Abstract
Post partum negative energy balance (NEB) in dairy cattle is associated with a delayed return to ovarian cyclicity and reduced fertility. This study compared the IGF system of pre-recruitment ovarian follicles between cows in mild (n = 6) or severe (n = 6) NEB during early lactation. Ovaries were collected in the second week post partum, when circulating concentrations of IGF-I and glucose were lower (P < 0.01) in severe NEB cows. mRNA expression for IGF-II, type 1 IGF receptor (IGF-1R) and IGF-binding proteins (IGFBP)-1 to IGFBP-6 was determined by in situ hybridisation in individual follicles using radiolabelled oligonucleotide probes. Follicles were classified as very small (1-2.5 mm) or small (2.5-5 mm) and healthy or atretic. Relative mRNA concentrations were measured as optical density (OD) units using image analysis. Thecal IGF-II mRNA expression was highest in very small, healthy follicles (P < 0.05). Granulosa cell IGFBP-2 was the only component to change with EB status, with higher mRNA expression in mild compared with severe NEB cows (P < 0.05). IGFBP-1 and IGFBP-3 mRNA expression were undetectable. IGF-1R, IGFBP-4 and IGFBP-5 mRNA expression were not significantly altered by follicle size or health, but IGFBP-5 tended to increase in atretic follicles. The pattern of IGFBP-6 mRNA expression in theca paralleled that of IGF-II mRNA, with higher (P < 0.05) levels in healthy, very small follicles. In conclusion, the reduced expression of IGFBP-2 mRNA in severe NEB cows may alter the bioavailability of circulating IGF-I and locally produced IGF-II to modulate the pre-recruitment stages of follicles required to maintain normal post partum ovarian cyclicity.
Highlights
Dairy cow fertility has declined over the last 50 years, representing a major source of economic loss to the agricultural industry worldwide (Royal et al 2000)
The metabolic hormones insulin-like growth factor-I (IGF-I) and insulin are both likely candidates for mediating the effects of negative energy balance (NEB) on reproductive function
We have recently shown in the liver that severe NEB attenuates expression of IGF-I and IGF-binding proteins (IGFBP)-3 to IGFBP-6 mRNA, while increasing IGFBP-2 synthesis (Fenwick et al 2006)
Summary
Dairy cow fertility has declined over the last 50 years, representing a major source of economic loss to the agricultural industry worldwide (Royal et al 2000). An early return to ovarian cyclicity after calving can improve reproductive efficiency by promoting a short interval to first service, coupled with good conception rates (Butler & Smith 1989). One effect of NEB on fertility is to delay the interval from calving to first ovulation (Beam & Butler 1999). The initiation of follicle waves after calving occurs regardless of NEB (Beam & Butler 1999), but the number of follicles growing and their development to q 2007 Society for Reproduction and Fertility ISSN 1470–1626 (paper) 1741–7899 (online) larger sizes is decreased when poor body condition is combined with low nutritional levels (Perry et al 1991). IGF-I enhances the ovarian action of gonadotrophins (Adashi 1998) and stimulates follicle cell proliferation and oestradiol
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