Zinc ions in the endocrine and exocrine pancreas of zinc deficient rats.

OBJECTIVE: Zinc deficiency is a problem world-wide. Zinc and insulin are intimately related, and a reduced zinc intake may affect glucose metabolism. The present study investigates how subclinical zinc deficiency in rats affects glucose metabolism and zinc distribution in the pancreas. METHODS: Glucose metabolism was evaluated by blood-glucose, serum insulin, homeostasis model assessment (HOMA), and intraperitoneal glucose tolerance tests. Immersion zinc-sulphide autometallography (iZnSAMG) was used to describe zinc ion distribution. RESULTS: After 4 weeks on a zinc deficient diet (<10 ppm), the zinc deficient rats had a slightly impaired glucose metabolism characterized by significantly increased blood-glucose levels. No differences in serum insulin, insulin resistance, beta-cell function were observed. The zinc deficient rats had significantly decreased serum zinc without any clinical signs of zinc deficiency. Zinc ion staining intensity of the islets of Langerhans was unaffected by the zinc deficiency. In contrast, the acinar cells in the exocrine pancreas appeared depleted of iZnSAMG grains in the zinc deficient rats when compared with their controls. Though statistically non-significant, a reduction in total zinc of the pancreas was found. CONCLUSIONS: The present findings suggest that the endocrine pancreas is able to compensate for the subclinical zinc deficiency as it maintains an adequate zinc ion level in the secretory vesicles for insulin storage. The exocrine pancreas lacks this ability; it exhibits decreased levels of zinc ion staining as a consequence of 4 weeks of reduced zinc intake.

Zinc fluxes during acute and chronic exposure of INS-1E cells to increasing glucose levels.

Zinc in beta-cell secretory vesicles is essential for insulin hexamerization, and tight vesicular zinc regulation is mandatory. Little is known about zinc ion fluxes across the secretory vesicle membrane and the influence of changes in the extracellular environment on vesicular zinc. Our study aim was to investigate the effect of acute and chronic exposure to various glucose concentrations on zinc in secretory vesicles, the relation between zinc and insulin, and the presence of two zinc transporters, ZnT1 and ZnT4, in INS-1E cells. Zinc ions were demonstrated and semi-quantified using zinc-sulfide autometallography. Insulin content and secreted insulin were measured. Measurements were made on INS-1E cells after exposure to 2.0, 6.6, 16.7, and 24.6 mmol/l glucose for 1, 24, and 96 hours. 1h: Increasing glucose resulted in no changes in intravesicular zinc ions at 2, and 24.6 mmol/l glucose, but a slight increase at 16.7 mmol/l glucose. 24 and 96 h: Increasing glucose led to decreased vesicular zinc ion content accompanied by a decrease in insulin content. ZnT1 and ZnT4 were present in the cytoplasm. Our results demonstrate that intra-vesicular zinc ions respond to changes in the extra-cellolar glucose concentration, especially during chronic high glucose concentrations, where the content of vesicular zinc ions decreases.

Zinc ions in beta-cells of obese, insulin-resistant, and type 2 diabetic rats traced by autometallography.

Zinc ions in the secretory granules of beta-cells are known to glue insulin molecules, creating osmotically stable hexamers. When the secretory granules open to the surface, the zinc ion pressure decreases rapidly and pH levels change from acid to physiological, which results in free insulin monomers and zinc ions. The released zinc ions have been suggested to be involved in a paracrine regulation of alpha- and beta-cells. Since zinc is intimately involved in insulin metabolism and because zinc homeostasis is known to be disturbed in type 2 diabetes, we decided to study the ultrastructural localisation of zinc ions in insulin-resistant and type 2 diabetic rats as compared to controls. By means of autometallography, the only method available for demonstrating zinc ions at ultrastructural levels, we found zinc ions in the secretory granules and adjacent to the plasma membrane. The membrane-related staining outside the plasma membrane reflects release of zinc ions during exocytosis. No apparent difference was found in the ultrastructural localisation of zinc ions when we compared the obese Zucker (fa/fa) rats, representing the insulin resistance syndrome, and the GK rats, representing type 2 diabetes, with controls. This suggests that the ultrastructural localisation of zinc ions is unaffected by the development of type 2 diabetes in rats in a steady state of glycaemia.

Autometallography allows ultrastructural monitoring of zinc in the endocrine pancreas.

Zinc is intimately involved in insulin metabolism, its major known role being the binding of insulin in osmotically stable hexamers in beta-cell granules. To investigate the anatomical distribution of zinc ions necessary for insulin binding we examined the rat pancreas by autometallography (AMG). AMG demonstrates chelatable zinc and is a sensitive marker for zinc in vesicles and also a surrogate marker for recently described zinc pumps regulating intravesicular zinc metabolism. Zinc ions were found in alpha- and beta-cell granules, primarily in the periphery of the granules. Only occasionally was zinc seen in other islet cell types. AMG allows the study of the microscopic and ultrastructural localisation of free zinc ions in the pancreas. The applicability of the method at the ultrastructural level in particular makes AMG a very sensitive tool in future studies on the role of zinc ions in the pancreas.