ROLE OF INSULIN IN THE REGULATION OF MAMMARY SYNTHESIS OF FAT AND PROTEIN

Five lactating Holstein cows were subjected to a hyperinsulinemic-eugl ycemic clamp to examine the effects of insulin on milk yield and compo sition. Of special interest was the evaluation of the glucogenic-insul in theory of milk fat depression. Cows were fed every other hour to mi nimize postprandial effects, and blood samples were obtained via an in dwelling jugular catheter every 4 h for 2 d to establish baseline gluc ose concentrations. For the 4-d clamp, insulin was infused continuousl y (1 mu g/kg of BW per h) into the contralateral jugular vein, and cir culating insulin was increased approximately fivefold. Blood was sampl ed frequently, and blood glucose was maintained within 10% of baseline concentrations by infusion of exogenous glucose at variable rates (($ ) over bar X = .15 g/kg of BW per h). Dietary intake declined on the 4 th d of the insulin clamp (23.0 vs. 16.3 kg/d). Milk yield, however, d id not change (32.4 vs. 33.6 kg/d) in support of the lack of sensitivi ty of the mammary gland to insulin. Milk fat percentage (3.85 vs. 3.66 ) and yield (1.26 vs. 1.22 kg/d) did not change during the insulin cla mp. Milk protein yield increased (.98 vs. 1.05 kg/d), and milk protein percentage tended to increase (3.03 vs. 3.14), during the insulin cla mp. This modest increase in milk protein yield may have been constrain ed by a lack of available amino acids, as indicated by a decrease in c irculating concentrations of essential amino acids, urea nitrogen, and plasma proteins. Overall, results offer no support for the glucogenic -insulin theory of milk fat depression but do indicate that the insuli n infusion, either directly or indirectly, enhanced secretion of milk protein.