Mechanisms of control of the free Ca2+ concentration in the endoplasmic reticulum of mouse pancreatic β-cells: interplay with cell metabolism and [Ca2+]c and role of SERCA2b and SERCA3.

OBJECTIVE

Sarco-endoplasmic reticulum Ca(2+)-ATPase 2b (SERCA2b) and SERCA3 pump Ca(2+) in the endoplasmic reticulum (ER) of pancreatic β-cells. We studied their role in the control of the free ER Ca(2+) concentration (Ca(2+)) and the role of SERCA3 in the control of insulin secretion and ER stress.

RESEARCH DESIGN AND METHODS

β-Cell Ca(2+) of SERCA3(+/+) and SERCA3(-/-) mice was monitored with an adenovirus encoding the low Ca(2+)-affinity sensor D4 addressed to the ER (D4ER) under the control of the insulin promoter. Free cytosolic Ca(2+) concentration (Ca(2+)) and Ca(2+) were simultaneously recorded. Insulin secretion and mRNA levels of ER stress genes were studied.

RESULTS

Glucose elicited synchronized Ca(2+) and Ca(2+) oscillations. Ca(2+) oscillations were smaller in SERCA3(-/-) than in SERCA3(+/+) β-cells. Stimulating cell metabolism with various [glucose] in the presence of diazoxide induced a similar dose-dependent Ca(2+) rise in SERCA3(+/+) and SERCA3(-/-) β-cells. In a Ca(2+)-free medium, glucose moderately raised Ca(2+) from a highly buffered cytosolic Ca(2+) pool. Increasing Ca(2+) with high [K] elicited a Ca(2+) rise that was larger but more transient in SERCA3(+/+) than SERCA3(-/-) β-cells because of the activation of a Ca(2+) release from the ER in SERCA3(+/+) β-cells. Glucose-induced insulin release was larger in SERCA3(-/-) than SERCA3(+/+) islets. SERCA3 ablation did not induce ER stress.

CONCLUSIONS

Ca(2+) and Ca(2+) oscillate in phase in response to glucose. Upon Ca(2+) increase, Ca(2+) is taken up by SERCA2b and SERCA3. Strong Ca(2+) influx triggers a Ca(2+) release from the ER that depends on SERCA3. SERCA3 deficiency neither impairs Ca(2+) uptake by the ER upon cell metabolism acceleration and insulin release nor induces ER stress.