EXPRESSION OF INSULIN-LIKE GROWTH-FACTOR-I IN COWS AT DIFFERENT STAGES OF LACTATION AND IN LATE LACTATION COWS TREATED WITH SOMATOTROPIN

Relative amounts of IGF-I mRNA were measured in livers of Holstein cow s at different stages of lactation (6 early, 6 mid, 6 late lactation, 6 dry) and 6 late lactation cows treated with bST for 1 wk. Milk yield was greater for early lactation cows than for mid and late lactation controls. All cows except those in early lactation were in positive en ergy balance. Serum IGF-I increased as lactation progressed and was gr eatest during the dry period. Liver IGF-I mRNA was less in cows in ear ly than in late lactation but greater in lactating than in dry cows. T reatment with bST increased milk yield and concentrations of serum IGF -I, hepatic IGF-I mRNA, and serum IGF-binding protein-3, decreased con centration of serum IGF-binding protein-2, and did not alter abundance of mammary IGF-I mRNA. These parallel changes in serum IGF-I and hepa tic IGF-I mRNA suggest that exogenous bST increases IGF-I synthesis in liver of cows during late lactation and that IGF-I synthesis is depre ssed during early lactation when cows are in negative energy balance. We conclude that IGF-I may play an endocrine role in mediating galacto poietic effects of exogenous bST during late lactation. However, the r ole of IGF-I during early lactation remains unclear.