1. The metabolic pattern of [U−14C]glucose in the isolated rat heart has been studied, with both retrograde aortic (Langendorff) and atrially (working) perfused preparations in the presence and absence of insulin, in normal animals, animals rendered insulin-deficient (by injection of anti-insulin serum 1hr. before excision of the heart) and animals rendered diabetic by streptozotocin injection 7 days before use. 2. Radioautochromatograms of heart extracts show that the pattern of glucose metabolism in heart muscle is more complex than in diaphragm muscle. In addition to 14CO2, glycogen, oligosaccharides, phosphorylated sugars and lactate (the main metabolites formed from [14C]glucose in diaphragm muscle), 14C label from [14C]glucose appears in heart muscle in glutamate, glutamine, aspartate and alanine, and in tricarboxylic acid-cycle intermediates. 3. By a quantitative scanning technique of two-dimensional chromatograms it was found that a mechanical work load stimulates glucose metabolism, increasing by a factor of 2–3 incorporation of 14C into all the metabolites mentioned above except lactate and phosphorylated sugars, into which 14C incorporation is in fact diminished; 14CO2 production is equally stimulated. 4. Addition of insulin to the perfusion fluid of the working heart causes increases in 14C incorporation, by a factor of about 1·5 into 14CO2, by a factor of about 3–5 into glycogen, lactate and phosphorylated sugars, by a factor of about 2–3 into glutamate and tricarboxylic acid-cycle intermediates and by a factor of about 0·5 into aspartate, whereas incorporation into alanine and glutamine is not affected. The effect of a work load on the pattern of glucose metabolism is thus different from that of insulin. 5. Increasing the concentration of glucose in the perfusion fluid from 1 to 20mm leads to changes of the pattern of glucose metabolism different from that brought about by insulin. 14CO2 production steadily increases whereas [14C]lactate and glycogen production levels off at 10mm-glucose, at values well below those reached in the presence of insulin. 6. In Langendorff hearts of animals rendered insulin-deficient by anti-insulin serum or streptozotocin, glucose uptake, formation of 14CO2 and [14C]lactate, and 14C incorporation into glycogen and oligosaccharides are decreased. In insulin-deficient working hearts, however, glucose uptake and 14CO2 production are normal, whereas incorporation of 14C into glycogen and [14C]lactate production are greatly decreased. 7. Insulin added to the perfusion fluid restores 14C incorporation from glucose into 14CO2, glycogen and lactate in the Langendorff heart from animals rendered insulin-deficient by anti-insulin serum; in hearts from streptozotocin-diabetic animals addition of insulin restores 14C incorporation into glycogen and lactate, but 14CO2 production remains about 50% below normal. 8. The bearing of these results on the problem of the mode of action of insulin is discussed.
Characteristics of alanine: glyoxylate aminotransferase from Saccharomyces cerevisiae, a regulatory enzyme in the glyoxylate pathway of glycine and serine biosynthesis from tricarboxylic acid-cycle intermediates