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.