Metabolic disposition of glucose carbon by sensory ganglia of 15-day-old chicken embryos, with new dynamic models of carbohydrate metabolism.

Dorsal root ganglia from the lumbar region of chicken embryos 14--16 days old were incubated at 37 degrees in modified McCoy's culture medium containing [1-(14)C]glucose or [6-(14)C]glucose and sometimes 32Pi. A volume of 10 microliters of medium was used for four ganglia (dry weight approx. 90 micrograms). The output of 14CO2 was measured continuously. Incorporation of 14C into tissue constituents and into products released to the medium was measured after incubation for 3--17 h. Among nine radioactive components resolved in paper chromatograms of ganglion constituents, the most 14C was found in lipids and on materials remaining at the origin. Only the lipids and the origin materials were detectably labeled by 32Pi. Only relatively small amounts of 14C from [6-(14)]glucose were found in chromatographic regions that should contain intermediates of the pentose cycle. At least five labeled products were released to the bathing medium. Among these, the largest amount of 14C was found in lactic acid. A second component released, possibly alanine, also received considerable 14C. No 32P was detected in products in the medium. The rate of glucose uptake remained relatively constant as the concentration of glucose in the medium declined nearly 10-fold during prolonged experiments. Two new dynamic models of glucose metabolism successfully explained the time courses and magnitudes of previously reported 14CO2 outputs from [1-(14)C]glucose, [2-(14)C]-glucose, and [6-(14)C]glucose. These models are based on the assumption that glucose carbon was delayed on its way to CO2 in a pool of intermediates early in the metabolic chain and in a second pool either in or before the citric acid cycle. Both models assigned the pentose cycle to one cellular compartment, and incorporation into slowly-turning-over substances to another cellular compartment. According to both models, not more than one-half of the glyceraldehyde-P produced by the pentose cycle was converted to fructose-6-P, while at least half of this and other fructose-6-P from the pentose cycle was recycled into it. These conclusions differ from those from a previous model, which assumed that glucose carbon was delayed in a pool related to the pentose cycle; that model had suggested full recycling of both the glyceraldehyde-P and the fructose-6-P produced by the pentose cycle. In the citric acid cycle the efficiency of recycling was over 80%, according to all models. All models demonstrated the large differences that can occur in the metabolic handling of carbons 1, 2, and 6 of glucose. These differences need consideration in any description of the partitioning of glucose metabolism between alternative pathways.