In Vivo Observations on Reversible Binding of Estradiol-17β,6,7-³H in Rat Uterus. Measurement of a “Biological Exchange Constant” and of the “Tissue Capacity”1

Abstract

Binding of estradiol-17β,6,7-3H to rat uterus and endometrium in vivo has been investigated by long-term intravenous infusions with labeled hormone. Four-hr infusions, preceded by the intravenous injection of a priming dose, were shown to be sufficient to allow equilibrium to be established in plasma and uterus of adult rats, at low and high rates of infusion. In immature animals, equilibrium was not reached at the 4th hr of infusion at low infusion rates. Reversible binding was further studied by chase experiments. It was shown that unlabeled estradiol-17β, infused during 4 hr, was able to displace up to 90% of previously bound estradiol-17β,6,7-3H whether this binding had occurred 4 or 12 hr previously. A curvilinear relationship was found between the 4th hr estradiol- 170,6,7-3H concentration in uterus and endometrium and infusion rate, from 0.5 up to 920 ng/hr in adult and from 0.26 up to 677 ng/hr in immature animals. In adults, a steep increase was seen, which secondarily leveled off to about 5 ng/g in uterus and 7.5 ng/g in endometrium. In immature animals the concentration in the uterus plateaued to about 8 to 9 ng/g. The pattern of the uptake curves was found to resemble a hyperbolic function. Hence, 2 constants were calculated: the “tissue capacity” (NmSu and NHISE) (equal to Y max) and the “biological exchange constant” (1/KPU and l/KPE) (equal to X at Y=Y max/2). NmSU and NmSE were equal to 2.1±0.3 and 3.2 ± 1.7 (SD) × lO-8 moles/ kg, respectively. 1/KPU and l/KpE were equal to 2.9 ± 0.4 and 3.2 ±2.0 (SD) ×10-10M, respectively. The same constants were calculated for the uterus of immature animals considering only results from infusion rates higher than 25 ng/hr, at which equilibrium was obtained. NmSu was equal to 3.6±O.2 × 1O-8 moles/kg and 1/KPU was equal to 1.8 ±0.3 × 10-10M. “Tissue capacity” was similar in value to the concentrations of binding sites as reported by others. The value of the “biological exchange constant” agrees with reported estimations of the endogenous estradiol- 17β concentration in rat plasma. Hence, implication of this parameter in the physiological activity of the hormone at a particular target site is put forward. (Endocrinology88: 165, 1971)