Glycogen accumulation in rat pancreatic islets: in vitro experiments.

Under conditions of sustained hyperglycemia, glycogen accumulates in pancreatic islets, but not so in acinar pancreatic cells. Advantage conceivably could be taken from such a situation in the perspective of the noninvasive imaging of the endocrine pancreas. The present experiments aim, therefore, at characterizing the time course for glycogen accumulation in pancreatic islets cultured at a high concentration (30 mM) of D-glucose in the presence of tracer amounts of either D-[U-14C]glucose or 2-deoxy-2-[18F]fluoro-D-glucose. The 14C-labeled glycogen content of the cultured islets increased with time (150 min to 72 h), exceeded that found in acinar tumoral cells, and did not decrease over 60 min of incubation at 30 mM D-glucose in the absence of D-[U-14C]glucose. Glycogenolysis was observed, however, when the concentration of D-glucose was decreased to 2.8 mM and, in such a case, was further enhanced by forskolin and theophylline. Such a glycogenolysis coincided with the generation of 14CO2 from radioactive intracellular precursors and alteration of the B-cell secretory response to D-glucose. The radioactive glycogen content was higher in islets exposed to 2-deoxy-2-[18F]fluoro-D-glucose than D-[U-14C]glucose. Prior exposure of the islets to streptozotocin suppressed the accumulation of glycogen during their subsequent culture at high D-glucose concentration. These findings may help to define the experimental conditions optimal for the labeling and accumulation of islet glycogen in vivo.