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 recombinant porcine somatotropin (rpST) on placental size, fetal growth, and maternal 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 were 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 length. 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 vs 16.44 +/- .53 g; rpST vs control, respectively; P < .05). Implantation lengths were partitioned into quartiles to determine the effect of rpST on fetuses with different implantation lengths. The effect of rpST of fetal weight was greater in the first quartile ( < 137.5 mm) than in 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 suggests that the effect of rpST treatment on fetal weight is equivalent to the effect of increasing implantation length by 58.8 mm. Administration of rpST numerically raised IGF-I (P = .07) and IGF-II (P = .12) concentrations in fetal serum. Although maternal serum IGF-I concentrations were similar at d 30, treatment with rpST increased these concentrations 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 concentrations 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 pronounced effect of rpST on growth in fetuses with shorter implantation lengths suggests that rpST may increase uptake or utilization of nutrients by fetuses. In addition, nutrient transfer across placental membranes may be enhanced by rpST.