Adrenal and Urinary Catecholamines in Rats During Adaptation to Repeated Immobilization Stress

Adrenal and urinary catecholamine levels were measured in rats subjected to one immobilization stress or during adaptation to daily repeated immobilization. A decrease in adrenal epinephrine was apparent after 90 min of the first immobilization and persisted for 24 hr after the immobilization was terminated. Adrenal norepinephrine was not affected. Urinary epinephrine and norepinephrine (24-hr excretion) were increased by one immobilization; most of these catecholamines were excreted during the interval of immobilization. After daily repeated immobilization for 2.5 hr, "adaptation" of the adrenal medulla appeared to occur. Immediately after the ninth immobilization adrenal epinephrine was still decreased, but 24 hr later it was at the control level. After 40–350 times repeated immobilization, adrenal epinephrine was not decreased and adrenal norepinephrine was increased. Urinary excretion of epinephrine was greater in rats subjected to repeated immobilization than in unstressed or once-immobilized rats. These results suggest that the "adaptation" of the adrenal medulla to repeated immobilization stress in rats is the result of an enhanced ability to replace the released catecholamine rather than to a diminished catecholamine release after repetition of this stress. (Endocrinology87: 738, 1970)