Lactose synthesis in the rat, and the effects of litter size and malnutrition.

1. The rate of lactose synthesis per g of mammary tissue, measured in vivo by a radioisotopic technique, rose 13-fold between parturition and day 16 of lactation in the rat, but was unaffected by wide variation in litter size. 2. The increase reflected a greater tissue content of galactosyltransferase (EC 2.4.1.22), and was augmented by a rise in the total weight of mammary tissue. Superimposed on this were unpredictable changes in the functional efficiency of the enzyme. 3. Lactose synthesis in 14-day-lactating rats, permitted only 76% of the food intake of paired control rats over the previous 3 weeks, showed a pronounced diurnal variation at an overall rate markedly below that in control rats. 4. Such nutritional deficiency did not affect the tissue content of galactosyltransferase, but impaired its functional efficiency in a manner reversed by renewed feeding or by the preparation and incubation of acini in vitro. 5. Plasma insulin concentrations decreased at parturition and with increasing litter size, and remained relatively unchanged during lactation and malnutrition.