We have previously demonstrated changes in the episodic pattern of LH secretion in female rats as they approach first ovulation. In the present study, ovaries were taken from peripubertal rats and perifused in vitro with a medium containing FSH and LH. The concentration of FSH in the medium was kept constantly low, whereas the concentration of LH was modulated, simulating as closely as possible the various episodic secretory profiles previously observed in vivo. Medium from the perifusion chambers was collected every 10 min for 5 h and assayed for progesterone (P) and 17β-estradiol (E2). Exposure of the ovaries to low amplitude LH pulses (20 ng/ml; comparable to those found in the plasma of peripubertal rats during the morning) did not elicit a significant steroidogenic response. On the other hand, exposure to large amplitude LH pulses (80 ng/ml; as seen during the afternoon) led to a marked increase in the output of both P and E2. Similarly, increased steroid secretion occurred when ovaries were exposed to a 2-h minisurge of LH (160 ng/ml) or to a minisurge preceded by three large amplitude pulses (as seen in some animals during the afternoons of the peripubertal period). Continuous exposure to LH at concentrations similar to either the large amplitude pulses or the LH minisurge increased P and E2 release. However, the increase was not significantly larger than that produced by the discontinuous LH perifusion patterns, even though the total amount of LH reaching the ovaries during the 5-h perifusion period was considerably greater. The results show that episodic LH secretion is functionally much more efficient at eliciting release of P and E2 from immature ovaries than is continuous LH secretion. Furthermore, they strongly suggest that changes in the LH secretory pulse patterns, as seen in vivo close to the time of puberty, are fundamental for the activation of ovarian steroidogenesis that leads to the first preovulatory surge of gonadotropins.
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