EFFECTS OF RECOMBINANT PORCINE SOMATOTROPIN ON PLACENTAL SIZE, FETAL GROWTH, AND IGF-I AND IGF-II CONCENTRATIONS IN PIGS

The objective of this study was to determine the effects of recombinan t porcine somatotropin (rpST) on placental size, fetal growth, and mat ernal and fetal IGF-I and IGF-II concentrations. Twenty-four pregnant gilts received daily injections of either 1 mL of saline (control) (n = 12) or 5 mg of rpST (n = 12) from d 30 to 43 of gestation. Gilts wer e slaughtered on d 44 of gestation, reproductive tracts were removed, and fetal weight and length, placental weight, and implantation length were recorded. There was no effect of rpST on fetal or implantation l ength. Placental weight increased with rpST administration (71.20 +/- 3.52 vs 58.35 +/- 3.41 g; P <.02), as did fetal weight (18.06 +/-.55 v s 16.44 +/- .53 g; rpST vs control, respectively; P <.05). Implantatio n lengths were partitioned into quartiles to determine the effect of r pST on fetuses with different implantation lengths. The effect of rpST on fetal weight was greater in the first quartile (< 137.5 mm) than i n the fourth quartile (> 240 mm) (16.04 vs 13.86 g compared with 19.47 vs 18.21 g, respectively). Analysis using a modified Brody curve sugg ests that the effect of rpST treatment on fetal weight is equivalent t o the effect of increasing implantation length by 58.8 mm. Administrat ion of rpST numerically raised IGF-I (P =.07) and IGF-II(P =.12) conce ntrations in fetal serum. Although maternal serum IGF-I concentrations were similar at d 30, treatment with rpST increased these concentrati ons over time (77.76, 247.75, 267.85 vs 82.59, 79.59, 77.97 ng/mL on d 30, 37, 43, respectively; P <.001, SE = 14.09). Maternal IGF-II conce ntrations were also similar at d 30 but decreased over time with rpST treatment (265.78, 219.61, 191.05 vs 285.44, 284.72, 283.05 ng/mL; P < .03, SE = 14.03). Increased maternal IGF-I concentrations may exhibit negative feedback on maternal IGF-II concentrations. The more pronoun ced effect of rpST on growth in fetuses with shorter implantation leng ths suggests that rpST may increase uptake or utilization of nutrients by fetuses. In addition, nutrient transfer across placental membranes may be enhanced by rpST.