INFLUENCE OF DIETARY-PROTEIN, ENERGY AND CORTICOSTEROIDS ON PROTEIN-TURNOVER, PROTEOGLYCAN SULFATION AND GROWTH OF LONG-BONE AND SKELETAL-MUSCLE IN THE RAT

1. We report here the extent to which changes in protein turnover cont ribute to the previously described inhibition of growth of rat tibial length and skeletal muscle mass in response to protein deficiency [1], energy restriction and corticosterone treatment [2]. Measurements of S-35 uptake in vivo also enabled the qualitative pattern of changes in proteoglycan synthesis in bone and muscle to be established. 2. Prote in deficiency was examined by ad libitum feeding of 20%, 7%, 3.5% and 0.5% protein diets with measurements at 1, 3 and 7 days (all diets), a nd 14 and 21 days (0.5% protein). In bone this induced delayed inhibit ion of tibial growth with parallel inhibition of protein synthesis, as measured by the phenylalanine flooding dose method. This was mediated by reductions in both ribosomal capacity (RNA/protein ratio) and acti vity (protein synthesis/RNA) in the 0.5% protein group. The pattern of inhibition of proteoglycan sulphation, measured as S-35 uptake 60 min after injection of a tracer dose of labelled sulphate, was similar to that of protein synthesis. 3. In muscle there was an intermediate gra ded inhibition of protein synthesis by protein deficiency, mediated by reductions in both ribosomal capacity and activity in the 0.5% protei n group, which preceded growth inhibition in the 7% and 3.5% groups, a nd which was progressive with time. Transient increases in proteolysis contributed to the growth inhibition is some groups, but the rate fel l eventually in the 0.5% group. The pattern of response of proteoglyca n sulphation differed from protein synthesis with a delayed inhibition , but with subsequent marked reduction. 4. Energy restriction was indu ced by diets fed for 4 or 8 days at 75%, 50% and 25% ad libitum intake s with protein intakes held constant, and corticosterone treatment inv olved a dose of 10 mg day(-1) 100(-1) g (subcutaneous) with ad libitum feeding. In bone this induced a pattern of length growth inhibition w hich was dissociated from inhibition of protein synthesis in the moder ately restricted (75% and 50%) groups. Only in the 25% group and in th e 8 day corticosterone group was protein synthesis inhibited, through reductions in ribosomal capacity and activity. S-35 uptake was also di ssociated from growth inhibition, with reduced S-35 uptake observed on ly after corticosterone treatment or 8 days of the 50% or 25% diets. 5 . In muscle the energy restriction and corticosterone treatment induce d parallel inhibitions of growth and protein synthesis, mediated by si milar graded reductions in the RNA/protein ratios and in the 25% group in the K-RNA. Proteolysis was unchanged in all except the 4-day corti costerone group (elevated by 25%) and the day 8 25% group (elevated by 40%) and corticosterone group (elevated by 60%). S-35 uptake was inhi bited in parallel to muscle growth and protein synthesis. 6. These dat a show that inhibition of protein synthesis and S-35 uptake is an inva riable element of muscle growth inhibition, and a usual but not invari able element of bone growth inhibition. Partial correlation analysis o f the interactions between dietary protein, bone growth and muscle pro tein and proteoglycan synthesis shows that bone growth (as indicated b y epiphyseal cartilage width) is significantly correlated with muscle protein synthesis and especially S-35 uptake, suggesting that the regu lation of muscle growth by passive stretch consequent an bone lengthen ing includes muscle connective tissue growth as an important target.