MAMMARY SENSITIVITY TO PROTEIN RESTRICTION AND RE-ALIMENTATION

The present study tested the influence of protein undernutrition and r e-alimentation on mammary gland size and secretory cell activity in la ctating rats. During gestation, female Sprague-Dawley rats were offere d a high-protein diet (215 g crude protein (N x 6.25; CP)/kg DM; H); l itters were standardized to twelve pups at parturition. During lactati on, two diets were offered ad libitum, diet H and a low-protein diet ( 90 g CP/kg DM; L). Lactational dietary treatments were the supply ad l ibitum of either diet H (HHH) or diet L (LLL) for the first 12 d of la ctation, or diet L transferring to diet H on either day 6 (LHH) or 9 ( LLH) of lactation. On days 1, 6, 9 and 12 of lactation, rats from each group (n greater than or equal to 6) were used to estimate mammary dr y mass, fat, protein, DNA and RNA; the activities of lactose synthetas e (EC 2.4.1.22) enzyme and Na+,K+-ATPase (EC 3.6.1.37) were also measu red. Rats offered a diet considered protein sufficient (H) from day 1 of lactation showed a decrease in mammary dry mass and fat but an incr ease in DNA, RNA and protein on day 6, after, which there was no furth er change, except for mammary protein which continued to increase. How ever, rats offered diet L showed a steady loss in mammary mass and fat throughout the 12 d lactation period and no change in mammary DNA, RN A or protein. Rats previously protein restricted for either the first 6 or 9 d of lactation had their mammary dry mass and mammary fat loss halted and showed a rapid increase in mammary DNA, RNA and protein on re-alimentation. Lactose production in group HHH, as measured by lacto se synthetase activity, was similar on days 1 and 6 of lactation, afte r which a significant increase was seen. Protein-restricted rats showe d no change in lactose synthetase activity during the 12 d experimenta l period. Changing from diet L to diet H led to a significant increase in lactose synthetase activity to levels comparable with those offere d diet H from day 1. These results show that rats offered a protein-re stricted diet during lactation suffer mammary underdevelopment, but th is may be rapidly reversed by realimentation with a high-protein diet.