A quantitative assessment of the contribution of individual plasma amino acids to the synthesis of milk proteins by the goat mammary gland

Abstract

1. Arteriovenous differences of plasma free amino acids across the lactating mammary glands of six goats have been measured. 2. In four experiments, measurements of blood flow, amino acid arteriovenous differences, milk yield and milk nitrogen showed that the uptake of nitrogen in the form of amino acids was sufficient to provide all the nitrogen of the milk proteins synthesized in the mammary gland. 3. In the same four experiments the uptake from the plasma and output into the milk of individual amino acids per unit time were compared. The uptakes of essential amino acids and glutamic acid were approximately equal to the corresponding output figures. The uptake of serine was consistently less than the output, and the uptake of other non-essential amino acids was very variable, in some experiments being approximately equal to the output figures and in others being considerably less. 4. As in cows, there was an uptake of ornithine in all experiments, though ornithine is absent from milk. In goats, though not in cows, the uptake of arginine was consistently greatly in excess of the requirement for arginine residues in milk protein. 5. The possible significance of the uptakes of arginine and ornithine for the synthesis of serine and other non-essential amino acids in the mammary gland is discussed. 6. The importance of clamping the external pudic vein, when sampling mammary venous blood from the caudal superficial epigastric vein, is indicated.

Amino Acid Uptake by the Mammary Gland of the Lactating Ewe

“…3) declining only to parallel the fall in milk yield. In accord with the data obtained in the goat (Mepham and Linzell 1966;Mepham and Linzell 1974) uptake of non-essential amino acids was variable, as evidenced by the higher coefficients of variation with amino acid extraction for this group (see Table 2). However, rather than reflecting a true variation in uptake it is felt that the low extraction of non-essential amino acids precludes the accurate estimation of A V difference.…”

“…B Values in parentheses are coefficients of variation of amino acid extraction. C Mepham and Linzell (1966). D Mean of Friesian cows 'J' and 'T' (Bickerstaffe et al 1974).…”

“…The arterio-venous concentration (A V) difference technique has been applied to the study of mammary substrate uptake in the lactating dairy cow (Bickerstaffe et al 1974), dairy goat (Mepham and Linzell 1966) and pig (Linzell et al 1969;Spincer et al 1969).…”

Mammary Glucose Uptake in the Lactating Ewe and the Use of Methionine Arterio-Venous Difference for the Calculation of Mammary Blood Flow

“…For example, it has been proposed that methionine is the amino acid first-limiting for milk protein synthesis in the ewe (Davis et al 1978) and the mean plasma concentration in the lactating ewe (1. 70 Jig/ml) is considerably less than that found in studies on the cow [mean 3· 50 Jig/ml (Bickerstaffe et al 1974)] and goat [mean 2· 70 Jig/ml (Mepham and Linzell 1966)]. …”

Arginine metabolism: nitric oxide and beyond

“…The output of proline in the milk of goats [255], sheep [256], cows [257] and pigs [258] greatly exceeds the uptake of proline by the lactating mammary gland, whereas the uptake of plasma arginine by lactating mammary glands greatly exceeds the output of arginine in the milk [255][256][257][258][259]. Studies with lactating mammary tissues have demonstrated arginine-dependent production of proline [249,257], but there was little or no synthesis of proline from glutamate [249] because of the absence of P5C synthetase [47,249].…”