Impairment by cytochalasin B of the inhibitory action of D-mannoheptulose upon D-glucose metabolism in rat pancreatic islets

D-mannoheptulose was recently found to inhibit D-glucose metabolism in hepa tocytes and pancreatic islets, whilst failing to do so in parotid cells, er ythrocytes and the exocrine pancreas. In the latter three systems, however, the hexaacetate ester of D-mannoheptulose efficiently inhibits D-glucose m etabolism It was proposed, therefore, that the transport of unesterified D- mannoheptulose into cells may be mediated by GLUT2. Since cytochalasin B is known to inhibit D-glucose transport into pancreatic islet cells, it was n ow investigated whether the mould metabolite (0.02 mM) also impairs the inh ibitory action of D-mannoheptulose (1.0 mM) upon D-glucose metabolism in ra t pancreatic islets. The relative extent of D-mannoheptulose inhibitory act ion on D-[5-H-3]glucose utilization and D-[U-C-14]glucose conversion to (CO 2)-C-14, as well as radioactive amino acids and acidic metabolites, was ind eed much less marked in the presence of cytochalasin B (13+/-4% inhibition) than in its absence (40+/-3% inhibition). A comparable situation was not o bserved, however, in the case of glucose-stimulated insulin secretion, cyto chalasin B augmenting insulin output to the same relative extent in the abs ence or presence of D-mannoheptulose. These findings support the view that the entry of D-mannoheptulose into cells may be mediated by a cytochalasin B-sensitive transport system, such as the GLUT2 carrier.