Glucokinase is the beta-cell glucose sensor, i.e., the site in glucose metabolism that determines the glucose set-point (sensitivity) for insulin secretion. Hexokinase is also present, but it normally contributes little to glucose metabolism because of end-product inhibition by glucose 6-phosphate. There is a lowered glucose set-point for insulin secretion in 90% pancreatectomized (Px) diabetic rats. We investigated the mechanism by measuring hexokinase and glucokinase activity in islet extracts. Glucokinase activity was minimally raised in Px islets (Vmax 125% of sham-operated control rats). In contrast, hexokinase Vmax was 250% of the control value, suggesting that the increased hexokinase activity caused the beta-cell glucose hypersensitivity. Additional evidence was obtained with a 40-h fast that was performed because of a previous observation that the inhibitory effect of fasting on insulin secretion was impaired in Px rats. Glucokinase activity fell normally in the Px rats (32 +/- 4% reduction in sham vs. 37 +/- 4% in Px rats) as opposed to hexokinase activity, which was unaffected in either group. In summary, a feature of hyperglycemia is upregulated islet hexokinase activity. The result is that hexokinase assumes partial control over the glucose set-point for insulin secretion. As such, regulatory effects on insulin secretion, such as fasting, that are mediated through glucokinase activity may be altered.
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Hosokawa et al. (1995) studied this question.