Progesterone Inhibits the Ability of Estradiol to Increase Basal and Luteinizing Hormone-Releasing Hormone- Induced Luteinizing Hormone Release from Bovine Pituitary Cells in Culture: Neither Progesterone nor Estradiol Affects Follicle-Stimulating Hormone Release*

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Abstract
The objective of this study was to determine whether progesterone affects: 1) basal and LHRH-induced LH release and 2) the ability of estradiol to increase basal and LHRH-induced LH release. Samples were also assayed for FSH in an effort to provide preliminary data regarding mechanisms controlling secretion of that gonadotropin in cattle. Bovine pituitary cells were grown in Dulbecco's medium and treated on day 5 of culture. Treatments were for a 4 × 3 × 2 factorial design, with concentrations of estradiol (0, 0.5, 5, and 50 ng/ml), progesterone (0, 3.1 and 31 ng/ml), and LHRH (0 and 100 ng/ml) as main effects. Cells were e×posed to these steroids throughout a 12-h incubation period on day 5, and LHRH was present during the last 6 h of this period. In the absence of progesterone, both basal and LHRH-induced LH release increased linearly (P < 0.05 and P < 0.01) with increasing concentrations of estradiol, but progesterone completely inhibited this effet of estradiol (P< 0.01). In the absence of estradiol, progesterone affected neither basal nor LHRH-induced LH release. Estradiol did not affect FSH secretion. In a second e×periment, the period of e×posure to estradiol and progesterone was increased to 36 h, with LHRH treatments applied during the last 6 h of the incubation period. The design was a 3 × 2 × 4 factorial, with concentrations of estradiol (0, 0.5, and 5 ng/ml), progesterone (0 and 3.1 ng/ml), and LHRH (0,1,10, and 100 ng/ml) as the main effects. LH and FSH release increased linearly (P< 0.001) with increasing concentrations of LHRH. Estradiol increased (P < 0.01) LHRHinduced LH, but not FSH, release. In agreement with the results of the first e×periment, progesterone in the absence of estradiol did not affect basal or LHRH-induced LH or FSH release, but completely inhibited (P < 0.01) the stimulatory effects of estradiol on LH release. We conclude that progesterone does not directly cause the pituitary to al er basal LH release or LHRHinduced LH release. However, it does completely inhibit the ability of estradiol to increase both basal and LHRH-induced LH release. FSH secretion was not affected by progesterone, estradiol, or a combination of these steroids.

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