Mechanisms of Control of the Free Ca2+ Concentration in the Endoplasmic Reticulum of Mouse Pancreatic β-Cells

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+)(ER)) and the role of SERCA3 in the control of insulin secretion and ER stress. RESEARCH DESIGN AND METHODS: β-Cell Ca(2+)(ER) 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+)(c)) and Ca(2+)(ER) were simultaneously recorded. Insulin secretion and mRNA levels of ER stress genes were studied. RESULTS: Glucose elicited synchronized Ca(2+)(ER) and Ca(2+)(c) oscillations. Ca(2+)(ER) 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+)(ER) rise in SERCA3(+/+) and SERCA3(-/-) β-cells. In a Ca(2+)-free medium, glucose moderately raised Ca(2+)(ER) from a highly buffered cytosolic Ca(2+) pool. Increasing Ca(2+)(c) with high K elicited a Ca(2+)(ER) 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+)(c) and Ca(2+)(ER) oscillate in phase in response to glucose. Upon Ca(2+)(c) 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.