EFFECT OF SOMATOTROPIN TREATMENT ON LIPOGENESIS, LIPOLYSIS, AND RELATED CELLULAR MECHANISMS IN ADIPOSE-TISSUE OF LACTATING COWS

The effect of bST on the metabolism of lipid in adipose tissue was stu died using tissue biopsies from lactating cows treated with bST for 8 d. Cows responded to treatment by increasing daily milk yield by 10.9 kg, although net energy intake was not changed. Thus, net energy balan ce was changed from highly positive to slightly negative (+7.7 to -1.1 Mcal/d). Consistent with these changes in net energy balance, lipogen esis rates were dramatically reduced (97%) in adipose tissue from bST- treated cows. Activities of acetyl-coenzyme A carboxylase (initial and total) and fatty acid synthase were also dramatically decreased. Ther efore, for cows in positive energy balance, reduced lipid synthesis in adipose tissue represents a major mechanism whereby bST alters nutrie nt partitioning to support greater milk synthesis. Treatment with bST had no effect on beta-adrenergic-stimulated lipolysis in adipose tissu e explants incubated in vitro with adenosine deaminase. However, bST t reatment reduced the ability of adenosine to inhibit lipolysis in adip ose tissue, which involved changes in both sensitivity and responsiven ess to adenosine. Therefore, the enhanced lipolytic response to catech olamine in vivo with bST treatment relates to relief in the adenosine inhibitory signaling cascade rather than to a direct effect on the sti mulatory signaling pathway.