Role of insulin in the regulation of mammary synthesis of fat and protein.

Five lactating Holstein cows were subjected to a hyperinsulinemiceuglycemic clamp to examine the effects of insulin on milk yield and composition. Of special interest was the evaluation of the glucogenic-insulin theory of milk fat depression. Cows were fed every other hour to minimize postprandial effects, and blood samples were obtained via an indwelling jugular catheter every 4 h for 2 d to establish baseline glucose concentrations. For the 4-d clamp, insulin was infused continuously (1 microgram/kg of BW per h) into the contralateral jugular vein, and circulating insulin was increased approximately fivefold. Blood was sampled frequently, and blood glucose was maintained within 10% of baseline concentrations by infusion of exogenous glucose at variable rates (mean = .15 g/kg of BW per h). Dietary intake declined on the 4th d of the insulin clamp (23.0 vs. 16.3 kg/d). Milk yield, however, did not change (32.4 vs. 33.6 kg/d) in support of the lack of sensitivity 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 clamp. Milk protein yield increased (.98 vs. 1.05 kg/d), and milk protein percentage tended to increase (3.04 vs. 3.14), during the insulin clamp. This modest increase in milk protein yield may have been constrained by a lack of available amino acids, as indicated by a decrease in circulating 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 insulin infusion, either directly or indirectly, enhanced secretion of milk protein.