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 b etween basal glycemia in the toad and the B-cell threshold for glucose-indu ced insulin release. Hence, pieces of pancreas from Bufo arenarum were incu bated with 2 to 20 mM glucose or preincubated with 2 mM glucose plus the he xokinase and glucokinase inhibitors (50 mM of 2-deoxyglucose and mannoheptu lose, respectively) followed by an incubation with different glucose concen trations. The maximal rate of insulin release occurred at 8 mM glucose, whi le 50% of the release (K-s50) was observed at 7 mM glucose. Regardless of t he glucose concentration employed, pancreas pieces preincubated with 2-deox yglucose released less insulin than the corresponding controls. On the othe r hand, mannoheptulose significantly inhibited the release of insulin at hi gh 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 p ancreas. Since the activity ratio of this system determines the glucose sen sitivity of the insulin secretory mechanism, it is concluded that the possi ble existence of a higher ratio of these enzymes in toad B cells could expl ain the particular characteristics of glucose sensitivity in this animal, w hich in turn may explain its low blood glucose concentration. (C) 2000 Acad emic Press.