The intermediary metabolism of the mammary gland. 3. Acetate metabolism of lactating mammary gland slices with special reference to milk fat synthesis

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Abstract
The respiration and the production or utilization of acid by lactating mammary gland slices from ruminants (sheep, goat, cow) and non-ruminants (rat, rabbit) were studied with acetate and/or glucose as substrates. Mammary tissue from ruminants, whether in early or late lactation, is by contrast with tissue from non-ruminants almost inert towards glucose as shown by low QO2 values and R.Q.< 1. Mammary slices from lactating ruminants markedly utilize acetate, Q02 being greater than values without substrate or in glucose and R.Q. >1. On the other hand, tissue from non-ruminants shows R.Q. < 1 in acetate, though some acetate is utilized by rabbit tissue and Q02 is as great as in glucose. Mammary slices from a well-lactating sheep showed a regular increase in -QO2, R.Q. and acetate utilization over the period 7-47 days postpartum, i. e., over the period of increasing milk yield. In glucose, -QO2 and R.Q. were the same as in acetate at the outset, but showed no subsequent increase. Glucose stimulates acetate utilization by mammary slices from rat, rabbit and sheep. In presence of glucose, rat mammary tissue utilized some acetate with R.Q. > 1 and indeed similar to the value in glucose alone; in rabbit tissue acetate utilization was much increased, again with the high R.Q. maintained; while in the sheep the main effect of glucose was to increase R.Q. over the already high value for acetate alone. In the rat and rabbit the increment in Q02 was insufficient to account for the increased acetate uptake in terms of oxidation. The results are regarded as evidence of the utilization of acetate for milk-fat synthesis by the mammary gland, and they are discussed from this point of view.