The metabolism of fructose by isolated rat diaphragms.

The elucidation of enzyme systems involved in the metabolism of fructose has done much in clarifying its metabolic pathway. Recent work (1, 2) on glucose and fructose metabolism by normal and diabetic rat livers has suggested that fructose can be metabolized by liver enzyme systems that are independent of glucose metabolism. Similar enzyme systems that act upon fructose have been reported to occur in muscle (3), but many of the existing reports on fructose utilization by intact muscle cells are conflicting. Haft and coworkers (4) demonstrated that insulin increased fructose uptake by diaphragms. Mackler and Guest (5) showed that a considerable amount of fructose was taken up by rat diaphragms in the presence of glucose. These latter authors attributed this fructose uptake to the action of fructokinase. On the other hand, Corkill and Nelson (6) found that little or no fructose was utilized by the spinal eviscerated cat. A more detailed study on fructose metabolism by peripheral tissues was carried out by Wick, Sherrill, and Drury (7). These authors found that the volume of distribution of fructose in the eviscerated nephrectomized rabbit corresponded to the extracellular compartment. This was true whether the amount of fructose given was large or small, and the administration of insulin had no effect on the fructose volume of distribution. Not only was the rate of oxidation of fructose found to be low, but also the oxidation of radioactive glucose was not affected by the injection of unlabeled fructose. These results are inconsistent with the idea that muscle cells contain an independent mechanism for fructose metabolism. Because of the persistent assumptions that, since such an independent pathway has been found in liver, a similar situation must ipso facto be true in muscle, a further examination of fructose metabolism by muscle cells seemed necessary. This report gives the results of such a study with the use of the isolated rat diaphragm.