Progesterone (P4) regulation of LH secretion in sheep was studied in vitro using dispersed pituitary cell cultures. Neither cell content nor basal secretion of LH were normally altered by P4, but maximal stimulation of LH secretion by LHRH was suppressed up to 70% with 10-7 M P4, without changing the ED50 of LHRH. The inhibitory effect of P4 appeared within 6 h and became maximal by 24 h but was partially reversed during continued treatment with P4. The inhibitory effect was completely reversed by 24 h after P4 withdrawal. The ED50 of P4 was 10-9 M. P4 also decreased the sensitizing effect of 17β-estradiol (E2) on LHRH-induced LH secretion in ovine pituitary culture. It partially inhibited the E2-induced increase in maximal response to LHRH and decreased the ability of E2 to lower the ED50 of LHRH. Our results clearly show that P4 can act directly on the ovine pituitary to inhibit both normal and E2-sensitized LH responsiveness to LHRH.
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