The intermediary metabolism of the mammary gland. 2. Respiration and acid production of mammary tissue during pregnancy, lactation and involution in the rat

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Abstract
The respiratory metabolism and acid production of slices of rat mammary gland in the presence and absence of glucose were studied during pregnancy, during lactation, and following weaning: QO2 is low at the end of pregnancy, but has increased considerably on the day following parturition; thereafter it rises to a max. of approx. 10 at mid-lactation. Interpretation of these changes is complicated by changes in the dry-matter content of the tissue, which by mid-lactation is less than half of the value at the end of pregnancy. However, calculation of the total respiration of the abdominal mammary glands indicates that initiation and subsequent increase in the intensity of lactation are accompanied by a true increase in respiration. The effect of glucose in increasing the respiration of mammary gland slices is seen only in lactating tissue. The R. Q. (in glucose is well below 1 at the end of pregnancy, but has risen to 1 on the day following parturition; thereafter it reaches a max. of approx. 1.6, which is maintained throughout most of the lactation period. In absence of substrate, the R. Q. remains below 1 throughout lactation. Lactating gland shows a higher value for QO/G2 than the gland at the end of pregnancy; the increase is largely an artifact due to the changes in the dry-matter content of the tissue. The extra energy requirements of the lactating gland seem to be met solely by an increase in respiration. Weaning is followed by a sharp fall, the change in Q02 being too great to be attributed to the relatively slight increase in the dry-matter content of the tissue. Further, the R. Q. decreases below 1 and the apparent aerobic glycolysis rises. The significance of these results is discussed in the light of possible mechanisms of fat synthesis in the mammary gland, and of relationships between anterior pituitary hormones and changes in mammary gland metabolism.
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