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Faculty Mentor

George Perry

Abstract

Cows detected in estrus around the time of fixed-time AI had increased pregnancy success and progesterone concentrations. Additionally, GnRH following onset of estrus influenced LH pulse frequency and CL formation/function. Therefore our objective was to determine steriodogenic enzyme expression within luteal tissue of cows that were or were not detected in standing estrus prior to an injection of GnRH. Cows were synchronized with the CO-Synch protocol (day -9 100 mg GnRH; day -2 25 mg PGF2α; day 0 100 mg GnRH). Estrus was detected with the HeatWatch system. Location and size of the ovulatory follicle was determined on day 0 at time of GnRH by transrectal ultrasonography; blood samples were collected on day 3,4,5,7, and 9; and luteal tissue was collected on d 10 (n = 3 estrus and n = 8 no estrus) from CL originating from similar sized follicles (13.5 to 16 mm). Total cellular RNA was extracted and relative mRNA levels were determined by real-time RT-PCR and corrected for GAPDH. There was no effect of estrus on CL weight (P = 0.83). There was no effect of estrus by time (P = 0.17) or estrus (P = 0.97) on progesterone concentrations, but there was an effect of time (P < 0.01). In addition, there was no effect of estrus, follicle size, or CL weight on LH receptor expression (P = 0.97, 0.94, and 0.85), StAR expression (P = 0.87, 0.92, and 0.86), CYP11A1 expression (P = 0.49, 0.27, and 0.99), or 3HSD expression (P = 0.49, 0.61, and 0.91). However, there was a correlation between follicle size and CL weight (P = 0.01; R2 = 0.51); for every increase of 1 mm in follicle size, CL weight increased by 1.1 g. In addition, there was an effect of CL weight by time (P = 0.01) on progesterone concentrations and an effect of time (P < 0.01) with a tendency for an effect of CL weight (P = 0.06). In summary, estrus did not influence CL weight, progesterone concentrations, or expression of steriodogenic enzymes. However, as follicle size increased, CL weight increased, and CL weight influenced progesterone concentrations.

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