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Effect of negative energy balance on the insulin-like growth factor system in pre-recruitment ovarian follicles of post partum dairy cows

¹ Reproduction, Genes and Development Group, Royal Veterinary College, Hawshead Lane, North Mymms, Hatfield, Herts AL9 7TA, UK, ¹ Animal Production Research Centre Mellows Campus, Athenry, Co. Galway, Ireland, ² Faculty of Agriculture, University College Dublin, Dublin, Ireland and ³ Teagasc Dairy Production Research Centre, Moorepark, Fermoy, Co. Cork, Ireland

Correspondence should be addressed to S Llewellyn; Email: sllewellyn{at}rvc.ac.uk

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.

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