Hepatic mRNA expression of enzymes associated with progesterone metabolism and its impact on ovarian and endocrine responses in Nelore (Bos indicus) and Holstein (Bos taurus) heifers with differing feed intakes.

The aim of this study was to evaluate circulating progesterone concentration (P4), LH pulsatility and ovarian follicular dynamics in Nelore (B. indicus) and Holstein (B. taurus) heifers under high (HDMI) and low (LDMI) dry matter/energy intakes. In addition, the effects of dry matter/energy intake and breed on hepatic expression of six genes associated with P4 metabolism (AKR1C4, AKR1D1, CYP3A4, CYP2C19, SRD5A1, and SRD5A3) was evaluated. Heifers received an intravaginal P4 device (1 g), 2 mg of estradiol benzoate (EB) i.m. and 500 µg of PGF2α at the begging of the synchronization protocol (D0). Eight days later, the P4 device was removed and all heifers received 1 mg of EB 24h later. Regardless of dry matter/energy intake, the number of recruited follicles was greater in Nelore than in Holstein heifers. In contrast, the maximum diameter of the dominant follicle was greater in Holstein than in Nelore heifers. Circulating P4 concentrations were greater in Nelore than in Holstein from D2 to D9, and in heifers receiving LDMI than those receiving HDMI from D1 to D8 of hormonal protocol. In addition, Holstein heifers had greater LH pulsatility and area under the curve of LH peaks compared to Nelore heifers. However, no effects were observed for LH values between feed intake levels. Interestingly, Holstein heifers had higher expression of SRD5A1, AKR1C4, AKR1D1 than Nelore heifers; whereas, for Nelore heifers, only the expression of CYP3A4 was higher compared to Holstein heifers. In conclusion, there are important differences in the follicular dynamics, circulating P4 and LH pulsatility concentrations that need to be considered during synchronization protocols for Nelore and Holstein breeds. More importantly, these differences appear to be at least partially modulated by the level of feed intake and the contrasting enzyme system in the liver involved with P4 metabolism between these cattle breeds.

The effect of circulating progesterone on magnitude of the GnRH-induced LH surge: Are there any differences between Bos indicus and Bos taurus heifers?

This study evaluated the effects of differing circulating progesterone (P4) levels on the luteinizing hormone (LH) surge profile following treatment with gonadotropin releasing hormone (GnRH) in B. indicus (Nelore, n = 13) and B. taurus (Holstein, n = 16) heifers. All heifers were synchronized with a hormonal protocol to induce either a Low or High circulating P4 environment at the time of GnRH treatment. Heifers were randomly assigned to a 2 × 2 factorial design composed by two genetic groups (B. indicus and B. taurus) and two levels of circulating P4 concentrations (Low or High). Blood samples were collected every 30 min from -30 to 210 min and at 270 min after GnRH treatment. As expected, mean P4 concentration was greater for cows in the High than in the Low P4 group (P = 0.0008) and in Bos indicus than in Bos taurus heifers (P = 0.06). Despite genetic group, the area under the curve of LH release was greater in the Low-P4 than in High-P4 concentration group (P < 0.0001). Interestingly, it appears that High P4 concentrations had a more pronounced effect on LH peak in B. indicus than in B. taurus heifers, as indicated by the interaction (P = 0.01) between genetic group and P4 levels. In conclusion, circulating P4 concentration have a great impact on the GnRH-induced LH surge of both B. indicus and B. taurus heifers, but it does not explain the much lower LH peak in B. indicus with low circulating P4. Thus, more studies are essential to uncover some of the underlying physiological factors other than circulating P4 that are limiting LH release following a GnRH treatment in B. indicus cattle.

Plasma antimullerian hormone as a predictor of ovarian antral follicular population in Bos indicus (Nelore) and Bos taurus (Holstein) heifers.

In Bos taurus cattle, antimullerian hormone (AMH) has been demonstrated to have a high degree of correlation with ovarian antral follicle count and the number of healthy follicles and oocytes. To document the correlation between the plasma concentration of AMH and follicular number in Bos indicus and Bos taurus heifers, Nelore (Bos indicus, n = 16) and Holstein heifers (Bos taurus, n = 16) had their ovarian follicular waves synchronized. After synchronization, ovarian antral follicular population (AFP) was evaluated three times at 60-day (d) intervals (T-120 d, 120 days before plasma AMH determination; T-60 d, 60 days before; and T0, at the time of plasma AMH determination). The plasma AMH concentration was positively correlated with the number of ovarian follicles on the day of the follicular wave emergence in Bos indicus (Nelore) and Bos taurus (Holstein) heifers at each evaluation time (p < 0.05). The AFP was higher in Bos indicus (Nelore) than in Bos taurus (Holstein) heifers (p < 0.05). Similarly, the AMH concentration was higher in Bos indicus (Nelore) than in Bos taurus (Holstein) heifers (p < 0.0001). When heifers were classified as to present high or low AFP according to the mean of the AFP within each genetic group, high-AFP heifers presented a greater (p < 0.0001) AMH concentration than low-AFP heifers, regardless of the genetic group. In conclusion, the AFP is positively correlated with plasma AMH concentration in both Bos indicus (Nelore) and Bos taurus (Holstein) heifers. Furthermore, Bos indicus (Nelore) heifers presented both greater plasma AMH concentrations and AFP than Bos taurus (Holstein) heifers.