Metabolic clearance rate of inhibin in mature and immature male rats

Abstract

The half-life and metabolic clearance rate of human seminal plasma inhibin after injection into male rats was studied. Labeled inhibin was injected i.v. into 27-day-old rats (10 ng, 3.5 × 10⁵ cpm) and 75-day-old rats (15 ng, 5.5 × 10⁵ cpm). In both age groups, rats were killed at intervals between 1-5 and 480 min later, and samples of blood and tissues were taken for radioimmunoassay. [¹²⁵I]-hSPI disappeared from blood plasma in a multiexponential fashion. The first exponential curve, which represents the actual half-life of hSPI, was 2.34 min in adult rats and 90 min in 27-day-old rats. The slower component of each curve had values of 231 and 365 min, respectively. The initial distribution volume was 13.6 ml and 4.13 ml for the adult and immature animals. The metabolic clearance rate of inhibin in adult rats was 0.12 ml/min and 0.019 ml/min in immature rats. In both groups, only the pituitary and the pineal gland showed specific uptake of inhibin. The pituitary and the pineal uptake (tissue/plasma) was maximal at 120 min (3.9 and 3.4) in immature rats, while in adults pituitary uptake was maximal (1.55) at 10 min and the pineal gland showed uptake (2.4) only at 8 min. In vitro binding of labeled inhibin to the pituitary plasma membrane supported the above observations. We conclude that the metabolic clearance rate of inhibin was fasten in adult rats, whereas specific uptake of [¹²⁵I]-hSPI was greater in the pituitary and pineal glands of immature rats.