The stimulus-secretion coupling of glucose-induced insulin release. Fasting-induced adaptation of key glycolytic enzymes in isolated islets

Abstract

The rate of glucose and fructose 6-phosphate phosphorylation in islet homogenates is reduced by prior fasting of the donor rats. In fed rats, the velocity of glucose phosphorylation at increasing glucose concentrations (0.1 to 100 mM) is compatible with the presence of two enzyme activities. A preferential effect of fasting upon the high Km enzyme activity can be documented either at low ATP concentration which enhances the fractional contribution of the high Km enzyme activity, or in the presence of glucose 6-phosphate, which suppresses the low Km enzyme activity. Islet phosphofructokinase activity was characterized by inhibition by citrate or high ATP concentrations, and relief from ATP inhibition by AMP. Fasting reduces the activity of phosphofructokinase without altering its sensitivity to ATP and AMP. Cyclic AMP fails to overcome the effect of fasting upon phosphofructokinase. The activity of phosphoglucoisomerase is unaffected by fasting. The fasting-induced adaptation of key glycolytic enzymes could account, in part at least, for reduced metabolism of glucose in islets from fasted rats.