Development of heterologous down-regulation of lactogen receptors in the rat testis

Gonadotropin-induced loss (down-regulation) of testicular lactogen receptors was studied in 5-60-day-old rats. An i.m. injection of 600 IU/kg of hCG elicited in 5-day-old animals a 38-56% increase in testicular lactogen binding, measurable between 6 and 72 h after the hormone injection. In contrast, at the age of 60 days the same hCG dose decreased lactogen receptors by 75% at 24 h, which loss recovered in 2-3 days. When the amount of lactogen receptors was measured 24 h after the hCG injection (600 IU/kg) at different ages, the binding increased on average by 50% between ages 5 and 20 days, no effect on binding was seen at day 30, and a loss of binding by 50% was evident in animals of 40 days of age and older. The present results indicate that the plasma-membrane events elicited by gonadotropin binding to neonatal Leydig cells are clearly different from those occurring in the adult, and that down-regulation and functional coupling of LH and lactogen receptors are accompaniments of functional differentiation of the Leydig cell during pubertal maturation.