MATERNAL PROTEIN RESERVES AND THEIR INFLUENCE ON LACTATIONAL PERFORMANCE IN RATS .2. EFFECTS OF DIETARY-PROTEIN RESTRICTION DURING GESTATION AND LACTATION ON TISSUE PROTEIN-METABOLISM AND NA-ATPASE (EC-3.6.1.3) ACTIVITY(,K+)

Changes in tissue protein synthesis and an associated membrane transpo rt system in rats were investigated during lactation and under conditi ons of dietary protein restriction. Following mating, female Sprague-D awley rats (second parity) were caged individually and offered a high- protein diet (H; 215 g crude protein (N x 625; CP)/kg dry matter (DM)) ad lib. until day 12 of gestation. Subsequently half continued to rec eive diet H, whilst the remainder were offered a low-protein diet (L; 65 g CP/kg DM) until parturition. On day 1 of lactation females were t hen allocated to either diet H or another low-protein diet (L(2); 90 g CP/kg DM) which were offered ad lib. until day 13 of lactation, givin g four lactation groups HH, LH, HL(2) and LL(2). On days 1 and 13 of l actation groups of females were used in the estimation of tissue prote in synthesis (flooding dose of [H-3] phenylalanine) and Na+, K+-ATPase (EC 3.6.1.3) activity (polarographically) in skeletal muscle, mammary gland, liver and duodenal mucosa. By day 1 of lactation diet L had re duced fractional and absolute synthesis rates (FSR and ASR) of muscle protein (P < 0.05) and the O-2 consumption associated with Na+, K+-ATP ase, although not significantly (P < 0.10). Rates of protein synthesis in the other tissues studied were not affected on day 1 of lactation by the gestation dietary treatment. By day 13 of lactation the feeding of diet L(2) had reduced muscle FSR and ASR of group HL, to rates tha t were lower than those on day 1 (P < 0.05), comparable to those of gr oup LL(2) and lower than those of groups HH and LH (P < 0.05). Diet II had allowed group LH to increase their muscle protein synthesis compa red with that on day 1 (P < 0.05). Muscle Na+, K+-ATPase activity on d ay 13 of lactation was also lower in groups offered diet L(2) (P < 0.0 5). Mammary protein synthesis was increased during lactation with the feeding of diet H (P < 0.05), which was prevented by diet L(2) such th at rates of groups HL(2) and LL(2) were lower than those of the two hi gh-protein groups on day 13 (P < 0.01). Mammary respiration and in par ticular Na+, K+-ATPase activity was increased during lactation by the feeding of diet H (P < 0.05). Rates of protein synthesis and respirati on in liver and duodenal mucosa were not significantly affected by the gestational or lactational dietary treatments. Calculated rates of mu scle protein degradation suggest that whilst the loss of muscle protei n in group HL(2) during lactation might have been promoted by the decl ine in synthesis, the increase in degradation may have been quantitati vely more important.