Slow binding of progesterone to plasma proteins

The uptake of 3H- and 14C-progesterone (P) from bovine plasma into the wall of a 1 m PVC-catheter was measured. Fifty ml of bovine plasma (22 degrees) was pumped through the catheter (mean transit time 2 or 10 sec. per m). The 14C-P was added one hour previously while the 3H-P was continuously added at the inlet. The uptake of the isotopes into ten cm sections of the catheter was measured by liquid scintillation counting. The uptake of 3H-P was always higher than that of 14C-P indicating that the binding to plasma proteins does not reach equilibrium within 10 sec. Preincubation with 100 ng/ml cold P further increased the difference in uptake. The increased fraction of accessible steroid hormone may be of physiological importance, because a high fraction of hormone exchanged from the ovarian vein to the ovarian artery may reach the ovary and the tube in a physiologically active form.