Characterisation of the β-cell glucose sensor in diabetic Goto Kakizaki-rats regarding ontogenetic and circadian variations

The Goto Kakizaki (GK)-rat is an established model of spontaneous non insulin dependent diabetes mellitus. The glucose transporter 2 (GLUT2) and glucokinase (GCK) form the β-cell glucose sensor complex, which is essential for adequate glucose stimulated insulin secretion (GSIS).

Objectives: The aim of our study was to characterise the diabetic situation and the β-cell glucose sensor complex in GK-rats of different ages with regard to circadian variations.

Methods: Samples (blood and pancreata) of 1, 3, 6, 24 and 42 week old GK- and Wistar rats (metabolically healthy controls) were taken either at mid light or within a 3-h interval (L: D=12:12, light on at 0700h). Plasma insulin was determined with a commercial RIA. β-cell GLUT2, GCK and insulin were investigated by real-time RT-PCR and immunohistochemistry. Perifusion experiments were performed with isolated islets of neonate rats.

Results: Body weight of GK-rats was lower at all investigated ages. Hyperglycemia was first seen at an age of 3 weeks and increased with age. We found light-time hypoinsulinemia in GK-rats up to 6 weeks of age. In the latter group a dark-time hyperinsulinemia became clear. Older GK-rats were significant hyperinsulinemic. There is a progressive loss of pancreatic GLUT2- and GCK-mRNA. We report that both show significant diurnal variation in the pancreas of 6 week old rats. A gain of soluble, cytoplasmatic GLUT2-protein was observed in GK-rat islets. GCK-immunoreactivity was reduced, more heterogenous and showed perinuclear accumulation in GK-rat β-cells. The intensity of insulin immunolabeling was diminished in GK-rat β-cells. Perifusion experiments showed a reduced GSIS with an impaired first phase in GK-rat islets.

Conclusion: Reduced islet-insulin content, impaired glucose sensing and GSIS are early events in the GK-rat. Diabetic metabolic state is early manifest and progressive with age. The circadian expression patterns of pancreatic glucose sensor implicates a higher β-cell glucose phosphorylation capacity and by that a stronger GSIS within the time period of main food intake.