Effects of bovine somatotropin (bST) and ß-adrenergics on lipolysis and hormone-sensitive lipase in lactating cows

Abstract

The effects of bST on hormone-sensitive lipase (HSL), the rate-limiting enzyme in lipid mobilization, were studied. Five multiparous Holstein cows received daily 40 mg injections of n-methionyl-bST or of excipient, in a single reversal design. Treatment with bST increased milk production from 19.2 to 27.9 kg/d (P<0.01), but feed intake was unchanged and thus the cows went from a positive energy balance (3.6 Mcal/d) to 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) activity or degree of activation of HSL in explants incubated with isoproterenol (10^-5 M) and adenosine deaminase (0.75 U/ml). Although energy balance and milk fat content suggest fat mobilization increased, maximal lipolitic rate and HSL, activity were unchanged. Results agree with previous results suggesting the increase in lipid mobilization after bST treatment is the result of a relief of the inhibition of lipolysis.