Interactions between environmental temperature and porcine growth hormone (pGH) treatment in neonatal pigs

Age-dependent interactions between environmental temperature and porcine gr owth hormone (pGH) treatment on the function of the somatotrophic axis were evaluated in the neonatal pig. At 3 d of age, 40 Landrace x Yorkshire x Du roc piglets received intraperitoneal implants containing either recombinant pGH (0.5 mg/d; n = 20) or vehicle (control; n = 20). Piglets were maintain ed at either a low (21 degrees C, 50% relative humidity; n = 20) or high (3 2 degrees C, 50% relative humidity; n = 20) temperature. At 4 and 6 wk of a ge, 5 pGH-treated and 5 control piglets from each thermal group were sacrif iced for tissue collection. Blood samples were collected at the time of sac rifice and analyzed for serum concentrations of CH, insulin-like growth fac tor 1 (IGF-1), and IGF-2. Liver RNA was analyzed for mRNAs specific for the GH receptor, IGF-1, IGF-2, and IGF binding protein 3. There was no effect of pGH treatment (P = 0.4) on average daily gain; however, both age (P = 0. 002) and temperature (P = 0.001) had an effect on average daily gain such t hat those animals maintained at a low temperature and those sacrificed at 6 wk had greater average daily gains. Serum concentration of GH was elevated (P = 0.003) by pGH treatment and was lowest in the 4-wk-old group (P = 0.0 08). Serum concentration of IGF-1 was elevated (P = 0.007) by pGH treatment and increased with age (P = 0.01). Liver GH receptor mRNA was unaffected ( P > 0.5) by pGH treatment, but was greater in the 6-wk-old group (P < 0.000 1) and in piglets maintained at the high temperature (P = 0.04). IGF-1 mRNA was enhanced by pGH treatment (P = 0.0003) and by exposure to the high tem perature (P = 0.04), but did not differ (P > 0.5) between age groups. IGF-2 mRNA was greater (P = 0.0009) in the 4-wk-old group and in piglets maintai ned at the high temperature (P = 0.007), but was unaffected (P = 0.5) by pG H treatment. IGF binding protein 3 mRNA increased with age (P = 0.0004) and was stimulated by pGH treatment in the 6-wk-old group (P = 0.034). The rel atively lower level of GH receptor and IGF mRNAs in conjunction with greate r growth in the cold environment suggests that somatotrophic gene expressio n in the liver is not rate limiting for growth in the neonatal pig. (C) Els evier Science Inc. 1999.