Arterial plasma composition during compensatory increase in milk secretion in the goat: relation to rate limitation

Colchicine was injected into one mammary gland of goats at peak lactation. Milk secretion in the treated gland decreased to 40% of the previous level while that in the other gland increased to reach 126% of its previous level. The concentrations of tyrosine and glucose in arterial plasma increased while those of lysine, arginine asparagine, glycine, alanine and ornithine decreased; the concentrations of other amino acids and of acetate did not change significantly. The results are discussed in relation to factors which normally limit the rate of milk secretion and to the mechanism by which compensatory increases occur in one gland during the inhibition of secretion in the other. These and previous results are not compatible with the view that arterial plasma concentrations of essential amino acids are rate-limiting for milk secretion.