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Substrate Utilization by Fetal Pig Skeletal Muscle

U.S. Department of Agriculture¹ Athens, GA 30613

Abstract

The ability of fetal pig skeletal muscle (biceps femoris) to metabolize glucose, fructose, lactate, acetate and palmitate in vitro was examined at 70, 90 and 110 d of gestation. Even though succinate dehydrogenase activity increased as fetal age increased (P<.01), the rate of oxidation of glucose, fructose, acetate and palmitate to CO² was not influenced by fetal age (P>.05), but each rate was dose-dependent (P<.01). At higher concentrations, lactate oxidation to CO₂ proceeded at a faster rate in the muscle of 70-d fetuses when compared with 90- or 110-d fetuses. Muscle glycogen content increased (P<.01) from 70 to 110 d of gestation. The rate of glucose incorporation into glycogen increased over this same time frame (P<.06). Glucose-6-phosphate dehydrogenase activity did not change with age when activity was expressed per unit wet weight of muscle (P>.05). The ratio [1-¹⁴C] glucose/[6-¹⁴C] glucose oxidized to CO₂ was independent of age and substrate concentration, and indicated significant pentose cycle activity in fetal muscle. Incorporation of lactate and palmitate into phospholipid was greatest at 70 d of gestation, a time period that coincides with establishment of mature muscle fiber number and fiber hypertrophy. The rate of palmitate incorporation into triacylglycerol was dependent on concentration of substrate (P<.01) but not on age (P>.05). The rate of fructose oxidation to CO₂ was lower than the rate for glucose, lactate and acetate when compared at similar concentrations. Acetate carbons were not incorporated into free fatty acids or lipid. In conclusion, fetal pig skeletal muscle is capable of oxidizing a variety of substrates. Fetal muscle is capable of generating energy stores in the form of glycogen and triacylglycerol.

¹ R. B. Russell Agr. Res. Center, P. O. Box 5677.