Abstract
At a low concentration of D-glucose (3.3 mM), the phosphorylation rate of this hexose in rat pancreatic islet homogenates incubated at 8 degrees C is higher with the beta- than with the alpha-anomer, as expected from the anomeric specificity of hexokinase. In the presence of a high concentration of glucose 6-phosphate (3.0 mM), which inhibits hexokinase but not glucokinase, the phosphorylation rates of the two anomers are not significantly different from one another. Nevertheless, in intact islets exposed at 8 degrees C to the same low concentration of D-glucose, the alpha-anomer augments, more than the beta-anomer, the production of lactic acid and net uptake of 45Ca. At the same concentration (3.3 mM), the alpha-anomer is also more potent than the beta-anomer in enhancing insulin release from perfused pancreases stimulated at 37 degrees C by L-leucine or by the combination of Ba2+ and theophylline. It is concluded that the participation of glucokinase is not essential for the anomeric specificity of glycolysis and insulin release in rat pancreatic islets.
Highlights
At a low concentration of D-glucose (3.3 mM), the The latter findings led us to investigate whether the glycophosphorylation rate of this hexose in rat pancreatic lytic and secretory responses of pancreatic islets would not islet homogenates incubated a8t O C is higher with the display a-stereospecificity even when the anomers of D-glu
Glucose werewashed in ethanol and ether andeventually dried under vacuum at 50 "C for a few hours
Pilot experiments performed with pancreatic islets incubated for 60 min at 37 "C indicated that 3.3 mM Dglucose failed to stimulate insulin release in theabsence of another secretagoguebut augmented insulin output in the presence of L-leucine (10.0mM)
Summary
Comparable results were obtained mM, the rateof glucose phosphorylation by islet homogenates, in the presence of L-leucine (10.0mM),which itself augmented as measured by a nonradioisotopic procedure, was higher with &Canet uptake to 516 & 13 fmol/islet ( n= 37). In the absence of the finding that D-glucoseor L-leucine stimulates 45Cauptake glucose 6-phosphate, the a//3 ratio in phosphorylation rate by islets incubated at 8 "C is not surprising since cooling averaged 0.705 ? In thepresence (see below) was qualitatively comparable to that previously of glucose6-phosphate, the a/@ratio in phosphorylation rate established when the two variables were measured at 37 "C. Of @-D-glUCOSein the absence of glucose 6-phosphate. Such a reference value averaged 34.5 -C 5.0 pmol/60 min/islet (h = 6)
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