Adrenal Medullary Responses to Repeated Hemorrhage in the Anesthetized Dog*

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Abstract
Adrenal secretory rates of catecholamines in response to a repeated stimulus have not been described. We have used repeated small hemorrhages spaced 24 h apart in pentobarbital anesthetized dogs prepared with a chronic adrenal venous catheter to study this question. Our results confirm earlier reports that an initial 7.5 ml/kg hemorrhage is a mild stimulus to the adrenal medullary secretion of epinephrine and norepinephrine. However, when this stimulus is repeated 24 h later, a greatly potentiated secretory response of both hormones is seen. Cardiovascular changes after hemorrhage did not differ on the 2 days. Repeated 3.75 ml/kg hemorrhages did not lead to augmented adrenal medullary secretion of catecholamines on day 2. These data indicate that there is a factor associated with hemorrhage that potentiates the adrenal medullary response to a second small hemorrhage. This factor appears to be subject to a threshold that lies between a 3.75 and a 7.5 ml/kg hemorrhage. A significant catecholamine response on day 1 was not essential for the potentiated response on day 2. Thus, this phenomenon does not appear to be the result of adrenal enzyme induction as a consequence of prior adrenal medullary stimulation.