ROLE OF INSULIN IN THE REGULATION OF MILK-FAT SYNTHESIS IN DAIRY-COWS

Five lactating Holstein cows were fitted with rumen fistulas and subje cted to a hyperinsulinemic-euglycemic clamp and abomasal casein infusi on to examine the effects on milk fat synthesis and the composition of milk fatty acids. The experiment consisted of two periods of abomasal infusions (water or 0.5 kg/d of casein); each period was divided into three 4-d intervals. The initial interval allowed for acclimation, an d baseline measurements were established during the second interval. D uring the third 4-d interval, a hyperinsulinemic-euglycemic clamp was maintained, and insulin was infused continuously at the rate of 1 mu g /kg of body weight per h. Circulating concentrations of insulin were i ncreased more than fourfold, and euglycemia was maintained by infusion of glucose at variable rates. Insulin had no effect on milk fat yield but casein infusion increased milk yield and tended to increase fat y ield. A trend toward higher milk yield during the clamp, combined with a slight numerical decrease in milk fat yield, resulted in decreased fat percentage. Calculated net energy balance was positive throughout the study, although feed intake decreased during the insulin clamp, pa rticularly for the water infusion period. Minor changes occurred in th e composition of milk fatty acids during the clamp when the balance be tween de novo and preformed fatty acids shifted slightly toward de nov o. Overall, results demonstrated that a relatively constant rate of mi lk fat synthesis was maintained during chronic hyperinsulinemia. Effec ts on milli fat yield and composition of fatty acids offered no suppor t for the role of insulin on milk fat depression.