Regulation of ovarian follicular growth and steroidogenesis by low-amplitude LH pulses.

A perifusion culture system controls the delivery of regulatory agents such that their concentration in the culture chamber is known at each and every point in time. The concentration in the culture chamber is predicted by the following equation: C(t) = Cf + (Ci - Cf)e(-R.t)/V, where C(t) is concentration in culture chamber (CC) at time t, Cf is concentration in holding flask (HF) (final CC concentration at t = infinity), Ci is initial CC concentration (t = 0), R is rate of media delivery from HF to CC (ml/h), V is volume of medium in CC (ml), and t is time (h). With this perifusion culture system, metestrous rat ovaries were exposed to tonic levels of follicle-stimulating hormone (FSH) and either tonic levels or hourly pulses of luteinizing hormone (LH). Both groups, however, received the same amount of FSH and LH. Compared with tonic levels, hourly pulses of LH increased estradiol-17 beta and suppressed testosterone secretion. In addition, pulsatile LH caused 1) a reduction in the atresia of small and midsized antral follicles, 2) an increase in atretic large follicles, and 3) an increase in the percent of growing midsized follicles. These results clearly demonstrate that ovarian responses depend on the pattern of LH stimulation. Since the LH pulses used in this study mimic in vivo metestrous levels, the LH pulses may be important in controlling ovarian function in vivo. The mechanism by which the ovary can distinguish between tonic and pulsatile stimuli remains to be determined.