitivity of the baroreflex heart rate response to phenylephrine hydrochloride and sodium nitroprusside before, during and after ACTH administration in sheep. During ACTH administration there was a sustained rise in blood pressure within 24 hours, whereas heart rate rose gradually. There was a sustained fall in baroreflex sensitivity to sodium nitroprusside within 24 hours, whereas baroreflex sensitivity to phenylephrine fell gradually over the first three days. The different time course of the change in sensitivity suggests that two different mechanisms are influenced by ACTH administration, for instance, changes in function in both cardiac vagal efferents and sympathetic pathways.