An increased sensitivity to glucose was observed in islets pre-exposed for 1 h to glibenclamide (0.1 micromol 1(-1)), but not to tolbutamide (100 micromol l(-1)), as indicated by a shift to the left of the dose-response curve (EC(50) at 5.8+/-0.3 mmol l(-1) glucose vs 10.6+/-0.8 in control islets; n=11, P<0.005). According to this secretory pattern also glucose utilization at 2.5 and 5.0 mmol l(-1) glucose was higher in islets exposed to glibenclamide. Since binding to mitochondria results in an increased enzyme activity, we measured hexokinase (HK) and glucokinase (GK) activity both in a cytosolic and in a mitochondrion-enriched fractions. Cytosolic hexokinase activity was similar in islets exposed to glibenclamide and in control islets but mitochondrial hexokinase activity was significantly increased after exposure to glibenclamide (124+/-7 vs 51+/-9 nmol microgram prot(-1) 90 min(-1), P<0.01), with no change in the enzyme protein content. In contrast, glucokinase activity in the two groups of islets was similar. When in islets < exposed to glibenclamide hexokinase binding to mitochondria was inhibited by the addition of 20 nmol l(-1) dicyclohexylcarbodiimide (DCC), no increase of glucose sensitivity was observed (EC(50) 10.9+/-1.3 mmol l(-1) glucose, n=3, similar to that of control islets). These data indicate that a 1 h exposure to glibenclamide causes the beta cell to become more sensitive to glucose. This increased sensitivity is associated with (and may be due to) an increased hexokinase activity, in particular the mitochondrial-bound, more active, form. This mechanism may contribute to the hypoglycemic action of this drug.