INCREASED CORTICOSTEROID-BINDING CAPACITY OF PLASMA-ALBUMIN BUT NOT OF CORTICOSTEROID-BINDING GLOBULIN CAUSED BY ACTH-INDUCED CHANGES IN FREE FATTY-ACID CONCENTRATIONS IN SNOWSHOE HARES AND RABBITS

Free fatty acids (FFAs) are rapidly mobilized by ACTH and have been sh own to be potent endogenous modulators of steroid-protein interactions . We increased FFA in lagomorphs by ACTH and then separated the transi ent increase in glucocorticoid binding capacity of plasma into that ac counted for by changes in binding to albumin and to corticosteroid-bin ding globulin (CBG). Sequential injections of dexamethasone and ACTH i nto both snowshoe hares and laboratory rabbits resulted in the rapid m obilization of FFA only after the ACTH injection. The maximum corticos teroid binding capacity increase paralleled that of the FFA increase i n both species. In rabbits, CBG levels remained constant over the dura tion of the experiment. Corticosterone binding by rabbit albumin incre ased in a dose-dependent fashion in response to increases in FFA (olei c and linoleic acid) concentrations. Finally, by stimulating FFA relea se in snowshoe hares with ACTH and separating the increase in corticos teroid binding capacity through selective denaturing of CBG by heat, w e determined that the increase in plasma binding capacity was a respon se to changes in binding by albumin, not CBG. Thus FFA released in res ponse to stressors in lagomorphs may effect short-term increases in st eroid binding.