GLUCOSE REGULATES THE MAXIMAL VELOCITIES OF GLUCOKINASE AND GLUCOSE-UTILIZATION IN THE IMMATURE FETAL-RAT PANCREATIC-ISLET

The cause of the poor secretion of insulin in response to glucose by t he beta-cell in the fetal rat pancreas is thought to be immaturity of the metabolism of glucose. Glucokinase (GK), a key enzyme in glycolysi s, is the glucose sensor that maintains glucose homeostasis in the adu lt beta-cell; its role in the fetal beta-cell has not been determined. The aim of this study was to examine whether GK was functional in pho sphorylation of glucose in the fetal islet, and if so, to determine wh at factors regulated this activity. Similar K-m values were found in b oth fetal and adult islets: 7.4 vs. 7.7 mmol/l. The maximal GK velocit y (V-max) of the fetal islet and the contribution of GK to total gluco se phosphorylation were also not significantly different from their ad ult counterparts. Western blot analysis of protein extracts from fetal and adult islets confirmed the presence of GK at 52 kDa. To determine if glucose had any effect on the V-max of GK, islets were cultured fo r 7 days in medium containing low (1.4 or 2.8 mmol/l), normal (5.6 mmo l/l), or high (11.2 or 16.8 mmol/l) concentrations of glucose. The max imal GK velocity increased linearly with increasing concentrations of glucose (r = 0.93; P < 0.01). To determine whether it was possible to up- and down-regulate V-max of GK, islets were cultured in either a lo w (1.4 mmol/l) or high (30 mmol/l) concentration of glucose for 7 days and then switched to the opposite concentration for a further 3 days. The V-max of GK in the fetal islet was upregulated 3.8-fold when the glucose concentration was raised. Conversely, the V-max was downregula ted 3.6-fold when the glucose concentration was lowered. The same phen omenon was also observed in the adult islet. These data indicate that GK is the glucose sensor for the fetal rat islet, just as it is for th e adult islet. Since glucose did not cause insulin secretion from the fetal islet, it was important to examine whether this substrate had an y effect on its own metabolism, Glucose utilization was estimated, and its V-max was found to increase linearly with increasing concentratio ns of glucose (r = 0.96; P < 0.01). We conclude that the inability of the fetal rat beta-cell to secrete insulin in response to glucose cann ot be explained by immaturity of GK or the glycolytic pathway.