Intracellular calcium ion response to glucose in β-cells of calbindin-D_28k nullmutant mice and in βHC13 cells overexpressing calbindin-D_28k

This article describes studies on the glucose-induced responses of intracellular Ca²⁺ concentration ([Ca²⁺]_i), insulin release, and redistribution of calbindin-D_28k, a calcium-binding regulatory protein, in β-cells of pancreatic islets of calbindin-D_28k knockout (KO) and wildtype mice (C57BL6) as well as in βHC-13 control cells and βHC-13 CaBP40 cells (β-cell line overexpressing calbindin-D_28k). Upon increasing the glucose concentration from 2.8 to 30 mM, islets of KO mice showed a significantly greater increase in [Ca²⁺]_i (mean increase in [Ca²⁺]_i, i.e., Δ[Ca²⁺]_i, was 296 nM) compared with wild-type mice (Δ[Ca²⁺]_i = 97 nM). βHC-13 CaBP40 cells showed little change in [Ca²⁺]_i upon elevation of glucose from 5.5 to 32.7 mM, whereas βHC-13 control cells exhibited significant increases in [Ca²⁺]_i (Δ[Ca²⁺]_i = 510 nM). Similarly, upon addition of 30 mM glucose, the rate of insulin release increased from 25.2 (basal rate) to 145.2 pg/mL/min in βHC-13 control cells, whereas in βHC-13 CaBP40 cells the rate of insulin release was only 27.5 pg/mL/min in high glucose. Thus, levels of calbindin-D_28k in β-cells affect both [Ca²⁺]_i and insulin secretion in response to glucose. The three-dimensional reconstruct of confocal immunofluorescent images showed that glucose caused redistribution of calbindin-D_28k resulting in co-localization in the region of L-type voltage-dependent calcium channels (VDCC). This colocalization may be an important regulatory function concerning Ca²⁺ influx via L-type VDCC and exocytosis of insulin granules.