Studies on follicular atresia: lysosomal enzyme activity and gonadotropin receptors of granulosa cells following administration or withdrawal of gonadotropins in the rat.

The changes that occur during follicular growth and atresia in the lysosomal enzyme activities and in gonadotropin receptors of isolated granulosa cells were studied. At different intervals after an injection of PMSG to 21-day-old female rats, the granulosa cells (GC) were isolated and the total activity of cathepsin-D, a representative lysosomal enzyme, and FSH receptor activity in terms of binding of [125I]oFSH to GC and the steroidogenic response of GC to FSH in vitro were determined. During the period of both follicular growth and atresia there was an inverse correlation between the lysosomal enzyme activity and FSH receptor activity of granulosa cells, the former being low during follicular growth or tropic phase when the levels of FSH receptors were increasing. During the atretic phase, beyond 48 h of PMSG treatment (perhaps due to the normal metabolic clearance of the injected PMSG), the cathepsin-D activity showed a significant increase while the FSH receptors were on the decline. That lack of the hormone was responsible for atresia was confirmed when an antiserum to PMSG was injected during the tropic phase (24 h after PMSG); this resulted in a sharp increase in cathepsin-D activity. Injection of excess amounts of either FSH or LH, along with or soon after PMSG antiserum, was found to prevent the increase in cathepsin-D activity, and also resulted in the maintenance of gonadotropin receptors. This suggested that gonadotropins can 'rescue' follicles from undergoing atresia. In the unprimed immature rat FSH, but not LH or hCG, mimicked the effects of PMSG, in terms of bringing about a reduction in cathepsin-D activity of GC. Also in such rats, neutralization of endogenous FSH with an antiserum to FSH resulted in an increase in the cathepsin-D activity of GC, suggesting that even the prepubertal rat ovary is dependent on the tropic support of FSH for prevention of atresia.