JENNINGS, D.H.*; RUYS, J.D.; MOORE, M.C.; ORCHINIK, M.: Corticosterone regulation of plasma steroid-binding globulin levels and free steroid hormone levels in Tree lizards, Urosaurus ornatus.

In vertebrates, steroids circulate bound to specific plasma steroid-binding globulins. Binding globulins potentially regulate steroid actions at a number of levels including steroid metabolism, availability of free steroids, and tissue-specific delivery of steroids. The affinity and capacity of steroid-binding globulins changes seasonally, and under different social conditions, suggesting that binding globulins may play an active role in mediating the effects of circulating steroids. In tree lizards, Urosaurus ornatus, the capacity of steroid-binding globulins is greater in territorial males than in nonterritorial males. This difference results in territorial males having lower levels of free corticosterone than nonterritorial males. To determine if binding capacity differences between male types results from differences in free corticosterone levels, we measured the affinity and capacity of steroid-binding globulins in males with experimentally elevated levels of corticosterone. Elevated corticosterone levels had no effect on steroid-binding globulin affinity. In contrast, steroid-binding capacity increased in animals with long-term corticosterone elevation. However, increased binding capacity had little effect on free hormone levels in either territorial or nonterritorial males. Our results differ from previous work in mammals, where elevated corticosterone levels resulted in decreased steroid-binding globulin levels. Differences in the hormonal mechanisms that regulate the production and binding characteristics of steroid-binding globulins suggest that the functions of steroid-binding globulins also may differ among taxa.