EFFECTS OF GROWTH-HORMONE ADMINISTRATION AND DIETARY-PROTEIN INTAKE ON INSULIN-LIKE GROWTH-FACTOR-I AND GROWTH-HORMONE RECEPTOR MESSENGER-RNA EXPRESSION IN PORCINE LIVER, SKELETAL-MUSCLE, AND ADIPOSE-TISSUE

The effects of growth hormone (GH) and dietary protein on expression o f 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 G H). The pigs were restriction-fed one of four diets, which differed on ly 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 wit h either porcine GH (50 mu g . kg(-1). d(-1) of rpST) or vehicle for t he last 7 d. Pigs were slaughtered 4 h after the final injection. Tota l RNA was extracted from all tissues and then RNase protection assays were performed to measure expression of IGF-I and GHR genes. Expressio n of IGF-I mRNA was found to be GH responsive in the liver, semitendin osus (ST), and adipose tissue (P < .01) but not in longissimus muscle (LD). Dietary protein increased IGF-I expression only in the adipose t issue (P < .01). Expression of class 2 transcripts of IGF-I were obser ved only in the livers of GH-treated pigs, with no effect of dietary p rotein. Expression of GHR mRNA was found to increase with GH administr ation in liver and skeletal muscle (LD and ST, P < .05) but not in adi pose tissue. There were diet x GH interactions on GHR expression in li ver, ST, and adipose tissue, resulting in the highest GHR expression b eing in the high protein-fed, GH-treated group for liver, but in the l ow protein-fed, GH-treated group for muscle and adipose tissue. This s tudy demonstrates tissue-specific control of expression of the two gen es and also tissue-specific promoter usage (IGF-I exon 2 in liver) in response to GH administration.