Porcine somatotropin (pST) increases IGF-I mRNA abundance in liver and subcutaneous adipose tissue but not in skeletal muscle of growing pigs.

The present study was undertaken to determine the effects of exogenous porcine somatotropin (pST) on IGF-I gene expression in liver, skeletal muscle (longissimus dorsi), and s.c. adipose tissue of growing pigs. Twenty prepubertal gilts (approximately 60 kg BW) were allotted to four treatment groups (n = 5) and treated with either 0, 35, 70, or 140 micrograms/kg BW of recombinantly derived pST by daily i.m. injection for 7 d. Serum concentrations of IGF-I were determined by RIA and IGF-I mRNA levels were determined by direct counting of individual samples on slot blots. Administration of pST increased IGF-I concentration in serum. This was accompanied by significant increases (P < .05) in IGF-I mRNA abundance in liver and s.c. adipose tissue; the effects were maximal at the lowest dose of pST. Insulin-like growth factor I mRNA levels were increased 2.5- and 4.5-fold, respectively. Levels of IGF-I mRNA were very low in longissimus muscle and were unaffected by administration of pST. When expressed as picograms of mRNA/10 micrograms of total RNA, IGF-I mRNA levels were highest in s.c. adipose tissue. Levels of IGF-I mRNA were 1.9-fold higher in s.c. adipose tissue than in liver of control animals, and pST administration increased this difference to 3.2-fold. Our results suggest that 1) the effects of pST administered by daily i.m. injection on IGF-I gene expression in pigs are tissue-specific and 2) the stimulatory effects of pST administered in this manner on muscle growth in pigs are not associated with increased expression of the IGF-I gene in skeletal muscle.