Stimulation of protein synthesis and expansion of pig blastocysts by insulin in vitro

Abstract

Present evidence indicates that insulin may act as a growth factor during preimplantation development. This hypothesis has been tested on pig blastocysts by determining the effect of insulin on protein synthesis and blastocyst expansion over 24 h. Blastocysts were collected from superovulated gilts or sows on Day 5 or 6 and incubated overnight in a modified BMOC2 medium. Those that were cultured with 1.7 nM insulin had 14% larger radii, and were 36% more active in their incorporation of [3H]leucine (protein synthesis) than those that had been cultured in non-supplemented medium. There was a significant linear correlation between the rate of protein synthesis and the radius of blastocysts when all blastocysts and only those cultured with insulin were examined, but the correlation for the blastocysts in non-supplemented medium was just outside statistical significance. The regression coefficient for the insulin-treated blastocysts was 132% of that for blastocysts cultured in unsupplemented medium; this suggests that insulin increased the size of blastocysts and the rate of protein synthesis per unit size. The results indicate that pig blastocysts respond to physiological levels of insulin in similar fashion to those of mice and cattle, supporting the hypothesis that insulin may act as a general embryonic growth factor. Because of the cross reaction between the insulin receptor and the ligands, insulin and insulin-like growth factor 1 (IGF-1), the results also suggest that IGF-1, reported to be present in pig uterine fluid, could be involved in this stimulation in utero.