Abstract
It is generally assumed that steroid hormones are carried in the blood free and/or bound to plasma proteins. We investigated whether blood cells were also able to bind/carry sex-related hormones: estrone, estradiol, DHEA and testosterone. Wistar male and female rats were fed a cafeteria diet for 30 days, which induced overweight. The rats were fed the standard rat diet for 15 additional days to minimize the immediate effects of excess ingested energy. Controls were always kept on standard diet. After the rats were killed, their blood was used for 1) measuring plasma hormone levels, 2) determining the binding of labeled hormones to washed red blood cells (RBC), 3) incubating whole blood with labeled hormones and determining the distribution of label between plasma and packed cells, discounting the trapped plasma volume, 4) determining free plasma hormone using labeled hormones, both through membrane ultrafiltration and dextran-charcoal removal. The results were computed individually for each rat. Cells retained up to 32% estrone, and down to 10% of testosterone, with marked differences due to sex and diet (the latter only for estrogens, not for DHEA and testosterone). Sex and diet also affected the concentrations of all hormones, with no significant diet effects for estradiol and DHEA, but with considerable interaction between both factors. Binding to RBC was non-specific for all hormones. Estrogen distribution in plasma compartments was affected by sex and diet. In conclusion: a) there is a large non-specific RBC-carried compartment for estrone, estradiol, DHEA and testosterone deeply affected by sex; b) Prior exposure to a cafeteria (hyperlipidic) diet induced hormone distribution changes, affected by sex, which hint at sex-related structural differences in RBC membranes; c) We postulate that the RBC compartment may contribute to maintain free (i.e., fully active) sex hormone levels in a way similar to plasma proteins non-specific binding.
Highlights
Steroid hormones are found free in blood plasma, but a variable proportion of these hormones is carried bound to protein, in a few cases with high affinity, as is the case of SHBG (Sex HormoneBinding Globulin) [1] in humans, binding both 17b-estradiol and testosterone, or CBG (Corticosteroid-Binding Globulin) [2], essentially binding cortisol/corticosterone and testosterone to a lower extent [3]
There are a number of other steroid binding proteins in plasma [4], but other proteins bind non- these hormones with a varying degree of affinity [5]; i.e. the large amount of albumin in plasma may be responsible for a considerable proportion of this binding [6] not because of high affinity but because of its bulk
There were no effects of sex or diet in protein percentage with respect to body weight, neither of sex for lipids, but exposure to cafeteria diet resulted in significantly higher percentages of body lipids
Summary
Steroid hormones are found free in blood plasma, but a variable proportion of these hormones is carried bound to protein, in a few cases with high affinity, as is the case of SHBG (Sex HormoneBinding Globulin) [1] in humans, binding both 17b-estradiol and testosterone, or CBG (Corticosteroid-Binding Globulin) [2], essentially binding cortisol/corticosterone and testosterone to a lower extent [3]. It has been found that there is a direct interchange of a few chemical substances between the RBC surface and that of the endothelial lining [13,14,15,16]. Other molecules, such as amino acids (and probably glucose) are loosely bound to the red blood cell surface, favoring a rapid interchange with the capillary endothelial cells [13]
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