Zinc-Induced Molt: Evidence for a Direct Inhibitory Effect on Granulosa Cell Steroidogenesis

Abstract

Results from previous studies indicate that the use of dietary zinc may provide an effective means to initiate an induced molt in laying hens. Although much evidence indicates that high concentrations of zinc (10,000 to 20,000 ppm) cause the cessation of lay primarily by depressing feed intake, recent data suggest that lower concentrations (2,800 ppm) in a calcium-deficient diet may act via a direct action on the ovary. Therefore, a series of in vitro studies was conducted to evaluate whether zinc can affect granulosa cell progesterone production. Incubation of granulosa cells from the largest preovulatory (F1) follicle with zinc as zinc sulfate (.1 to 10 μM) had no effect on basal progesterone production. By contrast, ovine luteinizing hormone-stimulated progesterone production was inhibited (P<.05) in a dose-related fashion by zinc in both the sulfate and acetate forms (1 to 10 μM). Furthermore, zinc attenuated oLH- and forskolin-induced cyclic adenosine monophosphate (cAMP) formation, and inhibited 8-bromo-cAMP- and calcium ionophore (A23187)-induced progesterone production. Such results indicate both pre- and post-cAMP sites of action for zinc's inhibitory actions on progesterone production in F1 granulosa cells. Finally, ovine follicle-stimulating hormone-stimulated cAMP accumulation and progesterone production in granulosa cells collected from 9- to 12-mm follicles (a stage of development representing the early, rapid growth phase) were suppressed (P<.05) by co-incubation of cells with zinc. From these data, it can be concluded that the effectiveness of zinc to induce the cessation of lay is due, at least in part, to a direct inhibitory action on ovarian granulosa cell function both in differentiating and in preovulatory follicles.