A new method for measurements of plasma protein steroid-binding kinetics in human plasma at 37 C

The equilibrium kinetics in vivo of free and protein-bound steroid hormone were examined at 37 C. Human plasma was pumped through a 1 m PVC catheter and tritiated steroid hormone continuously added at the inlet. The plasma was collected and rerun through a new catheter 24 h later. A significantly (P less than 0.05) higher uptake of steroid hormone was observed in the first parts of the catheter during the first passage when compared to the second passage. The results indicate that non-equilibrium conditions existed for testosterone (greater than 10 sec), estradiol (4 sec), estrone (greater than 2 sec), and estriol (greater than 2 sec) while no delay was observed for progesterone. The results indicate that the counter-current transfer between the testicular and ovarian vessels, respectively, may create a physiologically important, temporarily increased concentration of available hormones in the arterial blood supply to the organs.