Responses of the bovine mammary glands to absorptive supply of single amino acids

Abstract

Responses of the bovine mammary glands to absorptive supply of single amino acids. Can. J. Anim. Sci. 83: [341][342][343][344][345][346][347][348][349][350][351][352][353][354][355]. In this review, we discuss the mechanisms of responses of various tissues of the lactating dairy cow, particularly the mammary glands, to perturbations in supply of single amino acids that result in observed milk protein yields. Additions of methionine, lysine, histidine or leucine to the absorptive supply cause arterial concentrations of these amino acids to increase, mammary extractions to drop and mammary blood flow to decrease. Single subtractions of essential amino acids have the opposite effect. Changes in mammary blood flow that have been recorded can be explained as attempts by the mammary glands to restore intracellular ATP balance in the face of altered concentrations of energy metabolites in the general circulation. In a quantitative sense, milk protein yield is relatively insensitive to fluctuations in arterial amino acid concentrations but can be stimulated by any one of a number of amino acids. In this context, it is suggested that the designation of a limiting amino acid is not appropriate to the purpose of predicting milk protein yield. Rather, milk protein synthesis appears to operate at a predetermined rate set by external communications of milk withdrawal rate, physiological state and overall nutritional status. Utilization of amino acids (AA) by splanchnic and peripheral tissues, in coordination with the mammary setpoint, offsets imperfections in the dietary AA supply. How strongly an individual AA influences the mammary setpoint, arterial concentrations of energy metabolites, and mammary AA transport capacity will determine the magnitude of the milk protein yield response when its absorptive supply is changed. Dans cet article, les auteurs examinent comment certains tissus de la vache laitière en lactation, principalement la glande mammaire, réagissent à une modification de l'apport de certains acides aminés entraînant un changement du rendement du lait en protéines. L'addition de méthionine, de lysine, d'histidine ou de leucine aux acides aminés absorbés augmente leur concentration dans le sang artériel et la diminue dans les sécrétions et le sang de la glande mammaire. La soustraction d'un de ces acides aminés a l'effet contraire. On explique les variations observées au niveau de la circulation sanguine dans la glande mammaire par les efforts que cette dernière déploie pour rétablir l'équilibre de l'ATP après modification de la quantité de métabolites porteurs d'énergie dans la circulation générale. Sur le plan quantitatif, le rendement du lait en protéines est relativement insensible aux fluctuations de la concentration d'acides aminés dans le sang artériel, mais il est stimulé par plusieurs acides aminés. On estime qu'il est impossible de prévoir le rendement du lait en protéines à partir d'un acide aminé limitant. En effet, les protéines laitières semblent être synthétisées à un taux préétabli par les rappo...

Immediate and residual effects of heat stress and restricted intake on milk protein and casein composition and energy metabolism

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