Effects of growth hormone administration and dietary protein intake on insulin-like growth factor I and growth hormone receptor mRNA Expression in porcine liver, skeletal muscle, and adipose tissue

The effects of growth hormone (GH) and dietary protein on expression of IGF-I and GH receptor (GHR) genes in liver, muscle, and fat of pigs were investigated. Forty-eight intact male Large White x Landrace pigs were allotted to eight treatment groups (four diets with or without GH). The pigs were restriction-fed one of four diets, which differed only in their protein content (9.9, 13.1, 16.2, and 19.4%, as-fed basis), for a total of 3 wk. The pigs were then injected intramuscularly with either porcine GH (50 μg·kg-1·d-1 of rpST) or vehicle for the last 7 d. Pigs were slaughtered 4 h after the final injection. Total RNA was extracted from all tissues and then RNase protection assays were performed to measure expression of IGF-I and GHR genes. Expression of IGF-I mRNA was found to be GH responsive in the liver, semitendinosus (ST), and adipose tissue (P < .01) but not in longissimus muscle (LD). Dietary protein increased IGF-I expression only in the adipose tissue (P < .01). Expression of class 2 transcripts of IGF-I were observed only in the livers of GH-treated pigs, with no effect of dietary protein. Expression of GHR mRNA was found to increase with GH administration in liver and skeletal muscle (LD and ST, P < .05) but not in adipose tissue. There were diet x GH interactions on GHR expression in liver, ST, and adipose tissue, resulting in the highest GHR expression being in the high protein-fed, GH-treated group for liver, but in the low protein-fed, GH-treated group for muscle and adipose tissue. This study demonstrates tissue-specific control of expression of the two genes and also tissue-specific promoter usage (IGF-I exon 2 in liver) in response to GH administration