Comparison of the effects of D-mannoheptulose and its hexaacetate ester on D-glucose metabolism and insulinotropic action in rat pancreatic islets.

It was recently, and surprisingly, found that D-mannoheptulose did not affect D-glucose metabolism and insulinotropic action in pancreatic islets incubated at a low concentration of D-glucose. To explain this finding, the metabolism and secretory response to the hexose were investigated in rat islets exposed to D-mannoheptulose hexaacetate, which was recently found to inhibit D-glucose catabolism in cells that are otherwise fully resistant to the heptose. At a high concentration of D-glucose (16.7 mmol/l), the utilisation of D-[5-(3)H]glucose and oxidation of D-[U-14C]glucose, as well as the insulinotropic action of the hexose, were affected less by D-mannoheptulose tetraacetate than by unesterified D-mannoheptulose. This coincided with a reduced uptake of the ester by intact islets and a lower rate of hydrolysis of the ester in islet homogenates compared with findings in other monosaccharide esters such as D-glucose pentaacetate. At a low concentration of D-glucose (2.8 mmol/l), D-mannoheptulose hexaacetate was slightly more efficient than the unesterified heptose in reducing D-glucose catabolism, but still failed to suppress the secretory response to the hexose. These findings do not necessarily mean that unesterified D-mannoheptulose enters beta-cells more efficiently at high than at low extracellular D-glucose concentrations, especially if possible differences in the respective contributions of distinct islet cell types to the overall catabolism of D-glucose by whole islets is allowed for. These data do not rule out the possibility that D-glucose phosphorylation is more resistant to D-mannoheptulose in beta cells incubated at a low than a high concentration, independently of any difference in the intracellular concentration of the heptose. However, the mechanism of this resistance is still not explained.