Maternal nutrient restriction during early to mid-gestation, the period of rapid placental growth (30-80 days) in the sheep, is associated with altered fetal and placental development (Heasman et al. 1998). Consequently the resulting offspring may be at increased risk of diseases, in adulthood, including hypertension and obesity. The insulin-like growth factors (IGF) are important regulators of growth pre- (IGF-2) and postnatally (IGF-1). In particular IGF-2 hormone levels regulate fetal growth with down-regulation of its receptor being linked to fetal overgrowth (Young et al. 2001). This study aimed to determine the extent to which maternal nutrient restriction at specific stages of early and mid-gestation might result in altered placental IGF-2 receptor mRNA abundance.

Thirty-one Scottish Blackface singleton-bearing ewes of similar liveweight and body condition were individually housed from day of mating. Ewes were then randomly assigned to one of five nutritional groups (n = 4-8 per group). Group 1, control animals, were fed 100 % (8 MJ day^-1) of metabolisable energy (ME) requirements to maintain liveweight from day of mating to 110 days gestation. Groups 2, 3 and 4 were fed 50 % (4 MJ day^-1) of ME requirements, from 0-30, 31-65 and 66-110 days gestation, respectively, and 100 % at all other times. Group 5 received 50 % of ME requirements from mating to 110 days gestation. At 110 days gestation, all ewes were humanely killed with a barbiturate overdose (Euthatal; 500 mg ml^-1, 30 ml, I.V.), to enable placental and fetal sampling. All procedures were carried out according to UK legislation. The samples collected were frozen at -70 °C until later analysis. Total RNA was extracted from the placentomes and IGF-2 receptor mRNA abundance was examined by RT-PCR, using oligonucleotide primers specific to the IGF-2 receptor (forward 5Ô-ACCGGCAC-TTCAACTACACC-3Ô and reverse 5Ô-ACTCAGAATGA-CGGCTTCGT-3Ô). Results are expressed as mean values and standard errors in arbitrary units (a.u.) as a ratio of an 18S rRNA internal control. Statistically significant differences between groups were assessed using a Kruskal-Wallis H and Mann-Whitney U tests (P < 0.05).

There was no difference in fetal and placental weights between groups. At 110 days gestation placental IGF-2 receptor mRNA abundance was greater for ewes nutrient restricted between 31 and 65 days compared with the control group 1 (Group 1, 0.14 ± 0.02; Group 3, 0.19 ± 0.01 a.u.). There was no significant difference in IGF-2 receptor mRNA abundance between the other groups nutrient restricted during all other gestational periods.

Maternal nutrient restriction specifically during 31-65 days gestation up-regulates IGF-2 receptor abundance in the ovine placenta, to limit growth by the fetus. This coincides with the period of maximal placental growth and suggests that adaptive responses within the placenta may be dependent on the stage of nutritional manipulation.

G. Gopalakrishnan is supported by a BHF studentship.