Regional adrenal blood flow during hypoxia in anesthetized, ventilated dogs

The effect of hypoxic hypoxia (HH) and carbon monoxide hypoxia (COH) on adrenal medullary (MQ) and cortical (CQ) blood flow (radiolabeled microsphere technique) was studied in pentobarbital sodium-anesthetized, mechanically ventilated dogs. Animals were exposed to 60 min of hypoxia (arterial O2 content 8 vol%) induced by adding either nitrogen (HH, n = 6) or carbon monoxide (COH, n = 6) to the inspired gas. Whole adrenal Q and CQ increased by 70 and 50%, respectively, with HH but were unchanged during COH. MQ, however, increased threefold during both HH and COH. HH and COH both increased arterial levels of epinephrine, corticosteroids, and adrenocorticotropic hormone (ACTH). To determine whether the increase in CQ during HH was because of HH-induced increases in mean arterial blood pressure (MAP, approximately 20 mmHg), an additional group of animals (n = 6) was exposed to HH but had MAP maintained at control levels using a pressurized-bottle system. MAP control did not alter the CQ response to HH. We conclude that MQ appears to be associated with medullary secretory activity during hypoxia and that HH and COH stimulate adrenal medullary secretion equally. In contrast, CQ increases only with HH, despite similar increases in ACTH and corticosteroid levels during HH and COH, suggesting that an alternative mechanism is responsible for increased cortical blood flow during HH.