Metabolism, 45Ca uptake and insulin releasing capacities of glucose and mannose

A detailed dose-response study relating mannose and glucose oxidation with the induction of 45Ca uptake and insulin release was performed using in vitro incubation of collagenase digested rat islets of Langerhans. The threshold value for 14C-U-D-glucose oxidation, glucose-stimulation of 45Ca uptake, and insulin release was about 5.5 mM. The half maximal response for these 3 parameters occurred at 13.4 mM, 11.6 mM, and 12.2 mM, respectively. Their maximal responses were obtained at approximately 20 mM. The threshold level for mannose oxidation, induction of 45Ca uptake and insulin release was about 11.0 mM, with half maximal responses obtained at 24.6 mM, 20.5 mM, and 22.2 mM, respectively. The maximum response of the 3 parameters to mannose was obtained at 38.8 mM and appeared to reach the same level obtained for glucose. These results suggest that hexose degradation has a significant role in controlling Ca uptake and subsequent insulin release. A lower rate of mannose oxidation appeared to account for its weaker stimulating efficacy for Ca uptake and insulin release.