TeBG- and CBG-bound steroid hormones in rabbits are available for influx into uterus in vivo.

The metabolic clearance rate (MCR) of gonadal or adrenal steroid hormones in rabbits often does not bear the expected inverse relationship with hormone binding to testosterone-binding globulin (TeBG) or corticosteroid-binding globulin (CBG). This suggests TeBG or CBG may not impede steroid hormone delivery to tissues. The effects of rabbit plasma proteins on the influxes of 3H-labeled steroids from the circulation into the rabbit uterus were measured in vivo using a tissue sampling single-injection technique. In the absence of plasma proteins, estradiol (E2) and testosterone (T) were freely diffusible through the uterine microvasculature (i.e., extraction greater than 80%). The extractions of dihydrotestosterone (DHT) and corticosterone (B) ranged from 60 to 72%, while that of cortisol (F) was reduced at 40%. Rabbit serum exerted no inhibition of the influxes of the steroids tested. The influxes of T and B greatly exceeded the rates that would be expected if only the free and albumin-bound fractions estimated in vitro were diffusible in vivo. However, the extraction of [3H]corticosteroid-binding globulin or bovine [3H]albumin were low, consistent with little, if any, extravascular uptake of the plasma proteins. The results indicate both albumin-bound and globulin-bound steroid hormone are available for transport into the uterus in the rabbit in vivo without significant exodus of the plasma protein, per se.