Effect of progesterone on basal LH and episodic LH and FSH secretion in heifers

Abstract

Heifers between Days 6 and 10 of the cycle were allocated at random to groups of 8 and treated with a 4% progesterone-releasing intravaginal device (PRID) + oestrogen capsule for 12 days; 4% PRID for 12 days; 20% PRID for 12 days; 4% PRID for 14 days; or 20% PRID for 14 days. Blood was obtained daily during treatment and at 2- or 4-h intervals for 72 h after removal of PRIDs. Some animals were sampled every 20 min for 4.67 h on the 3rd day after PRID insertion, and 1 day before and 36 h after removal of the PRID. During progesterone treatment there was: no correlation between concentrations of progesterone and LH within days; a significant negative correlation between progesterone and days (P<0.01) and also between progesterone and LH over days (P<0.01); the overall correlation co-efficient between LH and days was positive (P<0.05). The amplitude of LH or FSH episodes was not affected as progesterone concentrations declined during PRID treatment, but the number of LH (but not FSH) episodes was increased (P<0.01). After PRID removal, the amplitude of both LH and FSH episodes increased (P<0.01). We suggest that progesterone is part of a negative feedback complex in LH secretion in cattle and that this effect is apparently mediated through frequency of episodic LH release.