Uptake of progesterone by red blood cells in the rat

This work examined the extent to which progesterone (P) in whole blood is transported in association with red blood cells (RBC) in the rat. The effects of plasma P concentration and changes in the source of whole blood on this association were also assessed. Initially, the RBC uptake of [3H]P was measured in vitro in blood from either adrenalectomized-ovariectomized (Ax-Ox) rats, thus rendered effectively P-free, and from rats at Day 16 of pregnancy. In blood from Ax-Ox rats, almost 10% of [3H]P was located within the RBC fraction, and this percentage increased with the addition of exogenous P (range of mean RBC uptake 9.4-18.9% over a plasma P concentration range of approximately 100-4500 ng/ml). A similar though less marked effect of exogenous P was observed in blood from pregnant rats (mean RBC uptake 11.7-14.8% over a similar plasma P concentration range), suggesting that plasma from pregnant rats had a greater capacity to bind P. To assess the RBC uptake of P in vivo, arterial blood samples were obtained during constant infusion of [3H]P in Day 16 pregnant rats with or without prior P supplementation (20 mg P per day over 4 days). The mean RBC uptake of P in control rats was 18.8 +/- 0.7%, but was reduced (p less than 0.05) to 16.2 +/- 0.5% after P supplementation. This reduction may have been due to the increase in the P-binding capacity of plasma known to occur after P supplementation.(ABSTRACT TRUNCATED AT 250 WORDS)