Specificity of insulin or oxytocin stimulation of protein synthesis in adipose tissue

Insulin stimulates glucose uptake, fat synthesis, and incorporation of amino acids into protein of rat adipose tissue. Other agents having insulinlike effects on glucose metabolism have now been tested for their ability to promote protein synthesis in this in vitro system. Rat epididymal fat pads were incubated in Krebs bicarbonate medium containing glucose or pyruvate, acetate-1-C14 as precursor for lipids, or (in separate experiments) histidine-2(ring)-C14 as precursor for protein. Glucose uptake was measured by the glucose oxidase method, and radioactivity of lipid and protein fractions was estimated. Synthetic oxytocin (Sandoz), 0.1–10 U/ml, stimulated glucose uptake, acetate incorporation into lipid, and histidine-C14 incorporation into protein when glucose was present; unlike insulin, oxytocin did not enhance protein synthesis when pyruvate replaced glucose in the medium. RNA (1 mg/ml), nicotinic acid (0.001 m), and protamine sulfate (1 mg/ml) each stimulated glucose uptake and acetate incorporation into lipid, but did not enhance histidine-C14 incorporation into protein. It is concluded that in adipose tissue insulin has a specific effect on protein synthesis which cannot be mimicked by other agents which stimulate glucose uptake or lipid synthesis.