Possible relationship between the B-cell threshold for glucose-induced insulin secretion and blood glucose concentrations in the normal toad.

The aim of this work was to gain information on the possible relationship between basal glycemia in the toad and the B-cell threshold for glucose-induced insulin release. Hence, pieces of pancreas from Bufo arenarum were incubated with 2 to 20 mM glucose or preincubated with 2 mM glucose plus the hexokinase and glucokinase inhibitors (50 mM of 2-deoxyglucose and mannoheptulose, respectively) followed by an incubation with different glucose concentrations. The maximal rate of insulin release occurred at 8 mM glucose, while 50% of the release (K(s50)) was observed at 7 mM glucose. Regardless of the glucose concentration employed, pancreas pieces preincubated with 2-deoxyglucose released less insulin than the corresponding controls. On the other hand, mannoheptulose significantly inhibited the release of insulin at high glucose concentrations, having no effect at low glucose concentrations. The blocking effect of these two inhibitors is the first indirect evidence of the existence of the hexokinase/glucokinase enzymic system in the toad pancreas. Since the activity ratio of this system determines the glucose sensitivity of the insulin secretory mechanism, it is concluded that the possible existence of a higher ratio of these enzymes in toad B cells could explain the particular characteristics of glucose sensitivity in this animal, which in turn may explain its low blood glucose concentration.