Insulin Enhancement of Luteinizing Hormone and Follicle-Stimulating Hormone Release by Cultured Pituitary Cells*

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Abstract
The role of insulin in the regulation of basal and gonadotropin-releasing hormone (GnRH) -stimulated release of LH and FSH was investigated in vitro using primary cultures of rat anterior pituitary cells from adult ovariectomized rats. Anterior pituitary cells were incubated for 2 days in the presence or absence of insulin in a serum-free medium. At the end of the insulin treatment, the cells were washed and reincubated in the presence or absence of GnRH, and the LH and FSH released into the medium were measured by RIA. Treatment with insulin (1.0 μg/ml) for 2 days resulted in significant increases in both the basal and the maximal release of LH and FSH, as well as a 3.2- and 6.3-fold decrease in the ED50 values for GnRH in terms of LH and FSH release, respectively. Treatment with increasing concentrations (0.1–10,000 ng/ml) of insulin, led to a dose-dependent increase in the GnRH (3 × 10-10 M)-stimulated release of both LH and FSH. This effect of insulin was significant (P < 0.05) at a physiological concentration of 1 ng/ml (24 μU/ml) with an ED50 value of 4.0 ng/ml. Increasing duration of exposure to insulin resulted in time-dependent increases in the GnRH (3 × 10-10 M)-stimulated release of LH, becoming significant at 24 h with maximal enhancement observed by 48 h. The effect of insulin was specific; epidermal or fibroblast growth factor did not enhance LH release. The augmenting effect of insulin was not associated with cellular proliferation or an overall change in protein or LH synthesis. Furthermore, the effect of insulin was independent of the ambient glucose concentration. Insulin was, however, without effect on gonadotrophs cultured in a serumsupplemented medium. Our findings suggest that the gonadotroph constitutes a target cell of insulin and that insulin may act directly on the anterior pituitary in the regulation of gonadotropin release.