Evidence for defective glucose sensing by islets of fa/fa obese Zucker rats

Abstract

The hypothesis that a defect in glucose sensing by islets of fa/fa Zucker rats contributes to hyperinsulinemia in these animals was tested. Islets from lean and fa/fa rats were isolated by collagenase digestion and step-density gradient purification and then cultured overnight in Dulbecco's modified Eagle's medium containing 12.5 mM glucose. Obese rat islets were more sensitive to hypoglycemic glucose levels with half-maximal effective concentration (EC50) of 5.6 mM compared with an EC50 of 8.2 mM for lean rat islets. In contrast, responsiveness of both phenotypes to alpha-ketoisocaproate and quinine was similar. Mannoheptulose did not inhibit insulin secretion from fa/fa islets, although inhibitors of later events in the stimulus-secretion coupling pathway were normally inhibited by iodoacetate and diazoxide. Finally, starvation in vivo and culture of islets in low glucose concentrations (5 mM) in vitro both decreased glucose-stimulated insulin secretion from lean but not fa/fa rat islets. We conclude that fa/fa rat islets have an exaggerated insulin response to hypoglycemic stimuli, possibly as a result of a defect in B-cell glucokinase function.

Effect of adrenalectomy on the development of a pancreatic islet lesion in fa/fa rats

“…Results in control fa/ fa rat islets confirm our previous results of a leftward shift in the K m and insensitivity to MH compared with lean controls [11]. A role for glucokinase is further supported by earlier results indicating no differ- (Table 3) ence in sensitivity to non-glucidic secretagogues including c~-ketoisocaproic acid, L-arginine, and quinine [9]. Glucokinase is a low-affinity glucose-phosphorylating enzyme found only in liver cells and pancreatic islet beta cells [10].…”

“…Failure of the insulin response to adapt to changes in ambient glucose is reported in starved adult fa/fa rats in vitro [9] and in vivo [15]. Furthermore, fa/fa rats exhibit fasting hyperinsulinaemia despite having blood glucose levels similar to lean rats [6].…”

“…Increased sensitivity to glucose of the pancreatic beta cells of fa/fa rats in both the preobese [3][4][5] and adult stages [6][7][8][9] is observed. This increased response offa/fa rats to glucose stimulation might be related to changes in the primary glucose sensing mechanisms of the pancreatic islet beta cells because sensitivity to other stimulants is not different from that observed in lean rats [9].Glucokinase, for which there is growing evidence as the primary pancreatic beta-cell glucose sensor [10], is reported to have increased sensitivity to glucose in fa/fa rat isolated islets [11]. The activity of glucokinase in pancreatic beta cells is regulated by glu-…”

“…Furthermore, fa/fa rats exhibit fasting hyperinsulinaemia despite having blood glucose levels similar to lean rats [6]. Mannoheptulose (MH), which competitively inhibits glucokinase to block glucose-induced insulin release [16], is found to have a reduced effect on insulin secretion in isolated islets from adult and weanling fa/fa rats [9,17], a phenomenon also reported in other genetically obese rodents, including corpulent (cp/cp) rats [18]. These changes in insulin secretory regulation may reflect early defects in pancreatic islet glucokinase function.…”

Increased neuronal nitric oxide synthase dimerisation is involved in rat and human pancreatic beta cell hyperactivity in obesity

“…The fa/fa rats compensate for insulin resistance partially through an increase in beta cell mass [3]. In addition, a primary defect in the beta cells also occurs, as neither in vivo starvation nor in vitro culture at low glucose concentration is able to decrease glucose-stimulated insulin secretion [4]. Furthermore, an innate hyperactivity of pancreatic beta cell response to feeding has been proposed to occur at least in some obese humans [5].…”