Tyrosine hydroxylase activity and catecholamine biosynthesis in the adrenal medulla of rats during stress

Chronic hypotension and hypoglycemia are known to increase the capacity for catecholamine biosynthesis in the rat adrenal medulla by increasing the maximal velocity of the rate-limiting enzyme, tyrosine hydroxylase (TH). The present report indicates that the gradual increase in maximal TH activity is preceded by a more rapid increase in the affinity of TH for its pterin cofactor. These short-term alterations in adrenal TH activity are related to the severity of the stress, associated with parallel changes in catecholamine biosynthesis, and prevented by prior adrenal denervation. In contrast, cold exposure, which leads to comparable long-term increases in adrenal TH activity, does so without causing a prior activation of TH. However, adrenal TH is activated by acute cold exposure if the sympathetic nerves, that normally are stimulated during cold, are destroyed previously by 6-hydroxydopamine treatment. These and other observations suggest that alterations in adrenal TH activity vary according to the type and duration of physiological stress, and may be mediated by temporally distinct processes.