Mouse extensor digitorum longus muscle preparation as a tool in nutrition research: A quantitative comparison to in vivo and cell culture experiments

Incubated restrained and unrestrained extensor digitorum longus (EDL) muscl es from adult non-growing mice were evaluated as a tool in non-steady state nutrition experiments. Energy state was determined by nucleotide determina tions in muscles. Protein synthesis was estimated by the amount of L-[U-C-1 4]phenylalanine incorporated into proteins, and protein balance was measure d by tyrosine release from muscle proteins, Confluent cultured L6 rat muscl e cells served as a reference system in steady state without hypoxia being sensitive to growth factors and regulatory peptides at physiologic concentr ations. Irrespective of medium composition, incubated EDL muscles remained in negative protein balance, being unrelated to the resting tension of the incubated muscles. Energy-rich phosphates were not restored to normal level s during incubation, but protein synthesis was not attenuated by the declin e in energy State, Fractional protein synthesis (0.05-0.15%/h) remained con stant for up to 6 h of EDL incubation, and was comparable to protein synthe sis in cultured confluent non-proliferating myocytes (0.20-0.30%/h) and to mixed leg muscles measured in viva (0.10-0.20%/h). Protein synthesis in inc ubated EDL muscles reflected alterations in muscle peptide formation in viv o following either oral provision of food or parenteral injection of insuli n. EDL muscles were sensitive to in vitro exposure to both insulin (60-125 mu U/mL) and insulin-like growth factor 1 (IGF-1) (1000 ng/mL). The sensiti vity to insulin seemed to be modified by the nutritional state (starved/fed ) of the animals before sacrifice. Protein synthesis in EDL muscles was les s responsive to serum-containing growth factors (IGE-1, epidermal growth fa ctor [EGF], platelet-derived growth factor [PDGF]) compared to confluent L6 muscle cells, which probably reflected different receptor expression. Our results demonstrate that protein metabolism in incubated unrestrained mouse EDL muscles reflects in vivo protein metabolism. Nutrition 1999; 15:200-20 7. (C)Elsevier Science Inc. 1999.