Regulation of adrenal medullary and cortical blood flow

Before 1986, little was known concerning regulation of blood flow to adrenal medulla and cortex. With the introduction of a radiolabeled microsphere methodology, which permitted independent measurement of blood flow to the two regions, it became apparent that medulla and cortex regulate blood flow independently and that both regions receive levels of blood flow considerably in excess of that required for nutrient delivery. It is likely that abundant blood flow serves to speed entry of adrenal secretory products into the systemic circulation. Coupling of blood flow with secretion is particularly apparent in medulla, where severalfold increases in blood flow occur during catecholamine secretion elicited by hemorrhage, hypoxia, and splanchnic nerve stimulation. Both catecholamine secretion and medullary vasodilation are neurally mediated. The two responses can be pharmacologically dissociated, suggesting possible direct innervation of adrenal vessels. However, morphological confirmation of vascular innervation is not available, and effects of released chromaffin cell products on adjacent vessels are also possible. In contrast to medulla, cortical secretory activity is not associated with altered blood flow. However, neural elements are found in close proximity to cortical vessels, and the cortical vasculature responds to changes in O2 tension and to a number of pharmacological agents. In addition, several studies indicate that cortical secretory activity can be modulated by changes in blood flow. The response of both vasculatures to pharmacological agents is, in many instances, different from responses observed in other vascular beds. These data indicate that the adrenal has evolved unique vasoregulatory mechanisms, either because vasoactive compounds are present in such high concentration, or because blood flow plays an important role in the secretory process.