EFFECTS OF BOVINE SOMATOTROPIN (BST) AND BETA-ADRENERGICS ON LIPOLYSIS AND HORMONE-SENSITIVE LIPASE IN LACTATING COWS

The effects of bST on hormone-sensitive lipase (HSL), the rate-limitin g enzyme in lipid mobilization, were studied. Five multiparous Holstei n cows received daily 40 mg injections of n-methionyl-bST or of excipi ent, in a single reversal design. Treatment with bST increased milk pr oduction from 19.2 to 27.9 kg/d (P<0.01), but feed intake was unchange d and thus the cows went from a positive energy balance (3.6 Mcal/d) t o a negative energy balance (-2.6 Mcal/d; P<0.01). Consistently, milk fat content increased (3.6 to 4.2%; P<0.01). The HSL activities, both basal and activated by dibutiryl-cyclic AMP or dbc plus protein kinase -A were unchanged by bST (P>0.10). Also, bST did not alter (P>0.10) ac tivity or degree of activation of HSL in explants incubated with isopr oterenol (10(-5) M) and adenosine deaminase (0.75 U/ml). Although ener gy balance and milk fat content suggest fat mobilization increased, ma ximal lipolitic rate and HSL activity were unchanged. Results agree wi th previous results suggesting the increase in lipid mobilization afte r bST treatment is the result of a relief of the inhibition of lipolys is. Index terms: bovine, regulating enzyme, lipid metabolism, adipose tissue.