AJP - Endocrinology and Metabolism, Vol 254, Issue 1 E79-E83, Copyright © 1988 by American Physiological Society
TeBG- and CBG-bound steroid hormones in rabbits are available for influx into uterus in vivo
G. Chaudhuri, K. A. Steingold, W. M. Pardridge and H. L. Judd
Department of Obstetrics and Gynecology, School of Medicine, University of California, Los Angeles 90024.
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.
